Polyurethane nanocomposite based gas barrier films, membranes and coatings: A review on synthesis, characterization and potential applications

Accepted Manuscript Polyurethane nanocomposite based gas barrier films, membranes and coatings: A review on synthesis, characterization and potential applications Mangala Joshi, Bapan Adak, B.S. Butola PII: DOI: Reference:

S0079-6425(18)30054-9 https://doi.org/10.1016/j.pmatsci.2018.05.001 JPMS 515

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Progress in Materials Science

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29 December 2017 14 May 2018

Please cite this article as: Joshi, M., Adak, B., Butola, B.S., Polyurethane nanocomposite based gas barrier films, membranes and coatings: A review on synthesis, characterization and potential applications, Progress in Materials Science (2018), doi: https://doi.org/10.1016/j.pmatsci.2018.05.001

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Polyurethane nanocomposite based gas barrier films, membranes and coatings: A review on synthesis, characterization and potential applications

Mangala Joshi*, Bapan Adak, B.S.Butola Department of Textile Technology, Indian Institute of Technology Delhi, New Delhi, India-110016 *Corresponding email id: [email protected]

Abstract Polyurethane (PU) and its nanocomposites based gas barrier films and coatings have established a distinctive position among various technologically important materials due to their large-scale potential applications. This review aims at highlighting the gas barrier property of polyurethane nanocomposite (PUNC) based films, membranes and coatings containing platelet-shaped fillers such as clays and graphene in PU matrix. The other fillers such as CNT, POSS, metal nanoparticles and nanocellulose have also been reviewed for their contribution in improving gas barrier property of polymers. The probable transport-mechanism of small gas molecules through PU and PUNCs have been discussed. There is also a discussion on basic PU-chemistry and effect of structure and morphology of PU on its gas barrier property. Various factors which influence the gas permeability through PU and PUNC films and coatings have been scrutinized. Some aspects of improving the gas barrier property of PUNCs have also been discussed. An emphasis is given on various proposed models for prediction of gas permeability through polymer nanocomposites. It also reviews the existing literature related to modeling and prediction of gas permeability of different PUNC membranes, films and coatings. Finally, special attention has been paid to the potential industrial applications of PUNC based films and coatings.

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Keywords: Polyurethane nanocomposite, clay, graphene, gas barrier, film, coating 1. Introduction Polyurethanes (PUs) are extraordinary and versatile polymeric materials having exceptionally balanced properties such as excellent flexibility, elasticity, wide range of hardness, good tear strength, good abrasion resistance, good chemical resistance, heat sealability and wide processing window [1-2]. By virtue of their tailorable properties, PUs have a variety of applications such as coatings, adhesives, sealants, foams (flexible or rigid), paints, varnishes, leathers, rubbers, fibers, films, biomimetic materials and many more [1, 3-4]. Despite their versatile properties, a major drawback of PUs is their inherent permeability to gases and vapors including oxygen (O2), nitrogen (N2), carbon dioxide (CO2), helium (He), water and organic vapors. In comparison to other polymeric barrier systems, gas barrier property of PUs are much inferior as shown in Table 1. However, the permeability of different gases through PU films/membranes varies widely depending on structure and morphology of PU [5-9]. Recently, some polymers such as biaxially oriented polyester (BoPET, Mylar ®), ethylene vinyl alcohol copolymer (EVOH) and polyvinylidene Chloride (PVDC, Saran®) have been extensively used in gas barrier applications. However, most of these polymers, lack properties such as lowtemperature flexibility, weatherability, flex fatigue, adhesion etc. and also many of them are not heat sealable [3]. Polyurethane, on the other hand, scores well on all these aspects, though its gas barrier properties need to be improved. A good gas barrier property is one of the stringent requirements for numerous special applications where PU based membranes, films or coatings are extensively used such as coated or laminated envelop for lighter-than air (LTA) applications (hot air balloon, aerostat, airship 2

etc), food packaging, automotive applications (primer coating, tire etc), biomedical applications (dialysis membranes, wound dressings, encapsulating membranes, catheters etc), flame retardant coatings, anticorrosion coatings and so on [1-2, 10-11]. However, the high permeation of gases through PU based films and coatings seriously affect their service or performance in such applications. PU is a block copolymer consisting of hard and soft segments. Due to thermodynamic instability between these segments, a phase separation occurs, which is the main reason of higher gas permeability through PU films or coatings. The gas barrier property of PU can be improved to some extent by modifying the PU-chemistry or by changing its morphology, by proper selection of components, composition and preparation conditions [12-16]. However, these approaches do not impart satisfactory improvement in gas barrier property. The gas barrier property of PU can be improved significantly by incorporation of impermeable lamellar or nanoplatelet like fillers such as clay or graphene, having high aspect ratio [17-19]. Incorporation of these nanoplatelets alters the diffusion path of gaseous molecules. Thus, the permeant molecules follow more tortuous and longer path during diffusional passage through PUNC films, resulting in better gas barrier property [6-7, 20]. When these inorganic phases (clay, graphene etc) are well dispersed, a significant improvement (up to 500 times) in gas barrier property of host matrix is observed even at very small loadings. Such fillers, which increase the tortuosity for diffusion of gas molecules through the polymer, are often called passive fillers. The efficacy of a passive inorganic filler is strongly dependent on its volume fraction, aspect ratio, distribution and orientation. In addition to these passive fillers, there are some reactive fillers (such as functionalized graphene and CNT, POSS, nanocellulose) which can react or form crosslinks with host matrix, which leads in improvement of gas barrier property of polymers like PU [21-22]. The incorporation of these 3

nanomaterials into PU matrix is also found to be effective in improving its mechanical, physical, thermal and many other functional properties [23-25]. Some recent reviews have focused on the synthesis of polymer-clay [26-29] and polymergraphene [30-32] nanocomposites and few of them have highlighted gas barrier properties of polymer nanocomposite films/coatings. However, no review specifically covers the gas barrier properties of PUNC films/membranes/coatings in detail. In this context, a detailed discussion about the structure-morphology and gas barrier properties of PU and its nanocomposite is of special significance. This review aims at rendering a thorough understanding of – (i) transport mechanism of gases through polymer nanocomposites, (ii) the factors affecting gas barrier property of PUNCs, (iii) some aspects for improving this property, (iv) different models for predicting gas permeability through PUNCs, (v) comparison of experimental as well as modelpredicted gas permeability of PU nanocomposites and (iv) potential applications of PUNC based gas barrier films, membranes and coatings. 2. Mechanism of gas transport through polymer and polymer nanocomposites films/membranes The transport of gases through a polymeric membrane or film is a complex process. The gases are initially adsorbed or become soluble on the surface of the membrane/film and then diffuse through it. During adsorption on an amorphous polymeric membrane or film, the gas molecules are positioned in the free volume spaces created by the Brownian motion of polymer chains or thermal agitation. Whereas, during diffusion the gas molecules pass through the connected free spaces. Therefore, the diffusion depends on size and number of the free volume spaces in the polymer. The gas diffusivity depends on the dynamic free volume created due to the segmental 4

motion of the polymeric chains. While the solubility depends on the interaction between the gas molecules and polymeric chains [19, 33]. In case of semicrystalline polymeric membranes/films, the size and shape of the crystallites, their structure, orientation and degree of crystallinity influence the gas permeation process. PU is a semicrystalline polymer containing a very low fraction of crystalline portion or hard segments. With increasing soft segments in PU structure, more free volume is generated, resulting in increase in gas diffusion rate through the PU films/membranes. In respect of gas permeation, the polymer nanocomposites behave similar to a semicrystalline polymer [26]. In gas barrier polymer nanocomposites, impermeable nano-platelets are dispersed in a polymer matrix which decrease the diffusion of gas molecules by creating a more tortuous path for gas molecules. In comparison, the micro-additive reinforced polymeric composites are not as effective in enhancing the gas barrier property of polymer [11], as illustrated in Fig. 1. The most acceptable theory of gas transmission through a nonporous polymeric barrier is known as activated diffusion mechanism. Gas transmission by activated diffusion mechanism is affected by the type of plastic used, temperature, humidity and molecular size of mobile gases [34]. The quantity of gas flow is proportional to the biquadrate of the pinhole size. Generally, gas flow rate is also proportional to the number of pinholes present and the pressure difference on two sides of the film [35]. Interestingly, Madhavan et al. [36] observed that the permeabilities of nitrogen (N2) and oxygen (O2) through poly(dimethylsiloxane)-urethane (PDMS-PU) membranes were independent of pressure, but CO2 permeability increased with pressure. In another study [22], it was reported that N2 and O2 permeabilities of all PDMS-PU and polypropylene glycol-PU (PPGPU) membranes decreased with increasing penetrants pressure. Compression of the polymer matrix with increasing pressure might be responsible for reduction of free volume for gas 5

diffusion, resulting in reduced permeabilities. Generally, gas flow rate is inversely proportional to the viscosity of the gas and film thickness [37-39]. 3. Measurement of gas permeability through polymeric membrane/films Gas permeation through a polymeric membrane or film is a very complex procedure and consist of three steps as follows (see Fig. 2) [26]: i)

Adsorption of gas molecules on the surface of membrane/film

ii)

Diffusion of gas molecules through the membrane/film

iii)

Desorption of the gas molecules from the other surface of the membrane/films

Concerning the theories, the transmission of gas through polymeric films or membranes follows diffusion models related to Henry’s and Fick’s law. Fig. 2 illustrates diffusion through a polymeric film or membrane of uniform thickness l, where the permeant concentration is c (c1 and c2 are the higher and lower permeant concentrations, respectively, c1>c2) and the permeant pressure is p (p1 and p2 are the higher and lower permeant pressures, respectively, p1>p2). Now, according to Fick’s first law [40]: ………………………………………………………………… (3.1) Where, J is the permeant (gas or vapor) flux (mol.cm–2.s–1), D is the diffusion constant (in cm2.s– 1

), Δc= concentration difference (in mol/cm3) across the film or membrane of thickness l (in cm).

‘D’ express the diffusional speed of permeant through the polymer. In steady state condition, both ‘c’ and ‘p’ obey Henry’s law. However, it is more convenient to measure the gas pressure ‘p’ than ‘c’. Thus, by replacing Δc by Δp we get: ……………………………………………………………….. (3.2)

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Where, ‘Δp’ (in atm) is the pressure difference across the film or membrane and ‘S’ is the solubility or sorption coefficient (in mol.cm–3.Pa–1), which expresses the amount of permeant present in the polymer. If ‘S’ is independent of the concentration, the gas permeability coefficient, P (in mol.Pa–1.cm–1.s– 1

), can be expressed as below: ………………………………………..………………… (3.3)

The ratio of P/l is generally indicated by q and is named as permeance. This type of diffusion where ‘D’ and ‘S’ do not depend on the permeant concentration is called as Fickian behavior [41]. However, in some cases, the diffusional behavior of gas molecules depends on the permeant concentration and that is called non-Fickian behavior. For example, where some interaction between polymers and permeants take place, like the interaction of water vapor with hydrophilic film [42]. The gas permeability of a membrane/film can be measured by direct measurement in a cell or by indirect sorption/desorption experiment. In the direct measurement, the membrane or film separate the cell in two compartments, as shown in Fig. 3. On one side of the compartment, gas is introduced and a constant pressure is maintained. The gas flux density in the other compartment is monitored. Initially, it is a non-steady state process and the gas permeability coefficient is noted when it reaches a steady-state distribution [26]. Alternatively, it can be performed in a Wicke-Kallenbach device which is also similar to the cell shown in Fig. 3. However, in this method, a mixture of gases such as CO2 and N2 is introduced on one side, while on another side a reference gas such as N2 is kept and the permeation of test gas (CO2) is measured maintaining an equal pressure in both compartments [43]. The selectivity of two gases is measured by taking the ratio of their permeabilities into consideration. 7

In sorption/desorption experiments, both the diffusion coefficient and the solubility coefficient are measured and gas permeability is measured using Eq. (3.3). Initially, the membrane remains under vacuum and then a gas is introduced maintaining a constant pressure. The gas dissolves in and diffuses into the membrane/film and the weight gain is estimated as a function of time using a balance. The diffusion coefficient is calculated using a specific formula, assuming a uniform distribution of gas in a flat membrane of thickness ‘d’. While the solubility coefficient is calculated by measuring the concentration of a soluble gas as a function of pressure [44]. The researches have proposed different models to measure permeability by calculating the diffusion coefficient and solubility, which will be discussed later in detail. 4. Polyurethane Before delving into gas barrier property of PUs, a brief discussion on basic PU-chemistry, its raw materials and synthesis, structure-morphology and classification is pertinent. 4.1.Basic chemistry and raw materials PUs are basically block copolymers which are formed by poly-addition reaction of three main components: a diol or polyol, a diisocyanate or polyisocyanate and a chain extender [45]. Sometimes catalysts are also used in the synthesis. In practice, only few isocyanates are commonly used to prepare PUs, which are mostly aromatic compounds such as 4,4′methylenebis(phenyl isocyanate) (MDI), toluene diisocyanate (TDI) and some aliphatic compounds such as hexamethylene diisocyanate (HDI) or cycloaliphatic compounds such as 4,4′-diisocyanate dicyclohexylmethane, (H12MDI or hydrogenated MDI) [2]. More recently, because of the high reactivity of isocyanates, the blocked isocyanates are being preferred due to their better stability. Blocked isocyanates do not react until they are exposed to their deblocking 8

temperature and after deblocking, they react with hydroxyl or amine functionalized co-reactants to form thermally stable urethane or urea bonds, respectively [46]. In comparison to diisocyanates, the versatility of polyols is much higher in term of chemical structure, functionality and molecular weight. However, polyether and polyester polyols are most commonly used in the synthesis of PUs. The effect of structure and characteristics of polyols on properties of PU has been thoroughly investigated in many literature [12, 47-50]. Fig. 4 shows the basic reaction for synthesis of PU as well as the schematic of PU-structure containing blocks of hard and soft segments. In Fig. 4, the chemical structure of ‘R’ may be varied to obtain PUs of a wide range of properties. Moreover, by using reactants having three or more functionalities, branched or cross-linked PUs may be formed. The properties of PU can be tailored as per requirements using right di or polyol, di or polyisocyanate and chain extender (diol or diamine) as raw materials. During polymerization, the aromatic diisocyanates generally show higher reactivity than aliphatic or cycloaliphatic diisocyanates. Different diisocyantes have different contributions towards properties of PU. For example, the UV and oxidative stability of aliphatic PUs are better than aromatic PUs, but it is less rigid than aromatic PUs [46]. 4.2.Synthesis of PU Two main methods for preparation of PU are: i)

One-shot process: In this process, PU synthesis takes place in one step where the raw materials (polyol, diisocyanate, chain extender and catalyst) are mixed simultaneously. It is a very exothermic reaction and generally requires a similar reactivity for different hydroxy and isocyanate compounds.

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ii)

Prepolymer process: It is a two-step process. In the first step, the polyol and diisocyanate are reacted to form a prepolymer of intermediate molecular weight of about 20,000. The prepolymer can be OH-terminated or NCO-terminated depending on the stoichiometry of the raw materials. In the second step, the prepolymer is reacted with a chain extender (diamine or diol) to obtain a high molecular weight PU [45].

4.3.Classification of PUs PU is a unique category of plastics which may be thermosetting or thermoplastic, rigid and hard or flexible and soft, solid or open cellular type with great property variances. However, high elasticity, flexibility, reformability and versatile properties of thermoplastic polyurethane (TPU), makes it suitable for wide range of industrial applications [2]. PUs can be classified mainly in two ways. On the basis of type of diisocyanate used, PU can be classified into two categories: (i) aromatic PU and (ii) aliphatic PU. Aliphatic PUs generally provide good weather resistance, excellent adhesion property and excellent optical clarity. On the other hand, aromatic PUs have high strength and toughness, but weather resistance property is not good as aliphatic PU grades [46]. Based on type of polyol used, PUs are classified mainly into three categories: (i) polyester based, (ii) polyether based and (iii) polycaprolactone based. Polyester based PUs provide good mechanical properties, thermo-oxidative stability, abrasion resistance, chemical resistance, but show poor hydrolytic stability. On the contrary, polyether based PUs give excellent hydrolytic stability and flexibility, but their thermo-oxidative stability is not as good as that of polyester based PU [7,51]. Polycaprolactone based PUs combine inherent properties of toughness and resistance of polyester based PUs with low-temperature performance and a relatively high 10

resistance to hydrolysis of polyether based PUs [52-53]. In case of thermoset PUs more varieties of PU are available based on the type of polyol used such as acrylic based PU [54], polycarbonate based PU [55], polybutadiene based PU [56], castor oil based PU [57] etc. 4.4.Structure and morphology of PU To fulfill the requirements of specific applications, the structure-property correlations of PU and the influencing factors have to be well understood. The structure, morphology and properties of the PU films or coatings depend mainly on the nature of the raw materials, their stoichiometric ratio and the reaction conditions [12]. The PUs and polyurethane ureas (PUUs) are basically multi-block copolymers containing hard and soft segments (see Fig. 4). These two thermodynamically incompatible segments cause phase separation during polymerization reaction, forming a supermolecular structure where hard and soft segments remain partially soluble in each other. Their solubility strongly depends on the interaction of components [58-60]. The soft blocks of PU are built out of a polyol and an isocyanate, remain in a rubbery state that is responsible for the flexibility and elastomeric character of PU. The soft segments are much longer and flexible than the hard segments and exhibit very low intermolecular forces. Whereas, the hard blocks which are constructed from a chain extender and isocyanate, exist in a crystalline state or an amorphous glassy state, are responsible for rigidity and physical performances of PU. The urethane groups are capable of forming H-bonding with each other. The hard segments exhibit very high inter molecular forces such as hydrogen bonding and higher crystallinity than soft segments [47,61]. PU shows very interesting mechanical, elastic, thermal and gas barrier properties because of their chemical structure and the extent of phase separation among hard and soft segments [12,61]. 11

4.5.Effect of structure and morphology of PU on gas barrier property The transport behavior of any gaseous molecule varies through a polymer depends on its chemical as well as physical structure and morphology. The parameters which control the morphology of PU and therefore gas permeability of PU films or membranes arei)

Chemical composition and molecular weight of raw materials

ii)

Hard/soft segment ratio

iii)

Crystallinity

iv)

Orientation and packing of polymer chains

v)

Glass transition temperature (Tg) and segmental motion of PU chains

vi)

Free-volume content

vii)

Density of PU

viii)

Degree of cross-linking

ix)

Surface characteristics

The chemical composition in PU determines the degree of phase separation and nature of chain packing in PU. Specifically, many studies have been reported on polyester or polyether based PU or PUU films/membranes showing the relationship between the structure of PU and gas permeabilities through PU films [47-50]. The gas permeability through a PU based films/membranes can be controlled by tuning the ratio of hard and soft domains [51,62]. Generally, the gas barrier and mechanical properties of PU improve with increasing percentage of hard-segments [13-14,16,46,63]. Many researchers suggested that it is a result of the decrease of both free volume size as well as fractional free-volume content with the increase in hardsegment content which has a higher Tg than soft-segments [13,15,64-65]. PUs with higher Tg 12

possess lower segmental mobility and therefore it have lower diffusivity [66]. The hard segments act as filler particles as well as crosslinker, restraining the motion of soft segments which lead to better gas barrier property. In contrary, with increasing soft-segments in PU, the tendency of phase separation increase resulting in increase in gas permeability value [67-68]. The rigidity of PU is highly dependent on the type of isocyanate used in the synthesis. Aromatic isocyanates provide much higher rigidity in comparison to aliphatic ones [46]. In an interesting study, Madhavan et al. [36] studied the effect of different isocyanates (HMDI, TDI and MDI) on gas permeability through a series of Poly(dimethylsiloxane-urethane) (PDMS-PU) membranes (see Fig. 5). There are two symmetric and ordered aromatic rings in MDI whereas, TDI has only one aromatic ring with methyl substituent. As a consequence, the MDI based PDMS-PU membrane is more rigid, densely packed and has highly ordered structure as compared to TDI based PDMS-PU membranes. Therefore, although both TDI and MDI are aromatic isocyanates, the MDI based PDMS-PU shows lower permeability to gases (O2, N2 and CO2) as compared to the other one. On the other hand, compared to both aromatic PDMS-PUs, the aliphatic isocyanate (HMDI) based PDMS-PU shows lower permeability to gases mainly due to two possible reasons. The first reason might be that on evaporation of the solvent, the PU chains collapse more easily in case of HMDI based PU, resulting in less fractional free volume. The second possible reason is better compatibility between hard and soft segments due to the nonpolar nature of hard segments in HMDI based PU, which results in better gas barrier property compared to both aromatic PDMS-PUs [36]. The amount, type, chain length and molecular weight of polyols and chain extenders determine the degree of phase separation and nature of chain packing in PU which ultimately controls the gas permeation through PU films or membranes [51,61-62,67,69]. The degree of phase 13

separation between hard and soft segments, determines the gas permeability through PU films or membranes. Generally, phase separation increases with increasing molecular weight of longchain diols, resulting in inferior gas barrier properties [70-71]. Compared to polyether based PU, a better phase mixing is observed in polyester based PU, because of stronger hydrogen bonding of –NH group of urethane linkage with the carbonyl group (>C=O) of hard segments and ester group of soft segments [70-71], which leads to better gas barrier properties. Ulubayram et al. [14] examined the oxygen permeability through a series of PUs which were produced from toluene diisocyanate (TDI) and polypropyleneglycol (PPG) with varying hard/soft segment ratios. It was reported that with increase in soft segment content, the oxygen permeation increased, while the addition of chain extender caused a drop in oxygen permeability. In an interesting study, Park et al. [61] investigated the separation of gases such as O2/N2 and CO2/N2 through different PU and PUU based membranes. The addition of small amount of poly(dimethylsiloxane) (PDMS) into polyether-based PU matrices led to phase separation in both hard segment (4,4'-diphenlymethane diisocyanate (MDI)/1,4-butanediol (BD)) and soft segment (polyethers), as PDMS has a different solubility parameter . It resulted in higher gas permeabilities and selectivities of O2/N2 and CO2/N2. The increase in gas selectivity suggests the formation of a polyphasic microstructure of polyether-based PU membranes with incorporation of PDMS phase. Whereas, addition of small amount of polyethers into PDMS-based PUU matrix caused decrease in all gas permeabilities, but did not affect the gas selectivities [61]. Kaveh et al. [7] compared the helium gas permeability of different PUs produced by varying the ratio of two different polyols (polyether and polyester based), while the content of diisocyanate, chain extender and catalyst was kept constant. The gas barrier property was found to improve significantly with increasing concentration of polyester based polyol, due to superior 14

intermolecular hydrogen bonding and polar-polar interactions. Moreover, increase in polyester polyol and cross-linker content resulted in higher crystalline domains with increase in Tg (see Fig. 6) which also restricted the diffusion of helium gas through PU films. In another study, Hsieh et al. [47] claim that the degree of crystallinity is the most important variable over Tg or density to influence gas or vapor permeability through PU membranes. The gas permeability decreased consistently with increasing crystallinity in soft segments of all PUcompositions produced by using polyols of various types and molecular weights and having different hard/soft segment distributions. Huang et al. [62] demonstrated that stretching of multilayered PU films under microconfinement may cause the decrease of oxygen permeability due to the increase in polymer chain orientation forming denser “hard-segment domains”. However, stretching beyond 75% (i.e. up to 300%) resulted in increase in oxygen permeability, due to the formation of fragmented “hard-segment domains”, as shown in Fig. 7. The water vapor permeability (WVP) through PU membranes also depends on the hydrophilicity of the soft segment of PU. A lower level of hydrophilicity of soft-segment leads to decrease in WVP [15,69,72]. In an interesting study, Wang et al. (2012) observed that annealing of PU causes a lower storage modulus, lower soft-segment thermal transition temperature, reduction in mechanical properties and higher oxygen gas permeability which was a consequence of dedensification of the soft segment during crystal growth [16]. A similar observation has been made by Xiao et al. [49] for CO2 permeation through PU membranes. Transport phenomena of gases through PU films strongly depends on crosslinking density [73]. The presence of cross linkers causes formation of a cross-linked network based PU after curing which improves tensile strength, abrasion resistance, thermal stability, gas barrier property, mar

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resistance as well as chemical resistance. Thus, many researchers have focused on increasing the cross linking density in PU by using any one of the following options: i)

A short chain polyol of high functionality (greater than 3)

ii)

A trifunctional hydroxyl compound in place of the normal glycol chain extender

iii)

A polyisocyanate in place of diisocyanate

iv)

A Hyperbranched polyamide as chain extender

Cross-linked PUs having high Tg, densely cross-linked and rigid structures have a great potential in improving gas barrier property. In a very interesting study, Maji et al. [17] reported the helium gas barrier property of PU and PU/clay nanocomposites where novel PU structures were derived using different polyol structures (varying from first generation linear polyol to fourth generation hyperbranched polyol), as shown in Fig. 8. In case of fourth generation polyols (containing 64 – OH functional groups, PU40), helium gas barrier property enhanced by 80% compared to trimethylol propane (having 3 –OH functional groups, PUTMP), as shown in Fig. 9. It was a consequence of an increase in the crosslinking density or decrease in chain length between cross linking points in PU structure, with increasing number of functional groups in the hyperbranched polyol. Also, a significant change was observed in the lamellar microstructure of different cured PUs produced by changing the cross-linkers, as detected by AFM (Fig. 9). The domain width of the hilly lamellar structure reduced from 45-80 nm (for PUTMP) to 10-15 nm (for PU40), indicating better dispersion of hard segments in the final matrix of PU40, resulting in significant improvement in helium gas barrier property [17].

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5. Polyurethane nanocomposites (PUNCs) Polymer nanocomposites are two-phase materials which consist of a polymeric matrix and dispersed particles in nanometer scale as reinforcement. In case of PUNC, the polymer matrix is PU based and the reinforcing phase may be any inorganic or hybrid nanomaterial. 5.1.Nanomaterials for enhancing gas barrier property The gas permeability of polymers reduces significantly by incorporation of impermeable platelike nano-fillers such as layered-silicates (clay), graphene, graphene oxide etc. These nanomaterials act mainly as a physical barrier and improve gas barrier property of polymers by increasing ‘tortuous path’ and therefore length for diffusion of gas molecules [1,5-7]. Except these nano-platelets, some other nanoparticles [e.g. silicon dioxide (SiO2) nanoparticles [74-76], titanium dioxide (TiO2) nanoparticles [74], aluminum oxide (Al2O3) [77-78], carbon nanotube (CNT) [79-80], nano-hybrid materials (Polyhedral Oligomeric Silsesquioxane (POSS) [81-82] and nanocellulose [24,83-88] can also improve gas barrier property of polymers. These nanomaterials may react with polymer matrix and change the morphology of polymers to enhance their gas barrier property. More recently, the researchers have discovered that hexagonal boron nitride (h-BN) which is non-carbon, atomically thin 2D analogue of graphene [89-90], also has good potential to enhance the gas barrier properties of polymers [90,91]. However, still now no study reported the use of h-BN and above mentioned different nano-particles for improving the gas barrier property of PU. The discussion however will be confined to only those nanomaterials (clay, graphene, CNT and POSS) which have been used for enhancing the gas barrier property of PU.\

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5.1.1. Layered silicates Layered silicates or clay minerals are very promising nanomaterials for improving mechanical properties, gas barrier property, thermal stability, flame resistance and corrosion resistance of polymeric nanocomposites [92]. The most commonly used clays are 2:1 layered-silicates or phyllosilicates such as montmorillonite (MMT), saponite, bentonite and hectorite [93]. The crystallographic structure of 2:1 layered silicates consists of several layers, where, in a particular layer an octahedral sheet of alumina or magnesium is sandwiched between two tetrahedral sheets of silicate (Fig. 10). The clay layers are stacked by weak van der Waal forces and the claygalleries are occupied by some exchangeable cations or moisture [29,94]. 5.1.2. Modified layered-silicates Being hydrophilic in nature, the pristine clays are not compatible with most of the hydrophobic polymers. Therefore, clays are modified by long-chain cationic surfactants such as primary, secondary, tertiary or quaternary alkyl-ammonium or alkyl-phosphonium cations [93,95]. The layered-silicates treated by organic modifiers are called organo-modified clays. Owing to the incorporation of alkyl chains in the clay galleries, the inter-layer spacing (d-spacing) increases and inter-layer density decreases, which can be characterized by wide-angle X-ray diffraction (WAXD) [6,96-97]. The inter-layer spacing of modified clay depends mainly on packing density, temperature, alkyl chain length and arrangement of alkyl-chains with respect to silicate layers [29]. Depending on charge density of the alkylammonium surfactant and silicates layers, various arrangements of onium ions are possible such as a parallel monolayer, a lateral bilayer, a pseudo-trimolecular layer or an inclined paraffin structure, as shown in Fig. 11. Generally, the higher charge density of clay and longer surfactant chain length contribute towards increase in dspacing by pushing apart the clay layers [98]. 18

During preparation of nanocomposites, polymer chains enter into the clay-galleries, resulting in a greater increase in d-spacing, forming intercalated or exfoliated structures. Two very important characteristics of layered-silicates, which determine the degree of dispersion of clay in polymer matrix and final properties (including gas barrier property) of polymer/clay nanocomposites are – (i) ability to fine-tune the surface chemistry through ion exchange reaction with inorganic or organic cations and (ii) ability to separate out the silicate particles to individual layers [29]. Properly dispersed and exfoliated clay has the capability to increase the gas barrier property of polymer significantly by increasing ‘tortuous path’ length of gas diffusion even at low clay concentration [29,99]. Moreover, the presence of clay in the polymer matrix restricts the movement of polymeric chains, which also reduce gas permeability [17]. 5.1.3. Graphene and its derivatives Graphene is a one-atom thick, 2D, planar sheet of sp2 bonded carbon atom that is densely packed in a honeycombed crystal lattice, with carbon-carbon (C-C) bond length of 0.142nm (Fig. 12). Among different nano-fillers, graphene based fillers are considered as best nano-fillers for next generation applications due to their potential to improve electrical and thermal conductivity, mechanical properties, microwave absorption, dielectric performance, weather resistance and gas barrier properties of polymer matrices [30,100]. Due to plate-like structure and very small geometric pore size (0.064nm), graphene is very efficient in improving gas barrier property of polymers. The thickness of individual graphene sheet was found to be only 1.153 nm (Fig. 13) [101], but it impermeable even to helium atoms [100,102]. There are two main routes by which graphene can improve the gas barrier property of polymers. The first is through few-layered coating of ultrathin sheets of graphene or graphene derivatives (GO and rGO) on polymeric films [31,102-103]. The other is via preparation of 19

polymer/graphene nanocomposites by proper exfoliation of very small weight fraction of graphene sheets in the polymer matrix [7,18,31]. In this article, only the gas barrier performance of graphene based PUNCs will be reviewed. The physicochemical properties of the graphene based nanocomposites depend on the distribution of graphene layers in the polymer matrix and interfacial bonding between the graphene layers and polymer matrix. Pristine graphene, due to the absence of any functional groups, shows poor compatibility with most organic polymers and hence does not form homogeneous composites. In contrast, modified graphene such as graphene oxide (GO), reduced graphene oxide (rGO) and functionalized graphene (e.g. hydroxyl modified graphene) sheets are more compatible with organic polymers. These have attracted considerable attention as a nanofiller for producing gas barrier polymer nanocomposites [104]. 5.1.4. Synthesis and modification of graphene Graphene and its derivatives are synthesized by different top-down or bottom-up approaches, which have been summarized in Fig. 14 [105-120]. Among different bottom-up approaches, CVD and epitaxial growth are capable of producing better, defect-free and large-size graphene sheets in comparison to other methods like mechanical cleavage method. Thus, these are the most common and effective methods for preparation of graphene and are able to produce both single and bi-layered graphene sheets, of a specific area up to 1 cm2 [121-122]. Unfortunately, these methods are not suitable for bulk production of graphene due their time and resource intensive nature and lower productivity. However, recently researchers are exploring different approaches to overcome these shortfalls [117,119]. Among different top-down approaches, the most widely investigated technique is the synthesis of graphene from graphite oxide (GO). Practically, it is considered as one of the best routes for 20

large-scale synthesis of graphene [123]. During oxidation, the sp2 hybridized carbon sheets of graphite break into nanoscale sp2 graphitic domains which are surrounded by oxidized sp3 domains as defects. The oxidized graphitic product has a layered morphology, where the hexagonal basal planes contain hydroxyl or epoxide functionalities and the periphery is covered by carbonyl and carboxyl groups. Even after oxidation, the graphite oxide retains the stacked structure similar to graphite due to the presence of attractive forces between layers containing oxygen functionalities. However, it possesses much wider spacing, enabling superficial exfoliation even by sonication in a polar solvent. During or after exfoliation, the obtained GO should be reduced for restoring the sp2 hybridized carbon sheets of graphene, which produce some defects into the graphene structure, resulting in increase in intensity-ratio of D-band and Gband in Raman spectra (see Fig. 15(b)). The obtained product is called reduced graphene oxide (rGO) [124-125]. Different graphitic fillers can also be characterized by XRD. The sharp diffraction peak at 2ϴ= 26.5° (for graphite) was shifted to 2ϴ= 12.3° with a broader spacing (for GO) and no diffraction peak was observed for rGO (see Fig. 15(a)) [125]. Thermal reduction and exfoliation of GO is regarded as the most economical route for mass production of high quality rGO. However, during thermal reduction, few of the partially oxidized sp3 hybridized carbon atoms become fully oxidized to some carbonaceous gases (e.g. CO, CO2 etc) and the remaining get reduced to sp2 hybridized graphene product [126]. The GO or rGO are capable of reacting with active sites of polymers, forming a good interface between them. For example, the formation of chemical bond between hydroxyl group of unmodified rGO and isocyanate of PU may increase the degree of rGO-exfoliation, resulting in better properties of PUNCs [127].

21

Interestingly, a sub-micron thick, defect-free graphene or graphene oxide (GO) sheet is totally impermeable to all liquids, vapors and gases, including helium [128-129] and also highly impermeable to aggressive chemicals like hydrofluoric acid [103]. However, the GO-membrane is unimpeded to the permeation of water vapor (H2O permeation rate about 1010 times faster than helium) due to a low-friction flow of water vapor through two-dimensional capillaries formed by closely spaced graphene sheets. [129]. A tremendous improvement in water vapor permeability can be achieved by using reduced graphene oxide (rGO) [103]. 5.1.5. CNT Carbon nanotubes (CNTs) are nanostructured allotropes of carbon, where graphene sheets are rolled into cylindrical or tubular forms. Because of their exceptional mechanical properties, electro-activity as well as very good electrical and thermal conductivity, the CNTs have tremendous potential to improve the performance of polymers [130]. Besides these, some researchers have also utilized the long tubular shape or high aspect-ratio of CNTs to improve the gas barrier property of polymers [79-80]. 5.1.6. POSS Polyhedral oligomeric silsesquioxane (POSS) is one of the most promising and emerging hybrid nanostructured material. POSS molecules have a nanosized polyhedral or a cage-type structure containing an inorganic silicone-oxygen (Si-O) core based frame with an organic substituent (R) at the corners (Fig. 16). The inorganic Si-O core is chemically and thermally robust. The organic substituents (R) may be completely hydrocarbon chain structures (non-polar) or polar structures or functional groups. By controlling these organic structures, the property and the reaction chemistry of POSS can be tailored [93]. POSS is incorporated in polymers or polymer blends to 22

improve their thermal stability, flame retardancy, mechanical properties, rheological properties and sometimes even gas barrier properties. The gas barrier property of polymer/POSS nanocomposites increases with increasing cross-linking density which depends on the type of functional group present in POSS [22, 80-81]. 5.2.Structure and morphology of PUNCs The layered structure nanomaterials (layered silicates, graphene etc) have very thin layers (about 1nm thick) and have very high aspect ratio (generally 10-1000). A small fraction of these fillers disperse properly in the polymer matrix and create a much higher surface area for interaction with the polymer matrix, compared to micro/macro sized fillers [29]. Like other nanocomposites, the structure and morphology of PUNCs depend on the strength of interaction between nanomaterials and PU matrix. The physical mixture of nanomaterials and PU may form four different types of composite structures (see Fig. 17), which arei)

Phase-separated micro-composites

ii)

Intercalated nanocomposites

iii)

Flocculated nanocomposites

iv)

Exfoliated nanocomposites

Poor interaction between PU matrix and layered nanomaterials results in a phase separated micro-composite structure where the property of the composite is similar to conventional polymeric composites [131]. In intercalated PUNC structure, the PU chains enter into the galleries of layered nanomaterials in a crystallographically regular fashion, irrespective of the ratio of nanomaterials to PU. Generally, a few molecular layers of polymer invade into interlayer domains of layered nanomaterials present in intercalated nanocomposites. Conceptually, 23

flocculated nanocomposites are same as intercalated nanocomposites. However, in polymer/clay nanocomposites, sometimes the silicate layers are flocculated because of their hydroxylated edge-edge interaction, as shown in Fig. 17(c). In an exfoliated nanocomposite, the individual layers of layered nano-fillers are separated in a continuous polymer matrix where the average distances among different layers depends on filler loading. Generally, the filler content in an exfoliated nanocomposite is much lower than that of an intercalated nanocomposite for similar improvement in properties [19,29,132]. Different techniques are used for characterization of polymer nanocomposite structures or analysis of filler-dispersion in polymer matrix. These are mainly wide-angle X-ray diffraction (WAXD) [6,97,133], small angle X-ray scattering (SAXS) [134-135], rheological analysis [6,96], transmission electron microscopy (TEM) [17,96-97], scanning electron microscopy (SEM) [5] and sometimes even optical microscopy [136]. Fig. 18 shows different states of dispersion of layered silicates in polymer matrix characterized by WAXD and TEM. The improvement in gas barrier property of PUNCs is highly dependent on the degree of exfoliation of nanomaterials such as clay, graphene etc. Higher extent of exfoliation exfoliation results in a larger tortuous path length which would result in enhanced gas barrier properties [137]. 5.3.Synthesis of PUNCs The dispersion of nanomaterials (layered silicates, graphene etc) in PU matrix is the most important step in the preparation of PUNCs. The final properties, including gas barrier property of PUNCs change significantly with a high level of homogeneity in terms of dispersion. Among various factors influencing dispersion of nanomaterials in PU matrix, the most important one is a 24

method of synthesis which controls the interfacial interaction between filler and matrix. The process route of PUNCs is very wide and it can be synthesized by any one of the three routes, viz., melt intercalation, in-situ polymerization and solution mixing. 5.3.1. Melt intercalation Melt intercalation or melt mixing is the most economical and industry scalable route for the preparation of PUNCs. In this process, the naoplatelets are peeled apart by application of a high shear force and diffuse in PU matrix (see Fig. 19) [138]. The main advantage of this method is that there is no need of organic solvents, which nullify the environmental issue. However, the major drawbacks are- (i) poor dispersion due to the high viscosity of polymer melts in most of the cases and (ii) possibility of thermo-oxidative degradation of PU and additives (sometimes), during processing at high temperature. Because of dispersion issue, till now, there is very few literature [139] on gas barrier property of PU/clay and PU/graphene nanocomposites prepared by melt mixing route. Moreover, among different graphitic nano-fillers, more research has been reported on melt mixing of TRG than CRG, due to its better thermal stability [122]. 5.3.2. In-situ polymerization In case of in-situ polymerization, initially raw materials of PU (diisocyanate, polyol, chain extender and/or catalyst) or a pre-polymer are/is made to diffuse inside the interlayer spacing of nanoplatelets of clay or graphene in presence/absence of a suitable solvent. During subsequent polymerization, the polymer chains grow leading to expansion of inter-layer spacing and exfoliation of nanoplatelets in the polymer matrix (Fig. 20(a)). In this process, the intercalation

25

or exfoliation of the nanoplatelets is highly dependent on the compatibility between the interlayer modifier and raw materials or pre-polymer [6]. There is some literature which reports the gas barrier property of PU/clay [6,96-97], PU/graphene [18] and PU/POSS [22,139] nanocomposites produced by in-situ polymerization route. Unlike solution mixing, in this process, a high level of dispersion of nanoplatelets is obtained without prior exfoliation step which leads to tremendous improvement in gas barrier and other properties of the polymers. However, this method is also not free from limitations. The main drawbacks of this route are- (i) issue of toxicity due to use of organic solvents and (ii) sometimes, property enhancement is not significant due to the reduction in number of H-bonding in PUNCs [18]. 5.3.3. Solvent-mixing In this process, at first a polymeric solution is prepared by dissolving PU in a suitable solvent, which is used to disperse nano-fillers by simple stirring or mechanical/shear mixing. However, in order to get a better dispersion, the nano-fillers are often sonicated, before mixing with the polymer solution. By virtue of uniform and proper dispersion of nano-fillers in a low viscous polymer solution, an exfoliated nanocomposite structure is obtained (Fig. 20(b)). In this process, selection of proper solvent, sufficient stirring and sonication prior to mixing are very essential to achieve adequate dispersion of nano-fillers or nanoplatelets in the polymeric matrix. Generally, dimethyl formamide (DMF), tetrahydrofuran (THF), N-Methyl-2-Pyrrolidone (NMP) are used as solvents to produce PUNCs using this route. Extensive studies have been reported on gas barrier properties of PU/clay [5,17,39,133,140-141], PU/graphene [7,20] and PU/CNT [79] nanocomposites prepared by solvent mixing route.

26

However, in spite of lots of lab-scale research, due to high cost and toxicity of organic solvents, this route is not being considered as an industry viable process [18]. 6. General factors affecting gas barrier property of PUNC based films/coatings The permeation of gases through any polymeric membrane or film is strongly dependent on thermodynamics where main driving forces are pressure, concentration and temperature gradient [38]. The molecular diameter of gases, chemical structure and morphology of polymer as well as thickness of polymeric film/membrane/coating also control the gas permeation rate through it. The nanoplatelets like clay or graphene are used to improve the gas barrier property of PUs. There are many factors which control the gas permeability across the nanocomposite films/ membranes/coatings, there are: i)

Geometrical shape of nanomaterials

ii)

Size and aspect ratio of nanomaterials

iii)

Concentration of nanomaterials in PU matrix

iv)

Degree of dispersion and homogeneity of nanomaterials in the polymer matrix

v)

Orientation of nanomaterials in polymer matrix

vi)

Polarity of nanoparticles and PU

vii)

Chemical structure and morphology of PU

For proper exfoliation of nanomaterials in a PU matrix, it requires a compatibility or polarity match between the surface of nanomaterials and PU. Even the composition of nanocomposites and the nature of the permeant strongly affect the gas permeability [19,26]. The effect of these factors on gas barrier property and some tactics for enhancing the gas barrier property of PUNCs are discussed below. 27

7. Strategy for improvement of gas barrier property of PUNCs There are number of ways by which gas barrier property of PUNCs can be improved significantly, which mainly are- (i) improvement in filler dispersions and exfoliation, (ii) improvement in filler orientation in PU matrix (iii) use of nano-fillers with higher aspect ratio, (iv) use of clay with higher d-spacing and (v) matching solubility parameters. 7.1.Improvement in dispersion of nano-fillers Proper dispersion of nano-fillers in polymer matrix is the main key for improvement of gas barrier property of any polymer nanocomposite. There are some tricks to be followed during processing, for proper dispersion of nano-fillers in PU matrix. Some of which are discussed below: 7.1.1. Improvement in polymer-filler interaction Among different factors which influence the dispersion of nano-fillers in PU, the interaction between the polymer matrix and filler’s surface plays the prime role in controlling gas barrier properties of nanocomposites. Modification of clay, graphene or CNT helps to enhance the compatibility of these fillers with polymers, which helps to improve the dispersion of nano-fillers properly in the polymer matrix [19]. Thus, in all manufacturing techniques, the main focus has been imposed on the improvement of interfacial interaction by modification of fillers. 7.1.1.1.Effect of clay modification on dispersion in PU matrix and gas barrier property of PUNCs The cation exchange by organic onium ions in the galleries of clays not only serves to match with the polarity of the polymer, but also expands the inter-layer gallery spacing (see Fig 21(a)). 28

It facilitates easy penetration of polymeric chains in the clay galleries, resulting in intercalated or exfoliated clay-morphology [5-6,26,142]. Quin et al. [6] obtained highly intercalated and partially exfoliated PU/clay nanocomposites by incorporating modified vermiculite clay, as confirmed by TEM (Fig. 21(d-e)). In contrast, incorporation of unmodified vermiculite resulted in agglomeration of clay in PU matrix (Fig. 21(b-c)). In case of PU/modified vermiculite nanocomposites, an increase in hard segment relaxation temperature was observed by DSC and DMA tests, suggesting strong interactions between clay platelets and the hard segment of PU. In addition to this, dispersion of clay platelets in PU matrix as well as compatibility between the surfaces of the organomodified-clay to the polymer matrix were improved, which resulted in improvement in gas barrier property. Qian et al. [6] reported that the reduction in N2 and CO2 gas permeability coefficients were about 30% and 19%, respectively for the PUNCs produced with 5.3wt% vermiculite clay modified by cetyltrimethylammonium bromide (CTAB-VMT). In contrast, the permeability coefficient reduced by only 10.3% for N2 but increased slightly for CO2 with the incorporation of unmodified-VMT of same amount, due to poor dispersion of clay in PU matrix. In comparison to N2, the lower reduction in CO2 gas permeability might be due to the higher solubility of CO2 in the PU/clay interface than N2. In another study, Shamini and Yusoh [5] also observed a significant improvement in gas barrier property after modification of clay by copper and iron chloride. 7.1.1.2.Effect of modification of graphitic fillers on their dispersion in PU matrix and gas barrier properties of PUNCs Due to the presence of van der Waals forces between graphene layers, exfoliation of individual graphene sheets in the polymer matrix is a very challenging task. Therefore modification is 29

required for the creation of single or few layer thick graphene sheets. Graphene oxide (GO), reduced graphene oxide (rGO) or functionalized graphene (e.g. hydroxyl or carboxyl modified) all have good compatibility with PU and many other polymer matrices, which enhances dispersion and exfoliation of graphene in polymer matrices. Properly exfoliated and oriented graphene sheets have huge potential to improve gas barrier properties of the polymer by increasing the tortuosity of the path for gases to diffuse. Kim et al. [18] synthesized graphite oxide from graphite by treating it in a mixture of sodium nitrate (NaNO3), a concentrated acid (H2SO4) and KMnO4. It was further modified to thermally reduced graphene oxide (TRG) and phenyl isocyanate modified graphene oxide (Ph-iGO) [Fig. 22(i)]. These different graphitic materials were analyzed by X-ray scattering [see Fig. 22(ii)]. For pristine graphite, a sharp peak was found at 2ϴ=26.4° (d002 spacing= 0.34nm), which was shifted to 2ϴ=12.7° (d-spacing= 0.70nm) upon oxidation, due to intercalation by oxygen groups and moisture. After treating with phenyl isocyanate, the peak was further shifted to 2ϴ=9.3° with broadening of d-spacing (d=0.95 nm), due to disorder created by the reduction of average crystal size. The absence of any peak in case of TRG signified a complete exfoliation after superheating of GO. These different graphitic fillers were incorporated in PU matrix by different process routes as explained in Fig. 22(i). Improved dispersion and intercalation were observed after modification as seen in TEM images [Fig. 22(iii)]. It resulted in remarkable improvement in nitrogen gas barrier property of PU, after incorporation of modified graphene derivatives, in comparison to pristine graphene [18]. 7.1.2. High energy ball milling Ball milling is capable of delaminating clay-platelets very effectively which ensures a better exfoliation of clay-platelets in the polymer matrix [143-144]. However, there is a high chance of 30

reduction in aspect ratio of clays, if the clays are processed extensively. In a very recent study, Chatterjee et al. [143] demonstrated that the process parameters of ball milling (milling speed, milling time, number of balls used etc) have a significant role in controlling the dispersion of clay in PU matrix, which ultimately affected the helium gas barrier property of PU/clay nanocomposite based coatings. 7.1.3. Processing technique Selection of proper processing technique is very important for obtaining desired properties of PUNCs. Among three processing techniques (melt, in-situ and solution mixing), generally the best dispersion is obtained in case of solution mixing (see Fig. 23), resulting in tremendous property enhancement. In case of in-situ polymerization, almost a similar dispersion is obtained which results in improved gas barrier property. However, other properties of PU might be deteriorated slightly due to change in morphology by reduction of inter-molecular forces like Hbonding [18]. 7.1.4. Sonication Sonication of nano-fillers in a solvent before mixing breaks the flakes and influences its size, which ultimately helps to improve dispersion significantly. The permeability of water molecules through PU/organoclay nanocomposite coatings reduces significantly with increasing sonication time [136]. Quin et al. [6] used probe-sonication to disperse vermiculite clay homogenously in the polyol, which resulted in good intercalation of monomer in clay-galleries and formed an exfoliated PU/clay structure by in-situ polymerization. Herrera-Alonso et al. [133] reported a significant improvement in barrier property of PU/clay nanocomposites against gases and volatile organic compounds (VOCs) (Fig. 24), by preprocessing of organoclay with 30 min 31

sonication instead of plain stirring, which resulted in better dispersion of layered-silicates in PU matrix. 7.1.5. High-speed mechanical agitation or shearing Good exfoliation of nano-platelets in PU matrix can be obtained by applying high-speed external shear force or similar mechanical agitation [96,145-146]. Möller et al. (2010) observed that the cation exchange capacity (CEC), full width at half maxima (FWHM) and BET surface area of potassium- hectorite clay (K-Hec) improved significantly with increasing shearing cycles due to delamination of silicate layers (see Fig. 25) [146]. Heidarian et al. [136] demonstrated that sonication is more effective compared to 2h mechanical shearing to improve dispersion of organoclay in PU matrix (see Fig. 26(a-d)). Moreover, with increasing sonication time from 15 min to 30 min, the 2ϴ peak (on WAXD curves) of organoclay was shifted to the lower side with increase in d001 basal spacing, which was an indication of better dispersion and intercalation. No WAXD peak was found for 60 min sonication, suggesting complete exfoliation of clay-platelets in PU matrix, as shown in Fig 26(e). 7.1.6. Master-batch based mixing Masterbatch is generally a concentrated mixture of nano-fillers in the polymer matrix, which is further mixed with the neat polymer to obtain polymer nanocomposites with desired percentage of nano-filler loading. Many literature reported [96,139,147] a significant improvement in clay dispersion in the polymer matrices by master-batch based mixing. Park et al. [96] reported better CO2 gas barrier properties in case of PU/vermiculite based nanocomposites produced by in-situ polymerization of master-batches, compared to their counterparts produced by direct mixing with same clay loading. It was a consequence of better dispersion of clay in PU matrix, in case of 32

master-batch based mixing. In master-batch process, the dispersion of organomodified-VMT in MDI improved significantly by bulk in-situ polymerization with polyether based polyol-clay preblend (master-batch). The steps in the preparation of PU-VMT nanocomposites and different ways for efficient mixing of clay-platelets in PU matrix has been schematically explained in Fig. 27. Benali et al. [139] exploited two strategies for improving gas barrier and mechanical properties of PU involving – (i) poly(e-caprolactone) (PCL)/organoclay master batch and (ii) PCL-grafted organoclay hybrids, synthesized by in-situ intercalative grafting/ring-opening polymerization of e-caprolactone (CL), which were further mixed with an ester based PU by melt mixing. Significant improvement in water-vapor barrier properties of PU were obtained resulting from better dispersion and matrix/organoclay interaction. 7.2. Improvement of orientation Gas barrier property of polymer nanocomposites is strongly dependent on the alignment of nanofillers in polymer matrix [148]. It is established that both higher aspect ratio and perpendicular alignment of nanoplatelets can provide highly tortuous paths (Fig. 28), resulting in remarkable improvement in gas-barrier property of polymer [149]. The nano-filler’s orientation in PU matrix might be improved by following ways: a) Upstream stretching (in case of blown film) or uniaxial stretching (in case of extrusion casting film) increases the orientation of lamellar type fillers in the polymer matrix. b) Sometimes, a biaxial drawing system is used, for stretching of extrusion cast film which significantly improves the orientation of nano-fillers in the polymer matrix.

33

c) The orientation of rod or plate-like magnetic/conductive fillers might be assisted by application of magnetic force during processing of polymer nanocomposites [150-152]. Jiao et al. (2014) used a low magnetic field to orient the modified magnetic graphite (Fe3O4/Graphite) nanoparticles in an epoxy matrix which lead to highly ordered nanocomposite with remarkable improvement in gas barrier property [151].

7.3.Higher aspect ratio of nano-fillers Generally, the nano-platelets having higher aspect ratio provide better gas barrier property by creating a more tortuous path for diffusion of gas molecules [153]. Yano et al. [154] investigated the gas barrier property of polyimide-clay hybrid nanocomposites using four different clays (hectorite, saponite, montmorillonite and synthetic mica). All clays consisted of stacked silicate sheets, having same thickness (10Å) but varying lengths (hectrite-460 Å, saponite-1650 Å, mintmorillonite-2180 Å, and synthetic mica-12300 Å). As compared to other hybrid nanocomposites, the synthetic mica reinforced hybrid nanocomposite showed much better gas barrier property due to the very high aspect ratio of synthetic mica. With the incorporation of only 2wt% synthetic mica, the relative water vapor permeability of hybrid nanocomposite reduced to less than one-tenth of the unfilled polymer (Fig. 29). 7.4. Higher charge density and inter-gallery spacing of clay The effective intercalation of polymer-chains in the clay-galleries depends on the basal spacing (d-spacing) of the clay. Generally, a better intercalation or exfoliation is expected for the clay having higher d-spacing. Moreover, the higher charge density of clay facilitates in producing larger inter-gallery spacing. Pristine vermiculite (VMT) clay exhibits a broad major peak at d34

spacing of about 12Å, which increases to 26.6 Å and 28.5 Å, with incorporation of two different alkylammonium ions which are cetyltrimethylammonium bromide (CTAB) and octadecyl bis (hydroxyethyl)methylammonium chloride (OBMAC), respectively [6]. The d-spacing of these organomodified-VMT is higher in comparison to MMT-clay modified with similar modifiers [155]. Therefore, organomodified-VMT is more suitable for producing high aspect ratio than organomodified-MMT in the polymer matrix [6]. 7.5.Matching solubility parameters The solubility parameter (δ), gives an estimation of cohesive energy of any matter and used to evaluate the compatibility or miscibility of the constituents of a mixture. In solvent based processing, when the solubility parameter of solvent is close to the solubility parameter value of the clay surfactant, a better exfoliated state of dispersion is obtained. Therefore, selection of a suitable solvent of matching solubility parameters is very important for any solvent based synthesis of PU or mixing of organoclay in PU. Stratigaki et al. [21] observed a better dispersion of organoclay (δ=17 Mpa1/2) in acetone (δ=20.1 Mpa1/2) compared to ethyl alcohol (δ=26.6 Mpa1/2) for synthesis of waterborne PU/clay nanocomposite based coating formulation. 8. PUNC based gas barrier films or membranes PU and its nanocomposite based membranes are generally prepared by solvent casting followed by solvent evaporation or coagulation in a non-solvent. Whereas, PU and its nanocomposite based films are generally manufactured by any one of the following techniques, depending on raw materials or particular requirements [156]: i)

Solvent casting and evaporation of solvent

ii)

T-die casting 35

iii)

Slit-die extrusion

iv)

Film blowing

Extrusion die casting provides much higher productivity and uniform film-thickness compared to blown film technique. However, in blown film technique, much lower thickness can be obtained than that obtained in extrusion casting. During film blowing (in blown film technique) or during uniaxial or biaxial stretching (in extrusion casting), the hard segments of PU and the nano fillers are oriented along the direction of stretching, resulting in improvement in gas barrier property [157]. However, at lab scale, very frequently PU films are prepared simply by compression molding in place of extrusion casing. The gas barrier properties of different PUNC based films and membranes have been discussed below. 8.1.PU/clay nanocomposites based films and membranes In recent years, PU/clay nanocomposite based films have attracted a lot of attention as incorporation of clay in PU matrix imparts remarkable improvement in properties like strength and modulus, thermal resistance, gas barrier, flame retardancy and many others. A lot of literature [5-6,10,17,96-97,133,140,142,148,158] reports improvement in gas barrier properties of PU/clay nanocomposite systems. Osman et al. [148] compared the oxygen gas barrier property of epoxy and PU nanocomposites reinforced with OMMT at varying concentration. For both nanocomposites, the oxygen gas permeability decreased exponentially with increasing filler loading up to 3 vol%. However, for higher filler loadings (>3 vol%) gas permeability increased, probably due to the decrease in number of exfoliated layers or collapsing of layers which lead to the lowering of the average aspect ratio of fillers in both polymer matrices. The PUNCs showed better barrier properties 36

compared to epoxy nanocomposites because of the stronger interaction of filler with PU, leading to better exfoliation and higher aspect ratio of OMMT in PU matrix. Qian et al. [6] observed about 19% reduction in CO2 permeability in case of polyether polyol and MDI based PUNCs with incorporation of 5.3wt% modified vermiculite (CTAB-VMT). In a similar study [96], the authors reported about 40% reduction in CO2 permeability with incorporation of only 3.3wt% modified vermiculite (CTAB-VMT) in the PU by a master-batch based mixing. In the second case, the dispersion of clay in MDI was further improved by highintensity dispersive mixing with a master-batch (polyol-clay preblend), which resulted in better gas barrier property. With increasing clay concentration, improved gas barrier property was obtained which might be due to two main reasons- (i) increase in T g of PU and (ii) increase in storage modulus of PU-VMT nanocomposites, which resulted in a decrease in flexibility of polymer chains, causing lower permeability through PU matrix [96]. Chang et al. [140] discussed the effect of three different clay-modifiers on oxygen-gas barrier property of PU/clay nanocomposite films. Regardless of the type of organoclay, the oxygen gas permeability of PU/clay hybrid nanocomposites decreased linearly with increasing clay concentration from 0-4wt% due to increase in torturous path length for gas diffusion and also due to increase in rigidity of nanocomposite films. However, the performance of hexadecylamine-montmorillonite (C16–MMT), was much better than dodecyltrimethylammonium-montmorillonite (DTA–MMT) and Cloisite 25A in the reduction in gas permeability of PU/clay films, showing about 50% reduction of oxygen gas permeability with 4wt% loading of C16–MMT. In a similar study, Herrera-Alonso et al. [133] investigated the gas barrier properties of PU/clay nanocomposites films, where natural montmorillonite was modified by three different 37

alkylammonium surfactants and incorporated in PU matrix by solution mixing. The extent of barrier property of PU/clay nanocomposites against both He and CH4 gases increased in this order PU/Cloisite 30B > PU/Cloisite 10A> PU/Cloisite 20A (Fig. 30). Lowest increase in basal spacing (d001) was observed in case of cloisite 20A due to the presence of two tallow group in the modifier structure which resulted in lower dispersion and poorest gas barrier property in comparison to other clays. In contrast, due to the presence of only one tallow group in the modifiers of cloisite 30B and cloisite 10A, the increase in basal spacing was higher than cloisite 20A, resulting in better exfoliation in PU matrix and better gas barrier property. Over-all, the best gas barrier property was obtained with cloisite 30B. However, with higher clay concentration (50 wt%), both cloisite 30B and cloisite 10A showed almost similar barrier property against helium gas (Fig. 30). In all clay systems, gas permeability decreased gradually with increasing clay concentration up to 50wt%, providing much longer tortuous diffusion pathways. Shamini and Yusoh [5] developed a novel PU/clay nanocomposites by solution intercalation, using nanoclay (montmorillonite) modified by iron (III) chloride and copper (II) chloride. With incorporation of montmorillonite, significant improvement was observed in oxygen and nitrogen gas barrier property compared to virgin PU. Gas barrier property improved further after modification of clay, due to improved dispersion and reduced agglomeration tendency. A remarkable four-fold and three-fold decrease in oxygen gas permeability was observed with incorporation of only 1 wt% modified iron-clay and copper-clay in PU matrix, respectively. Similarly, nitrogen permeability decreased by 60 and 50% with incorporation of 1% modified iron-clay and copper-clay, respectively.

38

Xu and coworkers [158] reported a novel approach for preparation of biomedical poly(urethane urea) (PUU) based nanocomposite that resulted in a significant reduction in gas permeability along with increasing tensile strength and stiffness, without any loss of ductility. Water vapor permeability of Closite 15A (alkylammonium modified montmorillonite) reinforced PUU nanocomposites was reduced by fivefold at 6 vol% of nanoclays. In a similar study [10], two organically modified layered silicates (OLS), named as Cloisite 15A and Nanomer I.30TC were used for developing low gas permeable biomedical PUNCs. Here also a fivefold decrease in water vapor permeability was reported at the highest OLS content (20 wt%) owing to a proper nano-scale dispersion of clay in PUU matrix. In an interesting study, Maji et al. [17] reported a significant improvement in gas barrier property of hyper branched PU with incorporation of gas impermeable clay platelets, due to higher retardation against gas permeation by increasing tortuous path length. As a consequence, about 76% reduction in helium gas permeation was observed for third generation hyperbranched PU filled with 8wt% cloisite 30B. The improvement in gas barrier property was quite high in case of properly dispersed modified clay when compared with aggregated unmodified one. Table 2 highlights the gas barrier property of different PU/clay nanocomposite films, compiled from existing literature. In a very interesting work, Benali et al. [139] observed that the water vapor permeability of PU/clay nanocomposites prepared by direct melt mixing (TPU/MMT-OH2) was about 44% higher than neat TPU (Table 3). It was a consequence of the increase in sorption parameter due to fine dispersion of clay nano-platelets which caused high exposure of hydroxylated ammonium groups of clay to water molecules. In case of TPU/PCL binary blend also slight increase in water vapor permeability was observed, which indicates that presence of only low molecular weight 39

PCL in TPU system is not able to increase its barrier property. Whereas, for both cases of TPU/PCL-clay master batch (TPU/MMT-OH2/PCL) and PCL-grafted organoclay filled TPU nanocomposite (TPU/MMT-OH2-g-PCL) the permeability coefficient decreased by about 61 and 41%, respectively with respect to neat TPU, in spite of less complete clay-dispersion and presence of long intercalated tactoids in TPU matrix as shown in Fig. 31. It might be a combined effect of fine clay-dispersion and formation of hydrogen bonds between PCL and TPU, which protected the sorption of water molecules by the hydroxylated ammonium groups of MMT [139]. 8.2.PU/graphene nanocomposite films/membranes In an interesting study, Kim et al. [18] investigated the nitrogen gas permeability of PU/graphene nanocomposites reinforced with graphene oxide (GO), thermally reduced graphene oxide (TRG) and isocyanate modified graphene oxide (iGO). These graphitic additives were mixed with PU by all three routes viz. melt mixing, in-situ polymerization and solution mixing (see Fig. 22(i)). The best mechanical and gas barrier properties were obtained with solution mixing because of more effective distribution of thin exfoliated graphene sheets in PU matrix, compared to other processes. Whereas, the least improvement in gas barrier property was noticed in case of melt route. Although solvent was used in in-situ polymerization, enhancement in properties was not pronounced due to the reduction of H-bonding in processed PU. A remarkable 80 fold decrease in N2 permeability was observed with incorporation of 3wt% iGO via solvent mixing. It was a consequence of the barrier performance of highly exfoliated two-dimensional graphene platelets having high aspect ratio [18]. Kaveh et al. [1] studied the gas barrier property of polyester grade PU/graphene oxide nanocomposite produced by solvent blending. Interestingly, about 80% reduction in helium gas 40

permeability was observed with only 1wt% graphene oxide (GO) content. Presence of functional polar groups in GO facilitated the complete exfoliation of GO in PU matrix by simple sonication in DMF, resulting in tremendous improvement in helium gas barrier property. Gavgani et al. [1] prepared intumescent flame retardant polyurethane (IFR-PU) nanocomposites by incorporating reduced graphene oxide (rGO), microencapsulated ammonium polyphosphate and melamine. A good dispersion of rGO in PU matrix was achieved due to the strong interaction between rGO and PU. Incorporation of rGO in IFR-PU resulted in significant improvement in oxygen barrier property, thermal stability, electrical conductivity and storage modulus. About 90% reduction in oxygen gas permeability was obtained with incorporation of 2wt% rGO in PU matrix. Yousefi et al. [159] used a latex mixing method for preparation of PU/rGO nanocomposites, where in an aqueous dispersion of ultra-large size graphene was reduced by in-situ in a waterborne PU. The moisture vapor permeability of PU/rGO nanocomposites reduced significantly due to a synergistic effect of very high aspect ratio and high horizontal alignment of impermeable graphene sheets in PU matrix. Moreover, owing to the presence of polar groups such as urethane and carbonyl groups in PU-backbone, the PU and rGO interacted strongly mainly by hydrogen bonding (Fig. 32(b)). It resulted in complete disappearance of shouldered FTIR-peak for –NH stretching of urethane linkage, in PU/rGO nanocomposites (Fig. 32(a)). As a combined effect of these factors, the water vapor permeability of PU reduced about 76% with 3wt% rGO content, compared to neat PU film [159]. Xiang et al. [20] produced a unique, high gas barrier TPU/graphene nanocomposite film containing hexadecyl-functionalized low-defect graphene nanoribbons (HD-GNRs) by solution casting. The HD-GNRs (Fig. 33(c)) were produced by in-situ intercalation of sodium/potassium 41

alloy in multi-walled CNTs (MWCNTs), followed by quenching with 1-iodohexadecane. Compared to GO, the HD-GNRs preserved the graphite domains with lower defect, as confirmed by Raman spectroscopy (Fig. 33(d)). The hexadecyl groups on the edges assisted the ribbons to disperse easily in organic solvent (Fig. 33(e)), which led to its uniform dispersion even in TPU matrix. Additionally, the low defect GNRs structure was highly impermeable material which resulted in remarkable gas barrier performance. As a consequence, the nitrogen gas diffusivity though PU film reduced by three orders of magnitude with incorporation of only 0.5wt% HDGNRs. Over a short period of time (1000 sec), this film became almost impermeable as no pressure drop was detected (see Fig. 34). However, a little pressure drop was noticed over a longer period of time (60000 sec) [20]. Table 4 highlights the gas barrier property of different PU/graphene nanocomposite films, compiled from existing literature. 8.3.PU/CNT nanocomposites films To the best of our knowledge, till now there is only one research report on gas barrier property of PU/CNT nanocomposites. In this work, Ali et al. [79] studied the effect of multi-walled carbon nanotubes (MWCNTs) on physicochemical properties of castor oil based thermoset PUs (COPUs). The overall best properties were obtained with incorporation of only 0.3 wt% MWCNTs in the nanocomposite. The characteristic X-ray diffraction peak (2θ ≈20°) of COPU became broadened and intensity reduced with incorporation of MWCNT (Fig. 35(a)). It suggests the formation of short-range microstructural phases of both hard and soft segments, which might be due to the strong interfacial adhesion between COPU matrix and MWCNT [161-162]. Surface morphology studies revealed the presence of very few or no pores on the surface of the nanocomposites, which was in a good agreement with the reduction of nitrogen gas permeability 42

by 70% (with 0.5 wt% MWCNTs loading) over pure COPU. The increase in gas barrier property for PU-MWCNT nanocomposites might be due to good compatibilization, exfoliation, orientation, and lower reaggregation of purified MWCNTs in the PU matrix. However, at higher loading of MWCNTs (1 wt%) gas permeability increased slightly, as shown in Fig. 35(b) [79]. 8.4.PU/POSS nanocomposites The incorporation of POSS into PU provides improved flame retardancy, thermal stability and gas barrier property. The improvement in gas barrier properties of PU/POSS nanocomposites depends on cross-linking density based on reaction chemistry and functional groups available in POSS and PU [25,139,163-164]. Madhavan et al. [22] studied the effect of linear siloxane chain (PDMS and PPG) and caged silsesquioxane (POSS) on gas transport properties of PU based hybrid membranes. POSS-H was first functionalized to POSS-amine, which was further used as cross-linker for preparation of PDMS or PPG based hybrid PU membranes (see Fig. 36). It was observed that diffusion of gas was more favourable through PDMS soft segments than PPG in the PU membranes . However, with incorporation of POSS-amine, gas permeability reduced significantly compared to neat PDMS-PU. A decreasing trend in gas permeability was observed with increasing loading of POSS-amine, due to the increase in cross-linking density which resulted in decrease in the solubility and diffusivity of the gases. The permability of gases through POSS-amine incorporated hybrid film was in this order: CO2> O2 >N2 [22]. In another study, Madhavan and Reddy [139] measured the gas permeability through PDMS-PU based hybrid membrane by incorporating two different types of caged POSS; i.e., octakis(hydridodimethylsiloxy) octasilsesquioxane (POSS-H) and heptacyclopentyl 43

tricycloheptasiloxane triol (CyPOSS). It was observed that in presence of CyPOSS, the CO2, O2 and N2 gas permeability decreased significantly. However, the addition of small quantity of POSS-H with CyPOSS increased gas permeability of the membranes. It revealed that nature of POSS and compatibility between POSS and PU alters the gas permeability and the polarity of PU also played an important role in controlling the selectivity of gases through PU membrane. 9. PUNC based coatings There are many, one-pack/two-pack ASTM coating systems based on 100% solid PU or polyurea, which are applied on different substrates for improving their chemical resistance or corrosion resistance [165]. More recently, nanostructured PU composite based coatings are being applied on substrates for improvement of gas barrier properties as well as for many additional functionalities. Both, the solvent based TPU was well as waterborne PU resin (thermoplastic/thermoset) based formulations are very popular in this regard. The moisture cured, UV cured or electron beam (EB) cured PU or PUNC coatings are very popular especially for anti-corrosion applications [166-167]. In any PU nanocomposite coating following aspects are very important [21]: i)

The nano-filler should be dispersed and exfoliated in PU matrix

ii)

The whole mixture should be microscopically homogenous

iii)

The solvents have to evaporate completely after drying or curing of PU resin

iv)

Rate of evaporation should be controlled in such a way that the nanoplatelets should not collapse or aggregate

PUNC based barrier coatings can be applied by different techniques such as- direct hand coating, knife over roll coating, dip coating, spin coating, chemical vapor deposition (CVD), 44

layer-by-layer (LBL) self-assembly, sol-gel process, in-situ polymerization and electrophoretic deposition (EPD), followed by solvent evaporation and drying or curing [168]. 9.1. PU/Clay nanocomposite based gas barrier coatings and adhesives In an interesting study, Osman et al. [141] observed that water vapor transmission is much less than oxygen transmission through PU/organoclay nanocomposite adhesive with same amount of layered-silicates loading in both cases (Fig. 37). The higher barrier property against water vapor was due to the stronger interactions and hydrogen bonding between PU matrix and water molecules as well as their clustering. Furthermore, the water transmission rate was also influenced by the differences in the hydrophobicity of the clay-coating or organo-modifier. In this study three different clays were used, namely Nanofil 15, Nanofil 32 and Nanofil 804, where the organo-modifiers were dimethyl dihydrogenated tallow ammonium, alkylbenzyldimethylammonium (benzalkonium) and bis(2-hydroxyethyl) hydrogenated tallow ammonium, respectively and the tallow part constituted of a mixture of long alkyl homologues with octadecyl being the most prominent component. Surprisingly, an increasing trend in oxygen gas permeability was observed with increasing volume fraction of Nanofil 15, while a decreasing trend was observed for other two organoclays (Nanofil 804 Nanofil 32) [Fig. 37(a)]. The increase in oxygen gas transmission rate in presence of Nanofil 15, was owing to the change in morphology of the nanocomposite which resulted from phase separation at the interface of polar PU matrix and nonpolar hydrocarbon based filler coating. It lead to increase in free volume at the interfaces and also increased oxygen permeation rate. Interestingly, water vapor permeability gradually decreased with increasing volume fraction of Nanofil 15, similar to other organoclays (Fig. 37(b)). Again this dissimilarity in permeation rates also might be due to the size difference 45

of mobile units or permeants (oxygen and water vapor). Moreover, the water molecules tend to form cluster during diffusion through polymer, which may explain the difference in transmission rate of oxygen and water vapor through PU/Nanofil 15 composites [73,141]. Some researchers have patented the use of PU resin compositions as adhesive coating with excellent barrier property, for laminating a barrier polymeric film on to the packaging substrate [169-170]. Joshi et al. [39] observed a significant reduction in air and hydrogen gas permeability of PU/clay (3wt%) nanocomposite coated nylon fabric when compared with neat PU coated fabric. Another study reports 58.5% reduction in helium gas permeability through PU/clay nanocomposite coatings compared to neat PU based coating, where the clay (Cloisite 30B) were ball-milled in optimized conditions before mixing with PU [143]. In a very recent study, Chatterjee et al. [11] prepared a series of formulations using a mixture of conventional UV stabilizer (Tinuvin B75), graphene and organomodified nanoclay (Cloisite 30B) and applied a multi-layered coating on a woven polyester fabric. In presence of clay or graphene, deterioration of gas barrier properties was much less after exposure to weathering, compared to neat PU based coatings. Ma et al. [171] observed that 3-aminopropyltriethoxysilane functionalized graphene (AFG)/PU nanocomposite coatings have improved thermal stability compared to neat PU. Fig. 38 shows the schematic illustration of the preparation of AFG/PU nanocomposites, involving following steps(i) Treatment of graphene oxide (GO) with 3-aminopropyltriethoxysilane (AMEO), (ii) Reduction by hydrazine hydrate and (iii) incorporation of AMEO-functionalized graphene into PU matrix. Because of good interfacial interaction between AFG and PU, graphene was dispersed well in PU matrix. Here, the functionalized graphene sheets played the key role in increasing thermal stability by retarding the permeation of oxygen gas and also by increasing the ‘tortuous path’ of volatile degradation species. 46

9.2. UV-curable PU/clay nanocomposite based gas barrier coatings Möller et al. [145] developed a fast, simplistic and economical method for preparation of transparent, flexible and oxygen-barrier coatings by using cationic and UV-curable PU (ccPU) with two different clays- (i) natural montmorillonite (MMT) and (ii) a highly charged, coarsegrained, unique synthetic clay, Li-hectorites (HEC) (Fig. 39). With addition of the ccPU polycation to aqueous clay suspensions, hybrid materials (O-MMT and O-HEC) were formed. These polymeric modifiers acted as flexible matrices, were capable of filling the voids between the clay-platelets as well as reduce the free volume in the barrier films. Additionally, for curing of hybrid building blocks ethylenic, unsaturated moieties were included in the ccPU design. A highly flexible, transparent and oxygen-barrier nanocomposite was produced after hardening of the composite film via UV-irradiation (Fig. 39(vi/vii)). The gas barrier performance of O-HEC hybrid was better than O-MMT hybrid, by almost one order of magnitude. Moreover, the hectorite clay proved much better choice as filler than MMT, to prepare flexible and transparent barrier material because of its colorless appearance [145]. 9.3. Waterborne PU/clay nanocomposite based gas barrier coatings Stratigaki et al. [21] developed environment-friendly, economic and gas barrier coatings based on waterborne acrylic and PU emulsions with incorporation of different nanoclays by using different dispersion/suspension media. The waterborne resins formed colloids having a core-shell structure (hydrophobic core and hydrophilic shell). Significant improvement in CO 2 gas barrier property was observed with incorporation of both type of clays (modified and unmodified) due to the proper dispersion of clays in resins. The enhancement in gas barrier property was mainly affected by four parameters: (i) dispersion/exfoliation of clays in matrix, (ii) aspect ratio of clays, 47

(iii) formation of extra free-volume created at the clay/matrix interfaces during preparation and (iv) possible interaction between the permeating gas and the organic modifiers of the clay. The dispersion of organomodified clays in waterborne resin was controlled by selecting proper solvent (matching solubility parameter) to prepare a compatible solvent-based dispersion medium. The unmodified clay was able to disperse and exfoliate properly only in a relatively hydrophilic resin (acrylic) [21]. Rahaman et al. [172] coated a nylon fabric with a series of coating formulations based on waterborne PU (WBPU)/clay nanocomposite dispersions (clay concentration: 0-2 wt%), hardener (0.5 wt %) and thickener (0.5 wt %). Although the water vapor permeability (WVP) of coated nylon fabric increased with increasing temperature, the WVP as well as gas permeation rate decreased significantly with increasing clay concentration (Fig. 40(a)). This reduction in water vapor permeation through WBPU/clay nanocomposite coated fabric can be explained on the basis of increase in – (i) length of effective diffusion pathways (ii) T g and (iii) storage modulus, due to tight packing of PU chains, as confirmed by DMA (Fig. 40(b-c)) [66]. The barrier effect of dispersed nanoclay platelets also provides high corrosion resistance because of their ability to prevent the diffusion of corrosive species. Moradi et al. [172] utilized the barrier property of an amine functionalized nanoclay (cloisite 30B) in a novel two-component one-pack waterborne PU based anticorrosive coating prepared by the cathodic electrophoretic deposition method. Table 5 highlights the enhancement in gas barrier property of different PUNCs based coatings. 10. Theoretical models to predict the gas barrier property of polymer nanocomposites The improvement in barrier properties of polymer nanocomposites is attributed to the distribution of nanoplatelets such as clay, graphene etc in the polymer matrix and the formation 48

of a tortuous path for permeation of gas molecules [38]. Many researchers have proposed different models to determine the gas-transmission through polymer nanocomposites filled with inorganic nanoplatelets [173-176]. As per solution-diffusion model [38] permeability (P) of gases and vapors through polymer membranes is the product of solubility (S) of gas or vapor in the polymer matrix and diffusivity (D) through the polymer matrix, as shown in Eq. (3.3). The solubility of the gas molecules in the polymer nanocomposites depends on the volume fraction of the filler and these two are related as below: ……………………………….……………………....... (10.1) Where φ is volume fraction of fillers and S0 is the solubility of pure polymer matrix [31]. The diffusivity through the nanocomposite depends on tortuosity of diffusion-pathways through the membrane, which are related as: …………………………………….……….………………. (10.2) Where τ is tortuosity and D0 is the diffusivity of the pure polymer matrix [31]. The tortuosity (τ) is the ratio of the distance that gas has to travel across the polymer nanocomposite (l ) membrane/film and the membrane/film thickness (l), which can be expressed as [26]: ………………………………………………..……………….. (10.3) The value of τ depends on volume fraction and shape of the nano-fillers. Several researchers have calculated its value for different types of fillers. Actually, in presence of nanoplatelets in polymer matrix, the gas diffusion length increase and it can be calculated by following relation: …………………………………….……..……. (10.4) Where N= number of nanoplatelets =

, ‘L’ and ‘w’ are length and width of the nanoplatelets

(dispersed in a polymer matrix), respectively as shown in Fig. 41. Therefore, Eq. (10.3), i.e. the equation of tortuosity can be expressed as follows [177]: 49

…………………………………………...... (10.5) Where =L/w is the aspect ratio of the nanoplatelets. 10.1. Effect of nanoplatelets volume fraction and aspect ratio Many researchers have developed empirical models to calculate gas permeability through polymer nanocomposites based on tortuosity effect and structure of the nanocomposites. Nielson (1967) first developed a simple model to determine the relative permeability of polymer-clay nanocomposites with respect to neat polymer [173]. Nielson assumed a regular arrangement of two-dimensional rectangular nanoplatelets which are perfectly exfoliated and aligned perpendicular to diffusion direction. This model can be represented as below: …………………………………………………………… (10.6) Where P is gas permeability through polymer nanocomposites, P0 is the gas permeability of pure polymer matrix and P/P is the relative permeability (Rp) of polymer nanocomposites. Substituting for from Eq. (10.5), it becomes: ………………………………………………....……….. (10.7) From this equation, it is clear that relative permeability decreases with increasing aspect ratio and volume fraction of nanoplatelets. The predicted relative gas permeability for different

and

φ values have been shown in Fig. 42. In practice, the Eq. (10.7) is valid only when φ≤0.1 because most of the nanoplatelets tend to aggregate with increasing φ [26]. Later, Nielsen’s model was modified by Cussler et al. [175], assuming a system of well-aligned, randomly spaced flakes within a polymer matrix as shown in Fig. 43. Clussler’s model can be represented as follows: 50

………………………………….……… (10.8) Where μ is a geometric factor, characteristic of the random porous media which depends on the geometric shape of the plate-like particles, their shape and size irregularities and also on the extent of positional disorder [176]. The difference between these two models is that Nielsen’s model assumes a very low i.e. dilute filler concentration with regular arrangement of platelets, whereas Cussler’s model, also referred to as semi-dilute system with higher filler concentration, is more suitable for membranes with randomly sized, highly aligned and spaced flakes. There are many models which are based on the idealized flake-filled membrane as shown in Fig. 44. This membrane contains flakes of width ‘2d’ and thickness ‘a’, separated by a distance ‘b’, and extends infinitely into the plane of the page. The gap or distance between flakes is ‘s’. Eitzman et al. [178] developed an approximate derivation of Cussler’s equation for barrier membranes containing tipped flakes. This theoretical model was based on Monte Carlo simulations, considering the effect of flake’s orientation as follows: …………………………………..…………………… (10.9) Where

is the angle (in degree) of orientation of flakes. Using a similar flake-filled membrane

system, Falla et al. [179] further added two variations of Cussler’s model which are equivalent to Aris model [180] and Wakeham-Mason model [181], expressed by Eq. (10.10) and Eq. (10.11), respectively. -1

…….………………..….. (10.10)

-1

…….……………….….. (10.11)

51

Where σ= s/a (slit shape). Eq. (10.10) and Eq. (10.11) differ in the fourth term on the right-hand side. While the relative gas permeability as expressed by Aris Model is dependent on the aspect ratio , while in Wakeham-Mason model it is not. Lape and coworkers [182] proposed a modified model considering rectangular shape flakes/nanoplatelets of same aspect ratio placed parallel to each other and perpendicular to the direction of gas diffusion but arranged randomly. As per their model: ……………………………………………………… (10.12) Where Af and A0 are the available areas for diffusion in a flake-filled and homogeneous membrane, respectively; l is the distance that the gas molecules diffuses through the polymer nanocomposite, expressed in this case as below: ……………………………………………………….. (10.13) This equation is same as Eq. (10.4), the only difference being replacement of L/2 with ‘d’, the average distance the gas molecule has to travel to reach the edge of the platelet. Making some assumptions, the value of ‘d’ can be estimated considering the gas molecules randomly hit platelets along its length and the resistance to mass transfer is assumed proportional to traveled path length. So, l’ becomes: …………………………………………………. (10.14) The Eq. (10.14) is differes from Eq. (10.4) by a factor 2/3 due to the randomness of the positions of the nanoplatelets. Af can be calculated by dividing the available volume for diffusion by distance traveled through the membrane. Thus, …………………………………………………........… (10.15)

52

Where Vf is the total volume of all the nanoplatelets and Vm is the total volume of the membrane. Therefore, the relative permeability becomes as below: ……….. (10.16) For regular array of mono-disperse flakes this equation turns into [182]: ……….. (10.17) Fredrickson and Bicerano (1999) also modified both the Neilsen’s equation and Cussler-Aris equations for circular disk-like fillers of radius R and thickness 2W (see Fig. 45) [176]. These newly modified equations cover a wider range of φ and are applicable to both dilute and semidilute regimes. The ‘modified Neilsen’s equation can be expressed as: …………………………………………………..………. (10.18) and it is applicable for a dilute regime of disk concentration ( φ 1). The

Where k=

modified Neilsen’s equation is seen to be accurate at k φ

. Similarly, ‘modified Cussler-Aris’

equation can be expressed as follows: …………………………………………………….….. (10.19) Where μ=

2

/16ln and it is applicable to a semi-dilute regime of disk concentration (φ 1, but

φ 1). The modified Cussler-Aris equation is accurate for k φ

[176,183].

Gusev and Lusti (2001) developed a 3D periodic multi-inclusion computer model for random array of perfectly aligned circular discs [184]. This model is based on the solution of Laplaceequation for the local chemical potential: ………………………………………..…..… (10.20) Where P(r) is a position-dependent local permeability coefficient, whose value is assumed to be zero inside the platelets. This proposed model can be expressed as below: 53

………………………………………….....… (10.21) Where x0=3.47 and β=0.71, which are usually calculated by fitting experimental data [184]. 10.2. Effect of orientation of nanoplatelets It is noted that, the nanoplatelets are generally randomly dispersed in polymer matrix [185]. However, the gas barrier performance of polymer nanocomposites strongly depends on the orientation of nanoplatelets in polymer matrix. Therefore, in order to incorporate the effect of orientation of nanoplatelets on gas permeability of polymer nanocomposites, Bharadwaj (2001) further extended the Nielsen’s model by introducing an order parameter (S) which is defined as: ……………………………………..……… (10.22) Where

represents the angle between the direction of preferred orientation (n) and the sheet

normal (p) unit vector as shown in Fig. 46 [174]. When =0°, S=1 which indicates the case of perfect alignment as shown in the inset Fig 46. When =90°, S= -1/2 which indicates a perpendicular or orthogonal orientation of nanoplatelets showing almost no barrier to the diffusion of gas molecules in nanocomposites. Similarly, S=0, when =54.74°, indicating a random orientation of nanoplatelets in a polymer–matrix [174,185]. According to Bharadwaj model the relative permeability of a polymer nanocomposite can be represented as: …………………………….……… (10.23) This equation reduces to Eq. (10.7) when S=1, i.e., planner arrangement. In this case, the horizontally stacked layers of nanoplatelets provide maximum tortuosity, resulting in significant decrease in gas permeation rate through polymer nanocomposites [18,26]. On the contrary, Eq. (10.23) converges approximately to the permeability of the neat polymer when S= -1/2, i.e., 54

orthogonal arrangement as illustrated in Fig. 46. The effect of sheet orientation on the relative permeability of exfoliated polymer nanocomposites has been represented in Fig. 46 for different lengths of nanoplatelets at φ= 0.05 and W =1nm. In a different approach, Maksimov et al. [186] developed an empirical relation for measuring moisture permeability in a polymer nanocomposite containing randomly oriented 3D nanoplatelets. This model can be represented as below: )

…………………………………………… (10.24)

Where, P‖ is the permeability of the polymer nanocomposites as given by the Nielsen model for oriented nanoplatelets. Whereas, the second term represents the correction factor for permeability of the nanocomposites with nanoplatelets oriented along the diffusion direction. 10.3. Effect of the interfacial regions In many cases, the existence of interfacial regions which are created by the surfactant used in modification of nanoplatelets or by the formation of voids between two phases, affect the gas permeability. Sorrentino et al. [187] expressed an equation for relative diffusion coefficient as below: ………………………………………………………….… (10.25) Where Dc and Dm are the diffusion coefficients in composite and matrix, respectively; a' is a factor which quantifies the effect of the interfacial regions and can be calculated as below: ……………………………………………………..… (10.26) The factor

has a strong influence on the diffusivity at all the concentrations of the

impermeable phase (filler). It can be expressed as:

55

…………………………………………….….. (10.27) Where Ds and D0 are the diffusion coefficient in the interfaces and neat polymer, respectively. Vs and Vf are the volumes of the interfaces and the nanoplatelets, respectively. If Vs/V f is negligible, then

= -1 and

=1- . However, the value of Vs and Ds are very difficult to

measure [187]. 10.4. Effect of delamination state and dispersion of nanoplatelets in polymer matrix The much-expected improvement in gas barrier property of a polymer nanocomposites is strongly dependent on delamination state and dispersion of platelets in the polymer matrix. Actually, the model predicted gas barrier property values rarely match with experimental results with few exceptions because of the aggregation tendency of nanoplatelets especially at higher loading [133,174]. The effective width of nanoplatelets in polymer nanocomposites may vary depending on the extent of delamination, as illustrated in Fig. 47. With increasing delamination, the nanoplatelets exfoliate in a better way in the polymer matrix which is the most critical factor to obtain maximum gas barrier performance in polymer nanocomposites. However, aggregation of sheets or platelets result in a dramatic decrease in the tortuosity, showing a percolating path for the diffusing gas [174]. Therefore, relative permeability of a polymer nanocomposite gradually increases with increasing aggregate width, as shown in Fig. 47. 10.5. Effect of number of nanoplatelets layers and their aggregation It has been reported in many literature that with increasing number of layers in nanoplateletstacking (aggregate), the overall tortuosity of the polymer matrix becomes much low which results in the increase in gas permeability through polymer nanocomposites. Therefore, 56

Nazarenko et al. [188] modified the Neilsen’s equation considering the effect of number of disklike layers (N) in the stack (Fig. 48) as below: ……………………………………………….. (10.28) Where ‘h’ is disk thickness which is related to the width of the stack (W) and number of layers (N) as follows: ………. (10.29) Subsequently, a factor 1/3 was added in the denominator when the aggregates were dispersed randomly, as follows: ………………………….…. (10.30) The predicted gas permeability results for different N (at =500) and for different

(at N=10)

are shown in Fig. 49. When N=1, it simply implies complete delamination of layers or complete exfoliation resulting in maximum gas barrier performance of polymer nanocomposites. However, when the value of N is large, the delamination of layers is poor which results in poor gas barrier performance of polymer nanocomposites. 11. Experimental vs. theoretical models Almost all the theoretical models discussed in earlier section are dependent on volume fraction and aspect ratio of nanomaterials which are well dependent on each other. Generally, with increasing volume fraction, aspect ratio decreases, due to lower delamination tendency. Many researchers have tried to validate these models by comparing with experimental gas permeability results of PUNCs. Though in some cases, the model estimated results did not match with experimental results, good estimations could be found in many cases.

57

Xu et al. [10] applied ‘modified Cussler-Aris’ equation (Eq. (10.19)) for predicting water vapor permeability through PUU/clay nanocomposite films considering a semi-dilute regime (Fig. 50). The model predicted values demonstrated a good correlation with experimental values with change in aspect ratio from higher values for exfoliated single layers (at lower loading) to lower values for intercalated multilayer stacks at higher volume fractions. A similar study [158] reported that a better fit in the relative permeability results may obtained at higher volume fractions with a lower aspect ratio (α=300) compared to the higher aspect ratio (α=1000) of nanoplatelets. Maji et al. [17] also compared the experimental helium gas barrier properties of different PU/clay nanocomposites with gas barrier values predicted by seven different models (Fig. 51). A good correlation was established between gas barrier property and dispersion of clay nanoplatelets in PU matrix, as also characterized by TEM and AFM. A good correlation between actual and model-predicted gas barrier properties were also reported by Kim et al. [18] for PU/graphene nanocomposites (Fig. 52) produced by all three synthesis routes (melt, in-situ and solution mixing). In this case, relative nitrogen gas permeability was fitted using the model developed by Lape and co-workers assuming the graphene sheet as monodisperse flakes aligned to the polymeric film but arrayed randomly. About 90% reduction in nitrogen gas permeability was observed with only 3wt% iGO, which is comparable with gas barrier performance of unidirectionally aligned flat impermeable flakes having aspect ratio of 400-500. Similarly [96], a very good correlation of CO2 gas permeability of PU/Vermiculite clay nanocomposite films produced by both direct and master-batch routes was obtained with theoretically predicted values based on Fredrickson and Bicerano model (See Fig. 53). 58

In contrast, Stratigaki et al. [21] observed that reduction in gas permeability of PU/clay nanocomposite coatings did not follow the gas barrier model proposed by Nielsen. It is possibly because of change in free volume around the clay-interfaces in the system at higher clay loadings. Similarly, Herrera-Alonso et al. [133] reported that in none of the three different PU/clay nanocomposite systems, there was agreement between values measured experimentally and predicted by using Nielsen and Cussler models. In summary, it can be concluded that only in some cases a good correlation is obtained between experimental and model predicted gas permeability values. The main reason behind it that during prediction by model some ideal conditions are assumed such as – (i) all the platelets have the same aspect ratio and (ii) they are uniformly dispersed and oriented in the polymer matrix which is not the actual case. Achieving complete exfoliation and uniform dispersion of fillers in the polymeric matrix is very difficult (especially at higher loading) which creates a deviation between theoretical and experimental results. Therefore, recently, Wu et al. (2016) developed a simple and novel numerical method for predicting effective diffusion coefficients of gases and vapors through PUNC membranes. The two-dimensional (2D) nano-structure morphology of PUNCs was captured using TEM and the image was analyzed by an image processing software. The 2D image matrix data was further transformed into mathematical morphology data and numerical simulation models were developed for predicting gas and vapor permeabilities through PUNC membranes. A MATLAB software was used for distinct integration of mass of gases/VOCs in the whole domain as a function of time. The permeabilities of gases/VOCs were also predicted using Cussler’s model, but predicted aspect ratio of clay-platelets was found to be much less than the actual aspect ratios, which might be due to their agglomeration. However, the proposed numerical model 59

showed a good potential in predicting effective diffusion coefficients of gases/VOCs more precisely than conventional gas-permeability models [189]. 12. Potential applications of PUNC based barrier films and coatings Low gas permeable graphene or clay reinforced PUNCs based films, membranes, coatings or laminations have a potential for wide range of applications such as food packaging, medical applications (e.g. controlled release or encapsulating membranes, dialysis membranes or wound dressings, catheters etc.), lighter than air (LTA) systems, automotive tire application and coatings, sport balls, aerospace coatings, flame retardant and thermal insulation coatings etc [51,190]. 12.1. Packaging applications Restriction of gas migration through packaging materials to increase the shelf life of the products, is a great challenge for packagers. The migration of oxygen into the beer bottle can make the beer stale. Similarly, the migration of CO2 from the soda bottles can reduce its shelf life. Therefore, strong barrier properties against different gases (oxygen, carbon dioxide, water vapor, aroma etc) as well as good mechanical properties are very essential for packaging materials. Generally, multilayered film/foil based laminated structures are used for food packaging where PU based adhesives are extensively used to bind different layers [169-170]. The main advantages of PU based adhesive in producing packaging laminates are their flexibility and wide temperature range for applications [141]. However, the contribution of neat PU based adhesives to the barrier performance of laminates is not very significant. Therefore, PUNC based adhesives can be used to impart good barrier as well as strength to the packaging laminates [4].

60

Moreover, clay, graphene, or functionalized POSS reinforced PUNC based barrier coatings can be potentially applied for industrial packaging applications [81]. Osman et al. (2003) synthesized a gas-barrier adhesive nanocomposite using PU and organically modified montmorillonite. This PUNC adhesive was further used to join two corona treated polypropylene and polyamide foils as shown in Fig. 54. It was observed that with incorporation of only 3wt% organically modified nanoclays, the oxygen and water vapor transmission rate through PUNC reduced by 30% and 50%, respectively [141]. 12.2. Biomedical applications PUs, especially segmented polyether based PUs are used in wide range of applications in biomedical field because of its good physical and mechanical properties, abrasion resistance, hydrolysis resistance, flex fatigue resistance and biocompatibility [47,191]. Controlled gas or vapor permeability of PUs is an important feature for many biomedical applications such as for controlled release or encapsulating membranes, dialysis membranes, catheters, wound dressings etc. PU based segmented block copolymers, particularly poly(urethane urea) [PUU] are currently used in a variety of blood-contact applications in biomedical devices such as blood sacs in ventricular assist devices and artificial hearts [51,192194]. These biomedical PUU generally shows good biocompatibility and flex fatigue characteristics. However, their permeability to air and water vapor is relatively high due to their low Tg and relatively high concentration of soft segments [10]. Several approaches have been explored by researchers to improve air and water vapor permeability of segmented PUs [10,158,194-195]. Among these, one of the latest and most effective approach is the use of PUNC based gas barrier membranes. Xu et al. (2001 & 2002) have reported the development of 61

novel poly(urethane urea)/clay nanocomposites for biomedical applications with high gas and water vapor barrier properties [10,158]. 12.3. Material for envelope of lighter-than (LTA) air systems TPU based multilayered coated and laminated textiles are extensively used in different inflatables or lighter than air (LTA) systems such as parachutes, hot air balloon, aerostat and high altitude/ stratospheric airship [196]. Especially, flexible films based multilayered laminated fabrics are extensively used in the hull (outer envelope), fin (vertical stabilizer) and ballonet (internal barrier) materials of aerostat or high-altitude airship (see Fig. 55) [197-199]. These LTA applications impose significant challenges in maintaining high barrier property against containing lifting gases (hydrogen or helium), resistance against intense radiations (mainly UV and ozone) and flexibility at low temperature [3,200-201]. PU based coated or laminated textile materials are very useful especially for ballonet application because of their good tensile and tear strength, good low-temperature flexibility, fair to good resistance to UV and ozone, good abrasion resistance and heat/adhesive seal-ability [3]. However, hydrogen and helium gas barrier property of neat PU based coated and laminated textiles are not good. Therefore, clay or graphene filled PUNC based barrier films or coated textiles can be potential candidates for development of materials for ballonet and hull. Joshi et al. [39] explored the potentiality of PU/clay coated nylon fabric with good hydrogen gas barrier property for envelope of inflatables. PU has the best low-temperature toughness compared to all commercially available adhesives. Therefore, PU based additives are generally used in adhesive layers of aerostat/airship envelop [3]. Clay or graphene filled PUNC based adhesive have a potential to reduce the helium gas permeability compared to neat PU based adhesives. Furthermore, due to heat sealability of PU, 62

the PUNC based gas barrier layers can be joined effectively by applying heat, avoiding the use of an extra adhesive layer, with can reduce the total weight of LTA systems. Recently, Chatterjee et al. [11] explored the possibility of using a PUNC based coating on a high strength polyester fabric for aerostat envelope with improved gas barrier and weather resistance property, utilizing the synergistic effect of conventional UV resistance additives with nanomaterials such as nanoclay and graphene. 12.4. Automotive tire application Polyurethane rubbers are extensively used by some automotive tire companies owing to its excellent abrasion resistance, tear resistance, chemical resistance, and durability. Moreover, with advancement in rubber technology and nanotechnology, the reduction in gas permeability in case of PU rubber nanocomposites is considered to be one of the most important developments for tire-inner liner application [202]. PU/clay or PU/graphene nanocomposites having excellent air retention property can be utilized in the inner liner of a tube-less tire and in the inner tube of a tube-type tire. Use of barrier PUNCs may reduce the migration of oxygen through the bicycle, auto or truck tires which causes the steel to rust and thereby increase the tire’s life [203-204]. 12.5. Corrosion resistant coating Metals are generally coated with organic coatings to provide protection from corrosion [205]. As a primary method for corrosion protection of metals, the organic coatings act as an insulating barrier to prevent the transportation of oxygen and moisture from surrounding environment to the metal surface [136,206-208]. Recently, Moradi et al. [172] developed a facile method to improve anticorrosion property of mild steel by applying a waterborne PU and aminefunctionalized nanoclay based coat, utilizing cathodic electrophoretic deposition (CEPD) 63

technique. Generally, in automotive coatings, a PU based primer is used due to its very good adhesion property. Use of PU/clay nanocomposite based primer coating may also provide anticorrosion property by reducing the permeability of water vapor and O2. 12.6. Sport balls Gas barrier property is very essential in case of some sport balls such as soccer and tennis balls for feel and bounce due to air retention [209]. Sometimes, PU is used for preparation of bladder in soccer and tennis balls. Barrier properties of PUNCs can be effectively utilized for better performance and life of these sports balls. The basic technology lies in the application of PU based elastomeric barrier coating which contains highly exfoliated clay mineral/ graphene platelets [210-211]. In golf balls, top-two cover layers of the inner layer, are generally coated with a moisture barrier PU based formulation [212-213]. PU/clay or PU/rGO nanocomposite based coating formulations can effectively improve its moisture barrier property. 12.7. Thermal insulation Gas barrier property is a prime requirement also for rigid PU foams used for thermal insulation. One of the largest modern use of PU is as closed cell rigid foams for thermal insulation, due to their better thermal insulation properties and cost effectiveness, in comparison to other common insulation materials [214-215]. PU foam is generally prepared by in-situ polymerization reaction when a blowing agent expands the reacting mixture and finally form a solid, cellular foam. However, one major problem of PU based foams is the loss of blowing gases (such as hydrofluorocarbon, hydrocarbons, nitrogen, CO2 etc), which causes slow increase in its thermal conductivity. Presence of plate-like nano-fillers such as clay in rigid PU foam creates a diffusional barrier, which effectively reduces the gas transport and increases its thermal 64

insulation [96,216]. In case of neat PU foam, fewer cells with larger cell size are obtained. Clay acts as a nucleating agent as well as a gas barrier material in the foaming process. Therefore PU/clay nanocomposite based foams show higher number of cells with smaller cell size with increasing clay concentration. It results in decrease in gas permeability with increase in density and thermal insulation property [215]. 12.8. Other miscellaneous applications Transparent, flexible and barrier PUNC films or coatings have many other potential applications. Generally, in the protective printing systems of aerospace, a PU based top coat is used (Fig. 56). A PUNC (containing clay/graphene/POSS) based top coat can provide better adhesion property, barrier property, corrosion resistance, robust mechanical property, thermal stability and flame retardancy [217-218]. Super-flexible, transparent and gas barrier PU/clay nanocomposites are highly recommended for coating on high tech encapsulation applications (such as thin-film transistor (TFT) and organic light-emitting diode (OLED) displays) [219], lightweight mobile gas pipeline and gas storage containers [20]. Möller et al. [145] developed a cheap, fast and facile method for preparation of transparent diffusion-barrier coatings for encapsulation of optoelectronic devices, using UVcurable PU and Li-hectorites (HEC) in larger batches. In addition to gas barrier properties, the excellent solvent barrier properties of PUNCs can be utilized in surgical gloves to provide protection against chemical warfare agents and also for avoiding contamination from medicines [220]. POSS reinforced PDMS-PU hybrid membranes can be potentially used for gas separation due to their differential solubility for different gases [22,139]. The flame retardancy performance of PUNC based FR-coatings on textiles or polymeric materials can be improved by incorporating 65

nanoclay, graphene or POSS due to their potential for improving barrier property against diffusion of flammable gases or other volatile matters produced by burning. Therefore clay, graphene or POSS reinforced PUNCs have a great demand for FR-coatings. 14. Future scopes and challenges In most of the previous studies, solution mixing or in-situ polymerization was used for the synthesis of PU nanocomposite based gas barrier coatings/films/membranes, because these techniques facilitate better dispersion of nanofillers as compared to melt-mixing. However, the main disadvantage of solution mixing and in-situ polymerization are – (i) issue of environmental pollution due to the use of toxic solvents, (ii) complicated process and (iii) high processing cost. Therefore, these processing routes are not as much industry viable as melt processing. Although some study reports on melt processing of PU nanocomposites, almost no study has been reported on gas barrier property of PU nanocomposites produced by melt mixing routes. Therefore, concentrating on industrial needs, there is a great challenge as well as scope in preparation of gas barrier PU nanocomposites films or coating by melt processing. Additionally, a multilayered coextrusion by force assembly as developed by Baer and coworkers [221-223] may be useful in preparing gas barrier PU nanocomposite films in near future, where different nanomaterials can be incorporated in different thin layers of PU. Although, few studies have been reported for enhancement of gas barrier property of PUNC films by master-batch route, there is large scopes for research in this area, producing exfoliated nanocomposite based master-batches before further mixing with neat polyurethane. Molecular weight and viscosity which are inter-dependent, may also affect the morphology and hence the gas barrier property of PU and PUNC films and coatings. It has been observed by the 66

authors (unpublished work) that an optimum viscosity of PU solution, which is mainly dependent on molecular weight of PU, is needed for proper coating on a substrate for providing better gas barrier property. This particular aspect is yet to be explored. Presence of pin holes in films or coatings, significantly reduce its gas barrier property. Multilayered films or coatings may be the best solution for this. The PUNC based multilayered solution coatings will not only improve gas barrier property, but can also provide more uniformity. Similarly, multilayered PUNC films containing different gas-impermeable nanoplatelets in different layers, may provide better gas barrier property than a single PU film of same thickness and containing same type of fillers, by minimizing the number of pin holes. Moreover, multilayered nanocomposite films made of isocyanine free PU or food grade PU may be a potential replacement for the convention aluminum coated polyethylene packaging, solving the problem of recyclability of multilayered plastic packaging materials. Recently, due to increasing environmental concerns such as end-life disposal and sustainability, the renewable/recyclable resources are getting much interest. Therefore, biodegradable PU based green barrier films reinforced with nanocellulose may be a potential replacement of conventional non-biodegradable food packaging materials [83] as well as for many biomedical applications in future. Many models and experimental results of different studies show that with increasing aspect ratio of clay-layers in a polymer matrix, the gas barrier property of nanocomposites improves. Therefore, there is a huge scope for research for improving gas barrier properties by using clays having very high aspect ratios, such as synthetic mica. However, there is an additional challenge of retaining the high aspect ratio of clay-layers in the final PU nanocomposites. The gas barrier models indicate that the gas barrier property of polymer can also be improved by improving the 67

orientation of nanoplatelets in polymer matrix. However, almost no study has been conducted on this aspect. Also, most of the early studies have mainly focused on the use of clay and functional graphene/GO for improving the gas barrier property of PU. Although few studies have been reported on PU/CNT and PU/POSS nanocomposites for gas barrier applications, more intensive study is required to understand the potentiality of CNT and POSS for improving the gas barrier property of PU. Moreover, there are some other nanomaterials like boron nitride, nanocellulose and few metal oxide nanoparticles (TiO 2, SiO2, Al2O3 etc) which have been studied with other polymeric systems. Therefore, these nanomaterials may also be explored for improving the gas barrier property of PU. In this context, the h-BN, which has a hexagonal layered structure like as graphene, may be a potential material for making high-gas barrier PUNC films or coatings. 15. Concluding remarks The permeability of gases through PU membranes, films and coatings depends on various factors such as type, molecular weight, Tg, crystallinity, structure and morphology of PU, nature of crosslinks and crosslinking density, surrounding environment (temperature, pressure difference), size of the gaseous molecule etc. Among different factors, the structure and morphology of PU itself is one of the predominant factors which affects the gas permeability. The structure and morphology of PU is controlled by the nature and chemistry of raw materials (isocyanate, polyol, chain extender and catalyst), their compositions and preparation conditions. Generally, the gas permeability decreases with increase in cross-linking density and Tg. The molecular size and the shape of gas molecule also effect the transport rate through PU membranes or films. Generally, diffusion rates increase with reduction in molecular size of gases and increase in temperature. 68

Proper dispersion and orientation of plate-like nano-fillers such as layered silicates and graphene, results in significant improvement of gas barrier properties by lengthening the tortuous path for gas diffusion. This property can be improved further by increasing aspect ratio of nanoplatelets or by increasing volume fraction of passive nano-fillers up to a certain limit. Some reactive nanofillers (functionalized graphene and CNT, POSS, nanocellulose etc) are also capable of improving the gas barrier property of PU by increasing crosslink density. There are many proposed models for predicting transmission rates of gaseous molecules through polymers and their nanocomposites. Almost all models use aspect ratio, volume fraction and orientation of nano-fillers as critical parameters. In many proposed gas permeability predicting models, the assumptions like complete exfoliation of layered fillers and uniform dispersion in matrix are not realized in actual use, hence many times poor agreements are obtained between predicted and measured gas permeability values. Moreover, it is very difficult to measure the actual aspect ratio and orientation of fillers when they are distributed in the polymer matrix. Actually, structure and morphology of PUNCs are much more complicated than virgin PU. Because of the interactions between PU and fillers, there are changes in free volume fraction and segmental mobility of PU chains. The gas barrier property can be improved significantly by proper modification of fillers and dispersing them at nano-level in PU matrix. The process route has a strong potential for improving dispersion of nano-fillers in PU matrix as well as improving gas barrier properties. It also possible to improve the dispersion of nanofillers in polymer by using high energy ball milling, sonication, high-speed shear mixing or by master-batch based mixing. Though extensive research has been done on gas barrier property of nanoclay or graphene reinforced nanocomposites, there is a great possibility to explore the potential nano-fillers such 69

as POSS, CNT and nanocellulose for PU. PU/POSS hybrid nanocomposite gas barrier membrane can be used for gas separation. PUNC based barrier coating or lamination on textiles, metals or other polymeric materials imparts a wide range of properties to a diverse range of end applications such as flexible envelop for inflatables, packaging, fuel and water storage tanks, automotive coatings, thermal and flame retardant applications, sport balls and many more. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Reference [1]

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Figures with captions

Fig. 1. Gas permeation through (a) polymer, (b) polymer with micro-additives, and (c) polymer nanocomposite.

Fig. 2. Schematic representation of gas permeation through polymeric film, where c and p represent the concentration and pressure of the diffusing gas/permeant (c1 > c2 and p1 > p2).

89

Fig. 3. Experimental cell for the measurement of gas permeability. A membrane (thickness- d, area-A) divides the cell into two chambers. The gas is introduced in volume V 0 under constant pressure, P0. The concentration of the test gas is c0 on the left side of the membrane and cd(t) on the right side of the membrane. The concentration of the gas within the membrane is c(x,t) [26].

Fig. 4. Schematic of the basic reaction of polyol, diisocyanate and chain extender for synthesis of PU. Morphology of PU containing hard and soft segments has been represented by different colors.

90

Fig. 5. 1st step: synthesis of hydroxy-functionalized polydimethylsiloxane (PPS-C); 2nd step: Synthesis of cross-linked PDMS-PU membrane [36].

Fig. 6. (a) XRD spectra with crystallinity percentage (%Xc) and (b) DSC curves of three different PUs. Here, the weight ratios (with respect to total weight) of polyether polyol and polyester polyol are 20:0, 16:4, 12:8 in TPU1, TPU2 and TPU3, respectively [7].

91

Fig. 7. Schematic illustration of microconfinement effect on forming “hard-segment domain” in TPU B (consists of 100% hard-segments) layer, and microscopic fracture at very high deformation. The yellow layer is TPU B and the dark blue layer is TPU A (flexible soft TPU), orange boxes are the di-isocyanate and light blue spots are chain extenders [62].

92

Fig. 8. (a) Reaction scheme of the crosslinking of the PU prepolymer with hyperbranched polyol; (b) Chemical structure of 2nd generation hyperbranched polyol; (c) Chemical structure of 3 rd generation hyperbranched polyol; (d) Chemical structure of 4 th generation hyperbranched polyol [17].

93

Fig. 9. Helium gas permeability of different PUs produced by different cross-linkers and the lamellar microstructure of different cured PU, as detected by AFM. Here, PUTMP is trimethylol propane based PU (contains 3 –OH functional groups in polyol), PU20 is 2nd generation crosslinked PU (16 –OH functional groups in polyol), PU30 is 3rd generation cross-linked PU (32 – OH functional groups in polyol) and PU40 is 4th generation cross-linked PU (64 –OH functional groups in polyol) [17].

94

Fig. 10. Structure of 2:1 phyllosilicates clay [94].

Fig. 11. Orientation of alkylammonium ions (surfactant/modifier) in the galleries of layered silicates with different layer charge densities [98].

95

Fig. 12. Graphene lattice structure: (top) sp2 hybridized carbon atoms arranged in a 2D planar honeycomb lattice; (Bottom) the molecular structure with rough electronic density distribution. Very small geometric pores (0.064nm) of graphene make it practically impermeable to all molecules [100].

Fig. 13. AFM tomography of graphene showing line scan of the selected individual graphene [101]. 96

Fig. 14. Different techniques for synthesis of graphene and its derivatives

Fig. 15. (a) XRD patterns and (b) Raman spectra of graphite platelets, graphene oxide, and reduced graphene oxide [125].

97

Fig. 16. Basic chemical structure of POSS

Fig. 17. Schematic representation of different states of dispersion of layered nanomaterials in PU composites: (a) phase-separated, (b) intercalated, (c) intercalated-and-flocculated and (d) exfoliated.

98

Fig. 18. (a) WAXD spectra of organoclay and different type of polymer/clay nanocomposite structures; (b) TEM images showing the dispersion of clay-platelets in different types of polymer/clay nanocomposites [29].

99

Fig. 19. Mechanism of organoclay dispersion during melt processing of polymer [138].

Fig. 20. Schematic representation of (a) in-situ polymerization and (b) solution mixing of PU. 100

Fig. 21. (a) XRD patterns of different vermiculite clays before and after modification; TEM images of PU composites with 1.4wt% vermiculite clay (VMT) (b, c) and with 1.4wt% modified vermiculite clay (CTAB-VMT) (d, e) [6].

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Fig. 22. (i) Schematics of PU/graphene composite preparation routes: (a) Oxidation of graphite in concentrated acids; then Functionalization of graphene layers via either (b) Rapid Thermal Expansion (TRG, 2000°C/min to 1050°C in argon) or (c) Modification with isocyanate in DMF (iGO); Incorporation of different graphitic fillers into PU via (d) Melt compounding; (e) Solvent blending in DMF and (f) In-situ Polymerization in DMF. The black lines are indicative of graphitic reinforcements. Short, thick blue blocks and thin red curves represent PU hard and soft segment, respectively. (ii) X-ray diffractograms of graphite, GO, Ph-iGO and TRG. (iii) TEM images showing dispersion of graphite and TRG in PU, incorporated by melt compounding [18].

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Fig. 23. TEM images showing the dispersion of thermally reduced graphene oxide (TFG) in PU matrix processed by different routes: (a) melt-mixing, (b) in-situ polymerization and (c) solution mixing [18].

Fig. 24. Comparison of (a) O2 and (b) CH4 permeation through PUNCs: sonication vs. stirring of Cloisite® series with 28 wt% loading [133].

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Fig. 25. CEC and FWHM values (a) and the corresponding BET surface areas (b) for K-Hec with varying number of shearing cycles. The CEC was determined after a 10 min exchange with [Cu(triethylene tetramine)]2+ [146].

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Fig. 26. (a-d) Optical micrographs of 3 wt% organoclay/castor oil suspensions: (a) after 2-h mechanical agitation, (b) after 15-min sonication, (c) after 30-min sonication, and (d) after 60min sonication. (e) WAXD patterns of pure OMMT and different PU/OMMT (3wt%) nanocomposites [136].

105

106

Fig. 27. Stepwise mechanism of CTAB-VMT (modified Vermiculite clay) exfoliation in MDImaster-batch dispersion and PU nanocomposite: (a) layer expansion via organic modification, (b) clay particle break-up and intercalation through master-batch mixing and sonication, (c) high shear mixing leading to partial exfoliation in MDI dispersion, and (d) diffusion of selected polyol blends (PO200) due to high shear, resulting in platelet exfoliation and high intercalation in the PU nanocomposite [96].

Fig. 28. Illustration of formation of a ‘tortuous path’ of platelets for diffusion of gas molecules resulting in inhibited diffusion of gases through an elastomer composite (a) without alignment and (b) with alignment [149].

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Fig. 29. Effect of clay length on gas barrier property of different polymer-clay nanocomposites [154].

Fig. 30. Comparison of (a) CH4 permeation and (b) helium gas permeation through PU/clay nanocomposites prepared by sonication of different clays [133]. 108

Fig. 31. TEM images for PU melt-blended nanocomposites prepared by: (a) direct mixing of organoclay, TPU/MMT–OH2 (b) melt mixing with organoclay master-batch, TPU/(MMT– OH2/PCL); (c) ) melt mixing with PCL-grafted organoclay master-batch, TPU/(MMT–OH2–g– PCL) [139].

Fig. 32. (a) FTIR spectra of neat PU and PU/rGO nanocomposite having 1 wt.% graphene and (b) the chemical structure of PU and rGO and theeir possible interaction by hydrogen bonding [159].

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Fig. 33. Chemical structure of (a) graphene oxide (GO), (b) graphene nanoribbons (GNRs) and (c) hexadecyl-functionalized low-defect graphene nanoribbons (HD-GNRs); (d) Raman spectra of Go and HD-GNRs; (e) dispersion of GNR and HD-GNRs in chloroform (1mg/ml) [20].

Fig. 34. (a) Pressure drop of N2 gas through PU and PU/HD-GNRs films with time. (b) Pressure drop of PU/0.5 wt % HD-GNRs composite film over a longer time period [20]. 110

Fig. 35. (a) X-ray diffraction spectra and (b) nitrogen permeability of COPU and COPUNCs with different weight percentages of MWCNTs [79].

Fig. 36. (a) synthesis of POSS-amine; (b) synthesis of POSS incorporated hybrid PU membranes with PDMS and PPG [22].

111

Fig. 37. (a) Oxygen transmission rate and (b) water vapor transmission rate through different PU/clay nanocomposites produced by varying the inorganic volume fraction [141].

112

Fig. 38. Schematic illustration of AFG/PU nanocomposite coatings fabrication [171].

113

Fig. 39. Stepwise fabrication of UV-curable barrier coatings. Aqueous dispersions of clays (i) were flocculated by the addition of ccPU-dispersion (ii) Aggregates of obtained clay hybrids were washed (iii) and redispersed in THF (iv) Facile preparation of homogeneous composite films from clay hybrid dispersions by doctor-blading (v) UV-curing to cross-link ccPU to render the composite coating insoluble while improving its oxygen-barrier property (vi) . An idealized O-HEC hybrid-platelet consisting of a clay lamella with ccPU adsorbed on both sides (approximate height ~ 96 Å) (vii) [145].

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Fig. 40. (a) Change in water vapor permeability (WVP) with temperature and clay-concentration (in wt%, as specified in particular code) of WBPU/clay nanocomposite coated nylon fabrics; (b) storage modulus of WBPU/clay nanocomposite films and (c) tanδ of WBPU/clay nanocomposite films [21].

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Fig. 41. Schematic representation of tortuosity gas diffusion through a polymer nanocomposite.

Fig. 42. Predictions of Nielsen’s model for the relative permeability of polymer nanocomposites with respect to the volume fraction of platelets for different particle aspect ratios.

116

Fig. 43. Illustration for a model of barrier membranes with randomly spaced flakes or slits [175].

Fig. 44. An idealized model for flake-filled barrier membrane where the flakes are regularly spaced, extending infinitely into the plane of the page [179].

Fig. 45. Impermeable circular disks of radius R and thickness 2W. The disks are assumed to be positioned such that with unit normal aligned along the direction û [176].

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Fig. 46. Effect of nanoplatelet orientation on relative permeability of exfoliated nanocomposites at φ= 0.05 and W =1 nm. The illustrations show the direction of preferred orientation (n) of the silicate sheet normals (p) for three values of the order parameter (S) -1/2, 0, and 1 with respect to the film plane [174].

118

Fig. 47. Effect of incomplete nanoplatelets exfoliation on relative gas permeability. The illustrations show the effect of platelet width W, with one, two, and four sheet aggregates which were dispersed in the matrix. The plot shows the variation in relative permeability as a function of the aggregate width at varying lengths of the sheets at φ= 0.05 [174].

119

Fig. 48. Illustrations showing modified Nielsen tortuosity model modified for the nanocomposite structure consisting of homogeneously dispersed oriented layered stacks in the polymer matrix [188].

Fig. 49. Predicted relative gas permeability (using Eq. (10.30)) through polymer nanocomposites for parallelly aligned layer stacks: (a) effect of number of platelets (N) at =500, (b) effect of aspect ratio ( ) at N=10. 120

Fig. 50. Relative water vapor permeability for PUU/Cloisite 15A (▲) and PUU/Nanomer I.30TC (♦) nanocomposites. The solid trend lines depict predictions from ‘modified CusslerAris’ model (Eq. (10.19)) with aspect ratios = 200 and 800 [10].

Fig. 51. Comparison between experimental and predicted gas permeability results of different PU/clay nanocomposites at different clay loading. In the sample codes, PU 20, PU30, and PU40 represent cross-linked PUs of 2nd, 3rd and 4th generation, respectively which are reinforced with different clays such as Cloisite 30B (CB), Cloisite 15A (15A) and Na +-montmorillonite (Na+) [17]. 121

Fig. 52. Nitrogen permeability (PN2) of PU/graphene nanocomposites normalized by the permeability of unfilled PU (P0). Permeability data for PU/graphite composites are reproduced in the inset. The solid curves are the predictions based on the model of Lape et al. (2004) [Eq. (10.12)] using different aspect ratios (Af) [18].

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Fig. 53. Relative CO2 gas permeability of several PU/clay nanocomposite films with respect to the weight fraction of clay (VMT). The solid curves are for experimental values and the dotted curves are theoretically predicted values based on Fredrickson and Bicerano model (Eq. (10.18)) with aspect ratios of 40 and 100, respectively [96].

Fig. 54. Schematic representation of a packaging laminate

123

Fig. 55. Schematic structure of a typical stratospheric airship envelop [196].

Fig. 56. The layered structure of a typical aerospace material [217]

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Tables

Table 1. Gas permeability through different polymeric/elastomeric films at standard testing conditions [1, 3, 5-9] Permeability (cm3.mm/m2.day.atm) Polymer/ Elastomer

Polyvinyl fluoride (PVF) Polyvinylidene Fluoride (PVDF) Polytetrafluoroethylene (PTFE) Fluorinated ethylene-propylene copolymer (FEP) Vinylidene fluoridehexafluoropropylene copolymer Polyvinylidene Chloride (PVDC) Biaxially oriented polyester (BoPET) Nylon 6 Nylon 6,6 Ethylene vinyl alcohol copolymer (EVOH) Liquid crystal polymer (Vectran) Low-density polyethylene (LDPE) Polyurethane (PU) Silicon rubber

Nitrogen (N2)

Oxygen (O2)

Carbon dioxide (CO2)

Helium (He)

0.1 3-3.5 69-129

1.3 0.55-5.2 178-255

4.4 2.2-30 487-720

59.1 59-86 -

33-125

101-290

648-838

-

4.67-22

71-95

209-508

771

0.02-0.12 0.18-0.39 0.28-0.35 0.28

0.03-0.04 1-2.4 0.57-1.3 0.3-1.36

0.47-3.2 5.9-9.8 1.8-5.9 3-4.6

71 45.7 59.1

0.003-0.01

0.01-0.05

0.03-0.08

4-14

0.03-0.5 39-79 9-297 17280

0.05-0.1 69-274 3-1067 19685

394-959 175-2014 118110

36-2340 -

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Table 2. Overview of gas barrier property of PU/clay nanocomposite films and membranes Type of PU Polyether polyol and MDI based TPU Polyether polyol and MDI based TPU

Filler (Clay)

Filler loading

Preparation method

Permeant

% Reduction

Ref

CTABVMT

5.3 wt%

In-situ polymerization

CO2

19

[6]

CTABVMT

5.3 wt%

In-situ polymerization

N2

30

[6]

CO2

40

[96]

CO2

20

[96]

In-situ polymerization (master-batch mixing) In-situ polymerization (direct mixing)

Polyether polyol and MDI based TPU

CTABVMT

3.3 wt%

Polyether polyol and MDI based TPU

CTABVMT

3.8 wt%

Polycaprolactone based TPU

OMMT

20 wt%

In-situ polymerization

Water vapor

89.5

[97]

Polycaprolactone based TPU

OMMT

40 wt%

In-situ polymerization

Dichloromethane vapor

98.7

[97]

3rd generation hyperbranched PU

Cloisite 30B

8 wt%

Solution mixing

He

76

[17]

TPU

C16 –MMT

4wt%

Solution mixing

O2

50

[140]

TPU

DTA-MMT

4 wt%

Solution mixing

O2

15

[140]

TPU

Cloisite 25A

4 wt%

Solution mixing

O2

34.5

[140]

Polyester based TPU

O2

54

MMT-Fe

3 wt%

Solution mixing

Polyester based TPU

MMT-Cu

3 wt%

Solution mixing

[5] N2

62

O2

44

[5] N2 52 Note: C16–MMT= Hexadecylamine montmorillonite, Cloisite 25A= N-(hydrogenated tallow)-N,N,N-trimethyl ammonium montmorillonite, Cloisite 30B= Bis(2-hydroxy-ethyl)methyl tallow ammonium montmorillonite CTABVMT= Cetyltrimethylammonium bromide (CTAB) modified vermiculite clay (VMT), DTA-MMT= Dodecyltrimethylammonium montmorillonite, MDI= Methylene diphenyl diisocyanate, MMT= Montmorillonite clay, MMT-Fe= Na+ Montmorillonite modified by iron (III), MMT-Cu= Na+ Montmorillonite modified by copper (II), OMMT= Organically modified montmorillonite, TPU= Thermoplastic polyurethane.

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Table 3. Sorption coefficient S (g/100g.mm.Hg) of water vapor at P/P 0 ≤0.2 and zeroconcentration diffusion coefficients, D0 (cm2/s) and permeability, P (cm2 g/s.100g.mm.Hg) x 108 for different samples [139] Sample

S (g/100 g.mm.Hg)

D0 x 108 (cm2 /s)

P (cm2 g/s.100g.mm.Hg) x 108

TPU

0.051

7.74

0.395

TPU/PCL

0.060

7.04

0.422

TPU/MMT-OH2

0.118

4.84

0.571

TPU/MMT0.052 2.93 0.152 OH2/PCL) TPU/MMT0.036 6.42 0.231 OH2-g-PCL) Note: MMT= Montmorillonite clay, PCL= Poly(e-caprolactone), TPU= Thermoplastic polyurethane, TPU/PCL = Blends of TPU and PCL, TPU/MMT-OH2 = TPU/MMT nanocomposite prepared by direct mixing, TPU/MMTOH2/PCL = TPU/PCL-clay master batch, TPU/MMT-OH2-g-PCL= PCL-grafted organoclay filled TPU nanocomposite

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Table 4. Overview of gas barrier property of PU/graphene nanocomposite based films and membranes Type of PU

Filler

Filler loading

Processing

Permeant/ Gas

% Reduction

Ref

Polyester based TPU

GO

1 wt%

Solvent mixing

He

78.5%

[7]

Polyester based TPU

TRG

1.6 vol%

Melt intercalation

N2

52

[18, 160]

Polyester polyol and MDI based TPU

TRG

1.5 vol%

In-situ polymerization

N2

71

[18, 160]

Polyester based TPU

TRG

1.6 vol%

Solvent mixing

N2

81

[18, 160]

Polyester based TPU

iGO

1.6 vol%

Solvent mixing

N2

94-99

[18, 160]

Polyester polyol and MDI based TPU

GO

1.5 vol%

In-situ polymerization

N2

62

[18, 160]

IFR-PU

rGO

2 wt%

Melt mixing

O2

90.4

[1]

Aliphatic, polyetherbased TPU

HDGNRs

0.5 wt%

Solution mixing

N2

99.9%

[20]

Waterborne aliphatic PU

rGO

3 wt%

In-situ polymerization

Water vapor

~76%

[159]

Note: GO= Graphene oxide, TRG= Thermally reduced graphene oxide, rGO=Reduce graphene oxide, IFRPU=Intumescent flame retardant polyurethane, iGO= isocyanate treated graphene oxide, HD-GNRs= Hexadecylfunctionalized low-defect graphene nanoribbons

128

Table 5. Overview of gas barrier property of PUNC based coatings and adhesives Type of PU

Filler

Filler loading

Base material

Permeant/ Gas

% Reduction

Ref.

ccPU

MMT

-

PP-foil

O2

~96.5%

[145]

ccPU

Li-HEC

-

PP-foil

O2

99.8%

[145]

Waterborne PU

Cloisite 15A

3 wt%

Nylon fabric

Water vapor

~20%

[66]

Waterborne PU

Cloisite 10A

0.96 vol%

Glass microfiber filters

CO2

68%

[21]

Waterborne PU

OMMT (C93A)

0.97 vol%

Glass microfiber filters

CO2

72%

[21]

Polyester grade TPU

Cloisite 30B

3 wt%

Nylon 6,6 fabric

H2

36%

[39]

Aliphatic polyether grade TPU

Cloisite 30B

3 wt%

Polyester fabric

He

58.5%

[143]

Polyester based TPU

Water vapor

52.5%

Nanofil 804

5.42 vol%

PP and PA films

Polyester based TPU

Polyester based TPU

Nanofil 32

5.42 vol%

O2

33%

Water vapor

63%

PP and PA films

5.42 vol% Nanofil 15

[141]

[141] O2

36.5%

Water vapor

68.5%

PP and PA films 4.31 vol%

[141] O2

- 46.5%

Note: ccPU= Cationic and UV-curable PU, Cloisite 10A= Dimethyl benzyl hydrogenated-tallow ammonium montmorillonite ((2MBHT), Cloisite 15A= bis(hydrogenated tallow alkyl)dimethyl modified montmorillonite, Cloisite 30B= Bis(2-hydroxy-ethyl)methyl tallow ammonium montmorillonite, C93A= Montmorillonite clay modified by methyl, bis-hydrogenated tallow quaternary ammonium (M2HT), MMT= Montmorillonite clay, OMMT= Organically modified montmorillonite clay, Li-HEC= Lithium hectorites, Nanofil 804= Organoclay modified by bis (2-hydroxyethyl) hydrogenated tallow ammonium, Nanofil 32= Organoclay modified by alkylbenzyldimethylammonium (benzalkonium), Nanofil 15= Organoclay modified by dimethyl, dihydrogenated tallow ammonium, PA=Polyamide, PP=Polypropylene

129