Polymer Nanocomposites Based on Layered Double Hydroxides (LDHs)

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Polymer Nanocomposites Based on Layered Double Hydroxides (LDHs) Sajid Naseem, Andreas Leuteritz, and Udo Wageknecht

Abstract The development of nanocomposites based on layered double hydroxides (LDHs) has gained interest for applications ranging from catalysis and drug absorption to functional nanocomposites possessing flame retardancy, heat stability, and energy storage attributes, to name just a few. This chapter relates to LDH-based polymer nanocomposites, their structure, and synthesis methods. The preparation methods of LDHs for polymer nanocomposites are described in detail. Furthermore, properties of different combinations of LDH/polymer nanocomposites are discussed with relation to the processing variables. Finally, a novel approach for the preparation of polymer nanocomposites is depicted and discussed based on sonication assist masterbatch (SAM) melt mixing. The novel SAM melt mixing and conventional melt mixing methods are compared in the case of LDH/PLA nanocomposites, demonstrating that better dispersion is obtained when using SAM melt mixing.

„„12.1 Introduction to LDHs Layered double hydroxides (LDHs) are one of the most useful inorganic layered nanomaterials due to their structure, which can be altered at a microscopic level. The possibility of structure modification makes it a promising material for different applications because of its change in chemical composition, change in type of interlayer anions, amount of interlayer anions, crystallite size, and size distribution. LDHs are known as anionic clays and have been studied widely for the last few years due to their interesting and promising applications. Among the attributes that make LDHs interesting and useful materials is their ease of synthesis. The general formula of LDHs is and .

where

are divalent and trivalent metal cations, respectively, such as and is the interlayered anion, which can be , , , etc., and x is the

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molar fraction, which is

[1]. The structure and properties were first

explained by Allmann (1968) [2] and Taylor (1969) [3]. The structures of different LDHs are unique and provide many options to replace and modify their layers. The distribution of metal cations in the layers is uniform and they can be of different types, which is useful for different applications. LDHs can be easily tuned and prepared by different methods and can contain different metal cations or combinations of metal cations. Two metal LDHs, three metal LDHs, and multi-metal LDHs can be synthesized; interlayer anions can also be changed with organic and inorganic anions. The property of thermal and chemical stability as well as high biocompatibility make them useful for innovative materials. LDHs have been used in many applications. One of the most important is their polymer nanocomposites displaying improved mechanical strength, thermal stability, UV degradation protection, and flame retardancy. Different types of LDH/polymer nanocomposites have been synthesized based on different LDHs with multifunctional properties. Moreover, there are still many applications to explore and many questions that need to be answered, which makes LDHs interesting nanomaterials both for the research and development of advanced applications.

„„12.2 Structural Aspects of LDHs 12.2.1 Brucite Layers LDHs are made of positively charged brucite layers of divalent and trivalent cations with exchangeable anions that are located in the interlayers of LDHs and balance their positive charge [1]. Brucite layers [Mg(OH)2] are the basis of LDHs and contain Mg+2 ions octahedrally structured by hydroxide ions. These octahedral units form infinite layers with the O–H bond perpendicular to the LDH layers. Brucite-like layers can be stacked in different ways and lead to various polytype structures [4]. A typical LDH structure is shown in Figure 12.1.

12.2 Structural Aspects of LDHs

Figure 12.1 Structure of layered double hydroxides (LDHs)

12.2.2 Cation Substitution of LDHs The general formula for layered double hydroxides is . In this formula, di- and trivalent cations can be changed to form different types of LDHs. The bivalent cation M2+ could be Mg, Zn, Mn, Fe, Co, Ni, Cu, Ca, or Cd and the trivalent cation M3+ could be Al, Co, Fe, Mn, Cr, Ga, or In [4]. Different types of ternary and quaternary LDHs with different compositions of M+2 and M+3 can be prepared. The ionic radii of di-metal cations and tri-metal cations can play a significant role in making the different LDHs [4]. The most important characteristic of LDHs is that all of the divalent and trivalent cations should be distributed uniformly in the hydroxide layers [1]. A divalent ion with an ionic radius of 0.65 Å to 0.80 Å and a trivalent ion with an ionic radius of 0.62 Å to 0.69 Å or Al = 0.50 Å can enter into the LDHs’ hydroxide layers [1].

12.2.3 Interlayers of LDHs Interlayer galleries of LDHs contain water and interlayer anions with a network of water, interlayer anions, and hydroxyl groups. The bonding of LDH layers and interlayers involves a combination of hydrogen bonding and the electrostatic effect. The exact nature of the interlayers is complex because the hydrogen bonding is in a continuous state of flux. Columbic attraction also exists between layers of LDHs and interlayer anions [1, 5]. This kind of weak bonding between the host’s LDH layers and the guest interlayers make it easy to exchange different types of anions in the interlayers of LDHs. The amount of interlayer anion exchange depends

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­ irectly on the charge density of the hydroxide layer. The charge density of hydroxd ide can be controlled by controlling the ratio of M2+ and M3+. Other factors that can affect the replacement and arrangement of interlayer anions are temperature and the synthesis method of the LDHs. A large number of small to large anions is available: inorganic anions, organic anions, anionic complexes, and biomolecules [1]. The interlayer spacing can be changed with the intercalation of these anions and the distance of the LDH layers depending on the size and arrangement of these anions in the LDH layers. There are many organic anions that can be replaced for the different types of applications of LDHs. For example, the photostability of ­polymers can be enhanced by using organically modified LDHs as compared to un­ modified LDHs [6].

„„12.3 Synthesis of LDHs LDHs can be synthesized with multiple options with different properties depending on the metallic composition, cation combination, the anions in the interlayers, the combinations of different metals, and different synthesis methods [7–16]. The anion exchange capacity is the important factor that makes LDHs useful materials. A large number of anionic species can be introduced using one-pot synthesis methods. Interlayer anions could be carbonates, chlorides, nitrates, and larger organic anions such as carboxylates, sulfonates, or inorganic polyoxometalates [6, 15]. Due to the various alternatives, a variety of applications of LDHs were discussed. LDHs could be used as catalysts, nanomaterials for polymer nanocomposites, and as ­multifunctional materials [15]. The most common methods for the preparation of LDHs are based on co-precipitation, urea hydrolysis, an ion exchange process, reconstruction (the so-called memory effect), sol–gel synthesis, and the fast nucleation process [4, 15, 16]. Several researchers have discussed the different methods of synthesis [1]. Bravo-Suárez et al. (2004) reviewed the synthesis of LDHs using a thermodynamic approach. They studied the data of monovalent, divalent, trivalent, and tetravalent metals, which can be part of different LDHs. This study provides the theoretical data and possibilities to synthesize different types of LDHs [17]. The most common techniques for LDH synthesis are: 1. Co-precipitation method (direct method) 2. Anion exchange method (indirect method) 3. Calcination/reconstruction method

12.3 Synthesis of LDHs

12.3.1 Co-precipitation Methods Co-precipitation is a frequently used one-pot direct synthesis method for LDHs containing different cations  – divalent or trivalent  – and different interlayer anions. The anions could be inorganic (Cl-, NO3-, CO32-), organic molecules, or large molecules [1, 4, 16]. This method is used for preparing organic anion intercalated LDHs, which are difficult to prepare by other methods. The pH level is the decisive factor for the co-precipitation of the soluble hydroxide. A thermal treatment is used to enhance the yield and crystallinity of LDHs. A nitrogen environment is used to avoid the formation of CO32- as LDHs have a high affinity for carbonate ions [4]. Constant pH and variable pH are used in the co-precipitation method; a constant pH results in highly crystalline particles while a variable pH yields low crystalline particles [4, 15, 16]. In conventional co-precipitation it is difficult to control the particle size and size distribution of LDHs [18]. Zhao et al. (2002) used separate nucleation and aging steps (SNAS) to control the particle size of LDHs [19]. Highly crystalline MgAl-CO3 LDHs can be synthesized by a non-equilibrium aging method, which is complementary to the SNAS method [16]. LDHs with a wide range of MII/MIII ratios can be synthesized by the co-precipitation method [4, 18]. A variety of combinations have been employed using co-precipitation with di-metals and tri-metals such as MgFeAl [20], MgAlFe, MnMgAlFe [21], MgCuFe [22], CuMgAl [23], MgZnAl [24], CuMgAl, FeMgAl [25–27], CoMgAl, NiMgAl [27], ZnAl, ZnCuAl and ZnCoAl [28], MgCoAl, MgNiAl [29], MgAl, MgCoAl, MgNiAl, and MgFeAl [30]. Urea hydrolysis for the precipitation of LDHs (urea is a weak base) has been used as a reagent for homogenous precipitation [31]. In this method the temperature and time of the reaction is important. Hydrolysis of urea provides a pH of around 9 (depending on the temperature), which is suitable for the precipitation of many metal hydroxides. The method provides well-crystalline, larger sized particles, and a well-defined hexagonal shape due to the low supersaturation during the reaction [17]. Particle size can be changed by altering the reaction temperature; as the ­temperature affects the hydrolysis of urea, larger particles can be formed at low nucleation rates taking place at a lower temperature. LDHs prepared by urea ­hydrolysis contain a carbonate ion, which can be removed by treatment with HCl solutions, replacing it with a chloride ion [32–34]. This method is not suitable for LDHs containing CuII and CuIII [4] or the preparation of MgAl LDHs with a low charge density [15]. On the other hand, urea hydrolysis allows the synthesis of compounds with a high charge density, which are not easy to synthesize by other synthesis methods [15]. Different di-metal and tri-metal LDHs were synthesized previously by urea hydrolysis, such as MgCoAl and NiCoAl [35], ZnMg, ZnMgAl [36, 37], FeCoAl, and ZnCoAl [38].

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12.3.2 Anion Exchange Method (Indirect Method) The ability to intercalate specific anions in LDHs for different applications makes the anion exchange method a widely studied subject. Anion exchange is an indirect method for LDH synthesis and is mostly used when the co-precipitation method cannot be applied easily, for example, in the synthesis of LDHs where divalent and trivalent metal cations and anions are unstable in alkaline solutions. Anion exchange is also useful when an unfavorable direct reaction between metal ions and guest anions takes place [15]. The indirect method comprises a two-step synthesis, in which the first step is based on co-precipitation with different host anions (Cl-, NO3-, CO32-) and the second step involves exchange of the host anions with different guest anions depending on the LDHs’ end use. The process of anion intercalation is usually carried out in an inert atmosphere to avoid carbonate intercalation. The host–guest exchange usually depends on the electrostatic interaction of the LDHs’ host layers and exchangeable anions [1, 39]. Anions with high negative charges are most likely to be exchanged with anions with lower charges in the LDHs’ layer galleries. The ion exchange reactivity has the following order: CO32- > HPO42- > SO42- > OH- > F- > Cl- > Br- > NO3- > I-. Usually, NO3- and I- intercalated LDHs are used for the precursor of the ion exchange reaction [4]. The most important features of host-guest anion exchange are a suitable solvent, the composition of the brucite layers, temperature, and pH. The factors that favor the anion exchange process are: (1) higher temperatures, (2) a pH value of 4 or higher, and (3) an increase in the charge and a decrease in the ionic radius [1, 4]. There are many types of LDHs prepared by this method with different anion intercalations depending on the specific end use. The anion exchange method is chosen over the co-precipitation method in cases where a larger anion is difficult to intercalate. Several in­ organic anions, such as Cl-, Br-, NO3-, SO42- and organic anions such as adipate-­ terephthalate, succinate, p-hydroxybenzoate, benzoate, and dodecyl sulfonate can be intercalated by the anion exchange method [4].

12.3.3 Calcination/Reconstruction Method (Memory Effect) Calcination of LDHs at intermediate temperatures resulted in the formation of metal oxides, provided the metal oxides could be formed in the aqueous solution of the intercalated anion [4]. This interesting structural memory effect of LDHs can be used to modify the interlayers of LDHs. Consequently, LDHs with a volatile anion can be calcined into the mixture of oxides and then mixed in an aqueous solution containing the guest anion [39]. Miyata (1980) discussed the reconstruction of LDHs’ structure by hydration of calcined LDHs [40]. The parameters that control the intercalation of anions depend on the reaction medium, composition of layers, structure of anions, temperature of calcination, rate of calcination, and time of

12.3 Synthesis of LDHs

c­ alcination [4, 39]. The memory effect is lost when the calcination temperature increases too much (more than 500 °C). At high temperature, the solid-state diffusion of cations is increased. For example, for Cu, Co, Zn, and Al LDHs at 873 K, all divalent cations occupy tetrahedral sites and the memory effect is lost [15]. This method is mostly suitable for large organic anions that are not easy to intercalate by other methods [4, 39]. An inert nitrogen atmosphere is required during the reconstruction process to avoid carbonate intercalation [15]. Several LDHs synthesized by this method are ZnAl and MgAl LDHs with intercalation of salicylic acid [41], ZnAl LDHs intercalated by phenylalanine [42], organic chromophore-inter­ calated LDHs [43], surfactant-intercalated LDHs, amino acid- and peptide-inter­ calated LDHs [44], and several other anion-intercalated LDHs [4, 15].

12.3.4 Other Methods Several other methods have been used for the synthesis and modification of LDHs. The hydrothermal method is used for the intercalation of organic anions with a low affinity for LDHs. The pre-intercalation method or the pre-pillaring method is used when bulky guest anions have to be intercalated in LDHs. Dissolution and re-coprecipitation is a method in which LDHs are dissolved in organic acid solution and then added to an alkaline solution. This results in carboxylated intercalated LDHs. There are other methods that have been developed for specific types of LDHs such as the salt hydroxide method, electro synthesis, microwave aging method, template synthesis method, sol-gel method, and chimie douce method [4, 15].

12.3.5 Summary of LDH Synthesis Methods LDHs have many advantages because of their range of synthesis methods and options for different metallic combinations as well as anion exchange. One useful application is their use in treating water to remove harmful contaminants. There are some limitations of LDHs, for example, a specific ratio of divalent and trivalent metal cations is necessary to form the LDHs’ structure; the divalent/trivalent cation ratio should be within 2/1 to 4. LDHs are soluble in acidic solutions so the composites of LDHs are not stable in acidic media [1]. As discussed, there is a wide range of available methods depending on the applications of LDHs. For example, co-precipitation methods are generally required when LDHs are used as catalysts, as catalyst precursors, or when a higher yield is required [1, 15]. Anion exchange is preferred for the exchange of large anions [1]. A hydrothermal synthesis method is used when a pure phase with high crystallinity is required and it is possible to get an accurate structural composition [15]. For the

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fabrication of LDH films, suitable methods are colloidal deposition and the solvent evaporation method [4].

12.3.6 Thermal and Chemical Stabilities of Different LDHs The thermal and chemical stabilities of LDHs are important for a variety of applications. The important factors are summarized in Table 12.1: Table 12.1 Thermal and Chemical Stabilities of Different Synthesized LDHs Thermal stability

Chemical stability

Decrease of thermal stability of LDHs in the order MgCr ≈ MgAl > NiAl ≈ MgFe > CuAl ≈ ZnAl > CoAl [39]

Decrease of chemical stability of LDHs in the order Fe3+ > Al3+ > Zn+2 > Ni2+ ≈ Co2+ > Mn2+ > Mg+2 [39]

Highest decomposition temperature is for MgAl and MgCr – 400 °C, and lowest is for CoAl – 220 °C [39]

Calculated solubility of LDHs indicated that solubility of LDHs is strongly affected by interlayer anions [39]

Mg-based LDHs containing Fe3+ and Co3+ resulted in oxides [39]

Interlayer anions such as carbonate, borate, and silicate decrease the solubility of LDHs [39]

Mg-based LDHs are stable compared to Zn-based LDHs in general [45]

Interlayer anions such as nitrate and sulfate increase the solubility of LDHs [39]

Ni-based LDHs have intermediate thermal s­ tability between Zn and Mg [46] Co-based LDHs are less thermally stable ­compared to other LDHs [47]

„„12.4 Applications of LDHs Due to the wide range of compositions of LDHs, they have been widely used in ­different sectors as shown in Figure 12.2. LDHs are used in catalysts, catalyst supports, and catalyst precursors. Their use in catalysis is due to the ease of LDH synthesis, and their inexpensive and recyclable sources. Li et al. (2006) have discussed in detail the different applications of LDHs including catalysis [48]. Many types of LDHs have been used as catalyst supports in a variety of organic and inorganic reactions. LDHs are used as catalysts to reduce the environmental impact of SOx and NOx emissions from FCCUs (fluidized catalytic cracking units) in oil refineries. LDHs are also applied in natural gas conversion reactions of the partial oxidation of methane to obtain syngas [48]. LDHs are used as ion-exchange materials for removing negatively charged species by anion exchange and surface adsorption. A high surface area and high anion exchange capacity make LDHs useful

12.4 Applications of LDHs

materials for this type of application [48]. LDHs are used widely in the pharmaceutical industry for antacid formulations, as antipepsin agents, and to remove phosphate anions from gastrointestinal fluid. The controlled release of medicines from LDHs is extensively studied because of their biocompatibility, chemical composition, alkaline nature, and the intercalation of drug anions in LDHs. LDHs are used in different biomedical and biochemistry applications where they contain DNA, ATP enzymes, and vitamins in their interlayer galleries [48]. Finally, LDHs are used in photochemical reactions, containing photochemical guest molecules in their interlayer spaces [48].

Figure 12.2 Applications of LDHs in polymers and other materials

LDHs are widely used in functional polymeric materials for different purposes. PVC is being stabilized by LDHs to reduce an autocatalytic dehydrochlorination reaction when exposed to high temperatures and UV radiation [49, 50]. MgAl LDHs have a stabilizing effect on PVC because of their higher charge density providing the ability to transfer Cl- to the interlayers of LDHs [51]. LDHs are used as flame retardant materials for different polymers due to their inherent resistance to ignition, slower rate of flame spread, and reduced quantity of smoke produced during combustion [48, 52]. There are other useful applications in ceramics, agro industries, and in biomedical applications, which were recently discussed in detail by Mishra et al. (2018) [1]. The major applications are shown in Figure 12.2. Useful applications have been studied and more remain to be explored, due to LDHs’ ease of synthesis, low cost, and multi-functionality.

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„„12.5 Preparation of LDH/Polymer ­Nanocomposites 12.5.1 In-Situ Synthesis Method for Polymer Nanocomposites 12.5.1.1 In-Situ LDH Synthesis To prepare polymer nanocomposites, LDHs are usually synthesized in in-situ polymer solutions. Metal salts are dissolved in a polymer solution and are then co-precipitated in the form of LDHs. The polymer chains are intercalated in the layers of LDHs during LDH synthesis [53]. A schematic representation of this method is shown in Figure 12.3.

Figure 12.3 Schematic representation of an in-situ LDH synthesis method for LDH/polymer nanocomposites

Many researchers have investigated this technique over the last few years. A brief review is given in Table 12.2, which lists different polymers dissolved in different types of salts used as precursors for LDH preparation. Different characterization techniques were used to analyze the end nanocomposites. Table 12.2 LDH/Polymer Nanocomposites Prepared by In-Situ LDH Synthesis with Characterization Techniques Used* Polymer LDHs

Major characterization done

PVA

CaAl

XRD, FTIR, TGA, SEM, chemical analysis

[47]

PAA

MAlCo3 (M = Mg, Ca, Zn)

XRD, FTIR, elemental analysis, DSC, TGA, SEM

[54]

PEO

CuCr-Cl

Elemental chemical analysis, XRD, FTIR, SEM-EDX

[55]

PSS

MgAl

XRD, FTIR, TGA, electrophoretic mobility, TEM

[56]

PVS PSS

Ref

ZnM(OH)2 (M = Al, Cr)

PVS * PVA (poly(vinyl alcohol)), PAA (poly(acrylic acid)), PVS (poly(vinylsulfonate)), PSS (polysodium styrene-4-­ sulfonate), PEO (poly(ethylene oxide)), XRD (X-ray diffraction), FTIR (Fourier transform infrared spectroscopy), TGA (thermogravimetric analysis), SEM (scanning electron microscopy), DSC (differential scanning calori­ metry), TEM (transmission electron microscopy).

12.5 Preparation of LDH/Polymer ­Nanocomposites

12.5.1.2 In-Situ Polymerization The preparation method for LDH/polymer nanocomposites based on LDHs is the most widely used and is carried out in an aqueous solution; it is also known as the solution-based method. Basically, monomers are intercalated in the layers of LDHs and then the mixture is heated up to the required temperature to complete the polymerization reaction. A schematic diagram of the process is shown in Figure 12.4. There are various methods in which the intercalation of the monomer can be done by mixing LDHs in an aqueous solution of the monomer and then heating slowly [53].

Figure 12.4 Schematic representation of an in-situ polymerization method for LDH/polymer nanocomposites

The initial step is to intercalate the monomer into the LDHs and then heat or initiate the polymerization reaction as shown in Figure 12.4. The effective method for the synthesis of LDH hybrids is based on co-precipitation in a dissolved monomer. CO32- interference can be reduced by using a nitrogen environment. Different types of fillers have been incorporated in polymers to improve thermal, electrical, and structural properties [57, 58]. This method is suitable for LDH/PLA nanocomposites because it provides better dispersion compared to the melt mixing method for PLA-based nanocomposites [57]. Thermal stability is enhanced by the inclusion of the polymer chains into the LDHs’ interlayer space. The crystallinity and morphology of LDHs also change when polymer chains are introduced [59]. Messersmith et al. (1995) found an increase in thermal stability due to the nature of the interface formed between the inorganic and organic materials. The organoceramics hybrids transform into inorganic solids with compositions that are different from those obtained by pristine host materials [47]. Some previous polymer nanocomposites prepared by this method are shown in Table 12.3 with the characterization methods used.

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Table 12.3 LDH/Polymer Nanocomposites Synthesized by In-Situ Polymerization with ­Characterization Methods* Polymer LDHs/maximum loading

Major characterization done

Ref

PA6

CoAl-SDS/1 wt%

XRD, tensile testing, SEM, TEM, indentation test

[60]

PU

CoAl-DS/2 wt%

FTIR, XRD, SEM, elemental analysis, tensile test, TGA

[61]

PMMA

LDH-U/5 wt%

XRD, TEM, UV/Vis analysis, TGA, tensile ­properties

[62]

PANI

ZnAl, APTS grafted organo­ modified LDHs

XRD, electrochemical corrosion studies, FTIR, SEM, XPS, EIS

[63]

PVC

MgAl-DS

XRD, FTIR, TEM, particle size analysis, GPC, TGA, DMA, impact testing

[64]

VBS

ZnAl-sodium styrene-4-sulfonate NMR spectroscopy, FTIR, SEM

[65]

PIs

ZnAl-citric acid modified/4 wt%

FTIR, TGA, XRD, SEM, TEM, tensile properties

[66]

PS

MgAl-sodium dodecyl sulfate

XRD, FTIR, TEM, TGA

[67]

PSS

* VBS (vinyl benzene sulfonate), PSS (polystyrene sulfonate), PIs (polyimides), PS (polystyrene), PANI (poly­ aniline), PMMA (poly(methyl methacrylate)), PU (polyurethane), PA6 (polyamide6), APTS (g-aminopropyltriethoxysilane), XRD (X-ray diffraction), SEM (scanning electron microscopy), TEM (transmission electron microscopy), FTIR (Fourier transform infrared spectroscopy), TGA (thermogravimetric analysis), GPC (gel permeation chromatography), DMA (dynamic mechanical analysis), NMR (nuclear magnetic resonance).

12.5.2 Solution Intercalation The solution intercalation method involves addition of LDHs to a solution of a polymer while stirring to obtain intercalation of polymer into the LDH interlayer galleries. As the polymer chains are longer than the monomer or the oligomer, the LDHs should be organically modified to achieve a high level of intercalation. Organic modification increases the interlayer distance to enhance the intercalation of the polymer chains [53]. Modified and unmodified LDHs have been used to synthesize polymer nanocomposites by solution intercalation as shown in Table 12.4. The schematic diagram of solution intercalation for the preparation of LDH/polymer nanocomposites is shown in Figure 12.5. Table 12.4 summarizes the variety of LDH/polymer nanocomposites synthesized by the solution intercalation method. The table lists the different polymers, different LDHs, the maximum amount used in the different polymers and the resulting properties, and characterization. Qiu et al. (2005) studied the structural and thermal properties of exfoliated PS/ZnAl nanocomposites prepared by solution inter­ calation and concluded that thermal stability was enhanced in exfoliated nanocomposites [68]. Chiang et al. (2010) studied the different properties of poly(L-lactide)/ organically modified MgAl LDH nanocomposites prepared by solution intercala-

12.5 Preparation of LDH/Polymer ­Nanocomposites

tion. Mechanical (DMA) properties of poly(L-lactide) were enhanced when 1.2 wt% modified MgAl LDH was added to pure PLLA. However, the degradation temperature decreased as the amount of LDH increased due to the catalyzing effect of Mg and Al present in PLLA [69]. There are certain drawbacks of the solution method from a practical viewpoint as a result of the use of organic solvents, which may be present in the final product and can contaminate the polymer and affect the final properties [57].

Figure 12.5 Schematic representation of the solution intercalation method for LDH/polymer nanocomposites Table 12.4 Different LDH/Polymer Nanocomposites Synthesized by a Solution Intercalation Method with Major Characterization Methods* Polymer LDHs/maximum loading

Major characterization done

Ref

PE-g-MA

SRD, FTIR, TGA, DTA, TEM

[70]

Organomodified MgAl/5 wt%

LLDPE

ZnAl/20 wt%

XRD, TEM, TGA

[71]

PS

ZnAl(DS)/20 wt%

XRD, TEM, FTIR, TGA

[68]

PEO

MgAl-ASP/10 wt%

FTIR, TGA, XRD, SEM, ionic conductivity

[72]

PEO

MgAl-SDS

XRD, FTIR, photoelectron spectroscopy

[73]

Epoxy

MgAl/5 wt%

XRD, SEM, FTIR, TGA-DTG, tensile properties, flexural properties, UL94 test

[74]

PS

MgAl-(3,4-dihydroxybenzo-phenone)/ Particle size distribution measurement, 5 wt% SEM, TEM, cone calorimeter, oxygen index test, TGA-FTIR, rheological properties

[75]

PS

MgAl-(3,4-dihydroxybenzo-phenone)/ 5 wt%

[76]

Cone calorimeter analysis

PAI

Diacid-NiAl/8 wt%

FTIR, XRD, TGA, TEM, FE-SEM

[77]

PAI

Diacid 5-MgAl/8 wt%

XRD, TEM, DSC, FE-SEM

[78]

Diacid 7-MgAlAl/8 wt% PP

MgAl-NO3

MgAl-SDBS MgAl-AY36 MgAl-AR88

FTIR, WAXS, TGA, elemental analysis, SEM [79]

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Table 12.4 Different LDH/Polymer Nanocomposites Synthesized by a Solution Intercalation Method with Major Characterization Methods* (continued) Polymer LDHs/maximum loading

Major characterization done

Ref

sPS

CoAl/10 wt%

WAXS, FTIR, SEM, TEM, AFM, TGA, DSC, POM, MCC

[80]

PLLA

MgAl-(PLA-COOH)/10 wt%

XRD, FESEM, EDX, FTIR, ICP-AES, DMA, TG/DTA

[69]

* PE (polyethylene), MA (maleic anhydride), LLDPE (linear low density polyethylene), PS (polystyrene), PEO (polyethylene oxide), PAI (poly(amide-imide)), PP (polypropylene), sPS (syndiotactic polystyrene), PLLA (poly(L-lactide)), FE-SEM (field emission scanning electron microscopy), EDX (energy dispersive X-ray), FTIR (Fourier transform infrared spectroscopy), ICP-AES (inductively coupled plasma-atomic emission spectro­ scopy), DMA (dynamic mechanical analysis), MCC (microscale combustion calorimetry), POM (polarized optical microscopy).

12.5.3 Melt Compounding Melt compounding intercalation is preferred compared to solution intercalation for the preparation of polymer/clay nanocomposites. Table 12.5 depicts the different nanocomposite systems of LDH/polymers synthesized by melt compounding. Figure 12.6 displays schematically the process of synthesizing the nanocomposites. Accordingly, LDHs are mixed in the desired polymer by using conventional polymer processing equipment comprising: an internal mixer, extruder, two roll mixers, and other conventional equipment. The nanocomposite can be prepared in a single-step or two-step process when a masterbatch preparation step is involved prior to mixing in the final polymer [53].

Figure 12.6 Schematic representation of the melt compounding method for LDH/polymer nanocomposites

The melt compounding method is the most challenging especially in the case of polyolefins and non-polar polymers as a result of the different natures of non-polar polymers and polar LDHs [53].

12.5 Preparation of LDH/Polymer ­Nanocomposites

Polyolefin-based LDH/polymer nanocomposites are prepared by the two-step process as shown in Figure 12.6. Costa et al. (2007) synthesized LDPE/MgAl LDH nanocomposites where a two-step process was used. First a masterbatch of dodecyl benzenesulfonate (DBS)-modified MgAl LDH and maleic anhydride LDPE were blended and then the masterbatch was diluted in LDPE [81]. The single-step process is used mostly for polar polymers where the modified LDHs are compounded directly into the polymer. Leng et al. (2015) studied the structure–property relationship of PLA/organically modified MgAl LDH nanocomposites prepared by a one-step melt compounding process [82]. Leng et al. (2017) also studied the effect of different types of organically modified MgAl and NiAl LDHs on the structures and properties of PLA nanocomposites prepared by melt compounding. The degree of crystallinity, obtained from DSC measurements, increased as the LDH contents increased up to 6% for NiAl-LDH-based nanocomposites [83]. Wang et al. (2011) studied the combustion behavior of PP/O-MgAl LDH nanocomposites prepared by melt compounding. They showed that the flame retardancy was improved as the amount of organomodified MgAl LDHs increased from 1.2 wt% to 4.8 wt% in PP. The PP/O-MgAl LDHs showed lower pHRR (peak heat release rate), THR (total heat release), and HRC (heat release capacity) values as compared to pure PP [84]. Table 12.5 summarizes the different LDH/polymer nanocomposite systems prepared by melt compounding. Table 12.5 Different LDH/Polymer Nanocomposites Synthesized by Melt Compounding with Characterization Methods* Polymer

LDHs

Major characterization done

Ref

PLLA

SDBS-MgAl/12 wt%

SAXS, WAXS, DSC, BDS

[82]

PLLA

SDBS-MgAl

SAXS, WAXS, DSC, BDS

[83]

SDBS-NiAl EVA

MgAl-PAHPA/5 wt%

FTIR, HRMS, XPS, STEM, EDX, XRD, TEM, DMA, TGA, cone calorimeter

[85]

PP

Zn-Ti-SDBS/5 wt%

PXRD, TGA, FTIR, SEM, TEM, photo-­ oxidation study

[86]

ZnTi-LA/5 wt% XNBR

MgAL-CO3

XRD, BET analysis, rheological analysis, cross-link density analysis, SEM

[87]

EPDM

MgAl-sodium-1-decane­ sulfonate/10 phr

WAXS, FTIR, TEM, DMA, SEM

[88]

XNBR LLDPE

MgAl-DS/20 wt%

FTIR, XRD, TEM, TGA, DSC

[89]

PP

O-CoAl/6 phr

WAXS, FTIR, SEM, TEM, TGA, MCC

[90]

NR, NBR, BR, EPDM, CR, XNBR, S-SBR, IIR

ZnMgAl-stearic acid (SA)

WAXS, XRD, TEM, tensile test, ­mechanical tests

[91]

PP

MgAl-DBS/4.8 wt%

WAXS, FTIR, TEM, SEM, HRR, pHRR

[92]

PP

ZnAl/16.2 wt%

DSC, TEM, SAXS

[93]

357

358

12 Polymer Nanocomposites Based on Layered Double Hydroxides (LDHs)

Table 12.5 Different LDH/Polymer Nanocomposites Synthesized by Melt Compounding with Characterization Methods* (continued) Polymer

LDHs

Major characterization done

Ref

PP

CoAl-DBS/6 wt%

SEM, TEM, TGA, thermal degradation study by Friedman and FlynneWalleOzawa methods

[94]

EPDM

MgAl-SDBS/40 phr

WAXS, SAXS, TEM, SEM, MCC, cone calorimeter, mechanical testing

[95]

S-SBR

ZnAl-stearic acid

Tensile testing, DMTA, TGA, TEM, FTIR, UV-vis absorption spectroscopy

[96]

PP

A.O-MgAl

XRD, SEM, TGA, rheological characterization, photo-oxidation study, DMA

[97]

HDPE

Myristic acid, sorbic acid, stearic-acid-modified MgAl/3 wt%

XRD, TEM, DSC, tensile testing

[98]

PP

MgAl-metanil yellow/ 5 wt%

XRD, elemental analysis, SEM, TGA, FTIR, TEM, weathering test, tensile testing

[99]

XNBR

ZnAl/100 phr

Tensile testing, DMA, TGA, cone c­ alorimeter, FTIR, TEM

[100]

XRD, FTIR, TGA, LOI, SEM, tensile t­ esting, dual cone calorimeter

[101]

MgAl/100 phr PP

SDBS-MgZnAl SDBS-MgAl

EPDM

ZnAl/100 phr

WAXS, SEM, TEM, TGA, cone calori­ meter test, tensile testing, DMA

[102]

LDPE

NiAl/7 wt%

WAXS, SEM, DSC, FTIR, TGA-DTA

[103]

CoAl/7 wt% Carbonate, chloride, NaDDS, lauric acid, stearic acid, ­palmitic acid * PLLA (poly(L-lactide)), SAXS, WAXS (small- and wide-angle X-ray scattering), DSC (differential scanning ­calorimetry), BDS (broadband dielectric spectroscopy), EVA (ethylene vinyl acetate copolymer), PAHPA (N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-hexylacetamide-2-propyl acid), HRMS (high resolution mass spectrometry), XPS (X-ray photoelectron spectroscopy), STEM (scanning transmission electron micro­ scopy), EDX (energy dispersive X-ray), XRD (X-ray diffraction), SEM (scanning electron microscopy), TEM (transmission electron microscopy), DMA (dynamic mechanical analysis), TGA (thermogravimetric analysis), EPDM (ethylene-propylene-diene rubber), XNBR (carboxylated acrylonitrile-butadiene rubber), NBR (acrylonitrile-butadiene rubber), BR (polybutadiene rubber), IIR (butyl rubber), S-SBR (solution styrene butadiene ­rubber), CR (polychloroprene rubber).

12.5.4 Effect of Dispersion and Exfoliation of LDHs on LDH/ Polymer Nanocomposites The performance and properties of LDH/polymer nanocomposites depend strongly on the dispersion of LDHs in the polymer matrix. In addition, the aspect ratio of LDHs is crucial with respect to the final properties of the polymer nanocomposites. As discussed earlier, most LDH/polymer nanocomposites prepared by different

12.5 Preparation of LDH/Polymer ­Nanocomposites

methods contained LDHs that were organically modified to enhance the interaction between the polymer and the LDH. Modification of LDHs enhances the dispersion and exfoliation of LDHs in different polymers and as a result enhances different properties such as flame retardancy, and the thermal, optical, and mechanical properties of the polymer nanocomposites [104]. Highly exfoliated LDH/polymer nanocomposites are difficult to obtain in most cases and different techniques have been developed to enhance the dispersion and exfoliation of LDHs in polymers. The hydrophobic nature of polymers and the intrinsic hydrophilic nature of LDHs make them incompatible during polymer nanocomposite preparation. Carbonate-inter­ calated LDHs and borate-intercalated LDHs have excellent IR absorbing and flame retardancy properties, respectively. These two types are not compatible with surfactant modification based on dodecyl sulfate (DDS) [105]. Many researchers have studied ways to overcome this issue of miscibility. Commodity polymers, such as PE and PP, are used for many applications and since surface-modified LDHs are expensive, it leads to a major barrier for large-scale applications of these commodity polymer nanocomposites. Drying LDHs prior to melt compounding results in aggregation of LDHs and poor dispersion in polymers [105]. Wang et al. (2012) introduced a simple and cost-effective process for LDH/ polymer nanocomposites using solvent mixing. In this way, unmodified MgAl LDHs dispersed in PP were obtained using xylene as a solvent [105]. Yang et al. (2014) prepared hydrophobic LDHs by using aqueous miscible organic solvent treatment (AMOST) of MgAl LDHs. In this way, a highly dispersed monophasic system was obtained [106]. Furthermore, ZnAl-borate and MgAl-borate LDHs were used in unmodified PP up to 30 wt%. ZnAl-borate/PP nanocomposites showed better properties compared to MgAl-borate/PP nanocomposites at the same loading level. For example, the pHRR of pure PP was reduced by 63.7% at 15 wt% loading of  ZnAl-borate LDHs. The optimum loading was 15 wt% for ZnAl-borate where a highly dispersed nanocomposite was obtained. It was concluded that melt mixing demonstrated poor dispersion compared to solvent mixing at 6 wt% loading of LDHs resulting in a pHRR reduced by 23.8% in the case of melt mixing and by 29.9% in the case of solvent mixing [107]. Moujahid et al. (2002) prepared and compared the properties of polymer nanocomposites by two different methods. They used in-situ polymerization and a template method for the synthesis of vinyl benzene sulfonate (VBS) and polystyrene sulfonate (PSS) nanocomposites with ZnAl-Cl LDHs. Blending of PSS with ZnAl-Cl LDH caused the delamination of the LDH and better dispersion [65]. Zhang et al. (2008) synthesized and compared different properties of ZnAl/ethylene-vinyl acetate copolymer (EVA) nanocomposites by solution mixing and melt mixing. The tensile modulus and thermal properties were higher in the case of solvent mixing at the same loading level of LDHs. This was attributed to better exfoliation of LDHs in the case of the solvent mixing method [108]. Kafunkova et al. (2010) synthesized por-

359

360

12 Polymer Nanocomposites Based on Layered Double Hydroxides (LDHs)

phyrin-LDH/poly(butylene succinate) (PBS) and polyurethane (PU) by solvent mixing and melt compounding [109]. GuO et al. (2013) synthesized MgAl-N-­lauroylglutamate (LG)/polyester acrylate nanocomposites by blending the LDHs modified with LG with the polyester acrylate followed by UV curing. The properties are summarized in Table 12.6 [110]. Matusinovic et al. (2012) produced and compared the properties of CaAl-benzoate/polystyrene (PS) nanocomposites using in-situ bulk polymerization and melt blending. The fire retardancy and thermal properties were improved compared to pure PS. Optimal properties were observed in the case of in-situ bulk polymerization due to better dispersion of the LDHs using in-situ bulk polymerization compared to melt blending [111]. Table 12.6 displays the properties of the LDH/polymer nanocomposites synthesized by the various techniques compared with conventional methods. Table 12.6 LDH/Polymer Nanocomposites’ Properties, Synthesized by Various Methods* Polymer

LDHs/ loading level

Techniques

Results

Ref

PP

MgAl-borate/ 1,3,6,9,15,30%

1-Aqueous miscible organic solvent ­treatment (AMOST) Solvent mixing method

15 wt% is optimal loading for ZnAl-­ borate.

[107]

ZnAl-borate/ 1,3,6,9,15,30% VBS

ZnAl-Cl

PSS EVA

pHRR reduced by 63.7% for pure PP.

2-Melt mixing method

Solvent mixing is better for dispersion of LDHs compared to melt mixing.

1-In-situ poly­ merization method

PSS causes delamination of ZnAl-Cl LDH causes better dispersion.

[65]

1-Solution inter­ calation method/ 2,5,10 wt% LDH used

Thermal degradation temperatures increase from 435 °C to 470 °C for solution intercalation at 10 wt%.

[108]

2-Melt mixing method/ 2,5,10,20 wt% LDH used

Thermal degradation temperature increases from 435 °C to 461 °C for melt mixing at 10 wt%.

2-Templating method ZnAl

Tensile modulus is 66.7 MPa for melt mixing at 10 wt% and 68.1 MPa for solution method. Exfoliated structure is obtained in case of melt mixing at 10 wt% while intercalated structure is obtained in case of solution mixing.

PU

MgAl-TPPS

PBS

MgAl-PdTPPC ZnAl-PdTPPC

1-Solvent cast/cross-­ Used for photoactive coatings. linking technique/up Successful intercalation of different to 1.3 wt% (PU/LDH) compounds. 2-Melt-compounding/ Porphyrin remains intercalated when up to 1.3 wt% (PBS/ dispersed in PU. LDH) Similar dispersion in both polymers is observed. Photoactive surfaces obtained with precise control of porphyrin-LDH loadings.

[109]

12.5 Preparation of LDH/Polymer ­Nanocomposites

Polymer

LDHs/ loading level

Polyester MgAl-LG/ acrylate 0.1, 1, 2 wt%

Techniques

Results

Ref

Blending of MgAl-LG with polyester acrylate followed by UV curing method

Water resistance increased.

[110]

Different coefficients such as static friction, dynamic friction, and gloss of nanocomposite with 2 wt% of MgAl-LG decreased to 0.114°, 0.062°, and 87.5° compared with 0.856°, 0.758°, and 94.0° of the pure system, respectively. Thermal stability decreased c­ ompared to pure polymer. Exfoliated structure is obtained.

PS

CaAl-benzoate/ 1-In-situ bulk 1,3,5,7,10 wt% ­poly­merization 2-Melt blending

Thermal properties, fire retardancy, and dispersion improved in case of in-situ bulk polymerization method compared to melt blending.

[111]

* VBS (vinyl benzene sulfonate (styrene sulfonate)), PSS (polystyrene sulfonate), EVA (ethylene-vinyl acetate copolymer), PU (polyurethane), PBS (poly(butylene succinate)), TPPS (5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin), PdTPPC (Pd(II)-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin), LG-NA (N-lauroyl-glutamate sodium), PS (polystyrene).

12.5.5 Sonicated Assisted Masterbatch (SAM) Melt Mixing Most of the methods discussed that demonstrate good dispersion and intercalation are solution-based methods. Melt-mixing-based methods are used widely on an industrial scale. Considering the advantages of both methods, the authors developed a novel method based on a combination of both solution and melt mixing. Combining the advantages of melt mixing and solvent mixing, a new approach is developed with the addition of sonication assisted mixing. Sonication is used to prepare a masterbatch using solvents for the LDHs and polymers. The masterbatch is then diluted in the desired polymer by conventional melt blending machines (internal mixer, extruder, etc.). Figure 12.7 depicts a schematic diagram for the sonication assisted masterbatch (SAM) melt mixing method.

 Figure 12.7  Schematic represen­ tation of the sonicated assisted masterbatch (SAM) melt mixing method for LDH/polymer nanocomposites

361

362

12 Polymer Nanocomposites Based on Layered Double Hydroxides (LDHs)

SAM was compared to melt mixing of PLA/DBS-MgAl LDH nanocomposites. Different properties and dispersions were observed for the SAM-prepared nanocomposites. SAM melt mixing showed better properties compared to melt-mixing-prepared nanocomposites at the same loading level of LDHs. The dispersion in both techniques is shown in Figure 12.8. Transmission electron microscopy (TEM) images indicate better dispersion and exfoliated morphology in the case of SAM melt mixing techniques as compared to melt mixing techniques for 1.25 wt% LDH addition. This technique is relatively new and should be investigated for other types of polymer and LDH hybrids to obtain better intercalated LDH/polymer nanocomposites with enhanced properties.

A

B

Figure 12.8 TEM images of PLA nanocomposites with 1.25 wt% LDH: (A) melt mixing method and (B) SAM melt mixing method

„„12.6 Summary This chapter covers the processing and properties of LDH-based polymer nano­ composites. The first part encompasses the structure, synthesis, and modification methods affecting the LDHs. The different compositions of LDHs are analyzed indicating the derived applications. The second part deals with the preparation of LDH/polymer nanocomposites. The main techniques to prepare composites on an industrial scale are based on melt compounding, where the dispersion and exfoliation of LDHs in the polymer melt are the decisive factors. Modification of LDHs enhances the processing of LDH/polymer nanocomposites and affects their final end-use properties. The processing of LDH/polymer nanocomposites is discussed, including novel and non-conventional methods. In particular, a novel approach is conceptualized and applied to enhance the dispersion level of LDHs in a PLA ma-

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