Effect of nanoparticle on the mechanical and gas barrier properties of thermoplastic polyurethane

Effect of nanoparticle on the mechanical and gas barrier properties of thermoplastic polyurethane

Applied Clay Science 146 (2017) 468–474 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/cla...

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Applied Clay Science 146 (2017) 468–474

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Effect of nanoparticle on the mechanical and gas barrier properties of thermoplastic polyurethane

MARK

Shruti Pandeya, Karun K. Janaa, Vinod K. Aswalb, Dipak Ranac, Pralay Maitia,⁎ a b c

School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi 221 005, India Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Industrial Membrane Research Institute, Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON KIN 6N5, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords: Thermoplastic polyurethane Nanohybrid Structure Mechanical properties and gas permeability

Thermoplastic polyurethane (TPU) nanohybrids have been prepared through melt extrusion using ester type PU and different concentrations of Indian origin organically modified nanoclay as filler. The level of dispersion of nanoclay in TPU is found to be good and considerable intercalation occurs due to strong interaction between polymer matrix and filler. The interaction is shown through spectroscopic measurement from the shifting of peak position in FTIR and UV–vis. absorption spectra. Nanoclay induces crystallization in polymer while the blob size, as measured through small angle neutron scattering, decreases in nanohybrid (1.5 nm) as compared to pure TPU (1.7 nm) obtained after fitting the initial data point to Debye-Bueche model. Mechanical responses are much superior in nanohybrid as compared to pure TPU and stiffness values continue to increase with nanoclay concentration while the toughness reach ata maximum value at an optimum concentration of 4 wt% of nanoclay. Uniaxial stretching lead to the crystallization of segments and ordering of hard segments as verified through sharp melting points in stretched TPU vis-à-vis predominant amorphous nature before stretching. Nanohybrid membranes are prepared to investigate the gas permeation across the membranes and very high gas barrier of nanohybrid (449 Barrer) is found as opposed to pure TPU barrier of 169 Barrer. Critical assessment of permeability is performed in presence of nanoclay in different concentrations with a plausible mechanism of gas barrier.

1. Introduction During past decades, the development of nanohybrids is customized with different fillers and it has been the basis of production of many superior products. Polymers with inorganic clay particles with high aspect ratio with nanoscale dimension have yielded improved chemical and physical properties including thermal, mechanical, flame-retardant, electrical, biodegradability and barrier properties (Wang and Pinnavaia, 1998; Shah et al., 2004; Jana et al., 2012; Jana et al., 2013; Jana et al., 2015a, Jana et al., 2015b). Clays are layered silicate materials commonly used as reinforcing filler for the preparation of nanohybrids due to their easy availability, natural abundance and low cost (Barick and Tripathy, 2010). Indian origin NK75 is a montmorillonite (Mt) clay and has planar octahedral alumina layer sandwiched between two tetrahedral silicate layers (Nguyen and Baird, 2006). The structure repeats itself with a characteristic distance. Mt. clay has high cation exchange capacity (CEC), aspect ratio, surface area, surface reactivity and adsorptive properties (Salahuddin et al., 2010). Higher percentage of iron in nanoclay (NK75) helps in improving the



Corresponding author. E-mail address: [email protected] (P. Maiti).

http://dx.doi.org/10.1016/j.clay.2017.07.001 Received 4 March 2017; Received in revised form 3 July 2017; Accepted 3 July 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.

biocompatibility and, therefore, NK75 is used as a filler in biomedical applications as well (Kapusetti et al., 2014). The properties of nanohybrids improve when the silicate layers (inorganic phase) exfoliate or considerable intercalation occurs (Lebaron et al., 1999; Kim et al., 2003; Song et al., 2005). Improved nanohybrid preparation is very much reliant on the structural aspects of the polymer in addition to the compatibility of the clay with the matrix. Isomorphic substitution within the silicate layers generates negative charges which are counter balanced by Ca++ and Na+ cations (residing in interlayer gallery) resulting better miscibility between silicate layer and matrix and overall good dispersion of the silicate layers (Corcione and Maffezzoli, 2009). There are two fundamental processes to prepare nanohybrids (1) direct intercalation of polymer chain into layered clay, and (2) intercalation of monomer onto clay interlayer and consequent heat treatment for polymerization (Rehab and Salahuddin, 2005; Joulazadeha and Navarchiana, 2010a,Joulazadeha and Navarchiana, 2010b). The nanohybrid with large surface area can be used in a variety of applications such as food packaging (Qian et al., 2011), biotechnology (Singh et al., 2011), devices (Mitra et al., 2011) and fuel cell membrane (Jana et al.,

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3.3. Small-angle neutron scattering

2015a,b). Thermoplastic polyurethane (TPU) is a thermoplastic block copolymer characterized by a wide range of properties consisting of blinking soft segments (which is high molar mass polyether or polyester macro-diol) and hard segments (which is self-possessed of diisocyanate and chain-extender molecules that are either diol and diamine) (Koerner et al., 2008; Mishra et al., 2012). TPU bridges the gap between flexible rubber and rigid plastics and offers exceptional benefits. Unlike other thermoplastic elastomers, TPU provides numerous physical properties which makes it extremely flexible material adaptable to various uses. Polymer nanohybrids obtained from a polyurethane matrix and the adequate nanofillers offer a chance to produce new materials (Finnigan et al., 2004), where the properties of polyurethanes can be improved. Polyurethane elastomers have massive challenge in modern time as TPU possess good properties like high hardness, abrasion resistance, excellent mechanical properties and flexibility and it has a variety of universal applications caused by its distinctive properties, e.g. biotechnology, adhesives and packaging etc. (Osman et al., 2003; Choi et al., 2011; Mahanta et al., 2015). This work has been carried out to study the properties of elastomeric TPU with clay (NK75) with significantly improved properties as compared to pure polymer and the effect of varying concentration of fillers in the nanohybrids properties. Mechanical, morphological, thermal, gas barrier properties are used to study the structure and the performance of the nanohybrids.

Small angle neutron scattering experiments were performed on the spectrometer at the Dhruva reactor at Bhabha Atomic Research Centre in Mumbai, India. The data was collected for the scattering vector (q) range from 0.17 to 3.6 nm− 1. The scattering from the samples was modified after background correction. The characteristics length (Λc) was calculated using the equation:

Λc =

2π qm

(1)

where, qm is the scattering vector q consequent to the peak position of shoulder in the scattering patterns of the samples. The temperature was kept constant and stable at 30 °C during each measurement. 3.4. FTIR Fourier transform infrared (FTIR) spectroscopy was performed on a Thermo Scientific FTIR (NICOLET-6700) in the ATR mode in the wavenumber range of 650–4000 cm− 1. Each spectrum was recorded by accumulating 100 scans with a peak resolution of 4 cm− 1in reflectance mode. 3.5. UV–Vis spectroscopy

2. Experimental segment The UV–visible absorbance spectra of neat polymer and nanohybrid films were taken using Shimadzu 17,001 in the range of 200–800 nm wavelengths of light.

2.1. Materials Thermoplastic polyurethane (TPU) was obtained from Bayer Material Science AG: polyester based TPU (Desmopan 385S) having the shore hardness of 86A. A typical nanoclay (NK75; Indian origin high iron content Mt., ion-exchanged with dimethyl dihydrogenated tallow ammonium; CEC = 70 meq/100 g) was used as a filler for nanohybrid preparation (Kapusetti et al., 2014). Tallow contains ~65% C18, ~ 30% C16 and ~ 5% C14 alkyl chains.

3.6. TGA Thermogravimetric analysis (TGA) was performed on a MettlerToledo TGA under nitrogen atmosphere. Thermal degradation of TPU and NH specimens were studied with a heating rate of 20°/min in the measurement range of 40to 600 °C and the sample weight of nearly 5 mg was used for TGA experiments.

2.2. Preparation of nanohybrid 3.7. DSC measurement Melt extrusion technique was used for the preparation of nanohybrid (Dan et al., 2006; Grande and Pessan, 2017). Nanoclay (NK75) was mixed with TPU in a Haake twin-screw extruder (Hakke Mini Lab) for 10 min excluding the feeding time at 220 °C with the shear rate of 100 rpm. Henceforth, the nanohybrid of TPU will be termed as “NH” with 4 wt% of nanoclay in the pristine polymer. Nanohybrids containing 2, 6 and 10 wt% of nanoclay were also prepared to understand the effect of nanoclay loading. Films (~ 120 μm thick) of pristine polymer (TPU) and extruded nanohybrids (NHs) were prepared using a compression molding machine (S. D. Scientific Ltd.).

Differential scanning calorimetry (DSC) measurements were performed on a Mettler832 instrument. The DSC was calibrated with indium and zinc before actual experiment. The heat of fusion and melting temperature were evaluated from the melting endotherms using a software and computer attached to the instrument. Weight of the samples was in the range from 3 to 10 mg range and a10°/min heating rate was employed for all experiments. 3.8. Mechanical properties Tensile tests were carried out on a universal testing machine (Instron 3369). The strain rate was 5 mm/min at room temperature. Dog bone shaped specimens with gauge length 20 mm, thickness 2.12 mm and width 4 mm were prepared using Haake micro-injector.

3. Characterization 3.1. TEM Bright field images of nanohybrid specimens were obtained using TEM (Technai G2) operated at an accelerating voltage of 100 kV. A thin layer, around 70 nm thick, from the sample was sectioned at −85 °C using a Leica ultra-microtome equipped with a sharp diamond knife.

3.9. Measurement of gas Permeability Gas permeability of the specimens (neat TPU and NH) was examined at a constant pressure using three parallel cells method (Savoji et al., 2012). A spherical film of 11.044 cm2 area and 0.003 cm thick sample was placed in the cell and was fixed using aluminum foil, glue and cello tape. The feed pressure was set at 80 psi while permeate side was maintained at the atmospheric pressure. The calculation of gas permeation rate was resolved using soap bubble flow meter (Rana et al., 2012). Each experiment was performed triplicate for reproducibility and the averages of those results are reported. The gas permeability was calculated from the following Eq. (2);

3.2. XRD The change in d-value of silicate layers of the nanoclay was determined using Bruker AXS D8 Advance wide-angle X-ray diffractometer with Cu-Kα radiation with a graphite monochromator (λ = 0.154 nm). The samples were scanned at 1°/min in 1 to 10° range and at 3°/min for 10 to 40° 2θ range. The basal spacing of the nanoclay, d001 was calculated using Bragg's law. 469

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a.

Fig. 1. (a) TEM micrograph of TPU/nanoclay nanohybrid, (b) Wide-angle XRD patterns of neat nanoclay and its nanohybrid, (c) FTIR spectra of the pristine TPU and NH in the stretching frequency range 3500–1400 cm− 1; and (d) UV–Vis absorption spectra of pure TPU and its nanohybrid.

b. 4 nm

Intensity / a.u.

NH Clay

2 nm

2

c.

d. Reflectance / a.u.

0.4 3325

Absorbance

1725

NH TPU

322 nm 390 nm

0.3

0.2

0.1

3300 1728

1800

1703 -1

Wavenumber / cm

Gas flow rate (GFr) =

8

288 nm

TPU

3400

6

2θ / deg

0.5

NH

3330

4

1500

10 ml t min × 60

0.0 200

400

600

nature of interactions between polymer and nanoclay (Vaia and Giannelis, 1997) and to some extent on the preparation method of nanohybrid (Wang et al., 2002). Fig. 1c exhibits FTIR spectra of pure TPU and NH showing the carbonyl peaks around 1703 and 1728 cm− 1 for pure TPU assigned as hydrogen-bonded carbonyl group and free carbonyl group (eC = O) of the urethane linkage (Pattanayak and Jana, 2005). However, a single stretching frequency at 1725 cm− 1 has been observed in NH and the shifting of peak position from 1728 → 1725 cm− 1 along with considerable suppression of hydrogen bonded peak is presumably due to the interaction between polymer and nanoclay in nanohybrid. Similarly, pure TPU shows two eNH stretching frequency at 3330 and 3300 cm− 1 assigned as non-hydrogen bonded and hydrogen bonded eNH peak, respectively, while a single peak at 3325 cm− 1 is observed in nanohybrid suggesting interactions between the components as evident from shifting of peak and suppression of hydrogen bonded peak. A strong and wide UV–visible absorption band at 288 nm is observed in pure TPU, accredited as π → π⁎ electronic transition from carbon–oxygen double bond (C]O) (Wang et al., 2013) (Fig. 1d). It is interesting to note that a significant red shift along with greater absorption has been observed for nanohybrid at 322 nm arising from enhanced interaction of organic modifier and polymer. Further, a wider tail appeared for NH at 390 nm, preassembly duo to filler. Therefore, the interactions, as evident from FTIR and UV–vis studies, between polymer and nanofiller are responsible for good dispersion and superior intercalation as observed in TEM micrograph and XRD patterns. However, slight shift of peak position occurs in FTIR (1728–1725 cm− 1) led to inference that there are interaction but that is not very strong rather some sort of interaction do exist. That is the reason behind moderately good dispersion in TEM arising from weak interaction.

(2)

where, tmin is the time (sec) of gas flow rate of the permeate gas passing through the membrane (cm3/s). Gas flux was calculated from the Gfr per unit area where, gauge pressure was considered as 80–14.69 psi. Gauge pressure indicates the absolute pressure difference between the feed side and permeates side (cm Hg). Then, the normalized gas flux was calculated using the Eq. (3);

Normalized gas flux (NGF) =

Gas flux Guage pressure

800

Wavelength / nm

(3)

and gas permeability coefficient (GPC) = NGF × membrane thickness, where, permeability was expressed in barrer (1 barrer = 10− 10 cm3(STP) cm cm− 2 s− 1 cm Hg− 1). 4. Results and discussion 4.1. Dispersion and interactions Organically modified nanoclay is dispersed in TPU and the dispersion is found to be uniform as evident from the bright field TEM image (Fig. 1a) showing stacks of nanoclay with the correlation length, ξ of 239 ± 10 nm. High resolution image of TEM also indicates the intercalated structure of the nanohybrid (supplementary document) (Cai et al., 2007). The nanostructure analysis through XRD pattern showed a strong reflection at 2θ ~ 2.2°, corresponding to d001 value of 4 nm for nanohybrid as opposed to the value of 2 nm of organically modified nanoclay showing significant level of intercalation of polymer within the gallery of platelets of nanoclay (Fig. 1b) (Kapusetti et al., 2014). Further, the higher intensity of the intercalated reflection is presumably due to ordered pattern of the platelets against the relatively disordered nature of nanoclay powder. It should be noted that the level of dispersion and the extent of intercalation is strongly correlated with the 470

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Intensity / a.u.

20.9 20.1

b.

Clay

c.

4.8 4 3.2 ΔΤ = 22.5 nm c

2.4 10

15

20

25

30

2θ / deg

35

40

1.6 Δc = 14.9 nm

15

20

25

2θ / deg

30

35

2.8 ξ = 1.58 2.4 ξ = 1.70

2.0

0.8

10

TPU NH

3.2

NH TPU

I(q) / a.u.

NH TPU

I(q) / a.u.

a.

Intensity / a.u.

S. Pandey et al.

40

0.25

0.5

0.75 1 -1

q / nm

1.6 0.15

2.5

0.20

0.25

0.30

0.35

-1

q / nm

Fig. 2. (a) XRD patterns of the pure TPU and NH (the inset figure show the XRD pattern of pure nanoclay), (b) Small-angle neutron scattering plot I(q) vs. q (wavevector) of indicated TPU and nanohybrid, and (c) Debye-Bueche fitting for TPU and NH considering the value at lower q range.

degrades at around 336 °C (Strankowskil et al., 2012) (temperature corresponding to 5% mass loss) while nanohybrid degradation takes place at 320 °C. The slight faster degradation of nanohybrid is presumably due to nanoclay while two stage degradation is noticed in both the cases for the degradation of hard segment (lower temperature) and soft segment (relatively high temperature).

4.2. Structure of nanohybrid The structural alteration has been verified through wide angle X-ray scattering. Fig. 2a shows broad reflections at 20.1° due to amorphous hallow of pure TPU (Koerner et al., 2008). This reflection has been shifted to 20.9° in presence of filler in addition to other smaller humps indicating a crystalline phase in the nanohybrid. Fig. 2b represents the small angle neutron scattering (SANS) patterns of pristine TPU and its nanohybrid on a log-log scale. A distinctive shoulder is clear at wavevector, q ~ 0.65 nm− 1 for pure TPU, corresponding to characteristic length, Ʌc of 14.96 nm which has been shifted to q ~ 0.53 nm− 1 in nanohybrid, corresponding to Ʌc of 22.41 nm. These peaks are indicative of a stacked lamellar pattern inside the crystallites. Further, the initial data points are fitted with Debye-Bueche model to calculate the correlation length to understand the blob sizes which are found to be 1.70 and 1.58 nm for TPU and NH, respectively (Fig. 2c). These aggregates are formed due to self-assembly of greater hydrogen bonded clusters (Patel et al., 2015) and lower size in nanohybrid indicate that smaller aggregation size is enough for nanohybrid to form self-assembly because of stronger interactions between filler and TPU as opposed to only hydrogen bonded interaction exist in pure TPU. This result is in agreement with the literature reported values of different variety of polyurethanes (Mishra et al., 2010).

4.4. Mechanical responses Filler present in polymer matrix is expected to enhance the mechanical responses of polymer and the stress-strain curves of the samples under tensile condition are presented in Fig. 4a showing much higher elongation at break for nanohybrid as compared to neat TPU. Further, continuous increase of stress with increasing elongation suggests possible strain induced crystallization. The variation of mechanical properties with respect to varying filler concentrations in different nanohybrids is shown in Fig. 4b. The modulus was calculated from the linear regime while the area under stress-strain curves is considered for toughness calculation. The nanohybrids show improvement in elastic moduli consistently with increasing nanoclay concentration due to reinforcing effect of the filler while toughness increases up to 4 wt% of nanoclay followed by a decreasing trend raising an optimum concentration of 4 wt% of filler concentration both in terms of stiffness and toughness. Halpin–Tsai model (Ashton et al., 1969; Halpin, 1969; Halpin and Kardos, 1976) is widely used to estimate the reinforcement effect of filler particle in nanohybrids, following the equation (Fornes and Paul, 2003; Joulazadeha and Navarchiana, 2010a,b):

4.3. Thermal properties including stability Fig. 3a shows the DSC thermograms obtained by heating the sample from room temperature to 300 °C. A prominent glass transition temperature, Tg of TPU is found at 128 °C. The glass transition temperature of the nanohybrid has increased considerably to 148 °C and higher Tg of nanohybrid is attributed to the interactions between the organic and inorganic phases. The thermal stability of the TPU and NH has been investigated by using thermogravimetric analyzer (TGA) and the mass loss as a function of temperature has been shown in Fig. 3b. Pure TPU

a.

b.

o

148 C

o

128 C

(4)

where, ξ is a shape parameter dependent upon filler geometry, orientation and loading direction, and η is given by: NH TPU

1.0

Weight fraction

Heat flow / a.u.

NH TPU

1 + ξηVf Ec = Em 1 − ηVf

0.8 0.6 0.4 0.2 0.0

100

150

T / ∞C

200

200

300

0

400

Temp / C

471

500

Fig. 3. (a) DSC thermograms of TPU and nanohybrid, and (b) TGA thermograms of neat TPU and NH.

Applied Clay Science 146 (2017) 468–474

b.

NH TPU

20

287 288

5

0

c.

NH

0

300

600

900

ε/%

2.0

NH ξ=9.0

1.8

16 12

-3

10

Toughness / MJ m

σ / MPa

15

20

Ec/Em

a.

Modulus / MPa

S. Pandey et al.

1.6

120

1.4

80

1.2

1.0 0.00

40 0

1200

2

4

6

8

0.02

10

wt. of filler / %

0.04

0.06

0.08

Vf

Fig. 4. (a) Stress-strain curves of injected specimens at a rate of 5 mm/min for pristine TPU and TPU nanohybrid (NH) showing the dramatic increase in elongation at break for NH, (b) Variations of tensile modulus and toughness of nanohybrids as a function of nanofiller concentration, and (c) Young's modulus vs. loading of nanoclay (volume fraction) in nanohybrids, fitted with Halpin-Tsai model.

η=

Ef Em Ef Em

exhibit any melting peak in the DSC thermograms (Fig. 3a). The effect of uniaxial stretching has been shown through schematic in Fig. 5c where amorphous soft segments get crystallized after stretching while disordered hard segment transform in to an ordered state under the influence of stretching. Uniaxial stretching usually converts the coiled state of polymer chain into uncoiled configuration and the thermodynamically unfavorable low entropic condition is overcome through its crystallization to minimize the overall Gibbs energy. Relatively lower heats of fusion for hard segment is presumably due to their poor ability to crystallize in presence of aromatic diisocyanate used to prepare TPU.

−1 +ξ

(5)

where, Ef, Em represent Young's modulus of the filler and matrix, respectively. The model is based on micromechanics contributed by Herman and Hill (Hill, 1964; Hermans, 1967). Halpin-Tsai fit for TPU/ nanoclay nanohybrids has been shown in Fig. 4c considering Em = 8.6 MPa and Ef = 200 GPa. The modulus predictions for nanoclay volume fraction range from 2 to 10% which is within the range for practical applications. As expected, Halpin-Tsai equation predicts well the development of modulus with increasing volume fraction of nanofiller with the value of ξ = 3 which fits the data point in a best possible manner. The value of ξ depends on the geometry and packing of the particles as well as on the direction of the load relative to the orientation of the nanoparticles. Increasing the aspect ratio results in higher reinforcement for a given filler modulus and volume fraction. For oriented lamellar shape filler particles, ξ is equal to 2 (w/t), where w and t are the width and thickness of the particles, respectively.

4.6. Effect of nanoclay on gas permeation Incorporation of filler particles in the polymer matrix increases the gas barrier properties in nanohybrids (Adame and Beall, 2009; Sheng et al., 2011; Tan and Thomas, 2017). The influence of nanoclay content on the barrier performance to nitrogen has been studied by investigating nitrogen permeability through TPU and its nanohybrid films. Nitrogen permeability results from testing of the neat TPU and its nanohybrid films of different filler concentrations are shown in Fig. 6a. All the nanohybrid films showed better nitrogen barrier than pure TPU film and the gas permeability decreases with increasing filler concentration while this effect is more pronounced in lower concentration arising from the uniform distribution of nanoplatelets. The efficiency of barrier effect is reduced for high concentration presumably due to agglomeration of nanoclay at higher concentration because of effective blockade is less as compared to increased concentration. The schematic representation of gas permeation has been shown in Fig. 6b where the path becomes tortuous in presence of two-dimensional platelets reducing the gas permeation to significant level. Geometrical influences like shape and state of exfoliation/intercalation of platelets and their

4.5. Effect of uniaxial stretching on structure The samples are stretched uniaxially using UTM at a particular rate. Interestingly, sharp reflections appear in XRD patterns both in TPU and NH at 2θ ~ 21.5 and 21.7° for TPU and NH, respectively (Fig. 5a). The shifting of reflection position for nanohybrid indicates greater crystallization vis-à-vis amorphous hallow at 2θ ~ 20.1° before stretching. Further, the appearance of new peak in DSC thermograms also consolidates the crystallization after stretching. Moreover, the higher heat of fusion for stretched nanohybrid (22.4 J·g− 1) as opposed to stretched TPU (7.0 J·g− 1) confirms the crystallization of both the soft segment (peak around 55 °C) and hard segment (small peak around 189 °C). This is to mention that unstretched samples (both pure TPU and NH) do not

b.

NH TPU

c. NH TPU

Intensity / a.u.

21.7

Heat flow / a.u.

a.

21.5

ΔH = 1.5

175.8 ΔH = 22.4 ΔH = 2.6

55.9

179.5

ΔH = 7.0

55.8

10

15

20

2θ / deg

25

30

50

100

T / ∞C

150

200

Fig. 5. (a) XRD patterns of TPU and its nanohybrid after stretching, (b) DSC thermograms of TPU and its nanohybrid showing the melting peak of soft and hard segments with respective heat of fusion values, and (c) schematic representation of chain configuration before and after stretching. Disorder hard segment along with amorphous soft segment is predicted against the order hard segment and crystallized soft segment after uniaxial stretching.

472

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S. Pandey et al.

Fig. 6. (a) Experimental (points and dashed line) and predicted (solid line following the Eq. (6)) gas permeability of nitrogen vs. nanoclay content of TPU-nanohybrid specimens at room temperature. (1 Barrer = 1 × 10− 10 [cm3 (STP) cm/cm2 s cm Hg]), (b) Schematic path of representative for neat TPU and nanohybrid (in case of TPU, nitrogen gas can permeate across the film thickness while gas has to follow a tortuous path in presence of two-dimensional platelets).

interactions. The interactions are visualized through spectroscopic measurement and considerable shifting of peak position both in FTIR and UV–vis. suggest strong interaction between TPU and nanoclay. Crystallinity develops in nanohybrid in presence of nanoclay as opposed to predominant amorphous nature of TPU. Greater assembly of tens of nanometer is observed through small angle neutron scattering of varying size of 22.5 and 14.9 nm for TPU and nanohybrid, respectively, with blob size of ~ 1.7 nm, arising from the Debye-Bueche fitting of the data points at lower wavevector region. Nanohybrids show remarkable improvement in mechanical strength and toughness increase up to 4 wt % of nanoclay followed by decreasing tendency while Young's moduli gradually increase with the nanofiller concentration. Gas barrier property of nanohybrid has significantly been improved in nanohybrid as compared to the matrix without nanoclay. Strain induced crystallization has been observed as revealed through sharp melting points both for soft and hard segment vis-à-vis the absence of any melting point before stretching leading to propose a model of ordering of hard segment and crystallization of soft segment under uniaxial stretching. The permeability of gas decreases in nanohybrids with increasing nanoclay loading because of delayed diffusion in tortuous path created by the dispersed nanoclay platelets. Gas permeation in nanohybrid has been critically evaluated using different models.

orientation in the polymer matrix is also reflected by the degree of tortuosity created by the filler particles. The permeability of the nanohybrids (Ps) is related to the permeability of the pure polymer (Pp) and the volume fraction of the sheets (Фs), length (L) and width (w) of the sheets (nanoclay particles) as shown in Eq. (6) following Bharadwaj's model (Sridhar, 2006). S is considered as filler order parameter and is found to be the value of 0.3 using best fitting of the Bharadwaj's equation to the experimental data points. The Eq. (6) reduces to the expression in the form of Eq. (7)considering the value of S = 1. Thus, the deviation of the order parameter from unity in this case is due to the retardation in the diffusion mechanism of the gas molecules through the matrix because of the tortuous path being created by the orientation of nanoclay platelets.

Ps = Pp 1+

(1 − Фs)

( )(S + )

L 2 Фs 3 2w

Ps (1 − Фs) = L Pp 1 + 2w Фs

1 2

(6)

(7)

where, L and W are the length and thickness of the filler platelets and Фs is the volume fraction of the filler (nanoclay). Volume fractions of the nanoclay platelets in NH are calculated from the weight fraction using the density of fillers used. Aspect ratio (L/2w) is likely to be changed due to considerable agglomeration of nanoclay platelets at higher concentration and, hence, slight distortion of permeability occurs from the prediction by Bharadwaj. However, theoretical prediction of permeability happens to a great extent in this case and more importantly, permeability can be reduced using the nanoclay. Now, it is pertinent to compare the results of literature reported data against our work to improve the properties and structure property relationship of TPU nanoclay nanohybrid. Most of the works are related to the dispersion using different types of nanoclay and subsequent improvement in mechanical properties mentioning the interaction behind the enhancement (Dan et al., 2006; Cai et al., 2007). This work emphasizes on the significant improvement in gas barrier properties using small amount nanoclay as compared to minimal increment reported by Sheng et al., 2011. Further, structural development during stretching is the unique feather suggesting crystallization of soft segmented zone and overall crystallite size determination through small angle neutron studies.

Acknowledgement The authors are thankful to Dr. D. K. Avasthi and Dr. Pawan K. Kulriya of University Accelerator Centre (IUAC), New Delhi, India for supporting in XRD measurement. Authors also acknowledge the Council for Scientific and Industrial Research, New Delhi (Grant No. 02(0074)/ 12/EMR-II). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.clay.2017.07.001. References Adame, D., Beall, G.W., 2009. Direct measurement of the constrained polymer region in polyamide/clay nanocomposites and the implications for gas diffusion. Appl. Clay Sci. 42, 545–552. Ashton, J.E., Halpin, J.C., Petit, P.H., 1969. Primer on Composite Materials: Analysis. Published by Technomic. Barick, A.K., Tripathy, D.K., 2010. Preparation and characterization of thermoplastic polyurethane/organoclay nanocomposites by melt intercalation technique: effect of nanoclay on morphology, mechanical, thermal, and rheological properties. J. Appl. Polym. Sci. 117, 639–654. Cai, Y., Hu, Y., Song, L., Liu, L., Wang, Z., Chen, Z., Fan, W., 2007. Synthesis and characterization of thermoplastic polyurethane/montmorillonite nanocomposites produced by reactive extrusion. J. Mater. Sci. 42, 5785–5790. Choi, J.T., Kim, D.H., Ryu, K.S., Lee, H., Jeong, H.M., Shin, C.M., Kim, J.H., Kim, B.K., 2011. Functionalized graphene sheet/polyurethane nanocomposites: effect of particle size on physical properties. Macromol. Res. 19, 809–814.

5. Conclusions Thermoplastic polyurethane nanohybrids have been prepared through melt extrusion using Indian based organically modified nanoclay. The dispersion level of nanoclay in TPU matrix is found to be good as observed through TEM bright field image. Intercalated nanostructure is noticed as the gallery spacing of nanoclay increases from 2 to 4 nm due to insertion of polymer chain into the gallery arising from specific 473

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