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Polyethylene terephthalate/calcined kaolin composites: effect of uniaxial stretching on the properties.


Polyethylene terephthalate (PET) is a commodity grade polyester, which has found many applications such as textile fibers and food packaging materials due to its good chemical resistance, thermal stability, and mechanical properties. A technique to further improve the properties of PET is through the formation of a composite, which can be achieved by the addition of fillers. Many physical properties such as modulus, ultimate tensile strength, and barrier properties can be enhanced by adding low contents of layered silicate fillers such as kaolin to polymer matrices.

Polymer/layered silicate composites, as a new class of composite materials, have received an extensive attention in the last 20 years [1-5]. Generally speaking, layered silicates improve mechanical performance, thermal stability, and barrier properties of polymer materials, at relatively low filler contents compared to that of conventional composites [6-10], However, the majority of investigations on polymer nanocomposites has been conducted on a limited number of fillers, mostly montmorillonite clay, silica, and calcium carbonate.

Kaolin is a type of clay primarily composed of the mineral kaolinite with minor amounts of impurities such as quartz, feldspar, and other clay minerals. Kaolinite ([Al.sub.2][O.sub.3].2Si[O.sub.2].2[H.sub.2]0) is a hydrated aluminosilicate with 1:1 layer structure consisting of an octahedral aluminum hydroxide sheet and a tetrahedral silica sheet [11], Kaolin is of relatively low-cost and readily available worldwide. It has a wide range of applications in the ceramic, cosmetic, and pulp and paper industries. Pristine kaolin is calcined by firing the powder to a temperature around 650[degrees]C to lose both surface-adsorbed and chemically bonded water (lattice water). Calcination at higher temperatures leads to the complete dehydroxylation and formation of partially crystalline metakaolin. Fully calcined products are produced above 950[degrees]C and have an amorphous spinel structure [12].

Since the surface of mineral fillers is generally hydrophilic, they are incompatible with most polymers. Chemical treatment of a filler surface can promote the compatibility and dispersion of the particles into a polymer matrix. Treatment of particles with silane coupling agent has been extensively applied to different types of fillers such as Ti[O.sub.2] [13] and clay minerals [14-16]. In this work, an epoxy functional silane was used to examine the effect of silane treatment of a calcined kaolin (CKao) on the properties of PET composites.

Many industrial polymer forming processes such as blow molding or thermoforming are associated with secondary processing involving uniaxial and biaxial stretching. Therefore, having a good knowledge of how these types of deformation can influence the polymer-clay composites is of great importance. It has been demonstrated that biaxial stretching of PET-silica can significantly affects the filler dispersion [17]. Litchfield and Baird studied uniaxial stretching of spun fibers above their glass transition temperature ([T.sub.g]) and concluded that secondary processing can enhance the mechanical properties [18].

Previous studies showed that chemically modified kaolin can be dispersed in a PET matrix [19, 20]; however, the existence of hydroxyl groups in the structure resulted in the degradation and molecular weight loss of the polymer matrix and, subsequently, spoiled the final properties of the composite. In this article, the effect of CKao particles on morphological, mechanical, optical, and gas-barrier properties of PET composites has been studied. The effect of secondary processing such as hot-stretching on the properties of PET-CKao composites was also investigated.



A commercial grade PET resin (Laser+[R] 7000) with an intrinsic viscosity of 0.84 dL/g provided by DAK Americas LLC was used as polymer matrix. A commercial calcined grade kaolin (Ultrex[R] 96), denoted hereafter as CKao, from BASF Corporation was used in this study. It is a dchydroxylated aluminosilicate with a density of 2.63 g/[cm.sup.3] (at 25[degrees]C) and an average particle size of 1.2 [micro]m. In order to raise the melt viscosity and molecular weight of the PET resin, an FDA-approved masterbatch of PET with a chain extender (denoted hereafter as ch), from Polyvel Inc., was added during the melt mixing. A silane coupling agent, 3-(glycidoxy-propyl) trimethoxysilane (denoted hereafter as SiE), supplied by Gelest was used for silanization of the particles.


Characterization and Testing

A Hitachi S4700 SEM instrument with a cold field emission gun under an acceleration voltage of 2 kV was used to examine the morphological characteristics of the composites. The samples were microtomed using an Ultracut FC microtome (Leica, Germany) with a diamond knife and gold coating was subsequently applied to make them conductive.

The structure and crystal size of the composites were investigated by wide angle X-Ray diffraction (WAXD). The WAXD patterns were recorded over scattering angles, 2[theta], range from 10[degrees] to 40[degrees] on a Philips X'pert diffractometer (CuK[alpha] radiation, [lambda] = 1.54056 [Angstrom]) operating at a voltage of 50 kV and current of 40 mA.

Rheological measurements were carried out under nitrogen atmosphere on a stress-controlled rheometer (Bohlin Gemini HR nano, Malvern) with parallel plates of 25 mm diameter and 1 mm gap. The rheometer was equipped with a convection oven to control the temperature at 270[degrees]C for all the samples.

Melting and crystallization characteristics of the samples were determined by differential scanning calorimetry (DSC) using a DSCQ1000 TA Instrument at a heating/cooling rate of 10[degrees]C/min under helium atmosphere. The degree of crystallinity ([X.sub.c]) was calculated using the following equations:

[X.sub.c] = [DELTA][H.sub.m]/(1 - [W.sub.f]) x [DELTA][H.sub.0]

where [DELTA][H.sub.m] is the melting enthalpy of sample, [DELTA][H.sub.0] is the melting heat of 100% crystalline PET which is taken as 140 J/g according to the literature [21], [W.sub.f] denotes the filler weight fraction.

Thermal gravimetric analysis (TGA) was carried out on a TGA500 TA Instrument. About 10 mg of the samples were heated at 10[degrees]C/min from 30 to 700[degrees]C under nitrogen atmosphere.

The mechanical properties were measured in tensile mode at room temperature using an Instron universal testing machine (Model 3365) with 500 N load-cell at a strain rate of 25 mm/min.

In order to determine barrier properties of the composites, oxygen transmission rates (OTRs) were measured at 23[degrees]C under a barometric pressure 700 mm Hg using an Ox-Tran oxygen permeability MD Module (Model 2/21) from Mocon. The permeability coefficients were calculated by normalizing (multiplying) measured OTR values by the films thickness.

Haze values were determined according to ASTM D1003 using a LAMBDA 1050 spectrophotometers from PerkinElmer.


Sample Preparation

Applying Silane Coupling Agent. Modification of CKao particles was accomplished via an aqueous alcohol solution method. An ethanol-deionized water (90/10 wt%) solution was adjusted to pH 4.5-5.5 with acetic acid. The required amount of silane coupling agent was added drop by drop to yield a 4% final concentration and the mixture was stirred for 10 min for hydrolysis and silanol formation. Then, the temperature was increased to at 80[degrees]C, CKao particles were added into the solution and the grafting reaction was completed under stirring for 2 h. The product was filtered and extensively washed with ethanol in order to remove the excess silane and finally dried at 50[degrees]C in a vacuum oven.

Preparation of the PET-CKao Composites. The PET powder (PET granules were ground in liquid nitrogen to obtain a fine powder of PET) was manually mixed with CKao for a few minutes and then the mixture was melt-blended via a twinscrew extruder (LE1STR1TZ Extruder Corp., L/D = 40) to obtain a 20 wt% masterbatch. The temperature profile was set in the range of 250-275[degrees]C and the screw rotating speed was 150 rpm. The masterbatch was eventually diluted with neat PET and the final films with 2-8 wt% filler content were prepared via cast film extrusion. Unless otherwise indicated, for all the composites, the filler loading was 2 wt% and the PET-CKao-ch composite contained 0.5 wt% chain extender. Prior to processing, all the materials were dried in a vacuum oven at 110[degrees]C for 24 hr to eliminate the moisture and prevent hydrolysis reactions during the melt compounding.


Uniaxial Stretching of Composites. Rectangular PET sheets (10 cm wide, 40 mm long, and 100 pin thick), cut from the extruded films, were uniaxially stretched in an Instron mechanical testing machine (ElectroPuls[TM] E3000) equipped with an environmental chamber. The stretching temperature was set in the range of 90-120[degrees]C and specimens were strained up to draw ratios of 24 and rapidly cooled down to room temperature with air flow.


Effect of Silane Coupling Agent and Chain Extender

Morphology. SEM images of as-received CKao, PET-CKao, silane-treated composites (PET-CKao-SiE), and PET-CKao-ch composites are shown in Fig. 1. As-received kaolin has a layered structure. During the calcination process at high temperature, dehydroxylation occurs, the layers collapse and a tightlystacked structure is formed (Fig. 1a). However, this process does not change the layered structure of the material. After blending with PET, large CKao agglomerates break up to form smaller particles (<1 [micro]m) uniformly dispersed in the matrix (Fig. 1b). However, very few particles in the range of 2-3 [micro]m can be still observed in the image. It seems that the silane treatment and addition of the chain extender has a small effect on the morphology of composites as well. The size of dispersed particles is slightly smaller in the case of samples treated with the silane coupling agent (Fig. 1c) or those containing a small amount of chain extender (Fig. Id). The chain extender can react with the end groups of PET chains and provide a chemical link between them, so increases the molecular weight and subsequently the viscosity of the PET during the melt process. Due to increased viscosity, the agglomerates in the extruder will undergo a higher shear Field, which promotes the breaking in smaller particle size.



Rheology. The rheological properties of neat PET and PETCKao with different filler loadings are presented in Fig. 2. From frequency sweep measurements, the neat PET and its composites show a Newtonian behavior in most of the frequency range examined. At high frequencies, the viscosity becomes slightly shear-thinning. The constant melt viscosity at low frequencies is known as the zero shear viscosity and is proportional to molecular weight of a polymer [22], Filled PET generally shows a complex viscosity lower than that of neat PET and increasing the filler loading results in even lower viscosities. This effect suggests that the presence of CKao particles intensify the degradation of PET chains, which lowers the molecular weight and the melt viscosity of the samples. Furthermore, the Newtonian plateau slightly extends to higher shear rates as the clay concentration increases. This could be due to either reduced molecular weight of PET in the presence of CKao particles or the lubricant effect of the filler, i.e., slippage between the particles and the PET chains.

The main degradations that can occur during PET melt processing include thermal degradation, hydrolytic degradation, and oxidative degradation. Generally, degradation is initiated at the ester link in the random chain scission; however, other degradation mechanisms may occur at chain ends. Thermomechanical degradation is more pronounced in the presence of kaolin particles possibly due to their catalytic effect. It seems that Bransted and Lewis acid sites on the filler surface and release of water molecules (trapped in the kaolin structure) at higher temperatures promote PET degradation [23].

To find the effect of chain extender and silane coupling agent on the viscosity of the samples, the frequency (co) dependence of the complex viscosity ([[eta].sup.*]) for PET-CKao, PET-CKaoch, and PET-CKao-SiE composites containing 4 wt% of filler are presented in Fig. 3. The modification of CKao with silane coupling agent does not seem to have a significant effect on the rheological properties of PET. However, the composite containing the chain extender displays higher viscosity and a pronounced shear thinning behavior. The chain extender acts as a bridge connecting the polymer chains and increases the molecular weight of PET. Thus, the shear-thinning behavior could be due to the formation of long chain branches and broadening of the molecular weight distribution of PET because of the possible reactions with the chain extender. Long chain branching develops an entanglement network that confines the mobility of the polymer chains and, consequently, affect the rheological behavior of PET [24, 25],

Thermal Properties. Table 1 shows non-isothermal DSC data of the neat PET and PET-CKao composites. The melting temperature ([T.sub.m]) and the glass-transition temperature ([T.sub.g]) of the composites remains approximately the same regardless of the CKao incorporation. However, the crystallization temperature ([T.sub.c]) shifts to higher temperatures, which is attributed to the nucleating effect of CKao particles [21]. Furthermore, this effect results in higher crystallinity ([X.sub.c]) in the composites. It is also shown that the addition of a chain extender has a hindering effect on the crystallization of the PET matrix, as is revealed by the decrease in the crystallization temperature. Apparently, chain extender molecules can act as branching point and reduces chain mobility and their rearrangement in the crystalline structure. The data presented in Table 1 also show that silane modification has a similar effect on the crystallization behavior of PET. It seems that the silane coupling agent molecules cover the particles, obstructing the migration and diffusion of PET chains to the surface of the CKao (as nuclei).

The thermal stability of the neat PET and the composites under inert atmosphere (nitrogen) was investigated by TGA and the results are presented in Fig. 4. A single decomposition step is observed for all samples. The decomposition of neat PET started around 390[degrees]C and an identical temperature was observed for all the composites. However, for the composites the temperature of maximum decomposition rate ([T.sub.d,max]) is slightly increased (by 4-7[degrees]C) compared to that of the neat polymer. These results suggest that the incorporation of CKao can slightly enhance the thermal stability of PET.


Mechanical Properties. Figure 5 exhibits tensile properties of the neat PET and PET-CKao composites. For the neat PET, the tensile modulus is 1930 MPa and with the addition of 2 wt% CKao, the modulus raises to 2050 MPa. It is well known that filler particles can diminish polymer chain mobility, resulting in less flexibility and higher modulus [26], The improvement of modulus is more pronounced for the PET-CKao-SiE composite. Silane treatment provides a stronger interaction (chemical bonds) between PET molecules and filler particles, and, thus, results in better stress transfer and more enhancement of the modulus. It is also shown that the addition of a chain extender has a reinforcing effect on the tensile properties of the composite, probably due to the increased molecular weight and formation of a long chain branching structure [27], On the other hand, the incorporation of CKao particles had an adverse influence on ductility of samples, thus the increment in modulus was accompanied with 130% and 200% decrease in the elongation at break of PET-CKao and PET-CKao-SiE, respectively, compared to that of the neat PET (Fig. 5b). Similar observations have been previously reported for other polymer/clay composites such as epoxy/organoclay, PLA/MMT, and PET/MMT [28-30].



Effect of Stretching on PET-CKao Composites

Mechanical Properties. Figure 6a displays the Young modulus of the stretched and unstretched neat PET and PET-CKao composites with different filler contents. In the case of unstretched samples, the modulus increases gradually with filler loading. However, this effect is more remarkable at higher filler contents. This improvement in the Young modulus can be attributed to the reinforcing effect of CKao particles dispersed in the PET matrix.

Two main parameters are considered to have the greatest influence on the mechanical properties of composite materials; the incorporation of particles, which have relatively higher elastic modulus compared to polymer matrix, and the relative crystallinity of the samples, since the crystalline domains act as high stiffness domains. The crystallinity of the neat PET and the PET-CKao composites was determined by DSC and the data are displayed in Fig. 7. It is shown that the crystallinity of the unstretched PET-CKao composites increases with filler content; however, it does not change for stretched films (Fig. 7a).

For composites with the same filler content, the hot-stretched ones have significantly higher tensile modulus. This is attributed to the increased molecular orientation and strain-induced crystallization of the PET chains during the stretching process. The only exception to this is the composite containing 8 wt% of CKao. One should note that during the melt mixing of samples with higher filler contents agglomerates are formed, which can significantly decrease the specific surface area of the filler and deteriorate the filler-matrix adhesion. Moreover, this can negatively affect the wetting of the filler by the polymer matrix, which subsequently gives rise to development of voids and micro-cracks in the composite [31]. The vicinity of these voids and flaws are considered as stress concentration points that impair the effective load transfer through the interface between the matrix and the filler. This phenomenon is not significant in the case of unstretched films compared to stretched samples, which have already undergone a debonding between the phases during the hot-stretching step.

Figure 6b illustrates the effect of stretching temperature on the tensile modulus of the neat PET and PET composites. Both neat and filled PET samples stretched at lower temperatures have significantly higher modulus, which could be assigned to the effect of crystals formed during stretching and higher orientation developed at lower stretching temperatures. As presented in Fig. 7b, the relative crystallinity of composite stretched at 90[degrees]C is much higher than that of the unstretched ones or those stretched at 110[degrees]C.

The WAXD patterns for the neat PET and the 4 wt% CKao filled composites are displayed in Fig. 8a and b, respectively. A characteristic diffraction pattern of PET is generally broad and composed of the amorphous phase and reflections of the crystalline phase. The diffractogram of samples stretched at 90[degrees]C shows a diffraction pattern with a new single peak centered at 2[theta] = 25[degrees], indicating the formation of the crystalline phase in the samples due to molecular orientation during the stretching process. The molecular orientation gives rise to a narrow distribution of both the amorphous and the crystalline phase [32]. In case of unstretched samples and those stretched at high temperatures (110[degrees]C), the diffractogram displays a very broad peak centered around 20= 20[degrees]. This is a characteristic of glassy PET and is attributed to the amorphous phase. It is also noticeable that the crystalline phase is formed for all samples stretched at 90[degrees]C regardless of the addition of CKao.

The effect of stretching ratio on the tensile modulus is displayed in Fig. 6c. For both neat PET and PET-CKao, the modulus increases with the stretching ratio. This is due to the increased orientation of PET chains and strain-induced crystallization at higher stretching ratios. In all cases, the stretched PETCKao films have higher modulus compared to the stretched neat PET. Further improvement in the modulus of PET-CKao, compared to the neat PET, is likely to be a result of combined effects of incorporation of CKao particles and enhanced crystallinity (due to the particles, as shown in Fig. 7). It has also been reported that this could be due to the better alignment of filler particles and the reduced agglomeration after stretching [33, 34],

Barrier Properties. Figure 9 displays the oxygen permeability of the neat PET and PET-CKao composites as a function of filler loading, temperature, and stretching ratio, respectively. As shown in Fig. 9a, the PET-CKao composites have better barrier properties (lower OTR) than the neat PET. The main reasons for this barrier improvement are: (i) decrease of the solubility of oxygen due to the reduced volume fraction of PET in the composites (PET volume fraction <1) compared to the neat PET samples and (ii) increase in the tortuosity of the gas diffusion path by CKao particles [9]. There is an 18% reduction in OTR at 4 wt% CKao loading. However, for filler loadings higher than 4 wt%, due to the agglomeration of particles, the filler content does not improve the barrier property; the permeability of the 8 wt% composite is slightly lower than that of the 4 wt% sample.


After stretching, an opposite trend is observed in the barrier properties. For stretched samples the permeability increases with the filler content. Three different parameters can affect the permeability of composites: the barrier effect of particles as impermeable obstacles, the amount of crystalline domains in the sample, and the agglomeration of particles. Increasing the CKao content enhances the relative crystallinity of PET (Fig. 7) and, at the same time, it facilitates the formation of agglomerates in the composites. For stretched samples with filler content up to 4 wt%, the effect of crystallinity is dominant. However, at higher filler loadings, agglomeration of particles, debonding at the interface, and formation of voids compensate for crystallinity effect and augment the OTR values.

Figure 9b reports the oxygen permeability of samples stretched at different temperatures. The lowest oxygen permeation is observed for samples stretched at 90[degrees]C and it was increased at higher stretching temperatures because the orientation and crystallinity of the samples stretched at 90[degrees]C are much higher than that of the others. PET-CKao composites stretched at temperatures higher than 90[degrees]C depict higher permeation compared to the neat PET, which again can be caused by the increased crystallinity and formation of micro-voids as discussed in the previous section.



The effect of the stretching ratio on the permeability of PET composites is reported in Fig. 9c. The permeation of neat PET decreases by applying higher stretching ratio up to 3, after which it shows an increase. However, the exact opposite trend is observed in case of filled PET.

Optical Properties. Figure 10 depicts the haze values of the neat PET and PET-CKao films as functions of filler loading, temperature, and stretching ratio, respectively. Haze describes the ability to divert light in a material (lower is the haze, more transparent is the material) and is defined as the percentage of light that is deflected by more than 2.5[degrees] from the incident light direction [35]. The haze is remarkably affected by the presence of the filler and increases with the CKao content. The higher the concentration of filler and larger are the agglomerates, the more haziness of the film would be observed. As shown in Fig. 10a, the haze jumped from 0.8% for the neat PET to 30% for PET-CKao 8%. This can be explained by the dispersion level and size of the CKao particles inside the PET matrix. This figure shows also that hot stretching has a significant influence on the haziness of PET-CKao films. After being stretched, the haze jumped from 10% to 45%, from 19% to 60%, and from 30% to 75% for PET-CKao composites containing 2 wt%, 4 wt%, and 8 wt% CKao, respectively.

Figure 10b reports the effect of stretching temperature on the haze. In the case of the neat PET, the haze value remains less than 1% regardless of the stretching temperature. By contrast, the haze of the filled samples depends on the stretching temperature. As the stretching temperature increases, the haze value decreases. According to the dynamic mechanical analysis (DMA) results (not shown in this paper), glass transition temperature ([T.sub.g]) of PET falls between 85[degrees]C and 90[degrees]C, therefore the modulus of PET would be remarkably decreased (more than 80%) when temperature is increased from 90[degrees]C to 120[degrees]C. Therefore, due to low modulus at higher temperatures debonding is less likely to occur in the interface. The stretching ratio is another parameter that can affect the haze of composites. As shown in Fig. 10c, the higher stretching ratio results in higher haze in the samples. But again for neat PET the haze value is not changed by the stretching ratio.

Figure 11 displays SEM micrographs of the cross-section of the stretched neat PET and PET-CKao composites. The size of the dispersed particles is of the order of magnitude of the wavelength of visible light, which can cause light diffraction and make the samples hazier [36]. One should note that the haze can be affected by many factors other than particle dispersion. The formation of voids and micro-cracks in the sample seems to be one of the main reasons. As discussed above, during the stretching process, debonding between filler particles and the PET matrix takes place and some voids and cracks are formed around the particles. These voids, that are elongated in the stretching direction (Fig. 11b), can cause additional scatter of light. The size of the voids in the film increases as a result of stretching and cause more haziness in the films.


PET and CKao were melt mixed using a twin-screw extruder to produce PET-CKao composites. SEM images revealed that the final average size of the dispersed particles was sub-micron. The rheological study proved that the degradation of PET occurred during the melt mixing with CKao and led to lower melt viscosity, hence a loss in the molecular weight. Nevertheless, Young modulus enhancements were observed despite the reduction in the molecular weight. It was demonstrated that the incorporation of CKao had a positive effect on the thermomechanical and barrier properties of PET; however, the presence of CKao impaired the optical properties of PET and led to more haziness. DSC results showed that the crystallization temperature of PET-CKao was shifted to higher values, compared to the neat PET, due to the nucleation effect of the filler particles. TGA analysis indicated a slight improvement of the thermal stability of PET after the addition of CKao particles. A silane treatment of particles and addition of chain extender were shown to promote the final properties of the composites as well.

The composites with different amount of filler were subjected to uniaxial stretching under various processing conditions. It was concluded that the stretching of samples above [T.sub.g] had a great effect on the neat PET and PET-CKao composites; however, this effect was more pronounced in case of filled samples. Furthermore, this improvement was attributed to a stress-induced crystalline structure and orientation of PET chains. Decreasing the stretch temperature significantly improved the mechanical and barrier properties of the samples while the stretch ratio had a smaller effect. SEM images showed that debonding between the filler and PET matrix was the main reason for the increased haziness in samples after stretching.


The authors gratefully acknowledge PepsiCo for the support of this work.


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Khalil Shahverdi-Shahraki, (1) Tamal Ghosh, (2) Kamal Mahajan, (2) Abdellah Ajji, (1) Pierre J. Carreau (1)

(1) CREPEC, Department of Chemical Engineering, Polytechnique Montreal, Montreal, Quebec, H3C 3A7 Canada

(2) PepsiCo Advanced Research--Beverage Packaging, Hawthorne, New York 10532

Correspondence to: Abdellah Ajji; e-mail:

DOI 10.1002/pen.24015

Published online in Wiley Online Library (
TABLE 1. DSC Data of neat PET and PET-CKao composites (second


                    [T.sub.g]      []    []
Samples            ([degrees]C)   ([degrees]C)     (J/g)

Neat PET               78.9             134.8         22.5
PET-CKao (2 wt%)       78.4             132.0          3.8
  (2 wt%)              76.1             135.9         19.8
  (2 wt%)              75.8             134.2         19.8


                    [T.sub.m]     [H.sub.m]    [T.sub.c]     [X.sub.c]
Samples            ([degrees]C)     (J/g)     ([degrees]C)      (%)

Neat PET                 245.3        32.6          180.7         7.2
PET-CKao (2 wt%)         247.0        36.4          201.5        23.5
  (2 wt%)                243.6        30.1          187.2         7.4
  (2 wt%)                245.2        34.2          190.7        10.4
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Author:Shahverdi-Shahraki, Khalil; Ghosh, Tamal; Mahajan, Kamal; Ajji, Abdellah; Carreau, Pierre J.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2015
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