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Effect of water-assisted extrusion and solid-state polymerization on the microstructure of pet/clay nanocomposites.

INTRODUCTION

Polyethylene terephthalate) (PET) is a low-cost engineering polymer that is used in a large variety of applications, due to its excellent transparency and good barrier properties [l]. One of the important applications of PET is in food and beverage packaging. Demands are increasing to improve the barrier properties of this semicrystalline polymer for use in bottles for beer and other oxygen-sensitive liquids. Recent studies show that the presence of organoclay platelets in PET can lower permeability to oxygen [2, 3], carbon dioxide, and water vapor [4, 5]. It also improves UV shielding [5].

The microstructure of polymer nanocomposites substantially plays a role in the macroscopic properties of final products. To achieve significant performance enhancements, good dispersion of the nanoclay in the matrix and thermodynamic compatibility between the organoclay and the polymer are required [6], Polymer nanocomposites can be prepared by in-situ or melt-mixing methods. While in-situ polymerization usually yields better dispersion of clay platelets than melt-mixing, the latter approach is more economical and environmentally friendly [6-9]. Preparation of PET nanocomposites by melt-mixing; however, faces challenges mainly related to the degradation of both PET and nanoclay modifiers at high processing temperatures.

Several efforts have been made to lower the rate of degradation and enhance dispersion of organoclays in PET, by using a more stable clay modifier [5, 10, 11]. Other approaches use a clay supported catalyst [12, 13] or a chain extender [14]. The use of an ionomer [3, 15], swelling agent or plasticization of PET with carbon dioxide was also evaluated [16], Other studies considered the effects on clay dispersion of screw speed, screw geometry and temperature profile in twin-screw melt extrusion of PET nanocomposites [2, 17]. All these efforts, however, led to only moderately enhanced mechanical and barrier properties.

Water assisted melt blending is a new method to prepare nanocomposites using the benefits of both solution and conventional melt-mixing methods [18, 19]. This method was successfully used to prepare polyamide (PA)--montmorillonite nanocomposites with unmodified nanoclays with water content of 5 to 50 wt% and more preferably from 10 to 40 wt% [20, 21]. It is possible to prepare PA nanocomposites by simultaneous feeding PA and pristine clay into the twin-screw extruder (TSE) then injecting water into the extruder. Another possibility is to prepare clay slurry with water and feeding this suspension into the extruder to blend it with the molten PA [21]. Some researchers also reported significant effects of water injection into the TSE during the preparation of nanocomposites based on polypropylene (PP) and poly(styrene-coacrylonitrile) [22, 23].

Solid-state polymerization (SSP) of PET nanocomposites is a practical route to overcome the polymer degradation caused by the melt-mixing processing. Different researchers demonstrated that SSP of PET in the presence of nanoclays and nano SI[O.sub.2] is feasible [24-26]. These studies have also shown a reduced rate of the SSP compared to the neat PET. SSP is carried out under moderate temperature conditions. Thus, SSP can raise the molecular weight ([M.sub.w]) of PET with less thermal degradation than melt phase polymerization and also can reduce the contents of by-products such as acetaldehyde and oligomers to acceptable levels. The normal SSP reaction temperature range is 200 to 230[degrees]C and this temperature range can be varied depending on the melting point of the PET [2730]. Understanding SSP and its utilization to achieve high [M.sub.w] polymer nanocomposites with tailored microstructure is still an open issue.

It is assumed in this work that water/steam can diffuse during extrusion between organoclay layers and act as a swelling agent, expanding the gallery spacing and reducing the interlayer interactions. Conversely, the reduction of [M.sub.w] of PET by hydrolysis with water can increase the PET chain mobility. Thus, both effects facilitate the diffusion of PET chains into the organoclay galleries. In raising the [M.sub.w] by SSP we should recover the critical properties of PET. To the best of our knowledge, this is the first attempt to prepare PET nanocomposites using water-assisted extrusion. In this article, morphology, rheological, mechanical, thermal, and barrier properties of processed PET and PET nanocomposites are presented and discussed.

EXPERIMENTAL

Materials

A general purpose PET [PET 9921, Eastman Co, (Kingsport, TN)] with intrinsic viscosity of 0.75 dL/g was used in this study. Three types of nanoclay: Cloisite [Na.sup.+], Cloisite 30B [Southern Clay Products, (Gonzales, TX)], and Nanomer I.28E [Nanocor, (Hoffman Estates, IL)] were used as the nanoparticles without further modification.

The surface modified clays are produced commercially by the substitution of interlayer sodium cations by methyl, tallow, bis-2-hydroxyethyl quaternary ammonium cations for Cloisite 30B and octadecyl trimethyl ammonium (trimethyl stearyl quaternary ammonium) for Nanomer I.28E. The chemical structures of the surfactant cations proposed by the suppliers are shown in Fig. 1.

Phenol, 1, 1, 2, 2--tetrachloroethane, chloroform-d (CD[Cl.sub.3]) and trifluoroacetic acid-d (TFA-d) supplied by Sigma Aldrich (Oakville, ON) were used without additional purification for the determination of the inherent viscosity and the nuclear magnetic resonance (NMR) analysis.

Melt Compounding

PET and PET nanocomposites were processed using a corotating TSE [Berstorff ZE25, (Hannover, Germany)] with a 25-mm diameter (D) screw and length-to-screw diameter ratio of 28, at a screw speed of 200 rpm. The temperature profile was 245, 265, 260, 255, 255, and 255[degrees]C from the hopper to the die. Fig. 2 illustrates the screw configuration. The melting section of the TSE (zones 1 and 2) contains three different types of kneading blocks (left hand 45[degrees], 90[degrees], and right hand 45[degrees] staggering angles) followed by a mixing element, a blister ring, and conveying elements (Zone 3). Zone 4 has one kneading block (90[degrees] staggering angles) and a blister ring. In zone 5, five short pitch conveying elements are used.

The nominal nanoclay content was 2 wt% in all the nanocomposites. Ground PET pellets were dry-mixed with clay before melt compounding. The mixture was fed into the TSE at two different rates of 0.6 and 3.3 kg/h. They are identified here by suffix L and H, respectively. A 2-L pressure vessel [Parr Instrument Co., (Moline, IL)] was utilized to produce and feed steam in zone 2 of the TSE (see Fig. 2) at a rate of 0.3 L/h, temperature of 160[degrees]C and pressure of 2.82 MPa. Finally, all the residual water was eliminated through the vent in zone 5 as shown in Fig. 2. The extrudate was collected after reaching steady state. In the notation used for sample identification, the code of samples processed with water is "W," which is not included when no water was used during extrusion.

After extrusion, the extruded samples were granulated at room temperature and compression molded at 270[degrees]C under a purge of nitrogen to obtain 25 mm disks. The disks were used for X-ray and morphological analysis, as well as rheometry. All the samples were dried under vacuum for 24 h at 80[degrees]C prior to melt processing and molding.

SSP

SSP was carried out to rebuild the reduced [M.sub.w] of hydrolyzed PET and PET nanocomposites. Before SSP, the PET and PET nanocomposites were ground and sieved to a powder of size less than 400 (pm). SSP was performed in a cylindrical stainless steel reactor at 215[degrees]C under N2 for 8 h. This temperature was the maximum value at which we could operate without encountering experimental difficulties. The flow rate of N2 into the reactor was 2 L/min. After 8 h, the heater was turned off, but the nitrogen purge was continued to lower the temperature of the reactor to 100[degrees]C and collect the resulting polymer or nanocomposites.

Characterization

Viscometry tests and NMR analysis were used to characterize the PET samples before and after SSP. The inherent viscosity,i?jnh, of PET samples was obtained based on the ASTM D 4603-3 method by measuring the flow time of the solution at a single concentration. The relative viscosity of the samples was obtained from the ratio of average solvent flow time (f0) and average solution flow time (/) in a Ubblehode capillary viscometer, namely:

[[eta].sub.r] = t/[t.sub.0] (1)

According to ASTM D 4603-3, the inherent viscosity is obtained from the following equation:

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The average viscometric molar weight ([M.sub.v] [approximately equal to] [M.sub.w]) was calculated using the Mark-Houwink equation assuming that the inherent viscosity is equal to the intrinsic viscosity:

[[eta].sub.inh] = K[M.sup.a.sub.w] (3)

with k=2.29 x [10.sup.-4]dL/g and a = 0.73 [31].

The chemical structure of PET before and after SSP was analyzed by NMR. The spectra of [sup.13]C and [sup.1]H NMR were obtained using a Varian/Agilent VNMRS-500 spectrometer operating at 125 and 500 MHz, respectively. A mixed solvent of CD[Cl.sub.3] and TFA-d was used to observe the NMR peaks of PET at room temperature.

A wide angle X-ray diffractometer (WAXD) [D8 Discover, Bruker AXS, (Madison, WI)] with CuK[alpha] radiation ([lambda] = 1.54056 [Angstrom]) was used to estimate the basal spacing ([d.sub.0001) between silicate layers. The generator was operated at 40 kV/ 40 mA and the nanocomposites were scanned from 0.8 to 10[degrees] at 0.015[degrees]/s.

A field emission gun scanning electron microscope [FEG-SEM, S-4700, Hitachi, (Tokyo, Japan)] was used to investigate the distribution of clay in the PET matrix. The specimens were prepared using an Ultracut FC microtome [Leica, Wetzlar, Germany] with a diamond knife and then coated with platinum vapor. The quality of the clay dispersion was evaluated using transmission electron microscopy (TEM) [JEOL JEM-2100F (Tokyo, Japan) operating at 200 kV]. The samples were microtomed into approximately 50-80 nm thick slices, using an Ultracut FC cryomicrotome system at -100[degrees]C.

Rheological measurements were carried out at 265[degrees]C under nitrogen atmosphere using a Bohlin Gemini HR rheometer [Malvern Instrument, (Worcestershire, UK)] with a 25-mm parallel plate geometry. The samples were dried under vacuum at 80[degrees]C for 24 h before the rheological tests. Time sweep tests were performed at frequency 0.1 Hz and frequency sweep tests were done in the linear viscoelastic region for each sample determined by strain sweep tests. Frequency sweep tests over a frequency range of 0.3-100 rad/s were performed from low to high and high to low frequencies to make sure that the data were accurate.

The thermal properties of the neat PET and PET nanocomposites were determined by differential scanning calorimetry [DSC Q1000, TA instruments, (New Castle, DE] under N2 atmosphere using 10[degrees]C/min scanning ramp from 30 to 300[degrees]C. The crystallinity of the PET and PET nanocomposites used in the discussion of the mechanical and barrier properties was calculated using the following formula:

[DELTA]X (%) = [[[DELTA]H.sub.m] - [[DELTA]H.sub.cc] / [[DELTA]H.sub.0](1-[phi])] x 100 (4)

where [phi] is the weight fraction of clay, [[DELTA]H.sub.m] represents the enthalpy of melting, [[DELTA]H.sub.cc] is the enthalpy of cold crystallization and [[DELTA]H.sub.0] refers to the heat of fusion of 100% crystalline PET, which is 140 J/g [1],

Tensile measurements were conducted via an Instron 3365 universal tester using a 500 N load cell and according to the ASTM D882-10 standard. 10-mm wide and 100-mm long samples were cut from sheets (thickness of 450 pm) prepared by compression molding. The samples were tested at room temperature and a crosshead speed of 25 mm/min.

Oxygen transmission rates were determined using an Ox-Tran Model 2/21 oxygen permeability MD Module from Mocon at 23[degrees]C. Hundred percent dry oxygen was used and all the tests were done under a pressure of 93.3 kPa (700 mmHg). The test area of the samples was 5 [cm.sup.2] and the samples had a thickness of 450 pm. The oxygen permeability values reported in this work have been normalized by the film thickness.

RESULTS AND DISCUSSION

[M.sub.w] Determination and Structural Characterization

According to previous research reports [32, 33], the [M.sub.w] of PET decreases in the presence of water, especially under processing at temperatures higher than the melting point of PET. This is the result of the hydrolysis of PET according to the following equilibrium reaction in Scheme 1 [34],

The [[eta].sub.inh] and [M.sub.w] values of different PET samples are reported in Table 1. We observe important decreases of [[eta].sub.inh] and [M.sub.w] of PET as a result of hydrolysis during the water-assisted extrusion for various processing conditions. The extrusion in the presence of water led to a reduction of [[eta].sub.inh] from 0.75 to 0.38 (dL/g) with a corresponding decrease of the [M.sub.w] by a factor close to 3. Moreover, the feeding rate had a significant effect on the extent of hydrolysis of PET. [[eta].sub.inh] of the PET extruded at high feeding rate (W-PET-H) was 0.52 dL/g while for W-PETL it was 0.38 dL/g. As expected, the extent of the hydrolysis is larger for a longer residence time (lower feeding rate).

As shown in Table 1, SSP resulted in large increases of [[eta].sub.inh] and [M.sub.w] values of PET. [[eta].sub.inh] of the samples after SSP is the average values of the results from two batches. Moreover, to confirm its value, the viscometry tests were carried out at different concentrations based on ASTM D 2857 as well as ASTM D 4603-3. The results of both methods were in agreement. After SSP at 215[degrees]C for 8 h, [[eta].sub.inh] increased from 0.38 to 0.69 dL/g for W-PET-L and from 0.52 to 0.83 dL/g for W-PET-H. These results confirm that the [M.sub.w] of PET can be substantially rebuilt by SSP under these conditions. SSP of PET involves two reversible equilibrium reactions as presented in Scheme 2 [35].

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[sup.1]H and [sup.13]C NMR spectroscopy was used to confirm the presence of chemical entities in the "as received" PET, WPET-H, and SSP-W-PET-H samples. In the 'H NMR spectra, we observed the peaks at [delta]: 7.257 and 11.3 ppm corresponding to protons of chloroform (CD[CL.sub.3]) and those of TFA-d, respectively (not shown here). The peak position of protons of the "as received" PET is presented in Table 2 and Fig. 3. PET contains four equivalent methylene protons of the terephthalic acid ring and protons of the ethylene glycol (EG) segments. The other resonances are assigned to the methylene protons of diethylene glycol (DEG) and the hydroxyl end groups. This analytical technique does not allow the detection of aromatic carboxyl end-groups because they show a peak at the same position as TFA-d. As the "as received" PET is a copolymer, there are additional signals, which are characteristics of cyclohexanedimethanol (shown in Table 2).

In [sup.1]H NMR spectra, the area under each peak is related to the number of corresponding hydrogens in the molecules generating that peak [36]. The ratios of the integrals under the peaks assigned to C[H.sub.2] for EG, DEG, and hydroxyl end groups show the differences in the molecular structure of the different PETs. The ratios of the CH2 peak for EG to the CH2 attached to the hydroxyl groups are 100, 190, and 300 in W-PET-H, "as received" PET and SSP-W-PET-H, respectively. Therefore, as expected, the amount of CH2 attached to hydroxyl end-groups increases during the hydrolysis process, while a significant decrease is observed after SSP. The ratios of the CH2 peak for EG to the C[H.sub.2] attached to DEG were also determined. These ratios did not show significant changes for the different [M.sub.w] samples and the ratio for the "as received PET" is 15.6. Thus, the significant differences for the three PETs are for the ratio of methylene groups adjacent to hydroxyl end groups and those of EG.

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Generally, the number of nonequivalent carbons and types of carbon atoms are determined by [sup.13]C NMR [37]. An important parameter obtained from [sup.13]C NMR in solution is the chemical shift. Fig. 4 reports the [sup.13]C NMR spectra of the samples. The carbons of chloroform ([CDCL.sub.3]) exhibit four peaks from [delta]: 76.5 to 77 ppm and those of TFA-d show peaks from [delta]: 110.75-117.5 ppm and [delta]: 161-162.3 ppm. We observe peaks for carbons of EG at [delta]: 63.86 ppm, of aromatic CH at o: 129.98 ppm and of aromatic carbons at [delta]: 133.24 ppm; carboxyl end groups show a peak at [delta]: 167.89 ppm and carbonyl groups at [delta]: 162.3 ppm. The carbonyl peak is at the same position as TFA-d. Therefore, we cannot distinct from carbonyl groups in PET and carboxyl groups in TFA-d. These [sup.13]CNMR spectra of the "as received" PET and hydrolyzed PET before and after SSP show the same peak positions. Therefore, these different PETs have all the same carbon atoms.

XRD Results. The X-ray diffraction (XRD) patterns for the PET- Cloisite [Na.sup.+] nanocomposites processed with and without water show that the d-spacing of Cloisite [Na.sup.+] did not change after the melt-mixing process in either the dry extrusion process or with water (data not shown). This is due to the poor affinity between the pristine clay (Cloisite [Na.sup.+]) and PET as well as the collapse of clay galleries under the strong platelet-platelet cohesive force of Cloisite [Na.sup.+] [23]. Hence, water did not contribute to intercalation during the extrusion process.

The XRD results of PET-C30B and PET-I28E nanocomposites prepared under different conditions are presented in Fig. 5. For the nanocomposites obtained under dry extrusion (Fig. 5a), two distinct peaks are observed for the C30B nanocomposites (d-spacing ~3.45 and 1.7 nm) and for the I28E nanocomposites (d-spacing ~3.35 and 1.6 nm). As the d-spacing values for the pristine organoclays, C30B and I28E are 1.8 and 2.5 nm, respectively, the first peaks shown for the nanocomposites are indicative of some intercalation of organoclays by the PET chains. The second peaks suggest either possible degradation of the organo-modifiers during processing of PET or they could also be reflections of the first peaks according to Bragg's law. The WAXD results indicate that the dispersion of C30B in PET is better than that obtained with I28E, both d-spacing and increases in d-spacing are larger for C30B. This may be attributed to stronger interactions between PET and C30B, as suggested by the solubility parameters [38]. Finally, we note that the d-spacing is independent of the processing conditions. However, the first peak intensity for PET-C30B-L is smaller than for the others, indicative of better clay dispersion for the low feeding rate.

Fig. 5b presents the diffraction peaks of the PET nanocomposites prepared in the presence of water. Although the peak position of C30B in nanocomposites processed with water slightly shifted to lower angles compared to processing without water, the shape and intensity of the C30B peaks are significantly changed in the water-assisted process. The first peak of this organoclay became broader and more asymmetric as well as of smaller intensity, compared to the case of the dry extrusion (Fig. 5a). This indicates a higher degree of delamination in the presence of water. In W-PET-C30B-L, the first peak has almost disappeared, which suggests a high degree of exfoliation. Conversely, the XRD peaks of nanocomposites containing I28E prepared with or without water are similar. As also shown in Fig. 5a, the reduction of intensity in the first XRD peak for the low feeding rate confirms that the degree of delamination is increased under the process with a longer residence time.

The XRD results for the PET nanocomposites after SSP are shown in Fig. 5c. The nanocomposites exhibit peaks at the same 2[theta] values as before SSP (Fig. 5b). This suggests that the net diffusion of polymer in and out of the galleries during SSP is negligible. Moreover, possible SSP of PET chains intercalated inside the galleries of the organoclays do not have any significant effects [26].

SEM and TEM Images. SEM micrographs of PET nanocomposites containing Cloisite [Na.sup.+] prepared with and without water are presented in Fig. 6. The white areas represent the clay particles and the dark regions correspond to the PET matrix. Although the aggregates of Cloisite [Na.sup.+] in the nanocomposites prepared via water-assisted extrusion are smaller than those of nanocomposites prepared in the absence of water, they are still quite large. These results are in good agreement with XRD results that showed no change in gallery spacing of Cloisite [Na.sup.+] in the presence of water. Water is a good swelling agent for Cloisite [Na.sup.+] [20] and a hydrolysis agent for PET [32, 33]; but the poor affinity between Cloisite Na and PET as well as strong electrostatic forces between adjacent platelets of Cloisite [Na.sup.+] limit the diffusion of PET chains into the galleries of the pristine clay.

SEM micrographs of PET-C30B nanocomposites processed at different feeding rates are shown in Fig. 7. Smaller dispersed aggregates with a more uniform distribution are observed for the nanocomposites processed at the lower feeding rate. This may be attributed to the longer residence time of around 300 s compared to 50 s for the high feeding rate. The size of C30B aggregates is also decreased in PET-C30B nanocomposites processed with water before and after SSP, compared to those processed without water. Conversely, a comparison between the SEM micrographs of PET-C30B and PET-I28E shows that I28E is not distributed uniformly in the matrix. The latter has larger aggregate size compared to C30B. The micrograph of Fig. 7h shows that the presence of water in processing PET-I28E nanocomposites does not improve the particle distribution of I28E. In fact, it may have a negative effect.

Fig. 8 shows the TEM images of different PET nanocomposites. In the case of PET-C30B-L (Fig. 8a), the organoclay appears to be dispersed very well. In most cases, single layers of organoclay are observed. Conversely, PET-C30B-H (Fig. 8b) exhibits slightly poorer dispersion of C30B as compared to the nanocomposites processed under the low feeding rate. Also, in nanocomposites processed with water (Fig. 8c and d), better dispersion and good distribution of C30B particles are observed in nanocomposites, especially when processed at low feeding rate. Thus, lower feeding rate and accordingly longer residence time in the extruder lead to more breakup or delamination of stacks of clay platelets (tactoids). Figs. 8a and f show a better dispersion of C30B in PET compared to I28E. Most of the particles of C30B are broken down to single layers, while I28E particles are in the form of tactoids (Figs. 8e and 0 and the apparent particle concentration of I28E is lower than that of C30B. The quality of dispersion of I28E is deteriorated by the presence of water (Fig. 8g). This may be related to changes in the compatibility between PET and I28E by the hydrolysis reaction.

In order to obtain a quantitative estimate of the degree of dispersion of organoclay platelets in the PET matrix, the [D.sub.0.1] factor proposed by Luo and Koo [39] was calculated using between 600 and 700 measurements for each case. A value below 4% for [D.sub.0.1] suggests an immiscible system or microcomposite, and values over 8% indicate an exfoliated structure, while values between 4 and 8% indicate intercalation. The values of [D.sub.0.1] and of the aspect ratio, (p - 1/d, length over diameter of particles) are reported in Table 3 (the method proposed by Ghasemi et al. [2] was applied to determine p, using TEM images and 200 measurements for each case).

[D.sub.0.l] and p for PET-C30B-L are larger than those for PET-C30B-H, which is a sign of more delamination at low feeding rate. The values of [D.sub.0.1] and p show an improvement in the degree of dispersion for W-PETC30B-H compared to PET-C30B-H. Although, the shear stresses in the dry extrusion process are larger than for processing with water, the presence of water leads to swelling of C30B and to hydrolysis of PET. Both these weaken the cohesive forces among clay platelets and facilitate the diffusion process. [D.sub.0.1] is the same for PETC30B-L processed with and without water: however, the much larger aspect ratio (42 compared to 34) suggests more delamination for the water-processed nanocomposite. SSP did not affect the disordering of C30B nanoparticles, because the value of [D.sub.0.1] of W-PET-C30B-L before and after SSP were the same (data not shown). [D.sub.0.1] is significantly larger for PET-C30B-L, compared to PET-I28E-L, as expected from the solubility parameters reported in Ref. 38]. Ghanbari et al. [38] reported a [D.sub.0.1] value of 5.5% for a similar PET-C30B nanocomposite, while larger values (7.5% and 6.9 %) were obtained by Ghasemi et al. [40], using a higher Mw PET and a larger TSE.

In order to further quantify the extent of delamination of C30B in different PET/clay nanocomposites, the number of platelets per clay particle was manually counted using the TEM images of Fig. 9. In the case of PETC30B-L, the single and double layers represent 69% while in PET-C30B-H is 56%. This confirms the effect of the residence time on the delamination or breakup of the clay particles. In the presence of water at low feeding rate (W-PET-C30B-L), the single and double layers represent 72%, compared to 65% for the high feeding rate case (WPET-C30B-H). Furthermore, the number of platelets per particle for W-PET-C30B-L is not affected by SSP.

Rheology

The total time for frequency sweep tests of the neat PET and PET nanocomposites was 230 s and during that period, the viscosity variation due to possible thermal degradation was less than 5%, hence, within the experimental errors.

The results of small-amplitude oscillatory frequency scans are reported in Fig. 10. Fig. 10a shows the effect of water, residence time, and SSP on the complex viscosity of PET. The zero-shear viscosity ([[eta].sub.0]) for PET-L, which was processed at dry conditions and low feeding rate, is 200 Pa.s, while when water was injected into the system, [[eta].sub.0] decreases to 100 and 20 Pa.s for W-PET-H and WPET-L, respectively. Thus, the use of water and a longer residence cause significant reductions of the complex viscosity as a result of the hydrolysis reaction. Conversely, SSP helps to raise the [M.sub.w] of hydrolyzed PET significantly. The [[eta].sub.0] values are 400 and 1250 Pa.s for SSP-WPET-L and SSP-W-PET-H, respectively. It should be noted that [[eta].sub.0] for the "as received" PET is 630 Pa.s. Therefore, SSP-W-PET-H has a higher [M.sub.w] than the initial PET as shown in Table 1.

A Newtonian plateau is found in all PET samples irrespective of the processing method and [M.sub.w]. As expected, the Newtonian plateau region is reduced for the higher [M.sub.w] PETs obtained by SSP due to the increased number of chain entanglements. The value of 90[degrees] for the loss angle at low frequencies and its behavior with frequency are typical of linear polymer chains [41]. As [M.sub.w] increases, G' becomes more significant, and the value of the loss angle decreases with increasing frequency. The results show no indication that branching or cross-linking occurred during SSP. Thus, the linear structure of PET was maintained.

The results of frequency sweep tests for nanocomposites processed at high feeding rate with and without water are presented in Fig. 11. Fig. 11a shows that both neat PETs (PET-H and W-PET-H) have a pseudo-Newtonian behavior, while the PET nanocomposites are shear-thinning. The presence of organoclays influences the rheology of the polymer nanocomposites, due to polymer-particle and particle-particle interactions and possible changes of the molecular structure of the polymer molecules. The complex viscosity of the nanocomposites at high frequencies, where the behavior of the matrix is dominant, is lower than that of the neat PET. This reflects PET degradation in the presence of organoclays. The complex viscosity of nanocomposites containing I28E is larger than those containing C30B in the whole frequency range. TEM and SEM images show a better dispersion and distribution of C30B within the PET matrix compared to I28E. Therefore, PET is more exposed to the surface of C30B than I28E, which leads to higher level of polymer degradation. Moreover, C30B has hydroxyl groups and unsaturated tallow groups that accelerate the PET degradation compared to the hydrogenated tallow of I28E.

Interestingly, W-PET-C30B-H processed with water exhibits a larger complex viscosity at low frequencies compared to the W-PET-H, while PET-C30B-H exhibits a lower complex viscosity compared to that for the neat PET. This suggests that, in the presence of water, the dispersion of organoclay improves. It also appears to compensate for the large reduction of the matrix viscosity due to hydrolysis.

The storage modulus versus angular frequency for the PET and PET nanocomposites is presented in Fig. lib. Both W-PET-H and PET-H have a very low storage modulus. Significantly, the presence of 2 wt% (nominal) organoclay increases the value of G' and reduces its slope at low frequencies and the solid-like behavior reflects the interconnected structure and geometric constraints as a result of the presence of organoclays. At high frequencies, the role of the matrix is more prominent. Smaller G' values are found for the nanocomposites processed under dry conditions due to the matrix degradation, compared to the neat polymer, whereas for the nanocomposites processed with water the opposite behavior is found. It shows the strong contribution of nanoparticles interactions that compensates the low G' of the hydrolyzed PET.

Rheological data for the PETs and PET nanocomposites after SSP are presented in Fig. 12. The complex viscosity (Fig. 12a) of the PET nanocomposites is much smaller than that of the corresponding neat PETs after SSP. The situation is more complex for the storage modulus as reported in Fig. 12b: at low frequencies, the modulus of the nanocomposite is larger than that of the corresponding neat PET, but the trend is reversed at high frequencies. The behavior is clearly indicative of strong degradation due to the presence of the nanoparticles.

The smaller complex viscosity in the nanocomposites compared to the neat PETs after SSP indicates that the presence of organoclay in the matrix slows down the diffusion of by-products due to the increased tortuosity.

Barrier Properties

The SEM and TEM analysis show that the best dispersion and distribution were obtained for C30B compared to I28E. Therefore, only the properties of PET-C30B nanocomposites are presented in Fig. 13. The oxygen permeability values of the neat PET and PET-C30B nanocomposites, containing 2 wt% (nominal clay), processed with and without water are reported in the figure. The incorporation of C30B improves the barrier properties of PET nanocomposites, especially when processed with water. Although PET-C30B-L and SSP-W-PET-C30B-L show a better dispersion and distribution of C30B in the matrix compared to the nanocomposites processed at high feeding rate, the oxygen permeability of both nanocomposites is about the same. Ghasemi et al. [40] also showed that the feeding rate did not have a significant effect on the barrier properties of PET nanocomposites. The permeability of PET-C30B-L and SSP-W-PET-C30B-L shows 19% and 26% improvements, respectively, compared to the neat PET. The improvement can be attributed to the increased tortuosity in the presence of C30B and the lower oxygen permeability of SSP-W-PET-C30B is due to the better dispersion and distribution of C30B in the presence of water. Conversely, the presence of clay platelets changes the crystallinity of semicrystalline polymers that could affect the permeability. The percentage of crystallinity of PET-H, PET-C30B-H, and SSP-W-PETC30B-H, PET-C30B-L, SSP-W-PET-C30B-L was determined using Eq. 4 to be 5.9, 7, 4.8, 8.4, and 6.1%, respectively. Hence, the changes in crystallinity cannot account for the enhanced barrier properties, at least for the SSP-W-PET-C30B nanocomposites. Ghasemi et al. [2] showed that the presence of 3 wt% C30B into oriented PET films can improve by 23% the barrier properties compared to their neat PET, but the crystal content of their PET nanocomposites was almost 2 times larger than their neat PET, partly accounting for the barrier improvement. In another work, Shen et al. [42] reported a 27% reduction in oxygen permeability for biaxial oriented PET nanocomposites containing 6 wt% of nanoclay compared to their neat PET.

Mechanical Properties

The tensile modulus and elongation at break of the neat PET and PET-C30B nanocomposites are reported in Fig. 14. The presence of 2 wt% (nominal) of organo-clay increases the tensile modulus of PET nanocomposites compared to the neat PET (Fig. 14a). Two significant results are worth mentioning: on one hand, the PETC30B nanocomposite processed at low feeding rate has slightly smaller tensile modulus although the morphology suggests a better dispersion and distribution of C30B. This is explained by the more severe degradation of the PET matrix at low feeding rates compared to the high feeding. The effect of feeding rate on the tensile modulus was also reported by Ghasemi et al. [40], in good agreement with our results. Conversely, PET-C30B, prepared by conventional melt-mixing, exhibits a smaller modulus compared to the nanocomposites prepared by water-assisted melt-mixing and subsequent SSP. For example, the tensile modulus is improved by 15 and 20% in PET-C30B-H and SSP-W-PET-C30B-H. respectively. Further improvement in the tensile modulus of nanocomposites is obtained by water-assisted melt-mixing and SSP compared to the conventional melt-mixed PET; this is probably due to the better dispersion and distribution of C30B when processed with water compared to the conventional melt-mixing as well as the larger [M.sub.w] obtained by SSP.

Ghasemi et al. [2] reported a 20% larger tensile modulus for oriented PET nanocomposites containing 3 wt% of C30B compared to their neat PET, but, as mentioned before, the crystal content of their PET nanocomposites was almost 2 times larger than their neat PET. For samples prepared by compression molding other researchers [5] showed no improvement in the Young modulus in PET nanocomposites with 5 wt% of nanoclays compared to their PET matrix. Shen et al. [42] reported a 25% increase in the tensile modulus of non oriented films of PET nanocomposites containing 6 wt% of nanoclays compared to their neat PET.

Fig. 14b shows the effect of C30B and different processing conditions on the elongation at break of PET nanocomposites. As expected, the elongation at break significantly decreases in the presence of C30B in conventional PET nanocomposites. Moreover, the PET-C30B nanocomposites processed at low feeding rate have less elongation at break compared to those processed at high feeding rate. Surprisingly for nanocomposites obtained via the water-assisted extrusion and SSP, the elongation at break is reasonably high (the results were reproducible as shown by the error bars in Fig. 14b): the elongation at break is 130 and 180% for SSP-W-PET-C30B-L and SSP-W-PET-C30B-H, respectively, compared to 3 and 6% for PET-C30B-L and PET-C30B-H, respectively. It shows the strong potential of SSP for improving the ductility of PET nanocomposites.

The significant reduction of the elongation at break of nanocomposites compared to the neat PET can be attributed to aggregates of C30B, interfacial debonding of the clay particles at the PET matrix interface that could cause cavitations and microvoid formation. In the case of nanocomposites prepared by water-assisted melt-mixing and subsequent SSP (SSP-W-PET-C30B-L and H), a better dispersion and distribution of C30B into the matrix were obtained compared to the conventional PET-C30B nanocomposites, as shown in morphology images. Therefore, the better distribution of organoclay particles into the PET matrix results in an improvement of the stress distribution and smaller aggregates and, consequently, better mechanical properties. Conversely, increasing the [M.sub.w] of PET nanocomposites through SSP and reconnecting the PET chains could lead to a significant enhancement of the elongation at break. It is generally accepted that the elongation at break increases with increasing [M.sub.w] of linear polymers [43, 44].

CONCLUSIONS

There are significant advantages for the water-assisted melt-mixing process to produce partially exfoliated, well-dispersed, and delaminated PET-C30B nanocomposites compared to the conventional melt-mixing. The presence of water results in a larger number of single and double layers of C30B nanoparticles as well as an increased aspect ratio in PET nanocomposites. The effect of water on the microstructure of PET nanocomposites is strongly dependent on the nanoclay modifier. Processing with water has negative effects on the PET-I28E nanocomposites, because of its lower compatibility.

Results of small amplitude oscillatory rheology and inherent viscosity showed that the [M.sub.w] of PET increased significantly after SSP. The linear molecular structure of PET was maintained, as confirmed by [sup.1]H NMR and [sup.13]C NMR spectra as well as rheological measurements. It was also found that the extent of the SSP reaction in nanocomposites was lower than for the neat PETs, due to the barrier effect of clay platelets.

PET nanocomposites prepared by water-assisted extrusion followed by SSP (novel process) showed better mechanical and barrier properties compared to the nanocomposites prepared by the conventional melt-mixing process due to the only better dispersion and distribution of C30B in novel method since the percentage of crystallinity did not change significantly in different nanocomposites. In particular, the elongation at break for the SSPW-PET-C30B-H was appreciably improved to the order of 180% compared to ~6% for the conventional nanocomposites (PET-C30B-H). In addition, the extent of enhanced barrier properties in nanocomposites prepared by novel method compared to conventional melt-mixing was higher than using chain extender in the PET and PLA/C30B nanocomposites proposed by literature as well as observed brittle behavior in the nanocomposites with chain extenders. Importantly, by using this novel method, enhanced barrier and mechanical properties were obtained without using any chemicals which would cause side reaction and changing the structure of PET.

If we consider the typical shelf life of carbonated soft drinks as 90 days, it will increase to 107 and 113 days for the PET nanocomposites prepared by conventional melt-mixing and by the water-SSP novel method, respectively.

ACKNOWLEDGMENTS

The authors acknowledge financial and infrastructure support received from The Natural Sciences and Engineering Research Council of Canada (NSERC), National Research Council of Canada (NRCC), Canada Development Bank (CDB). The authors would like to gratefully thank CREPEC members Mrs. W Leelapompisit, Mrs. M Hamdine and Dr. B. Esmaeili for their technical help.

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Maryam Dini, (1) Tahereh Mousavand, (2) Pierre J. Carreau, (1) Musa R. Kama (l,2) Minh-Tan Ton-That (3)

(1) Department of Chemical Engineering, CREPEC, Ecole Polytechnique, H3T 1J4, Montreal, Quebec, Canada

(2) Department of Chemical Engineering, CREPEC, McGill University, H3A 2B2, Montreal, Quebec, Canada

(3) Automotive Portfolio, National Research Council of Canada, J4B 6Y4, Boucherville, Quebec, Canada

Correspondence to: Pierre J. Carreau; e-mail: pierre.carreau@polymtl.ca

DOI 10.1002/pen.23685

Published online in Wiley Library (wileyonlinelibrary.com).

TABLE 1. Values of the inherent viscosities and MW.

Sample               [eta] inh (dL/g)      [M.sub.w] (g/mol)

As received PET    0.75 [+ or -] 0.020    65000 [+ or -] 2000
W-PET-L            0.38 [+ or -] 0.023    25000 [+ or -] 2100
W-PET-H            0.69 [+ or -] 0.033    58000 [+ or -] 4000
SSP-W-PET-L         0.52 [+ or -] 0.03    39000 [+ or -] 3200
SSP-W-PET-H         0.83 [+ or -] 0.04    75000 [+ or -] 5000

Suffix "W" means that the PET was extruded in the presence of
water (hydrolyzed samples) and "SSP" means the hydrolyzed
samples were solid-state polymerized in a reactor. L and H
stand for samples prepared at low and high feeding rates,
respectively.

TABLE 2. Protons numbering in PET and their peak position
in [sup.1]H NMR spectra.

                                       Peak position (ppm) of each
Units of PET copolymer                 numbered protons

[FORMULA NOT REPRODUCIBLE IN ASCII]    (1): 8.13, (2):4.8

[FORMULA NOT REPRODUCIBLE IN ASCII]    (3,4 (a)): 4.61,4.19

[FORMULA NOT REPRODUCIBLE IN ASCII]    (5,6): 4.14,4.65

[FORMULA NOT REPRODUCIBLE IN ASCII]    (7): 4.38, (8): 4.28 (9-14):
                                       1.2,1.6,1.73,1.91,1.88,2.14

(a) small amount of TFA-d leads to esterification of hydroxyl
end group. Thurs, signals of methylene end groups shift to the
higher frequencies and overlap with the 4.6-4.8 ppm region.

TABLE 3. Aspect ratio and [D.sub.0.1].

Sample           Aspect ratio    [D.sub.0.1] (%)

W-PET-C30B-L          42               6.5
PET-C30B-L            34               6.5
PET-I28E-L            34               4.3
W-PET-C30B-H          38               6.0
PET-C30B-H            30               4.9
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Publication:Polymer Engineering and Science
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Date:Aug 1, 2014
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