Studies on the effect of equi-biaxial stretching on the exfoliation of nanoclays in polyethylene terephthalate

European Polymer Journal 45 (2009) 332–340

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Macromolecular Nanotechnology

Studies on the effect of equi-biaxial stretching on the exfoliation of nanoclays in polyethylene terephthalate R.S. Rajeev *, E. Harkin-Jones, K. Soon, T. McNally, G. Menary, C.G. Armstrong, P.J. Martin School of Mechanical and Aerospace Engineering, Queen’s University, Belfast BT9 5AH, UK

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a r t i c l e

i n f o

Article history: Received 25 May 2008 Received in revised form 2 September 2008 Accepted 16 October 2008 Available online 6 November 2008

Keywords: Nanoclays Nanocomposites Polymers Transmission electron microscopy (TEM) Equi-biaxial stretching Polyethylene terephthalate (PET)

a b s t r a c t TEM image analyses of PET nanocomposites were done to study the effect of equi-biaxial stretching and stretch ratio on the exfoliation and other tactoid properties. Though XRD spectra did not show any evidence of exfoliation, TEM image analysis revealed 10% exfoliation of clay platelets in the unstretched sheets. Stretching further improved the exfoliation as the concentration of thinner tactoids in the matrix increased due to stretching. Stretching also caused an increase in the concentration of longer tactoids and tactoids having higher aspect ratio, the reason for which was assumed to be due to the slippage of the platelets while stretching. It was found that stretch ratio affected the nanocomposite properties as increase in stretch ratio improved the exfoliation of clay platelets. Equi-biaxial stretching imparted preferential orientation of the tactoids in the matrix. Dynamic mechanical and barrier property measurements confirmed the observations made by the TEM image analysis. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Polyethylene terephthalate (PET) has many applications including those in textiles, automobiles, rubber goods, and in food and beverage packaging. One of the important applications of PET is in food and beverage packaging where, apart from mechanical, ageing and high temperature properties, the polymer needs good barrier properties for reduced permeability. Recent studies have shown that the barrier properties of PET are improved by the addition of nanofillers like modified clay [1]. Several reports are available on the processing and properties of PET nanocomposites [2–6]. Like many other polymers, nanocomposites based on PET are also processed by either an in-situ method or by the melt intercalation technique. In-situ methods generally lead to good exfoliation of clay platelets in PET matrix [7,8] whereas melt mixing is more environmentally friendly and industrially viable method for pro* Corresponding author. Address: Raj Nivas, Railway Station Road, Ettumanoor (P.O.), Kottayam, Kerala, India. Tel.: +91 9495110733. E-mail address: [email protected] (R.S. Rajeev). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.10.036

ducing nanocomposites [2,9,10]. A proper surfactant-clay system is needed to overcome the problem of degradation of surfactant due to the high operating temperature of PET [11]. Calcagno et al. [12] studied the effect of different modifiers on the intercalated and exfoliated morphologies of PET/MMT nanocomposites. Based on XRD and TEM analyses, they found that the maximum distance between the platelets was 33.9 Å for all intercalated nanocomposites. Above this distance, the platelets were assumed to be exfoliated. They also observed that the shear forces in the extruder helped in exfoliation and the clay acted as a nucleating agent for crystallization of the PET molecules. Like modified montmorillonite clay, synthetic fluoromica is another type of clay which is used in the melt processing of PET [13]. Imai et al. [3] prepared PET/expandable fluorine mica nanocomposites having high modulus using a reactive compatibilizer. Recently Ammala et al. [14] improved the dispersion of Somasif MEE and Cloisite 10A nanoclays in PET matrix by dispersing clays in water together with an aqueous dispersion of an ionomer having a high level of compatibility with PET. Melt mixing of the components was done after the removal of water. They

found that ionomer helped in improving the exfoliation of clay platelets in the PET matrix. In the manufacturing of PET bottles for packaging applications, the preform is subjected to biaxial orientation during blow moulding, resulting in enhanced mechanical properties and low gas permeability [1]. Biaxial orientation is performed either by sequential biaxial stretching which is done by stretching the sheet in one direction and then again at right angles to the original stretch or by simultaneous or equi-biaxial stretching where stretching is done to both the axis simultaneously [15]. Several reports are available on the effect of biaxial stretching on the crystallinity and orientation of crystals in PET matrix [16–19]. Fan et al. [16] studied the effect of stretch ratio in the machine direction and in the transverse direction on the permeability of oriented PET film. They analysed the influence of annealing on the barrier properties of the film. They also observed that the influence of stretch ratio on the barrier properties both in the machine direction and in the transverse direction depends on the size of the penetrant gas. More recently Je´ol et al. [17] studied the effect uniaxial and biaxial stretching on the dispersion of two types of silica nanoparticles (fractal silica and spherical silica) in a PET matrix. They found that the high shear during injection moulding of the preform created an isotropic morphology of the composite as was evident from the TEM images. Further, based on the TEM image analysis, they concluded that uniaxial stretching had no significant influence on the dispersion of silica in the PET matrix. However, biaxial stretching significantly affected the state of dispersion of both types of silica fillers in PET. The behaviour of PET chains subjected to biaxial stretching varied depending on whether fractal or spherical silica was present in the matrix. In the above study, Je´ol et al. [17] synthesized the PET nanocomposite by the in-situ polymerization method. However, melt mixing of PET is an industrially viable and environmentally friendly process. This paper reports some interesting observations made when PET nanocomposites, prepared by melt mixing synthetic fluoromica with PET pellets, are equi-biaxially stretched at different stretch ratios. TEM studies of the samples are done by collecting specimens from different portions of the unstretched and equi-biaxially stretched nanocomposite sheets in order to compare the results of XRD and TEM image analyses. To the author’s knowledge this is the first report on the influence of biaxial stretching of melt mixed PET-clay nanocomposites on the exfoliation of clay platelets. This paper also confirms the observations made by other researchers that XRD alone may not give reliable information on the exfoliation of clay platelets in polymer nanocomposites. Absence or presence of a peak in XRD spectrum does not confirm the absence or presence of exfoliated clay platelets [18–20].

2. Experimental 2.1. Materials and processing PET grade T74F9IV080 supplied by Tergal Fibre (density of 1.4 g/cm3 and an intrinsic viscosity of 0.8 dl/g)

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was mixed with Somasif MAE nanoclays supplied by UNICOOPJAPAN (Now CBC Co. Ltd.), Japan, in a twin screw extruder. Somasif MAE nanoclays are synthetic clays based on sodium fluoromica, modified by di-methyl di-(hydrogenated tallow) ammonium chloride [13]. The mixing of the dried PET and clay was done in Colin ZK25 twin screw extruder. Neat PET was also extruded for comparison purpose. The temperature profile of the extruder was 230, 275, 270, 270, 265, 260 °C from the feeder to the extruder end and the die temperature was 260 and 255 °C at the inlet and exit, respectively. The extruded sheet was cooled on a pair of chilled rolls rotating at 1 m/min at 80 °C. The extruder speed was set at 150 rpm. A dry nitrogen flow was supplied to both the hopper and the feeder to aid in maintaining dry materials. The resulting sheet was cut into plaques having dimension 76 mm  76 mm  1 mm (length  width  thickness) and then equi-biaxially stretched in a biaxial stretching machine at a stretch ratio of 3, strain rate of 8/s and at 100 °C to obtain thin sheets having thickness 0.1 mm. Neat PET was also processed in the same way. In order to study the effect of stretch ratio on exfoliation and other nanocomposite properties, the extruded sheets were stretched at three different stretch ratios; 2, 2.5 and 3 keeping all other parameters the same. 2.2. Characterization Wide Angle X-ray Diffraction (WAXD) studies were done in a Philips PW3040 machine using Cu Ka radiation with wavelength 1.54 Å at a step size of 0.01° and scan rate of 0.3°/min. 2h range was from 2° to 10°. The nanocomposite samples for TEM analysis were prepared by microtoming the unstretched and equi-biaxially stretched sheets to 70 nm thickness in a Reichert Jung Ultracut-E ultramicrotome using a diamond knife and collecting the samples on a 200 mesh copper grid. For cross-section analysis, microtoming was done along the machine direction of the sample cross-section. In order to study the orientation of platelets and tactoids on the surface of the samples, a small piece of the sample was glued on to epoxy blocks, which was then trimmed to the required size and shape before microtoming. In this case, microtoming was done on the surface of the sample, by placing the glued surface parallel to the diamond knife. Fig. 1 shows the portions from where samples were collected for microtoming. For the unstretched sheet (Fig. 1(a)), because of the higher thickness (3 mm) and also because of the size limitation of the microtomed specimen to be placed on the copper grid, the cross-section was divided into three portions with respect to the machine direction (top, middle and bottom). Similarly, the width of the sheet was also divided in to three portions (left, middle and right). For the present study, microtoming was done on the middle portion of the sheet (with respect to both cross-section and width) as shown in Fig. 1(a). For stretched sheet, samples were collected from the middle of the sheet in the machine direction, along the cross-section (Fig. 1(b)). Since the thickness of stretched sheet was only 0.1 mm, microtoming was done through the whole

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Fig. 1. Diagram showing the portions of the sample from where specimens for TEM image analysis were collected: (a) unstretched sheet and (b) equibiaxially stretched sheet. MD, machine direction; Trans, transverse to the machine direction; L, length of the sheet; W, width of the sheet; and T, thickness of the sheet.

thickness of the sample. TEM analyses were done using a Philips CM 100 TEM operating at 100 kV accelerating voltage. TEM image analysis was done by following the method proposed by Vermogen et al. [21]. Images were taken at different magnifications depending on the nature of the analysis to include different types of clay platelets (individual sheets, thinner tactoids, thicker tactoids and agglomerates) in the analysis to provide a good statistics of the distribution of clay platelets in the matrix [22]. In order to calculate the tactoid thickness, number of platelets in a tactoid and tactoid aspect ratio, images at magnification 28.5 k was used as this magnification gives the resolution of 1.1 nm in the image analysis software. For tactoid length and orientation studies, images with magnification of 8.9 k were used. The number of platelets in a tactoid is calculated using the formula [21]

Npi ðtÞ ¼ ðt particle þ d001  t platelet Þ=d001

ð1Þ

where N is the number of platelet per particle, tparticle is the thickness of the particle measured by TEM, d001 is the d-spacing measured by WAXD and tplatelet is the estimated thickness of a single platelet (1 nm). The detailed image analysis procedure is described by Vermogen et al. [21].The thickness, length, aspect ratio, orientation and number of platelets in a tactoid were measured by using the image analysis software JMicroVision version 1.25 (www.jmicrovision.com). Around 600 tactoids/platelets were considered for each measurement. Dynamic mechanical thermal analysis (DMTA) was done by using Rheometric Scientific DMTA V in tensile mode. The DMTA experimental conditions were: frequency 1 Hz, strain 0.5%, heating rate 3 °C/min and temperature range 30–200 °C. Oxygen barrier tests were conducted on a OX-TRAN model 2/21 ML (Mocon, Minneapolis) at room temperature and 0% relative humidity. The test area of the samples was 50 cm2. Permeability data were taken after the oxygen permeation change rate was less than 1% for a 45 min test cycle. Samples were conditioned in the test environment before test.

3. Results and discussion 3.1. Effect of equi-biaxial stretching (with stretch ratio 3) on the dispersion of 2 wt% nanoclay in PET matrix Fig. 2 shows the XRD spectra of the unstretched and equi-biaxially stretched PET nanocomposites having 2 wt% MAE. The corresponding spectra of the unfilled PET and that of neat MAE are also given for comparison. Based on the XRD spectra, it may be concluded that the polymer chains are not well intercalated between the clay layers and the platelets are not exfoliated in the matrix as there is no significant change in the d-spacing of the clay layers in the nanocomposite. However, the TEM image analysis gives different information. The TEM photomicrographs of the cross-sections of the unstretched and equi-biaxially stretched sheets are shown in Fig. 3(a) and (b), respectively. Five tactoid parameters are analysed here: thickness, number of platelets, length, aspect ratio and orientation. Table 1 shows the distribution of the above parameters in both unstretched and equi-biaxially stretched sheets. For this part of the analysis, only the unstretched sheet and equi-biaxially stretched sheet stretched at stretch ratio 3 in Table 1 are considered. 3.1.1. Effect of stretching on the tactoid thickness Contrary to the information obtained based on XRD analysis, TEM image analysis shows that equi-biaxial stretching improves the exfoliation of the nanoclays. This is evident in the increase in concentration of clay tactoids having thickness in the range 1–2 nm and 2–3 nm in the stretched sheet (stretch ratio 3) compared to that in the unstretched sheet. As given in Table 1, the concentration of thinner platelets in the thickness range 1–2 nm increases from 10% to 30% and that in the 2–3 nm range increases from 10% to 25% after equi-biaxial stretching at stretch ratio 3. At the same time, thicker tactoids, for example, tactoids in the thickness range of 10–15 nm, are more in the unstretched sheet (21%) compared to those in the stretched sheet (6%). The table also shows that both

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Fig. 2. XRD spectra of (a) Somasif MAE clay; (b) neat PET sheet unstretched; (c) neat PET sheet stretched at stretch ratio 3; (d) PET with 2 wt% Somasif MAE unstretched and (e) PET with 2 wt% Somasif MAE stretched at stretch ratio 3.

unstretched and stretched sheets contain some larger tactoids in the thickness range of 20–50 nm. Tactoids having thickness above 50 nm are found only in the unstretched sheet. Fig. 4 is the graphical representation of the thickness distribution of clay platelets in the unstretched and equibiaxially stretched sheets. The effect of stretching on exfoliation is evident here. The number of platelets in a tactoid also follows similar trend. Sixty-four percentage of tactoids in the stretched sheet contain 1–2 platelets compared to 30% in the unstretched sheet. At the same time, only 7% tactoids in the stretched sheet contain number of platelets between 5 and 10 in them whereas in the unstretched sheet, the concentration is 11%. There is a possibility that

3.1.2. Effect of stretching on tactoid length and aspect ratio Analysis of platelet length and aspect ratio leads to some interesting observations. Stretching reduces the concentration of shorter platelets and tactoids. At the same time, the concentration of longer platelets and tactoids in the composite are increased due to stretching. As explained above, though there is a possibility of inducing an artificial increase in length in stretched sheets due to geometric effect, the effect of equi-biaxial stretching on the increase in length of the tactoids is significant. For example, the percentage of nanoclays in the length range 1–150 nm is reduced from 36 to 18 and those in the length range 150–300 nm is reduced from 45 to 32 due to stretching at stretch ratio 3. At the same time, clay platelets having length above 300 nm are more in the stretched sheets. As shown in Table 1, platelets having length above 750 nm are found only in the stretched sheet. This shows that stretching increases the length of the tactoid. Boyaud et al. [23] and Fenouillot et al. [24] observed a fibrillar dispersed polymer phase when polymer blends are elongated at the die exit of an extruder. Hu et al. [25] reported that when a PET/poly(m-xylylene adipamide) blend was biaxially elongated, the polyamide spherical domain was deformed into high aspect ratio platelets. Je´ol et al. [17] observed that fractal silica particles were organized in the shape of flat agglomerates or platelets, which were elongated in the direction of deformation when injection moulded PET/fractal silica nanocomposites were biaxially stretched. One of the reasons for the increase in tactoid length is thought to be due to the slippage of the platelets within

Fig. 3. TEM images of the unstretched (a) and equi-biaxially stretched (b) PET nanocomposites. Stretch ratio is 3. Samples are collected from the crosssection of the sheets.

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the orientation of the tactoids due to stretching induces automatically an artificial decrease in their thickness due to geometric effect so that in the stretched sheet, the tactoids appear thinner compared to those in the unstretched sheet. However, as shown in Fig. 1, the microtoming direction of both unstretched and equi-biaxially stretched sheets is the same so that when the TEM images of the sheets are compared, the only difference between them is the effect induced by equi-biaxial stretching. Therefore, the process of equi-biaxial stretching has a significant effect on the decrease in thickness of the tactoids.

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Table 1 Results of TEM image analysis showing the effect of stretch ratio on the tactoid and platelet properties. US denotes unstretched sheet; SR 2, SR 2.5 and SR 3 denote equi-biaxially stretched sheets, stretched at stretch ratios 2, 2.5 and 3, respectively. Tactoid thickness range (nm) 1–2

2–3

3–4

4–5

5–10

10–15

15–20

20–50

>50

10 21 21 25

11 13 15 9

13 10 12 6

28 20 27 16

21 10 7 6

4 5 1 3

5 8 4 7

1 0 0 0

% frequency US SR 2 SR 2.5 SR 3

10 15 16 30

Number of platelets in a tactoid 1–2

2–3

3–4

4–5

5–10

10–15

15–20

20–50

>50

26 23 28 15

15 9 11 8

14 8 6 5

11 9 3 7

4 3 2 3

1 0 0 0

0 0 0 0

0 0 0 0

150–300

300–450

450–600

600–750

750–1000

1000–1500

>1500

45 34 28 32

13 34 22 23

5 13 16 15

2 3 5 6

0 2 7 6

0 1 5 1

0 1 0 0

10–20

20–30

30–40

40–50

50–100

100–200

200–300

>300

6 9 3 4

11 27 14 7

22 9 19 9

15 6 10 13

37 23 36 37

4 20 14 21

2 3 4 4

4 1 0 4

% frequency US SR 2 SR 2.5 SR 3

30 49 51 64

Tactoid length range (nm) 1–150

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% frequency US SR 2 SR 2.5 SR 3

36 14 19 18

Tactoid aspect ratio range 0–10 % frequency US SR 2 SR 2.5 SR 3

0 3 0 0

Fig. 4. Graph showing the effect of equi-biaxial stretching (with stretch ratio 3) on tactoid thickness. Y axis represents the frequency distribution of tactoid thickness.

the tactoid due to stretching as illustrated in Fig. 5. This is confirmed by the corresponding decrease in thickness of the tactoids. At the same time, the stretching conditions are such that in all the cases, the tactoids are not completely separated out due to stretching. Thus the effective length of the tactoids is increased. This may be the reason for the higher concentration of longer tactoids in the stretched sheet. It is also likely that some platelets may completely separate out from the tactoids. This phenomenon may also be a reason for the increase in the concentration of individual platelets and thinner tactoids in the stretched sheet.

Fig. 5. Schematic representation of the possible mechanism of increase in length of the tactoid due to stretching. Effective length of the tactoids is increased by dx units due to stretching.

There is a possibility that the present nanocomposite consists of flocculated clay layers as well [2,26]. Flocculation occurs due to the hydrogen bonding between the clay and the polymer. In flocculated nanocomposites, the length

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3.1.3. Effect of stretching on tactoid orientation On comparing the TEM images of the cross-section of the unstretched and equi-biaxially stretched sheets (Fig. 3(a) and (b), respectively), it can be seen that the stretched sheets show preferential tactoid orientation. Fig. 6(a) and (b) is the histograms of the orientation distribution of the unstretched and equi-biaxially stretched sheets. The orientation of the tactoids in the unstretched sheet is random whereas the stretched sheet shows a narrow distribution of tactoid orientation which implies the preferential orientation of the platelets. It should be noted that the orientation angle does not have much significance in the given histograms because the angle may vary depending on the placement of the TEM specimens in the chamber of the microscope.

Fig. 7(a) and (b) shows the TEM photomicrographs of the unstretched and equi-biaxially stretched sheets, respectively where the TEM specimens are collected from the surface of the sheets. Microtoming of both the sheets was done here in the direction parallel to the machine direction. As observed in the case cross-section of the samples, the surface also shows random orientation of the tactoids for the unstretched sheet and preferential orientation for the stretched sheet. Thus it is evident that compared to unstretched sheets, equi-biaxial stretching causes preferential orientation of the tactoids when observed through both the cross-section and surface of the sheets. There is a possibility that some of tactoids in the stretched sheets lie perpendicular to the direction of microtoming. However, as given in Table 1, majority of the tactoids in the stretched sheet lie in the length range of 150–450 nm whereas the thickness of microtomed sheet is only 70 nm. Therefore, it can be assumed that equi-biaxial stretching causes preferential orientation of tactoids in the matrix. 3.2. Effect of stretch ratio on the dispersion of nanoclays in PET matrix Fig. 8(a)–(d) is the TEM images of the cross-sections of the unstretched sheet, equi-biaxially stretched sheets having stretch ratios 2, 2.5 and 3, respectively. Table 1 summarizes the effect of stretch ratio on different tactoid parameters. It is evident that stretch ratio has an effect on the dispersion of the nanoclays in the matrix. The percentage of individual exfoliated sheets (those in the thickness range 1–2 nm) is only 10 in the unstretched sheet. However, at a stretch ratio of 2, their concentration is increased to 15%. As stretch ratio increases from 2 to 2.5 and finally to 3, the concentration of platelets in the thickness range 1–2 nm increases to 16% and 30%, respectively. Tactoids in the thickness range 2–3 nm and the number of platelets in a tactoid also follow similar trend. The unstretched sheet contains 30% tactoids which are having 1 or 2 platelets in them. As stretch ratio increases from 2 to 3, the concentration of individual platelets or tactoids having only 2 platelets increases from 49% to 64%. This shows that stretching and increase in stretch ratio improve the exfoliation of the clay platelets in the matrix. This observation is confirmed in the dynamic mechanical analysis and barrier property measurements of the composites. Fig. 9 shows the effect of stretch ratio on the storage modulus of the equi-biaxially stretched sheets. Storage modu-

Fig. 6. Histograms of the orientation of the tactoids and platelets; (a) unstretched and (b) equi-biaxially stretched sheets stretched at stretch ratio 3. Samples are collected from the cross-section of the sheets.

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of the clay tactoids is increased significantly, resulting in a corresponding increase in the aspect ratio. If this is true, the morphology of the nanocomposites under investigation is a mixture of intercalated, exfoliated and flocculated clay particles. However, further studies are required to confirm this hypothesis. Aspect ratio is considered to be one of the most important parameters in determining the properties of nanocomposites. Osman et al. [27] mention that experimental determination of aspect ratio of nanoparticles in a polymer nanocomposite is difficult because of the geometry of the platelets in the matrix. Many of the tactoids are bent and folded in the matrix. In this study, the actual length and thickness of the tactoids were measured as accurately as possible by using the image analysis software. Here, the orientation of the tactoids and the way they are placed in the matrix may cause some error in the determination of aspect ratio. Thus, the width of the tactoid may sometimes wrongly be considered as length. Nevertheless, based on the image analysis, it is found that majority of the tactoids lie in the aspect ratio range 50–100 in both the unstretched and stretched sheets with stretch ratio 3 (Table 1). However, stretched sheets show a larger concentration of tactoids having aspect ratio above 100. It is found that the highest aspect ratio of the tactoid is 431 for the stretched sheet whereas the highest value is 377 for the unstretched sheet. It should be noted that the increase in aspect ratio of the tactoid is in line with the increase in length and decrease in thickness of the tactoids because of the probable slippage of the tactoids due to stretching.

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Fig. 7. TEM images of the surface of the nanocomposite sheets; (a) unstretched sheet; (b) equi-biaxially stretched sheet stretched at stretch ratio 3.

Fig. 8. TEM images of the cross-sections of the nanoclay filled sheets showing the effect of stretch ratio: (a) unstretched sheet; (b), (c) and (d), equi-biaxially stretched sheets stretched at stretch ratios 2, 2.5 and 3, respectively.

lus increases with increase in stretch ratio, especially that above the glass-to-rubber transition temperature (Tg) of PET. At lower temperatures, composites with low stretch ratio (that is, those with stretch ratios 2 and 2.5) show lower storage modulus than that of stretched virgin PET, which is stretched at a stretch ratio of 3. However, as temperature increases, the behaviour is changed and storage modulus of composites with stretch ratios 2 and 2.5 is higher than that of virgin PET, the cross-over temperature being 120 °C for the composite stretched at stretch ratio 2

and 85 °C for the composite stretched at stretch ratio 2.5. The composite with stretch ratio 3 shows the highest storage modulus in the whole temperature range of the experiment compared to the virgin PET (which is also stretched at stretch ratio 3) and to the composites stretched at stretch ratios 2 and 2.5. As the stretch ratio increases, the cross-over temperature with respect to storage modulus, in reference to virgin PET, decreases. The oxygen permeability is decreased with increase in stretch ratio, as shown in Table 2. It is well known that a well-exfoliated

Fig. 9. Plots of storage modulus vs. temperature for equi-biaxially stretched sheets: (A) virgin PET with stretch ratio 3; (B), (C) and (D), PET+2 wt% Somasif MAE with stretch ratios 2, 2.5 and 3, respectively.

nanocomposite will show improved barrier properties by creating a tortuous path which will decelerate the movement of gas molecules through the matrix [2,28,29]. The DMTA analysis and barrier property measurements show that stretch ratio has some effect on the exfoliation of clay platelets in the matrix. However, it should be noted that increasing stretch ratio orients the PET chains and increases the fraction of mesophase and crystals. Thus increases of storage modulus and barrier properties due to stretching at different stretch ratios can be attributed not only to the effect of the filler but also to changes of the crystalline structure of the PET. Nevertheless, if the storage modulus and O2 permeability of neat PET and clay-filled PET are compared (both having the same stretch ratio of 3, the only difference being the presence of clay), clay filled PET shows higher storage modulus and lower permeability, which shows the effect of clay in increasing the modulus and barrier properties of the sheets. Stretch ratio affects the tactoid length and aspect ratio as well. As shown in Table 1, shorter tactoids are found more in the unstretched sheet (those having length up to 300 nm).The effect of stretch ratio on tactoid length is pronounced in the 450–1000 nm length range. Longer tactoids in this length range are more in the stretched sheets. As stretch ratio increases, the concentration of tactoids in this length range also increases. However, there is no significant change in the length of the tactoid when the stretch ratio is increased from 2.5 to 3, compared to the increase in stretch ratio from 2 to 2.5. This is more significant for Table 2 Results of the barrier property measurements of equi-biaxially stretched virgin PET and 2 wt% nanoclay filled composites stretched at three different stretch ratios (SR). Type of sheet

Stretch ratio (SR)

Thickness (mm)

O2 permeability coefficient cm3-mm/[m2-day-bar]

Virgin PET PET + 2wt% clay PET + 2wt% clay PET + 2wt% clay

3.0 2.0 2.5 3.0

0.10 0.22 0.14 0.10

2.35 ± 0.01 2.41 ± 0.05 2.03 ± 0.03 1.82 ± 0.02

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tactoids having length above 450 nm. As observed previously for stretched sheets with stretch ratio 3, in different stretching conditions as well, the maximum number of tactoids lies in the aspect ratio range 50–100. As stretch ratio increases from 2 to 3, percentage of tactoids in the 50–100 aspect ratio range increases from 23 to 37. However, unstretched sheet is also having 37% tactoids in the aspect ratio range 50–100. In the lower aspect ratio range (for e.g., in the aspect ratio range of 20–30), stretch ratio 2 gives 27% tactoids whereas stretch ratio 3 gives only 7% tactoids. However, in the higher aspect ratio range (50–100), stretch ratio 2 gives 23% tactoids whereas a stretch ratio of 3 gives 37% tactoids. As in the case of tactoid length, the effect of stretch ratio on tactoid aspect ratio is pronounced at higher aspect ratio range. For example, in the 200–300 range, increase in the stretch ratio progressively increases the concentration of tactoid having aspect ratio 200–300. Irrespective of the above observations, if the whole aspect ratio range from 1 to 300 is considered, it is difficult to arrive at a definite conclusion on the effect of stretch ratio on tactoid aspect ratio. As seen in Fig. 8, the unstretched sheet shows random orientation of the tactoids and platelets whereas stretching results in preferential orientation of tactoids, irrespective of the stretch ratio. Though XRD spectra of unstretched and equi-biaxially stretched sheets stretched at different stretch ratios (spectra are not shown) indicate no significant change in the inter gallery spacing of the clay layers with respect to stretching (which is considered as evidence of intercalation), TEM image analysis, as explained above, gives evidence of intercalation and exfoliation due to stretching. In a comparative study on characterization of polymer/clay nanocomposites by TEM and XRD, Morgan and Gilman [19] conclude that XRD results alone are not adequate to describe the nanoscale dispersion of clay platelets in polymer matrix. In order to get a better understanding of the nanoscale as well as global dispersion of clay platelets in polymer matrix, TEM image analysis needs to be combined with X-ray analysis. In the present study also, it is confirmed that more than 30% clay layers are exfoliated in the matrix due to equibiaxial stretching whereas based on the XRD results alone, it may be wrongly concluded that the composites contain no exfoliated clay platelets. 4. Conclusions This paper reports the effect of stretching and stretch ratio on the exfoliation of clay platelets in PET/clay nanocomposites. It is observed that equi-biaxial stretching improves the exfoliation of clay platelets by at least 10% based on the increase in the number of tactoids/platelets in the thickness range 1–2 nm. The concentration of tactoids having 1 or 2 platelets in them also increases which confirms the extent of exfoliation due to stretching. Another interesting observation made in this study is the increase in length of the tactoids due to stretching. Longer tactoids, in the length range above 300 nm are found more in the stretched sheet. It is assumed that due to stretching, the platelets in the tactoids are able to slip past each other, and at the same time, not completely separate out, causing an increase in the effective length of the tactoid. More

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