Strain-Induced Crystallization and Conformational Transition of Poly

Aug 12, 2013 - development are dependent on the strain rate. The cold crystallization peak decreases with increasing draw ratio and disappears complet...
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Strain-Induced Crystallization and Conformational Transition of Poly(trimethylene terephthalate) Films during Uniaxial Deformation Probed by Polarized Infrared Spectroscopy Nadarajah Vasanthan* and Naga Jyothi Manne Department of Chemistry, Long Island University, One University Plaza, Brooklyn, New York 11201, United States ABSTRACT: The structure development during the drawing of PTT films immediately above the glass transition temperature at two different strain rates (8.33 × 10−3 and 8.33 × 10−4 s−1) has been investigated using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. It was found that both the melting temperature and the crystallinity development are dependent on the strain rate. The cold crystallization peak decreases with increasing draw ratio and disappears completely beyond a draw ratio of 2.5. DSC results showed that the decreasing strain rate delays crystallization and reduces the rate of crystallization. The bands at 1358 and 976 cm−1 were chosen to determine the gauche and trans conformations of methylene segments in PTT as a function of the draw ratios and strain rates. It was found that the crystalline gauche conformation increases at the expense of the amorphous trans conformation during the strain-induced crystallization of PTT. The conversion of the amorphous trans conformation into the crystalline gauche conformation is also delayed at lower strain rate, which is consistent with our DSC observation. Polarized IR spectroscopy was used to measure the crystalline and the amorphous orientation functions separately with draw ratios and strain rates, and it was demonstrated that the crystalline orientation develops rapidly with strain-induced crystallization and that the amorphous orientation stays constant up to draw ratio of 2.5 and increases slowly above a draw ratio of 2.5, which is typical behavior for flexible chain polymers.

1. INTRODUCTION Poly(trimethylene terephthalate) (PTT) has attracted less attention compared to other well-established terephthalicbased polyesters such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) because of the relatively high manufacturing cost of the monomer, 1,3propanediol.1,2 Nonetheless, in recent times, PTT has attracted interest because of its excellence mechanical, physical, and processing properties. PTT shows high resilience and elastic recovery compared to nylons and other aromatic polyesters, making it very suitable for carpet and textile applications.3,4 PTT has also found applications in the fields of optical communications, optical data processing, and nonlinear optics because of its high birefringence and luminous transmittance.5 In keeping with its increased applications, PTT is now synthesized via a more economical process. Furthermore, it is necessary to investigate the structure−property relationship of PTT. Numerous studies have focused on thermally induced crystallization and strain-induced crystallization of PET and PBT.6−10 Furthermore, some studies have focused on thermally induced crystallization of PTT.11−15 Various research groups have investigated the uniaxial deformation and strain-induced crystallization of PTT.16−18 Several research groups have studied the PTT crystal structure, but only one crystal form has been identified thus far.19−21 The unit cell of this crystal form is triclinic, with a = 4.637 Å, b = 6.226 Å, c = 18.64 Å, α = 98.4°, β = 93°, and γ = 111.5°, and it has a density of 1.432 g/ cm3. A single chain passes through each unit cell, and two monomer units are present in a unit cell. The trimethylene glycol unit of PTT (O−CH2−CH2−CH2−O) has a highly contracted tggt (trans−gauche−gauche−trans) conformation © 2013 American Chemical Society

with low conformational energy in the crystal whereas the ethylene glycol unit of PET (O−CH2-CH2−O) has a high energy ttt (trans−trans−trans) conformation in the crystal. PTT has been shown to have a very low crystal modulus compared to PET because of its helical conformation.22,23 The crystallization of PTT has been studied using infrared (IR) and Raman spectroscopy.24−30 IR band assignments were made, and bands associated with crystalline and amorphous phases were identified. The band at 976 cm−1 was attributed to the trans conformation whereas the band at 1358 cm−1 was attributed to the gauche conformation of the methylene segment. A method has been developed to quantify the trans and gauche conformations during the thermally induced crystallization of PTT.28,30 In addition to its use to study conformational changes, polarized Fourier transform infrared (FTIR) spectroscopy can be used to measure the crystalline and the amorphous orientation separately along with the overall orientation.31−35 Polarized IR spectroscopy has been used to determine the dichroic ratios and transition moment angles of various vibrational bands of PTT.16,17 In the present report, we examined the microstructure and conformational changes during strain-induced crystallization of PTT by a combination of differential scanning calorimetry (DSC) and FTIR spectroscopy. The amount of gauche and trans conformation of the methylene unit was quantified as a function of the draw ratios at two different strain rates. Polarized IR spectroscopy was also employed to determine the crystalline and the amorphous Received: Revised: Accepted: Published: 12596

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orientation functions as a function of the draw ratio at both strain rates. The development of crystallinity and molecular orientation during the drawing of PTT is compared with those during the drawing of PET.

by quenching from the melt in ice water and rectangular-shaped amorphous PTT films (3 cm × 4 cm) were drawn to different draw ratios at two different strain rates (8.33 × 10−3 and 8.33 × 10−4 s−1) at 50 °C. Figure 1 shows DSC heating scans of

2. EXPERIMENTAL SECTION 2.1. Materials. PTT pellets with an intrinsic viscosity of 0.85 dL/g (measured in dichloroacetic acid at 23 °C) were supplied by Shell Chemical Company. The weight and numberaverage molecular weights of PTT resin were 78 000 and 34 700 g/mol, respectively. The molecular weight distribution was ∼2.0. An isotropic amorphous film with a thickness of 25−30 μm was prepared by melt-pressing and subsequent quenching in the ice water bath. These films were cut into a rectangular shape with dimensions of 3 cm × 4 cm and then marked to 1 cm × 1 cm dimensions for stretching. These films were uniaxially stretched at two different strain rates (8.33 × 10−4 and 8.33 × 10−3 s−1) at 50 °C on a stretcher equipped with an environmental chamber, and the stretched films were used for strain-induced crystallization and structure development studies. The strain rate was estimated using the equation (L − L0)/(L0t), where L is the final length; L0, the initial length; and t, the time required to stretch the film for a particular draw ratio at a particular speed. The draw ratios of stretched films were estimated by taking the ratio between the final and the initial length. Eight different draw ratios (1−4) were prepared. 2.2. DSC. Thermal analysis was performed using a PerkinElmer DSC 7 under constant nitrogen flow to avoid thermal degradation of the material. DSC calibration was performed with indium and zinc standards. Samples of 3−5 mg weight were used for the DSC analysis. Each DSC scan was obtained for the PTT stretched film from 25 to 260 °C at a heating rate 10 °C/min. The glass transition temperature, cold crystallization temperature, and melting point of PTT were obtained by taking the onset values. The heat of fusion is directly proportional to the crystallinity, and it was determined from the area under the melting peak. The crystalline fractions were calculated by using the following equation.

Figure 1. DSC heating scans of PTT films after drawing at 50 °C at a strain rate 8.33 × 10−3 s−1 to different draw ratios.

amorphous PTT films stretched at a strain rate of 8.33 × 10−3 to different draw ratios along with a DSC scan of an amorphous PTT film annealed at 50 °C. DSC scans of PTT films stretched at 8.33 × 10−4 follow a similar trend and are not shown here. Three transitions can be seen for the amorphous PTT and the films drawn to a draw ratio below 2.5. These transitions are attributed to glass transition (Tg), cold crystallization (Tc), and melting transitions (Tm). Tg as well as Tc disappeared for PTT films stretched at higher draw ratios using both strain rates (draw ratio >2.5). Table 1 presents the effect of the strain rate Table 1. Thermal Properties of Drawn PTT as a Function of Draw Ratios and Strain Ratesa strain rate (s−1) 8.33 × 10

%crystallinity = [(ΔHs − ΔHcc)/ΔHo] × 100

where ΔHs is the heat of fusion of the sample; ΔHcc, the heat of cold crystallization; and ΔHo, the heat of fusion of 100% crystalline polymer. ΔHo was taken as 145.63 J/g.36 2.3. FTIR Spectroscopy. IR spectroscopic measurements were carried out in the region between 400 and 4000 cm−1 on a Nicolet Magna760 Spectrometer with a resolution of 2 cm−1 using a DTGS-KBr detector. At least 256 scans were coadded to achieve an adequate signal-to-noise ratio. The selected IR bands were resolved using a peak fitting program (Jandell) to determine the area under the peaks. The bands were assumed to have a Lorentzian shape with a linear baseline during peak fitting. Two different transmission spectra were collected for each sample using incident beams parallel and perpendicular to the draw direction. The IR dichroic ratio was calculated as D = A||/A⊥.

−4

8.33 × 10−3

a

draw ratio

Tcc (°C)

Tm (°C)

crystallinity (%)

1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5

68.8 64 54.9 48

227.8 225.8 223.5 222.2 221.3 220.7 227.8 224.7 222.5 221.3 220.5 220.3

12.4 16.5 21.8 25.2 28.6 32.8 12.4 21.5 29.2 34.6 35.5 37.2

68.8 64.3 60.4 55

All of the data points are averages of at least three determinations.

and drawing on the melting temperature and cold crystallization temperature, respectively. The cold crystallization exotherm decreases with increasing draw ratio at both strain rates and disappears completely at draw ratios exceeding 2.5. This observation clearly suggests that incomplete crystallization had occurred during deformation at 50 °C below a draw ratio of 2.5. It should be noted that Tc shifts to a lower temperature with increasing draw ratio at both strain rates, suggesting that strain-induced crystallization significantly influences the cold crystallization process. The mobility of the polymer chain in the

3. RESULTS AND DISCUSSION 3.1. Thermal Behavior. The strain-induced crystallization of PTT was investigated by DSC and FTIR spectroscopy. Because it was not possible to follow the strain-induced crystallization in situ in our laboratory, that of PTT was studied post-crystallization. The amorphous PTT films were prepared 12597

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density measurement, wide-angle X-ray diffraction, and DSC. The crystallinity of drawn PTT films was determined using the cold crystallization exotherm and melting endotherm in the DSC scans using the following equation.

amorphous region is reduced because of strain-induced crystallization that leads to a decrease in the cold crystallization and glass transition temperatures. The cold crystallization temperature cannot be observed in DSC scans of PTT films drawn at draw ratios exceeding 2.5 at both strain rates, suggesting that cold crystallization requires a longer time for these PTT films. Figure 2 shows DSC scans in the region from 100 to 260 °C; these scans show only Tm as a function of the draw ratios for

χc = (ΔHs − − ΔHcc)/ΔHo

where ΔHs is the heat of fusion of the sample; ΔHcc, the heat of cold crystallization; and ΔHo, the heat of fusion of 100% crystalline PTT. ΔHo was taken as 145.63 J/g.36 The crystallinity development for films drawn at strain rates of 8.33 × 10−4 s−1 and 8.33 × 10−3 s−1 versus the draw ratios is shown in Figure 3. It is clear from Figure 3 that the strain rate

Figure 3. Crystallinity development for PTT film as a function of draw ratios at various strain rates.

significantly influences the crystallinity development. It appears that the lower strain rate delayed crystallization and the level of crystallinity achieved is higher for the PTT films drawn at a higher strain rate within the window of draw ratios studied. It should be noted that the crystallinity increases slowly with draw ratio for films drawn at 8.33 × 10−4 s−1, and it increases rapidly up to a draw ratio of 2.5 followed by a small increase for films drawn at 8.33 × 10−3 s−1. The effect of the strain rate on the strain-induced crystallization of PET has been studied by various groups, and it has been shown that the lower strain rate and higher drawing temperature delay the crystallization of PET.41−43 However, the level of crystallization was independent of the strain rate in the case of PET whereas it appeared to be dependent in the case of PTT. It is well-known that the development of the molecular orientation and relaxation process will occur at the same time during deformation. The strain-induced crystallization occurs when the molecular orientation dominates the molecular relaxation process. It appears that the reducing strain rate provides more time for relaxation, thus leading to slower crystallization during the deformation of PTT films immediately above the glass transition temperature.8−10,45 3.3. FTIR Spectroscopy and Conformational Changes. The FTIR spectra of the amorphous and PTT films stretched to different draw ratios in the region between 400 and 1800 cm−1 are shown in Figure 4. The thermally induced crystallization of PTT has been successfully studied by FTIR spectroscopy by us and various other research groups.24−30 The bands at 933 and 947 (CH2 rocking); 1037 (C−C stretching); 1358 (CH2 wagging); and 1465 cm−1 (CH2 bending) were attributed to the ordered phase (crystalline) whereas those at 811, 976, and 1173 (CH in-plane bending of aromatic ring);

Figure 2. DSC melting endotherms recorded at 10 °C min −1 for PTT films after drawing at 50 °C to different draw ratios (a) at a strain rate 8.33 × 10−3 s−1, (b) at a strain rate 8.33 × 10−4 s−1.

both strain rates. It is apparent that Tm shifts to lower temperature with increasing draw ratio (see Table 1). For example an undrawn film melts at 228 °C, and a film drawn at a draw ratio of 3.5 at a strain rate of 8.333 × 10−3 s −1 melts at 220 °C. A similar shift was also observed for PTT films drawn to different draw ratios at a strain rate of 8.333 × 10−4 s −1. Tm observed for a drawn PTT film shows multiple peaks, and this behavior has been observed in many other semicrystalline aromatic polymers such as PET, PBT, and poly(aryl ether ketone) (PEEK).37−40 Multiple melting peaks in the DSC scans were explained by the melting of the original and the recrystallized crystals or by crystals with multiple lamellar thicknesses. It is clear from Table 1 that the melting temperature appears to depend on the strain rate. The decrease in melting temperature is higher for films drawn at a higher strain rate compared to those drawn at a lower strain rate. 3.2. Crystallinity Development during Deformation. The crystallinity of drawn PTT films can be determined by 12598

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Figure 4. FTIR spectra of the amorphous and PTT films stretched to different draw ratios at 50 °C at a strain rate 8.33 × 10−3 s−1 in the region between 400 and 1800 cm−1: (a) DR1, (b) DR1.5, (c) DR2, (d) DR3.

the unoriented absorbance using Aun = (A// + 2A⊥)/3, where Aun is the absorbance of the unoriented sample; A//, the absorbance under parallel polarization; and A⊥, the absorbance under perpendicular polarization. Figure 5 shows the

1328 and 1385 (CH2 wagging); 1452 (CH2 bending); and 1577 cm−1 were assigned to the amorphous phase. In the present study, strain-induced crystallization was investigated using polarized IR spectroscopy. Structural changes such as conformational changes and molecular orientation (crystalline and amorphous) were followed with draw ratios and strain rates. It can be seen in Figure 4 that the absorbance of crystalline bands at 933, 1024, 1037, 1358, and 1465 cm−1 increases and that of amorphous bands at 811, 976, 1173, 1328, 1385, 1452, and 1577 cm−1 decreases with the draw ratio at both strain rates relative to the absorbance of the band at 1504 cm−1, suggesting that the crystallinity increases with increasing draw ratios. The band at 1504 cm−1 is being used as a reference band to study structural changes during both thermally induced crystallization and strain-induced crystallization. It is generally accepted that semicrystalline polymers consist of crystalline and amorphous phases; however, it has been reported that strain-induced crystallized polymers, especially in the case of PET, have an additional phase called as a mesophase.44,45 To the best of our knowledge, a mesophase has not yet been reported for PTT. In this study, we assume a two-phase model for strain-induced crystallized PTT. The trimethylene glycol unit (−O−CH2-CH2−CH2−O−) of PTT adopts gauche and trans conformations via internal rotation through the C−C bond. In the crystalline phase, the −O−CH2CH2−CH2−O− unit adopts the gauche conformation, whereas in the amorphous phase, −O−CH2-CH2−CH2−O− is mainly in the trans conformation with some gauche conformation. The conformational changes during strain-induced crystallization were studied using the bands at 1358 cm−1 (gauche) and 976 cm −1 (trans). To obtain quantitative information, the absorbance of each band from unoriented spectra must be obtained. The absorbance of IR bands of oriented films under parallel and perpendicular polarization were used to calculate

Figure 5. Normalized absorbance of bands at 1358 and 976 cm−1 against draw ratios for the film drawn at a strain rate of 8.33 × 10−3 s−1.

normalized absorbance of bands at 1358 and 976 cm−1 against draw ratios for the film drawn at a strain rate of 8.33 × 10−3. A similar observation was also made for a film drawn at a strain rate of 8.33 × 10−4. It appears that the normalized absorbance of 1358 cm−1 increases and the normalized absorbance of 976 cm−1 decreases with increasing draw ratios, confirming the fact that the amount of gauche conformation increases and that of trans conformation decreases with increasing draw ratio. FTIR spectroscopy is used to determine the amounts of gauche and trans conformations of glycol units of PTT with increasing draw ratio, and these were correlated with the 12599

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crystallinity values determined by DSC measurement. The distributions of the gauche and trans conformations in the crystalline and the amorphous phases were determined using a previously described method.28,30 If we assume two conformational models, we can write the following equation P1(A1358 /A1504 ) + P2(A 976 /A1504 ) = 1

where P1 and P2 are constants related to the gauche and trans conformations, respectively. Figure 6 shows a linear fit between

Figure 6. Absorbance ratio 1504 cm−1/976 cm−1 vs 1358 cm−1 /976 cm−1 for the film drawn at a strain rate of 8.33 × 10−3 and 8.33 × 10−4 s−1.

the absorbance ratios obtained from IR measurements. From the graphs, the P1 and P2 values are obtained as 0.42 and 3.16, respectively. The fraction of gauche and trans conformations at various strain rates and draw ratios can be represented by P1(A1358/A1504) and P2(A976/A1504), respectively. The fractions of trans and gauche conformations and DSC crystallinity as a function of draw ratios and strain rates are listed in Table 2. If we assume that the crystalline gauche conformation is equal to the fraction of crystalline phase obtained by the DSC measurement, the amorphous gauche conformation can then be obtained by subtracting the crystalline gauche conformation from the total gauche conformation. Figure 7 shows the trans, crystalline gauche, and amorphous gauche conformations as a

Figure 7. Fraction of the amorphous gauche (○), crystalline gauche (●), and trans (▲) conformer against draw ratios: (a) at a strain rate of 8.33 × 10−3 s−1, (b) at a strain rate of 8.33 × 10−4 s−1. The data points are the mean of at least three determinations. The standard deviation was ± 0.02.

function of the draw ratio at both strain rates. It is observed that the amount of crystalline gauche conformation increases with increasing draw ratio irrespective of the strain rate whereas the amount of trans conformation decreases with increasing draw ratio. It can also be seen that the amorphous gauche conformation stays constant up to a draw ratio of 3 and increases significantly beyond. It also appears that the increase in the amorphous gauche conformation is higher for the sample drawn at a higher strain rate. The general deformation model proposed here is that the amorphous trans conformation converts into the crystalline gauche conformation at low draw ratios and that the amorphous trans conformation converts into the crystalline and the amorphous gauche conformations at high draw ratios. 3.4. Molecular Orientation Studies by Polarized FTIR Spectroscopy. The crystalline orientation of oriented semicrystalline polymers is usually measured using X-ray diffraction measurements.46−48 Polarized IR and Raman spectroscopy, sonic velocity, and birefringence have also been used to determine the crystalline orientation of semicrystalline polymers.49,50 Not many direct techniques are available to measure the amorphous orientation of oriented polymers, and indirect techniques used to measure the amorphous orientation are usually unreliable. The lack of a suitable method to directly measure the amorphous orientation of PTT has led us to explore the use of polarized IR spectroscopy for the orientation measurement. The IR spectra of PTT drawn to a draw ratio of

Table 2. Fraction of Crystalline Gauche, Amorphous Gauche, and Trans Conformations As a Function of the Draw Ratios and Strain Ratesa strain rate (s−1)

draw ratio

crystalline fraction

P1(A1358/ A1504) (total gauche)

P2(A976/ A1504) (trans)

amorphous gauche

8.33 × 10−4

1 1.5 2 2.5 3 3.5 1 1.5 2 2.5 3 3.5

0.12 0.17 0.21 0.25 0.29 0.33 0.12 0.22 0.29 0.34 0.36 0.37

0.16 0.21 0.24 0.28 0.33 0.43 0.16 0.28 0.33 0.38 0.42 0.50

0.87 0.79 0.76 0.75 0.64 0.59 0.87 0.70 0.70 0.64 0.61 0.50

0.04 0.04 0.03 0.03 0.04 0.10 0.04 0.06 0.04 0.04 0.06 0.13

8.33 × 10−3

a

All of the data points are averages of at least three determinations; standard deviation was ± 0.02. 12600

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Figure 8. IR spectra obtained for PTT films drawn to a draw ratio of 2 at 50 °C at a strain rate 8.33 × 10−3 s−1 under (a) unpolarized, (b) parallel, and (c) perpendicular polarizations.

3 were measured by increasing the polarization angle. The maximum and minimum absorbance was obtained at 0 and 90°, respectively. Figure 8 shows the IR spectra obtained for PTT films drawn to a draw ratio of 2 under parallel and perpendicular polarizations along with unpolarized spectrum. The parallel and perpendicular bands were identified. One of the practical difficulties is to identify IR bands that have an absorbance lower than 1 to satisfy the Beer−Lambert law for quantitative measurement. The bands at 1577, 1358, 1173, and 976 cm−1 appeared to have relatively low absorbances. The crystalline band at 1358 cm−1 and the amorphous bands at 1577, 1173, and 976 cm−1 exhibit strong dichroism and they are suitable bands for investigating the crystalline and amorphous orientation of PTT. In the present study, the band at 1358 cm−1 was used to characterize the crystalline orientation and the band at 976 cm−1 was used to characterize the amorphous orientation of drawn PTT films. The absorption of IR radiation depends on the transition moment vector and the applied electric vector. The dichroic ratio (D) of an IR band is obtained by taking the ratio of the absorbance measured under parallel polarization and perpendicular polarizations. The dichroic ratios of the bands at 1577, 1358, 1173, and 976 cm−1 are plotted as a function of draw ratios for the PTT film drawn at a strain rate of 8.33 × 10−4 in Figure 9. It is evident from Figure 9 that the dichroic ratios increase for the bands at 1577, 1358, and 976 cm−1 with increasing draw ratio, whereas the dichroic ratio of band at 1173 cm−1 decreases with draw ratio. It can be clearly noted that the dichroic ratios appear to be dependent on the strain rate and the draw ratio. Generally, the overall chain orientation is given by the following equation

Figure 9. Effect of drawing on dichroic ratios of bands at 1577, 1358, 1173, and 976 cm−1.

moment angles have been reported by various groups, and we adopted these angles for determining the orientation functions. The transition moment angle for the parallel bands at 1358, 1037, and 935 cm−1 were reported as 39, 42, and 32°, respectively, whereas those for the perpendicular bands at 2967, 2901, and 948 cm−1 were reported as 68, 64, and 62°, respectively.17 The band at 976 cm−1 is also a parallel band, but the corresponding transition moment angle is not reported. It can be seen that the transition moment angle for all parallel bands are relatively closer and can be averaged to 37°. Furthermore, the band at 976 cm−1 was attributed to the trans conformation of PET, and the transition moment angle for this vibration was reported as 34°. Making the very reasonable assumption that the transition moment angle for the 976 cm−1 band in PTT is 35°, we have calculated the amorphous orientation function. The crystalline and amorphous orientation functions determined using the gauche band at 1358 cm−1 and the trans band at 976 cm−1 as a function of the draw ratios and strain rates are shown in Figure 10. It should be noted that the amount of gauche conformation is slightly higher than the

P2(cos θ ) = f = [(D − 1)/(D + 2)]/[1/2(3cos 2 β − 1)]

To obtain the orientation function, ⟨P2(cos θ)⟩, the direction of the transition moment angle (β), is necessary. The transition 12601

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ratios, whereas the amorphous orientation develops much more slowly at higher draw ratios.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

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Figure 10. Variation in crystalline and amorphous orientation as a function of draw ratio at various strain rates. (○), crystalline orientation at a strain rate 8.33 × 10−4 s−1; (●)crystalline orientation at a strain rate 8.33 × 10−3 s−1 ; (Δ) amorphous orientation at a strain rate 8.33 × 10−4 s−1 ;(▲) amorphous orientation at a strain rate 8.33 × 10−4 s−1.

crystallinity obtained by DSC, and therefore, the crystalline orientation functions are overestimated. On the other hand, the trans conformation and some gauche conformations are present in the amorphous phase, and therefore, the amorphous orientation is underestimated. It is apparent that the crystalline orientation increases continuously up to a draw ratio of 3.5. No amorphous orientation was developed up to a draw ratio of 2.5 at both strain rates, and the amount increased slowly above a draw ratio of 2.5. The crystalline as well as amorphous orientation develop much faster for the film drawn at a higher strain rate compared to the one drawn at a low strain rate. The rapid development of the crystalline orientation to almost full fiber-axis orientation compared with the slower development of orientation in the noncrystalline regions is common behavior for flexible-chain polymers.

4. CONCLUSIONS Strain-induced crystallization of PTT after post deformation was studied using DSC and FTIR spectroscopy as a function of the draw ratios and strain rates. It was demonstrated that the crystallinity increases with increasing draw ratio at both strain rates and that the development of crystallinity is apparently dependent on the strain rate. The decrease in melting temperature is higher for the films drawn at a higher strain rate compared to those drawn at a lower strain rate. It was found that the cold crystallization exotherm decreases with increasing draw ratio and disappears completely beyond a draw ratio of 2.5. The structure development of PTT films was studied by IR spectroscopy, and it was shown that the bands at 1577, 1173, and 976 cm−1 can be used to characterize the amorphous phase whereas the bands at 1358 cm−1 can be used to characterize the crystalline phase. The conformational changes during the drawing of PTT were followed by FTIR spectroscopy, and it was shown that the amorphous trans conformation transforms into the gauche conformation as the draw ratio increases. This conversion appears to be dependent on the strain rate as crystallinity development. Crystalline and amorphous orientation functions were determined using the gauche band at 1358 cm−1 and the trans band at 976 cm−1 as a function of the draw ratios and strain rates, and it was shown that the crystalline orientation develops rapidly at lower draw 12602

dx.doi.org/10.1021/ie401783y | Ind. Eng. Chem. Res. 2013, 52, 12596−12603

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dx.doi.org/10.1021/ie401783y | Ind. Eng. Chem. Res. 2013, 52, 12596−12603