Article pubs.acs.org/IECR
Effect of Molecular Orientation on the Cold Crystallization of Amorphous−Crystallizable Polymers: The Case of Poly(trimethylene terephthalate) Nadarajah Vasanthan,* Naga Jyothi Manne, and Anusha Krishnama Department of Chemistry, Long Island University, One University Plaza, Brooklyn, New York 11201, United States ABSTRACT: The effect of molecular orientation on cold crystallization of amorphous crystallizable polymers was examined using poly(trimethylene terephthalate) (PTT) films drawn to different draw ratios and strain rates. A combination of differential scanning calorimetry (DSC) and polarized Fourier transform infrared (FTIR) spectroscopy was employed to examine structural evolution and nonisothermal crystallization kinetics. The cold crystallization temperature (Tc), cold crystallization exotherm (ΔHc), and subsequent melting temperature (Tm) were carefully correlated to the overall molecular orientation. For the first time, the overall molecular orientation was shown to have an inverse relationship to the cold crystallization temperature, as well as the cold crystallization exotherm. Nonisothermal cold crystallization has not occurred when the overall orientation exceeded the critical value of 0.43. The kinetics of nonisothermal cold crystallization of PTT with a different overall molecular orientation has been investigated and the Avrami equation has been applied to evaluate the kinetic parameters. An increase in the rate constant and a decrease in the Avrami exponent suggested that cold crystallization is faster for a PTT film with a high overall molecular orientation and a change in growth geometry with orientation. This study demonstrates that overall molecular orientation affects the cold crystallization kinetics of amorphous−crystallizable polymers.
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INTRODUCTION Poly(trimethylene terephthalate) (PTT) has combined physical properties of nylons and other aromatic polyesters, such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), and it is a promising material for engineering plastics and textile fibers.1−4 PTT is also expected to be used in the area of optical communications and nonlinear optics, because of its high birefringence and luminous transmittance.5,6 Isothermal crystallization studies of semicrystalline polymers are commonly used to derive the mechanisms associated with the crystallization process, because it is easier to analyze, compared to nonisothermal crystallization. Thermally induced crystallization of polymers can be carried out either by crystallizing from the melted state or crystallizing from the glassy state. The former is called melt crystallization and the latter is called either annealing or cold crystallization.7−10 Both crystallization studies have practical importance from a technical point of view, since heat settings and blow moldings are used during the processes. The physical and mechanical properties of these polymers are, directly or indirectly, controlled by crystallization.11−13 In the past decade, various researchers have investigated the crystallization and thermal properties of PTT.14−24 It has been reported that PTT crystallizes in only one crystal form and the crystal structure of PTT has been determined by several different groups.25−27 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°, γ = 111.5°, and a density of 1.432 g/cm3. Isothermal and nonisothermal melt crystallization kinetics of PTT have been studied in detail.14,15,18 The cold crystallization of PTT has recently received considerable attention, because of its gelationrelated morphology, but it is not well understood.28−30 It has been shown that the finely dispersed sheaflike crystallites © 2013 American Chemical Society
dominate in the early stage of cold crystallization, leading to gelation.28 Several techniques have been used to investigate cold crystallization of semicrystalline polymers. Among these, the most common are simultaneous small angle and wide angle scattering,31 differential scanning calorimetry (DSC),32,33 infrared (IR) spectroscopy,34 atomic force microscopy (AFM),35,36 and fluorescence spectroscopy.28 Luo et al.28 recently studied isothermal cold crystallization kinetics of PTT by fluorescence spectroscopy, and showed that kinetic parameters obtained by this method are in close agreement with kinetic parameters obtained via DSC. A four-stage mechanism of induction, nucleation, growth, and crystallization were proposed for isothermal cold crystallization of PTT. It has been shown that cold crystallization occurs during aging and changes the glass-transition temperature and mechanical properties.37−39 To our knowledge, nonisothermal cold crystallization of drawn PTT has not been conducted in detail, and the role of molecular orientation on cold crystallization of amorphous−crystallizable polymers remains unclear. In the present work, for the first time, PTT is investigated to address the effect of overall molecular orientation on cold crystallization and kinetics. This study will extend to other oriented polymers, and will provide valuable insight into the mechanisms associated with cold crystallization.
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 Received: Revised: Accepted: Published: 17920
August 30, 2013 November 24, 2013 November 25, 2013 November 26, 2013 dx.doi.org/10.1021/ie402860t | Ind. Eng. Chem. Res. 2013, 52, 17920−17926
Industrial & Engineering Chemistry Research
Article
(8.33 × 10−3 and 8.33 × 10−4 s−1) and 50 °C were investigated by DSC. Figure 1 shows DSC heating scans of amorphous PTT
supplied by the Shell Chemical Company. The weight- and number-average molecular weights of the 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 an ice water bath. 2.2. Drawing. The amorphous PTT films were cut into a rectangular shape with dimensions of 3 cm × 4 cm and then marked to dimensions of 1 cm × 1 cm 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. After stretching, each sample was air quenched to prevent any crystallization. The draw ratios of stretched films were estimated by taking the ratio between the final and the initial length. 2.3. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC) experiments were performed with a Perkin−Elmer DSC 7 system. The instrument was calibrated for temperature and heat of fusion using standard indium (Tm = 156.6 °C and ΔH = 28.5 J/g). All experiments were performed under nitrogen atmosphere with a flow rate of 20 mL/min. Samples of 3−5 mg were used for all measurements. The onset values were taken as cold crystallization and melting temperatures. The crystallinity of PTT was calculated using the following equation:
Figure 1. Differential scanning calorimetry (DSC) heating scans of PTT films before and after drawing at 50 °C at various strain rates: (a) undrawn amorphous PTT, (b) PTT drawn to a draw ratio of 3 at a strain rate of 8.33 × 10−4 s−1, and (c) PTT drawn to a draw ratio of 3 at a strain rate of 8.33 × 10−3 s−1.
films stretched at a strain rate of 8.33 × 10−4 and 8.33 × 10−3 s−1 to a draw ratio of 3, along with a DSC scan of an amorphous undrawn PTT film annealed at 50 °C. Three transitions were seen for the amorphous PTT film and the films drawn to a draw ratio below 3 (not shown here). 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 to a draw ratio of 3 using both strain rates. Figure 2 shows cold crystallization exotherm as a function of draw ratios at both strain rates. Note that the Tc value shifts to a lower temperature with an
⎛ ΔHs − ΔHcc ⎞ % crystallinity = ⎜ ⎟ × 100 ΔH0 ⎝ ⎠
where ΔHs is the heat of fusion of the sample, ΔHcc the heat of cold crystallization, and ΔH0, the heat of fusion of 100% crystalline polymer. The value of ΔH0 was assuemd to be 145.63 J/g.40 2.4. Nonisothermal Crystallization Kinetics by DSC. The nonisothermal cold crystallization kinetics were performed on a Perkin−Elmer DSC 7 under nitrogen atmosphere. All drawn and undrawn PTT films were heated at various heating rates from 10 °C/min to 25 °C/min up to 100 °C. The nonisothermal cold crystallization exotherms were analyzed to obtain kinetics parameters. 2.5. Fourier Transform Infrared (FTIR) Spectroscopy. Infrared (IR) spectroscopic measurements were carried out in the region between 400 and 4000 cm−1 on a Nicolet Model Magna 760 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 for orientation studies using a polarizer with an incident beam parallel and perpendicular to the draw direction. The infrared dichroic ratio was determined using the following equation: D=
A|| A⊥
where A|| is the absorbance parallel to the draw direction and A⊥ is the absorbance perpendicular to the stretching direction.
3. RESULTS AND DISCUSSION 3.1. Microstructure Changes. The cold crystallization of PTT drawn to different draw ratios at two different strain rates
Figure 2. DSC heating scan recorded at a rate of 10 °C min−1 for PTT films after drawing at 50 °C to different draw ratios at strain rates of (a) 8.33 × 10−3 s−1 and (b) 8.33 × 10−4 s−1. 17921
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much faster for the PTT film drawn at a higher strain rate than one drawn at a low strain rate.
increasing draw ratio, suggesting that strain-induced crystallization significantly influences the cold crystallization process. The percent crystallinity obtained by DSC for all PTT films stretched to different draw ratios at both strain rates are tabulated in Table 1. We previously reported that the lower Table 1. Crystalline Fraction, Fraction of Trans and Gauche Conformers, and Orientation Functions of Trans and Gauche Conformers, as Well as Overall Orientation Function as a Function of Draw Ratios and Strain Rates draw ratio 1.0 1.5 2.0 2.5 3.0 3.5 1.5 2.0 2.5 3.0 3.5
crystallinity
χg
χt
Strain Rate = 8.33 × 10−3 0.12 0.15 0.85 0.22 0.28 0.72 0.29 0.33 0.67 0.35 0.38 0.62 0.36 0.42 0.58 0.37 0.54 0.46 Strain Rate = 8.33 × 10−4 0.17 0.21 0.79 0.22 0.24 0.76 0.25 0.28 0.72 0.29 0.33 0.67 0.33 0.43 0.57
fg s−1 0.00 0.32 0.42 0.63 0.74 0.77 s−1 0.32 0.37 0.56 0.63 0.67
ft
fo
0.00 0.07 0.09 0.26 0.32 0.37
0.00 0.14 0.20 0.40 0.50 0.59
0.07 0.09 0.11 0.14 0.17
0.12 0.16 0.24 0.30 0.39
Figure 3. Overall molecular orientation of PTT film as a function of draw ratios at various strain rates ((●) 8.33 × 10−3 s−1 and (○) 8.33 × 10−4 s−1).
The effect of chain orientation on the crystallization of polymers has been extensively studied.13,44,45 In most of these studies, the amorphous orientation was correlated to straininduced crystallization. When amorphous polymers are stretched, the amorphous orientation increases. Once the amorphous orientation exceeds a critical value, some of the oriented amorphous phase transforms to a crystalline phase. These crystals have a very low crystal orientation; therefore, we believe that both the amorphous and the crystalline orientation influence the cold crystallization process. Cold crystallization can occur either by lamellar thickening or crystallizing from the amorphous phase. A correlation was made in this study for the first time between the overall orientation and cold crystallization kinetic parameters. Figures 4a and 4b respectively show the melting temperature and cold crystallization temperature as functions of the overall orientation function. Amorphous PTT films with zero overall orientation crystallized inside the DSC during the first heating melt at 228 °C. It appears that the value of Tm decreases with an increasing overall orientation of PTT films and levels off after the overall orientation becomes 0.43. On the other hand, a decrease in Tc was observed with an increase in the overall orientation, suggesting an increasing crystallization rate with orientation. The extrapolation of the line shows that the Tc becomes 45 °C, the same as Tg, at an overall orientation of 0.43. This implies that PTT would not be expected to cold crystallize once the overall orientation exceeds 0.43. Note that none of the PTT samples with an overall orientation of >0.43 showed cold crystallization exotherm in their DSC scans. The heat of cold crystallization (ΔHc) was plotted against the overall orientation in Figure 5, and extrapolation of this line shows that ΔHc becomes zero when the overall orientation exceeds 0.43, again confirming that cold crystallization does not occur when the overall orientation exceeds 0.43. 3.2. Nonisothermal Cold Crystallization Kinetics of PTT. It is well-known that cold crystallization is often used to improve the stiffness and tensile properties of semicrystalline polymers.28,37−39 A very few studies have been conducted on nonisothermal cold crystallization kinetics of PTT.14,32 Nonisothermal cold crystallization of PTT was investigated by heating PTT samples from 25 °C to 100 °C at various heating
strain rate delayed crystallization and the level of crystallization achieved is higher for the PTT films drawn at a higher strain rate within the draw ratios studied.41 The crystallinity of PTT was shown to increase rapidly up to a draw ratio of 2.5, followed by a small increase for the films drawn at 8.33 × 10−3 s−1, whereas a slow increase in crystallinity was found for the films drawn at 8.33 × 10−4 s−1 up to a draw ratio of 3.5. Fourier transform infrared (FTIR) spectroscopy was used to determine the conformational changes, the crystallinity, and the amorphous orientation development during drawing.41 The band at 1358 cm−1, assigned to the gauche conformation, and 976 cm−1, attributed to the trans conformation of the amorphous phase, were used to characterize crystallinity and the amorphous phases, respectively.42,43 PTT in the crystalline region assumes a gauche conformation of the glycol unit, whereas the amorphous region predominantly adopts a trans conformation with a small amount of gauche conformation. The fraction of trans and gauche conformation was determined as a function of strain rates and draw ratios using the bands at 976 and 1358 cm−1, and was described in detail in our previous report.41 The amount of trans and gauche conformations as a function of draw ratio is presented in Table 1. The gauche (crystalline) and the trans (amorphous) orientation functions were determined by polarized IR spectroscopy (see Table 1). An overall orientation averaged over both the crystalline and the amorphous orientation can be obtained by birefringence measurement. The overall orientation functions were determined in this study from the orientation of the gauche and trans conformations using the following equation, which is also listed in Table 1. foverall = fgauche χgauche + ftrans χtrans
where f represents the orientation functions and χ is the fraction of each conformer. The overall orientation of the PTT film drawn at both strain rates is plotted in Figure 3. Figure 3 clearly shows that the overall molecular orientation develops 17922
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which is released during the crystallization process. This relationship is depicted as t
χt =
∫0 (dH /dt ) dt ∞
∫0 (dH /dt ) dt
where the numerator represents the heat generated at time t, while the denominator represents the total heat generated for the entire crystallization process. The fraction of the relative crystalline phase (χt) for the sample at a particular time t was calculated. The relative percent crystallinity is plotted against time as a function of different heating rates in Figure 7 for undrawn PTT film. A similar observation was made for the PTT samples drawn to draw ratios of 1.5 and 2 at both strain rates. It appears that each DSC scan shows an induction period in which the DSC does not detect any crystallinity development. After the induction period, crystallization begins and crystallinity increases until it reaches equilibrium. It was found that the rate of cold crystallization increases with an increasing heating rate for both undrawn and drawn PTT films. The crystallinity development during cold crystallization of undrawn PTT film and PTT film drawn to a draw ratio of 2.0 at both strain rates with the same heating rate of 10 °C min−1 are compared in Figure 8. It was found that the nonisothermal cold crystallization rate is faster for the film drawn at a higher strain rate, suggesting that the rate of nonisothermal cold crystallization is influenced by the strain rate. The crystallization process for most of the PTT films finishes within
