Comparison of crystallization kinetics of miscible blends of syndiotactic polystyrene with atactic polysterene or poly.
Syndiotactic polystyrene (s-PS) is one notable product of stereospecific polymerization (1-3) with relatively high [T.sub.g] and [T.sub.m]. Optimization of structure-property relationships requires a fundamental understanding of the crystallization kinetics of this new type of stereoregular polymer, whose potential applications as an engineering thermoplastic have been extensively examined. However, some characteristic drawbacks need to be overcome. Owing to relatively fast crystallization of syndiotactic s-PS, melt processed s-PS can easily crack, and the properties of s-PS can be adversely affected if processing is not properly controlled. As neat s-PS is inherently brittle, applications of s-PS usually require miscible or compatible blends to balance the service and processing properties.
Polymorphism of s-PS has been widely studied and reported, and it is generally accepted that there exist two major types of crystals, [Alpha] and [Beta] forms, in most melt-processed s-PS (4, 5). These two types ([Alpha] and [Beta]) can be further sub-classified into [Alpha][prime], [Alpha][double prime], and [Beta][prime], [Beta][prime], respectively. Possibility of alteration of crystal forms induced by crystallization of melt s-PS in flow fields has been examined, in which injection-molded s-PS coupons were demonstrated to also exhibit these two same types of crystals, thus indicating little flow/orientation effects on alteration of crystal types (6), However, solvents have been shown to induce some alteration or generation of new types of crystals. It has been shown that s-PS in liquid solutions (with organic solvents) can exhibit two crystal forms ([Gamma] and [Delta]), in addition to two common types: [Alpha] and [Beta]. As miscible polymer blends can be viewed as one type of polymer solution in the solid form, it would be interesting to investigate whether s-PS in blends with miscible polymers might contain different crystal forms. Furthermore, if there is a difference, how may this microscopic factor of difference in crystal forms affect macroscopic crystallization kinetic behavior? An earlier report from our laboratory (7) revealed evidence showing that both [Alpha]- and [Beta]-crystal forms are found in neat s-PS, but only the [Beta]-form crystal exists in s-PS blends with a-PS or PPO. The behavior of s-PS chains in miscible states might be influenced by being in contact with other polymer chains. The amorphous polystyrene (a-PS) has long been known to form miscible blends with PPO (8). More recently, s-PS ([T.sub.g] = 95 [degrees] C). like a-PS, has been proven to be miscible with PPO ([T.sub.g] = 210 [degrees] C) (9). Subsequently, the polymorphism behavior of s-PS in its blend with PPO was investigated (10). Polystyrenes of different tacticity (atactic, isotactic, syndiotactic) can also form miscible pairs upon blending. For example, s-PS/a-PS blend forms a miscible system based on thermal transition, morphology, and melting point measurements, etc. (11). Using diffusion measurements of syndiotactic PS and deuterated atactic PS, Ermer et al. (12) concluded that there exists at least partial miscibility between a-PS and s-PS in the amorphous regions. It must be mentioned that in their study they did not claim positive identification of miscibility in s-PS/a-PS. In one of our earlier studies using the X-ray diffraction, the relative fractions of [Alpha] and [Beta] forms were found to be different if neat s-PS was annealed at different temperatures (7). Guerra, et al. (13) have reported that blending methods (melt vs. solution blending) may lead to s-PS with different polymorphisms. A difference in the crystal forms existing in s-PS vs. s-PS blend systems may not be a necessary condition for alteration of their crystallization behavior; however, it would be interesting to investigate the crystallization kinetics of s-PS polymer in the presence of miscible components.
Although crystallization of s-PS may have been extensively examined (14), s-PS in its miscible blends has been less studied. Thus, this study focused on experimental analysis of s-PS with miscible polymers (a-PS or PPO) as a model system. Understanding the crystallization kinetics is critical for providing information for optimal processing that might lead to optimal crystalline morphological structure and macroscopic properties of s-PS in blend forms. By using isothermal DSC analysis on the kinetics of melt crystallization of syndiotactic polystyrene and its miscible blends, we hoped to establish relationships for describing the crystallization behavior of s-PS in blends with two model polymers that have been known to be miscible with s-PS. For these purposes, crystallization kinetics of neat s-PS in comparison with the s-PS component in miscible polymer blends (solid solutions) with amorphous a-PS or PPO, respectively, were studied.
Materials and Preparation
Semicrystalline syndiotactic polystyrene (s-PS) was obtained as a courtesy research material from Idemitsu Petrochemical Co., Ltd. (Japan) with [M.sub.w] = 241,000 g/mol and PI ([M.sub.w]/[M.sub.n]) = 2.31. Two polymers that have been known to form miscible blends with s-PS were used in this study to investigate the melting behavior of s-PS in the miscible melt state with another component. Amorphous poly(2,6-dimethyl-p-phenylene oxide) (PPO) was purchased from a specialty-polymers supplier, Polysciences, Inc. (USA). Several different grades of atactic polystyrenes (a-PS) were used in this study for comparison. Two atactic polystyrenes (a-PS) were purchased from Polysciences, Inc. with [M.sub.w] = 125,000-250,000 g/mol and [M.sub.w] = 50,000 g/mol, Another a-PS of broader [M.sub.w] distribution was also used, (Chi-Mei, Corp., Taiwan), with GPC MW = 192,000 g/mol, PI = 5.1.
Hot-melt blending was used for preparing most blend samples of s-PS with intended polymer (PPO or a-PS). Prior to melt-blending, the polymers were first pulverized into fine powder, dried, and then pre-mixed. The mixed polymer powder was then placed into the miniature chamber (a small cylindrical cavity [approximately] 2-gram capacity) inside a laboratory-designed aluminum mold preheated to 320 [degrees] C. Temperature control was provided by placing the mold/mixing chamber assembly on a hot plate with thermostatted heating (set at 320 [degrees] C). Blending of the polymers (small quantities, [approximately] 1 g) could be easily accomplished by manually hand-stirring the mixtures of polymer melts inside the chamber. During melt-blending, a continuous purge of dry nitrogen was maintained to provide an inert-gas blanket on the mixing chamber in order to minimize possible thermal degradation/oxidation at high temperatures.
Apparatus and Procedures
Differential Scanning Calorimetry. The exotherms associated with crystallization of the blended samples were measured with a differential scanning calorimeter (DSC-7, Perkin-Elmer) equipped with an intracooler for quenching and cooling. In addition, the newly improved capability of the DSC instrument to heat and cool the polymer samples at extremely fast rates have helped to enhance the accuracy of the data by reaching and equilibrating at the targeted isothermal temperature quickly. Prior to DSC runs, the temperature and heat of transition of the instrument were calibrated with indium and zinc standards. For determining the transition temperatures and enthalpy of melting peaks, a dynamic heating rate of 10 [degrees] C/min was used. During thermal annealing or scanning, a continuous nitrogen flow in the DSC sample cell was maintained to ensure minimal sample degradation.
Procedure of Isothermal Experiments
The melt crystallization kinetics of s-PS sample and its blends were investigated by observing heat flow as a function of time at an isothermal temperature for time long enough for completion of the thermal process. Regarding the isothermal method for crystallization exotherm measurements, the procedures are as following. The sample was first melted by healing to 320 [degrees] C, held there for 5 min to eliminate residual crystals and for uniformizing the melt treatment for all samples. Then it was rapidly quenched (at -320 [degrees] C/min) in the DSC sample cells quickly to the designated isothermal temperatures to allow the polymer melt to start to crystallize at a temperature between 238 [degrees] C and 250 [degrees] C. As the temperature drop of quenching ([T.sub.melt] to [T.sub.c]) was not a big gap, any non-isothermal crystallization effects during the dynamic cooling transient could be ignored. Data acquisition was set to begin immediately as soon as temperature stability was reached at the designated isothermal temperatures. A continuous nitrogen flow in the DSC sample cell was maintained to ensure minimal sample degradation/oxidation. Isothermal DSC experiments at temperatures close to the maximum rate of crystallization were not attempted owing to greater inaccuracy in data acquisition. Therefore, isothermal melt-crystallization experiments were performed at several selected temperatures within an optimal window slightly beyond or below the maximum rate of crystallization.
For cold-crystallization exotherm measurements, the procedures were similar. Characterization of cold-crystallization kinetics was performed only on neat s-PS. Transparent amorphous film samples of neat s-PS was first prepared by quenching a compression-mold film into ice water. Quenching the specimens directly into ice water was found to be more efficient for producing truly amorphous s-PS samples for cold-crystallization experiments. The amorphous, glassy film sample was then transferred into a DSC sample pan, heated quickly to above [T.sub.g] in the DSC to the designated isothermal temperatures (125 [degrees] - 140 [degrees] C) to allow the glassy polymer to start to cold-crystallize.
Analysis of Crystallization Kinetics
The classical Avrami equation was used for analysis (15):
1 - [X.sub.t] = exp[-k [t.sup.n]] (1)
where k is the crystallization rate constant depending on nucleation and growth rates; and n is the Avrami crystallization exponent depending on the nature of nucleation and growth geometry of the crystals. [X.sub.t] is the relative crystallinity of the polymer sample at time t, defined as the ratio of the cumulative area under the exotherm peak up to t, [Delta][H.sub.c] (t) with respect to the total peak area of the crystallization exotherm, [Delta][H.sub.c], in the DSC curves.
[X.sub.t] = [Delta][H.sub.c] (t)/[Delta][H.sub.c] (2)
For purposes of accuracy, only the data of [X.sub.t] between 10 and 50% were used for analysis. Experimentally, the ideal design of small thermal mass in the DSC cells allowed quick equilibration and precise temperature control. The accuracy of the experiments depended on temperature control and quick equilibration at the designated isothermal temperature. Reproducibility of the results obtained from isothermal crystallization experiments will be discussed.
RESULTS AND DISCUSSION
Cold Crystallization. Figure 1 shows the Avrami plot for the cold crystallization of s-PS at 125, 130, 135, 138, and 140 [degrees] C. The maximum rate of cold crystallization is seen to occur at about 145 [degrees] C [+ or -] 5 [degrees] C (i.e., 50 [degrees] C above [T.sub.g]). Analysis of the cold crystallization of s-PS was based on the data obtained between 125 [degrees] C and 140 [degrees] C as the experiments and data acquisition near the temperature of the maximum rate of crystallization were found to be difficult and prone to error. It has been suggested that for a series of isothermal crystallization experiments, the Avrami exponent n is temperature-independent (16). Slight deviations of the values of n for different temperatures may be regarded as experimental errors. Thus, an average was taken in this analysis for estimating the values of n. The slopes yielded the values of the exponent n, and other kinetics parameters are listed in Table 1. Within an experimental error bound, the plots in this figure possess approximately the same slopes, suggesting a constant exponent for all temperatures investigated. If the average is taken as a representative value, the exponent is found to be n = 2.4 [+ or -] 0.1.
Melt Crystallization of s-PS. Figure 2 shows the Avrami plot for the melt crystallization of neat s-PS sample at 238 [degrees] C-252 [degrees] C. Note that the maximum rate of melt crystallization occurred at about 235 [degrees] C or below for neat s-PS. Data obtained at isothermal temperatures too near the temperature of the maximum crystallization rate was found to be prone to error. Analysis of the melt crystallization of s-PS was based on the data obtained with sufficient accuracy and reproducibility. The slopes yielded the values of exponent n, and other kinetic parameters are listed in Table 2. The plots in this Figure possess approximately the same slopes for all isothermal temperatures investigated. If the average is taken as a representative value, the Avrami exponent is 2.8. Wesson (17) has reported an Avrami exponent of 2.9-3.0 for s-PS ([M.sub.w] = 348,000, 562,000, and 805,000 g/mol) using nonisothermal DSC analysis. The exponent n = 2.8 obtained from the isothermal experiments in this study is slightly different but compares well with the value obtained from Wesson's nonisothermal DSC experiments, This value suggests that the crystal growth in melt-crystallized s-PS is nearly of homogeneous nucleation and that the growth pattern of the spherulites is 3-D spherical. In comparison, the Avrami exponent for the cold crystallization of s-PS was found to be n = 2.4. This value suggests that the crystal growth in cold-crystallized s-PS is of more heterogeneous nucleation, but with a similar growth pattern of 3-D spherical geometry. Optical microscopy results confirmed that the spherulites of cold-crystallized s-PS were similar to those of melt-crystallized s-PS. In general, cold crystallization exhibits a lower value of Avrami exponent than that of melt-crystallization of the same polymer.
Table 1. Kinetic Parameters of Cold Crystallization of s-PS. [T.sub.c]([degrees] C) n k ([min.sup.-n]) 125 2.4 0.00085 130 2.5 0.0041 135 2.3 0.0474 138 2.4 0.1297 140 2.2 0.3216 Avg. n = 2.4. Table 2. Kinetics Parameters of Melt Crystallization of Neat s-PS. [T.sub.c]([degrees] C) n k ([min.sup.-n]) 238 2.9 4.56 242 2.8 2.42 245 2.7 0.41 248 2.8 0.08 252 2.7 0.04 Avg. n = 2.8 [+ or -] 0.1.
Melt Crystallization of Blends. Figures 3 and 4 show the Avrami plots for the s-PS/a-PS blend samples of two compositions: 75/25 and 25/75, respectively, which were melt-crystallized at 238 [degrees] C-252 [degrees] C. The Avrami plots show that each of these two compositions consistently exhibits the same slopes. A comparison of these two figures shows that the average value of n decreases with decreasing fraction of s-PS (or increasing a-PS) in the blends. For the s-PS/a-PS (75/25) blend, the average exponent was 2.7, which is about the same as the neat s-PS, while the average exponent for the 25/75 composition was about n = 2.2. As the geometry of the spherulites remained roughly similar for all compositions, the decreasing exponent suggested that the nucleation mechanism probably became more heterogeneous with increasing contents of a-PS (i.e., decreasing s-PS) in the blends.
To evaluate effects of molecular weights of s-PS on kinetic behavior of s-PS, blends samples of s-PS/a-PS of different grades of a-PS ([M.sub.w] = 50,000, and 125,000-250,000 g/mol) were prepared and investigated. Figure 5 (Diagrams A and B) shows the Avrami plots for the s-PS/a-PS (50/50) blend samples containing two different grades of a-PS. The respective sample was melt-crystallized at several selected isothermal temperatures in the range of 238 [degrees] C-252 [degrees] C. The average value of the Avrami exponent remains about the same for these blend samples with the same composition but different grades of a-PS. For both samples, the 50/50 s-PS/a-PS blend exhibited an average exponent of 2.5, which is roughly in between those for the 75/25 and 25/75 compositions. The calculated values of n and k for all s-PS/a-PS composition are listed in Table 3.
As a direct visual aid for monitoring the progress of crystallization, Fig. 6 shows the relative crystallinity ([X.sub.t]) for s-PS/PPO blend (75/25) as a function of time at 238 [degrees] C, 242 [degrees] C, 245 [degrees] C, 250 [degrees] C, and 252 [degrees] C. The plot shows that the maximum crystallization rate of this s-PS/PPO blend occurs at near 238 [degrees] C, which is similar to the behavior of neat s-PS. From the plot, the crystallization half time ([t.sub.1/2]) could be easily read. Similar plots could be generated for other blend compositions; however, for brevity, they are not shown as they are similar in shapes and differ only in the location of crystallization half times. In general, at a specific isothermal temperature the crystallization half time increases with increasing PPO content in the s-PS/PPO blends, indicating that at higher PPO contents the s-PS component in the miscible blends would crystallize more slowly.
Table 3. Kinetic Parameters of Isothermal Melt Crystallization of Miscible s-PS/a-PS Blend. Composition [T.sub.c]([degrees] C) n k ([min.sup.-n]) 75/25 238 2.7 6.57 242 2.9 2.25 245 2.8 0.73 246 2.8 0.24 250 2.9 0.1 252 2.5 0.04 50/50 238 2.4 1.04 242 2.5 0.4 245 2.5 0.13 248 2.4 0.04 252 2.4 0.01 25/75 238 1.9 0.62 240 2.4 0.38 242 2.4 0.22 245 2.6 0.05 248 2.0 0.06 250 2.0 0.03
Figures 7 and 8 show the Avrami plots for the melt crystallization of s-PS/PPO of two compositions: 80/20 and 75/25, respectively, which were isothermally crystallized at 238 [degrees] C-252 [degrees] C. The straight-line plots in each composition possess similar slopes. For both compositions, the average value of n was found to be nearly 2.2 and does not vary much between different blend compositions. This value of the exponent for the s-PS/PPO blends is significantly lower than that for neat s-PS, indicating that the mechanism of crystallization might differ. Crystallization behavior of other compositions were also investigated. For details, the calculated values of n and k for all temperatures are listed in Table 4.
For comparison with different blending methods, one composition of s-PS/PPO (75/25) blend was prepared by using solvent (o-dichlorobenzene] blending. Figure 9 shows the Avrami plots for the solvent-blended s-PS/PPO (75/25) sample isothermally crystallized at 238 [degrees] C-252 [degrees] C. The nearly parallel straight lines in the Avrami plots demonstrated similar slopes for all temperatures. The average value of n for this solvent-blended sample was found to be nearly the same as that for the melt-blended s-PS/PPO of the same composition, indicating that the different blending method did not influence the result.
Table 4. Kinetic Parameters of Melt Crystallization of Miscible s-PS/PPO Blends. Composition [T.sub.c]([degrees] C) n k ([min.sup.-n]) 75/25 245 2.4 2.95 248 2.3 0.08 90/10 242 2.1 2.53 245 2.2 1.44 248 2.4 0.47 80/20 248 2.2 0.08 250 2.2 0.03 252 2.4 0.004 75/25 238 2.2 0.212 242 2.3 0.06 245 2.4 0.022 248 2.5 0.0056 252 2.3 0.0027 70/30 245 2.1 0.61 248 2.3 0.17 Avg. n = 2.3 for all compositions of s-PS/PPO blends.
In this study, all the samples were first melted by heating to 320 [degrees] C for 5 min before quenching and initiation of crystallization at a designated temperature. This temperature of melt holding (320 [degrees] C, 5 min) was chosen because it was regarded as high enough to eliminate most residual crystals while minimizing any thermal degradation. In order to investigate effects of melt temperature on the crystallization and/or nucleation of s-PS and its miscible blends, additional melt crystallization experiments were conducted by holding at several different melt temperatures between 290 and 340 [degrees] C. Several blend compositions were examined. As an example, results for one composition is presented here. Figure 10 shows the relative crystallinity ([X.sub.t]) for s-PS/PPO blend (75/25) as a function of time at an isothermal temperature of 245 [degrees] C. The samples of this blend composition were melted and held at 290, 300, 310, 320, 330, and 340 [degrees] C, respectively, before isothermal crystallization experiments were started at 245 [degrees] C. Apparently, if melted and held at higher temperatures, the initiation of crystallization is delayed to a later time. Other blend compositions were also examined; however, the plots are not shown as they are similar to this figure in behavior. By incorporation of the "incubation" time as an additional parameter, the original Avrami equation is given by (15, 18):
[X.sub.t] = 1 - exp[-k[(t - [Tau]).sup.n]] (3)
The incubation time ([Tau]) apparently is a function of melt temperature. By holding the polymer melts at the same temperature for same duration, the samples were uniformized and the effect of melt temperature could be ignored.
In summary, the crystallization kinetics behavior of miscible blends of s-PS/a-PS and s-PS/PPO has been compared to that of neat s-PS polymer. The results suggest that the crystal growth in melt-crystallized s-PS is nearly of homogeneous nucleation and that the growth pattern of the spherulites is 3-D spherical. In comparison, the Avrami exponent for the cold crystallization of s-PS was found to be lower at n = 2.4. This value suggests that the nucleation in cold-crystallized s-PS is less homogeneous and more heterogeneous than that in melt crystallization. The s-PS component in its miscible blends was found to crystallize with similar behavior to that of neat s-PS, but with lower Avrami exponents. Blending s-PS with a-PS led to slight but significant changes in the exponent. For the s-PS/a-PS (75/25) blend, the average exponent was 2.7, which is about the same as the neat s-PS. With increasing a-PS in the blends, the average exponent further decreases. For 50/50 composition, the exponent is n = 2.5. For the s-PS/a-PS (25/75) blend composition, the exponent is about n = 2.3. By comparison, the effect of blending s-PS with PPO was found to be more evident. At PPO contents of 20 or higher, the s-PS/PPO blends exhibited an average exponent of 2.3. As the geometry of the spherulites remained roughly similar, the decreasing exponent suggests that the nucleation mechanism is likely more heterogeneous with Increasing contents of amorphous polymer in the miscible blends.
Solvent blended vs. melt blended s-PS/PPO samples were investigated and the results indicated that the different blending method did not influence the bulk crystallization behavior. This study also investigated the effect of the maximum temperature at which the polymer samples were melted on nucleation and/or crystal growth. The result indicated that higher temperatures led to longer "incubation" times. That is, if the polymer samples were exposed to higher temperatures for melting, it would have taken longer times for the nuclei to be initiated and crystallization to be started.
The authors would like to thank Mr. M. Kuramoto of Idemitsu Petrochemical Co. Ltd. (Japan), who supplied the s-PS material for this study. This work was financially supported by a grant from National Science Council (NSC) of Taiwan, NSC87CPCE006017.
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|Title Annotation:||1,4-dimethyl-p-phenylene oxide|
|Author:||Wu, Fu Sun; Woo, E.M.|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 1999|
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