Miscibility, crystallization and morphologies of syndiotactic polystyrene blends with isotactic polystyrene and with atactic polystyrene

Two-component blends of differing polystyrene (PS), one syndiotactic (sPS) and the other isotactic (iPS) or atactic (aPS), were discussed. The phase behavior, crystallization and microstructure of binary polystyrene blends of sPS/iPS and sPS/aPS with a specific composition of 5/5 weight ratio were investigated using optical microscopy (OM), differential scanning calorimetry, wide-angle X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM). Based on the kinetics of enthalpy recovery, complete miscibility was found for the sPS/aPS blends where a single recovery peak was obtained, whereas phase separation was concluded for the sPS/iPS blends due to the presence of an additional recovery shoulder indicating the heterogeneity in the molten state. These findings were consistent with OM and SEM observations; sPS/iPS exhibits the dual interconnectivity of phase-separated phases resulting from spinodal decomposition.Both iPS and aPS have the same influence on the sPS crystal structure, i.e., dominant β-form sPS and mixed α-/β-form sPS obtained for melt-crystallization at high and low temperatures respectively, but imperfect α-form sPS developed when cold-crystallized at 175 °C. Co-crystallization of iPS and sPS into the common lattice was not observed regardless the thermal treatments, either cold or melt crystallization. Due to its slow process, crystallization of iPS was found to commence always after the completion of sPS crystallization in one-step crystallization kinetics. Segregation of rejected iPS component during sPS crystallization was extensively observed from TEM and SEM images which showed iPS pockets located between sPS lamellar stacks within spherulites, leading to the interfibrillar segregation, which was similar with that observed in the sPS/aPS blends. The addition of iPS (or aPS) component will reduce the overall crystallization rate of the sPS component and the retardation of crystal growth rates can be simply accounted by a dilution effect, keeping the surface nucleation intact. The phase-separated structure in the sPS/iPS blend shows a negligible effect on sPS crystallization and the signature of phase separation disappears after sPS crystallization. Depending on the relative dimensions of the segregated domains and iPS lamellar nucleus, subsequent crystallization of iPS can proceed to result in a crystalline/crystalline blend, or be inhibited to give a crystalline/amorphous blend morphology similar with that of sPS/aPS blends.     

Miscible blends of syndiotactic polystyrene and atactic polystyrene. Part 2. Depolarized light scattering studies and crystal growth rates

Crystallization kinetics and morphology in miscible blends of syndiotactic polystyrene (sPS) and atactic postyrene (aPS) have been investigated by means of time-resolved depolarized light scattering (DPLS), polarized optical microscopy (POM) and scanning electron microscopy (SEM). Two different weight-average molecular weight of aPS, i.e. M_w=100k and 4.3k, were used to prepare the blends and denoted sPS/aPS(H) and sPS/aPS(M), respectively. Owing to a dilution effect, addition of aPS reduces the crystal growth rate and the overall crystallization rate of sPS; the reduction is more significant in sPS/aPS(M) of which a depression of equilibrium melting temperature is found due to the enhanced mixing entropy. Linear crystal growth is always observed in sPS/aPS(H) at the temperatures studied (240-269 °C) and results in an interfibrillar segregation morphology revealed by SEM, whereas sPS/aPS(M) with high aPS content exhibits non-linear growth behavior at low supercooling and gives an interspherulitic segregation morphology. Based on the Lauritzen-Hoffman theory, the fold surface free energies (σ_e) of sPS lamellae derived from DPLS and POM are in fair agreement, being 15.1 erg/cm² from the former and 12.6 erg/cm² from the latter. The peculiarly low values of σ_e and the derived work of chain folding are discussed briefly. On addition of aPS, the lateral surface free energy of lamellae remains intact (9.9 erg/cm²) regardless of aPS molecular weight used, which is ascribed to the absence of specific interaction between sPS and aPS components. Moreover, it seems that the activation energy for sPS chains to diffuse from the miscible melt to the crystal growth front is slightly increased in sPS/aPS(M), plausibly attributable to the extra energy required for the demixing process.     

Miscible blends of syndiotactic polystyrene and atactic polystyrene. Part 1. Lamellar morphologies and diluent segregation

Lamellar morphologies of melt-crystallized blends of syndiotactic polystyrene (sPS, weight-average molecular weight ) and atactic polystyrene (aPS, M_w=100k) have been investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). sPS/aPS blends with various compositions were prepared and crystallized isothermally at 250 °C prior to morphological studies. Due to the proximity in the densities of the crystal and amorphous phases, a weak SAXS reflection associated with lamellar microstructure was obtained at room temperature. In addition, strong diffuse scattering at low scattering vectors was evidently observed and its appearance may obscure the intensity maximum associated with the lamellar features, leading to the difficulties in determining the microstructure of the blends. To enhance the density contrast, SAXS intensities at an elevated temperature of 150 °C were measured as well to deduce the morphological results with better precision. Based on the Debye-Bueche theory, the intensities of the diffuse scattering were estimated and subtracted from the observed intensities to obtain the scattering contribution exclusively from the lamellar microstructure. Morphological parameters of the sPS/aPS blends were derived from the one-dimensional correlation function. On addition of aPS, no significant changes in the lamellar thickness have been found and the derived lamellar thicknesses are in good agreement with TEM measurements. Segregation of rejected aPS components during sPS crystallization was evidently observed from TEM images which showed aPS pockets located between sPS lamellar stacks and distributed uniformly in the bulk samples, leading to the interfibrillar segregation.     

Isothermal crystallization, melting behavior and crystalline morphology of syndiotactic polystyrene blends with highly-impact polystyrene

Syndiotactic polystyrene (sPS) blends with highly-impact polystyrene (HIPS) were prepared with a twin-screw extruder. Isothermal crystallization, melting behavior and crystalline morphology of sPS in sPS/HIPS blends were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). Experimental results indicated that the isothermal crystallization behavior of sPS in its blends not only depended on the melting temperature and crystallization temperature, but also on the HIPS content. Addition of HIPS restricted the crystallization of sPS melted at 320 °C. For sPS melted at 280 °C, addition of low HIPS content (10 wt% and 30 wt%) facilitated the crystallization of sPS and the formation of more content of α-crystal. However, addition of high HIPS content (50 wt% and 70 wt%) restricted the crystallization of sPS and facilitated the formation of β-crystal. More content of β-crystal was formed with increase of the melting and crystallization temperature. However, α-crystal could be obtained at low crystallization temperature for the specimens melted at high temperature. Addition of high HIPS content resulted in the formation of sPS spherulites with less perfection.     

Thermal and structural properties of blends of isotactic with atactic polystyrene

Blends of isotactic polystyrene (iPS) with non-crystallizable atactic polystyrene (aPS) were studied by differential scanning calorimetry and small angle X-ray scattering. The iPS/aPS blends, prepared by solution casting, were found to be miscible in the melt over the entire composition range. Both quenched amorphous and semicrystalline blends exhibit a single, composition-dependent glass transition temperature, depressed from that of either of the homopolymer components. Addition of aPS causes a decrease in crystallinity and in the rigid amorphous fraction, and suppression of the reorganization/recrystallization of iPS during thermal scanning: only one melting peak is observed for blends with larger aPS content. Formation and devitrification of the rigid amorphous fraction of iPS are also affected by aPS addition. The annealing peak, which is due to the relaxation of rigid amorphous fraction in parallel with melting of a tiny amount of crystals, is retarded with an increase of the composition of aPS, resulting in the slow devitrification of RAF in parallel with the melting of large amount of crystals. X-ray scattering shows that the long period in the iPS/aPS blends is greater than in the iPS homopolymer, and long period increases slightly as aPS content increases. Comparison of the volume fraction of phase 1 with the volume fraction crystallinity from DSC suggests that more and more amorphous phase is rejected outside the lamellar stacks as aPS content increases. The effect of aPS addition is to reduce the confinement of the amorphous phase chains. The cooperativity length, ξ_A, which is calculated from thermal analysis of the T_g region, increases with aPS addition. The interlamellar and extra-lamellar amorphous chains both contribute to the glass transition relaxation process.     

Pre‐melting temperature effect on the isothermal melt crystallization and multiple melting behavior of syndiotactic polystyrene

This work examined how pre‐melting temperature (T_max) affects the isothermal melt crystallization kinetics, the resulting melting behavior and crystal structure of syndiotactic polystyrene (sPS) by using differential scanning calorimetry (DSC), polarized light microscopy (PLM) and the wide angle X‐ray diffraction (WAXD) technique. Experimental results indicated that raising T_max decreased the nucleation rate and the crystal growth rate of sPS. The Avrami equation was also used to analyze the overall crystallization kinetics. The Avrami exponent n and rate constant K were determined for different T_max specimens at various crystallization temperatures (T_c's). Our results indicated that the nucleation type of sPS is T_max and T_c dependent as well. Evaluation of the activation energy for the isothermal crystallization processes revealed that it increases from 375 kJmol^?1 to 485 kjmol ^?1 with an increase of T_max. From the melting behavior study, we believe that the T_max and T_c‐dependent multiple melting peaks are associated with different polymorphs as well as recrystallized crystals formed during heating scans. Moreover, the percentage content of α form in the crystals formed under different crystallization conditions was estimated through WAXD experiments.     

Simultaneous presence of positive and negative spherulites in syndiotactic polystyrene and its blends with atactic polystyrene

Crystal growth rates of syndiotactic polystyrene (sPS) and its blends with atactic polystyrene (aPS) at various temperatures (T_c) were measured using a polarized optical microscope (POM). In addition to the positively birefringent spherulites and axilites (P-spherulites and P-axilites) which are predominantly observed, small population of negatively birefringent spherulites (N-spherulites) is also detected in the neat sPS as well as in the sPS/aPS blends at a given T_c. Both P-spherulites and P-axilites possess a similar growth rate, whereas a smaller growth rate is found for N-spherulites at all T_c and samples investigated. Melting behavior of individual P- and N-spherulites was feasibly traced using hot-stage heating and a highly sensitive CCD through the decay of transmitted light intensity under cross-polars. Both P- and N-spherulites demonstrate exactly the same melting behavior under POM, which well corresponds to the differential scanning calorimetry measurements, suggesting no difference in lamellar thickness distribution or crystal perfection within P- and N-spherulites. Lamellar morphologies within spherulites were extensively investigated using transmission electron microscopy (TEM) as well as scanning electron microscopy (SEM). Results obtained from TEM and SEM show that the lamellar stacks within P-spherulites grow radially, whereas those within N-spherulites are packed relatively tangentially. The growth of P-spherulites is associated with the gradual increase of lamellae' lateral dimensions which follows the conventional theory of growth mechanism. However, the measured growth rate of N-spherulites is relevant to the gradual deposition of new lamellar nuclei adjacent to the fold surfaces of already-existing lamellar stacks. The difference in measured growth rate between P- and N-spherulites is attributed to the different energy barrier required to develop stable nuclei. Based on the exhaustive TEM and SEM observations, plausible origin of N-spherulites is provided and discussed as well.     

Effect of the crystallization conditions on the phase behavior of syndiotactic polystyrene/poly(phenylene oxide) blends

Syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends, miscible in the melt state, were crystallized from the melt and the quenched state at different temperatures. The effect of the crystallization temperature on the phase behavior of the blends and the polymorphic changes in sPS was investigated by dynamic mechanical analysis (DMA), wide‐angle X‐ray diffraction (WAXD), and density measurements. In most blends, the crystallization of sPS induced segregation into two homogeneous amorphous phases of different compositions. The temperatures of the DMA relaxations of the neat homopolymers and crystallized blends were fit by the Gordon–Taylor relation to calculate the compositions of these phases. In melt‐crystallized blends, with slower crystallization, the major amorphous phase became sPS‐rich, whereas the minor phase became PPO‐rich. These major and minor amorphous phases could be tentatively assigned to interfibrillar and interlamellar regions, respectively. In cold‐crystallized blends, slower crystallization decreased the sPS concentration in both phases, and the scale of segregation was much smaller. WAXD studies and density measurements indicated a complex polymorphic behavior of sPS after it was blended with PPO. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1975–1983, 2003     

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.     

Effect of HIPS on polymorphism,melting, and crystallization behavior of sPS crystallized dynamically from melting state

Syndiotactic polystyrene/highly‐impact polystyrene (sPS/HIPS) blends were prepared with a twin‐screw extruder. Differential scanning calorimetry and wide angle X‐ray diffractometry were used to investigate the effect of the maximal melting temperature, the content of HIPS and cooling rates on the melting and crystallization behavior and crystal forms of sPS. The experimental results indicated that the addition of low content of HIPS induced the formation of more α‐crystal, whereas the addition of high content of HIPS favored the formation of β‐crystal for sPS/HIPS blends crystallized dynamically from low melting temperature. Both sPS and its blends produced only β‐crystal as crystallized from high melting temperature. The crystallization temperatures of sPS and its blends decreased as the melting temperature increased, favoring the formation of β‐crystal. Higher temperature of sPS crystallization favored the formation of more content of α‐crystal while lower temperature of sPS crystallization produced more content of β‐crystal. Cooling rates showed no significant effect on the crystal form of sPS and its blends, but influenced the melting behavior of both sPS and its bends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3353–3361, 2007     

Miscibility, thermal properties and polymorphism of syndiotactic polystyrene/poly(styrene-co-α-methyl styrene) blends

This work examined the miscibility, crystallization kinetics, melting behavior and crystal structure of syndiotactic polystyrene (sPS)/poly(styrene-co-α-methyl styrene) blends. Differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction technique were used to approach the goals. The single composition-dependent T_gs of the blends and the melting temperature (T_m) depression of sPS in the blends indicated the miscible characteristic of the blend system at all compositions. Furthermore, the T_gs of the blends could be predicted by either of the Gordon–Taylor equation (with K=0.99) or the Fox equation with a slightly higher deviation. The dynamic and isothermal crystallization abilities of sPS were hindered with the incorporation of the miscible copolymer. Complex melting behavior was observed for melt-crystallized pure sPS and its blends as well. Nevertheless, the blends showed relatively simpler melting curves. Comparing with melt-crystallized samples, the cold-crystallized samples exhibited simpler melting behavior. The equilibrium melting temperature (T_m⁰) of β form sPS crystal determined from the conventional extrapolative method is 295.2 °C. The Flory–Huggins interaction parameter, χ, of the blends was estimated to be −0.27. The crystal morphology of sPS was disturbed in the blends. Only underdeveloped granular-like crystalline superstructure of sPS exhibited in cold-crystallized blends. Moreover, the existence of the copolymer in the blends apparently reduced the possibility of forming the less stable α form sPS crystals.     

sPS/PBA-aPS共混物的结晶与熔融行为

通过熔融共混法制备了间规立构聚苯乙烯/聚丙烯酸丁酯无规立构聚苯乙烯核壳乳胶粒子（sPS/PBA-aPS）共混物,采用差示扫描量热仪、X射线衍射仪和偏光显微镜研究了PBA-aPS对sPS结晶性能、结晶形态的影响,以及共混物在不同降温速率下、等温结晶条件下所得试样的熔融行为。结果表明, PBA-aPS的引入对sPS的结晶起阻碍作用,sPS及其共混物存在明显熔融重结晶再熔融现象,sPS平衡熔点为293.2 ℃,共混物的平衡熔点随PBA-aPS含量增加而降低,sPS形成β型大球晶完善性变差,sPS/PBA-aPS共混物的冲击强度明显提高,sPS/PBA-aPS质量比为80:20时,冲击强度提高了117 %。     

Morphology,structure, and photovoltaic properties of poly(3‐hexylthiophene) and [6,6]‐phenyl‐C61‐butyric acid methyl ester‐based ternary blends doping with polystyrene of different tacticities

The influence of the polystyrene of different tacticities on the morphology, phase structure, and photovoltaic properties of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blend has been extensively investigated. The atactic polystyrene (aPS) immiscible with P3HT tended to form the phase‐separated and columnar structure at low aPS weight ratio. Besides, the aPS could migrate to the surface of the films with PCBM phase distributing in the interfaces between P3HT and aPS domains at high aPS weight ratio of 75 wt %. The syndiotactic polystyrene (sPS) immiscible with P3HT could induce the crystallization of P3HT at low weight ratio of 3 wt %. The device based on aPS/P3HT/PCBM ternary blend showed of power conversion efficiency (PCE) of 1.2% even at aPS weight ratio of 50 wt %. However, the device based on sPS/P3HT/PCBM exhibited a sharp decrease in PCE value from 2.3% to 0.6% at sPS weight ratio of 3 wt %, due to the change in film morphology. The performance of the solar cell is believed to be determined by the morphology and phase structure of the ternary blends as revealed by the atomic force microscopy and UV‐vis spectra analysis. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41823.     

Process analysis of phase transformation of α to β-form crystal of syndiotactic polystyrene investigated in supercritical CO2

Mechanism of solid phase transformation of α to β form crystal of syndiotactic polystyrene (sPS) was investigated in supercritical CO₂. The phase transformation occurred in the original pure α and mixed (α+β) form sPS in supercritical CO₂ was traced as a function of temperature and pressure by means of wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC). At appropriate temperature and pressure, sPS underwent solid phase transitions from α to β form. Higher temperature or higher pressure favored this transformation. Compared to the original pure α form sample, the original β form crystal in the mixed (α+β) form sample acted as the nucleus of β form crystal, so that reduced the transition temperature and pressure.     

Thermo-mechanical properties of the blend syndiotactic/atactic polystyrene after crystallization of the syndiotactic polystyrene

The thermo-mechanical properties of the blend syndiotactic polystyrene (sPS)/ atactic polystyrene (aPS) are characterized by studying the concentration depending softening behavior with thermo-mechanical analysis (TMA) and the temperature depending Young's modulus for different concentrations with dynamic mechanical analysis (DMA).     

Effects of maleated syndiotactic polystyrene on the morphology,mechanical properties,and crystallization behavior of syndiotactic polystyrene/polyamide 6 blends

The compatibilization of syndiotactic polystyrene (sPS)/polyamide 6 (PA‐6) blends with maleic anhydride grafted syndiotactic polystyrene (sPS‐g‐MA) as a reactive compatibilizer was investigated. The sPS/PA‐6 blends were in situ compatibilized by a reaction between the maleic anhydride (MA) of sPS‐g‐MA and the amine end group of PA‐6. The occurrence of the chemical reaction was substantiated by the disappearance of a characteristic MA peak from the Fourier transform infrared spectrum. Morphology observations showed that the size of the dispersed PA‐6 domains was significantly reduced and that the interfacial adhesion was much improved by the addition of sPS‐g‐MA. As a result of reactive compatibilization, the impact strengths of the sPS/PA‐6 blends increased with an increase in the sPS‐g‐MA content. The crystallization behaviors of the blends were affected by the compatibilization effect of sPS‐g‐MA. A single melting peak of sPS in the noncompatibilized blend was gradually split into two peaks as the amount of the compatibilizer increased. A single crystallization peak of PA‐6 in the noncompatibilized blend became two peaks with the addition of 3 wt % sPS‐g‐MA. The new peak was a result of the fractionation crystallization. As the amount of sPS‐g‐MA increased, the intensity of the new peak increased, and the original peak nearly disappeared. Finally, the crystallization peak of PA‐6 disappeared with 20 wt % sPS‐g‐MA in the blend. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2502–2506, 2003     

Effects of crystallization temperature on the polymorphic behavior of syndiotactic polystyrene

The influence of crystallization temperature on formation of the α- and β-form crystals of syndiotactic polystyrene (sPS) was investigated by X-ray diffraction and non-isothermal differential scanning calorimetry analysis. For sPS samples without any thermal history, the crystallization temperature must be the intrinsic factor controlling the formation the α and β-form crystals. Being crystallized at different cooling rate from the melt, sPS forms the β-form crystal until the temperature cooled down to about 230 °C, and α-form crystal can only be obtained when the temperature was below about 230 °C.     

Non‐isothermal crystallization and multiple melting behavior of syndiotactic polystyrene—pre‐melting temperature effects

This work investigated how pre‐melting temperature (T_max) and cooling rate (C) affected the non‐isothermal melt crystallization, melting behavior and crystal structure of syndiotactic polystyrene (sPS) by using differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD) techniques. Experimental results indicated that raising T_max or C decreased the crystallization peak temperature (T_p) and crystallization initiating temperature (T_i). The crystallization kinetics was analyzed through the Ozawa equation. Although the Ozawa exponent n and cooling function K(T) were determined for T_max = 340°C and T_max = 315°C specimens, for T_max = 290°C specimens, the Ozawa equation was not applicable. Activation energies for the non‐isothermal crystallization processes of different T_max specimens were estimated to be approximately 418 kJ/mol. As T_max was raised the nucleation rate of sPS became slower. The multiple melting peaks were associated with different polymorphs as well as recrystallized crystals that formed during heating scans. The percentage content of α polymorph formed in the crystals under various crystallization conditions was estimated through WAXD experiments.     

Plastic deformation and subsequent crystallization of thin films of isotactic/atactic polystyrene (iPS/aPS) blends

Thin films of crystallized and non crystallized isotactic polystyrene (iPS) and its blends with atactic polystyrene (aPS) were deformed below their glass transition temperature T_g. Deformation occurs in a very narrow ‘deformation zone’ (λ = 4 nm) and the deformed material exhibits long range order independent of the crystallinity of the films till concentrations of aPS up to 15%. Films containing more than 20% aPS do not show long range order within the deformed material even after subsequent annealing above T_g. From these results, which were obtained by transmission electron microscopy and electron diffraction, the conclusion is drawn that the molecular processes of crazing in amorphous polymers and the high local deformation of polymer single crystals are obeying similar mechanisms.     

Effect of syndiotactic polystyrene on the crystallization behavior of isotactic polypropylene/syndiotactic polystyrene blends with and without β‐nucleating agent

Blends of isotactic polypropylene (PP) and syndiotactic polystyrene (sPS) with and without β‐nucleating agent were prepared using a twin‐screw extruder at 290 °C. Blends of PP/sPS with β‐nucleating agent mainly show β crystalline form, irrespective of high (20 °C min^?1) or low (2 °C min^?1) previous cooling rates. This suggests that the cooling rates have little effect on the polymorphic composition of PP in PP/sPS blends. The effect of sPS on the crystallization of PP is compared with that of polyamide 6 (PA6). The increase in crystallization temperature of PP is smaller in the presence of sPS than in the presence of PA6; the fold surface free energy of PP/sPS is larger than that of PP/PA6 blends. These results reveal that compared with PA6, sPS has much weaker α‐nucleation effect on the crystallization of PP. The weak α‐nucleation effect of sPS is attributed to the high lattice mismatch between PP and sPS crystals.