Isothermal and nonisothermal melt‐crystallization kinetics of syndiotactic polystyrene

Analyses of the isothermal and nonisothermal melt kinetics for syndiotactic polystyrene have been performed with differential scanning calorimetry, and several kinetic analyses have been used to describe the crystallization process. The regime II→III transition, at a crystallization temperature of 239°, is found. The values of the nucleation parameter K_g for regimes II and III are estimated. The lateral‐surface free energy, σ = 3.24 erg cm^?2, the fold‐surface free energy, σ_e = 52.3 ± 4.2 erg cm^?2, and the average work of chain folding, q = 4.49 ± 0.38 kcal/mol, are determined with the (040) plane assumed to be the growth plane. The observed crystallization characteristics of syndiotactic polystyrene are compared with those of isotactic polystyrene. The activation energies of isothermal and nonisothermal melt crystallization are determined to be ΔE = ?830.7 kJ/mol and ΔE = ?315.9 kJ/mol, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2528–2538, 2002     

Crystalline forms in melt‐crystallized syndiotactic polystyrene/clay nanocomposites

X‐ray diffraction methods and polarized optical microscopy have been used to investigate the structural change of syndiotactic polystyrene/clay nanocomposites. The nanocomposite has prepared by mixing an sPS polymer solution with organically modified montmorillonite. Both X‐ray diffraction and transmission electron microscopy results indicate that most of the swellable silicate layers are exfoliated and randomly dispersed into the sPS matrix. The X‐ray diffraction data also show the presence of polymorphism in sPS/clay nanocomposites, which is strongly dependent on the thermal history of the nanocomposites from the melt and on the content of clay. In this study, the effect of premelting temperatures and crystallization temperatures of sPS and sPS/clay nanocomposites on their crystalline phases is discussed.     

Crystallization behavior of syndiotactic polystyrene nanocomposites for melt‐ and cold‐crystallizations

Analyses of the effects of montmorillonite (clay) on the crystallinity and crystallization behavior of syndiotactic polystyrene (s‐PS) were investigated by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The dispersibility of the clay in s‐PS nanocomposites was studied by X‐ray and transmission electron microscopy (TEM). The clay was dispersed into the s‐PS matrix by melt blending on a scale of 1–2 nm or few tenths–100 nm, depending on the surfactant treatment. On adding clay, the crystallization behavior of the s‐PS tends to convert into the β‐crystal from the α‐crystal after being cold‐crystallized because the clay plays a vital role in facilitating the formation of the thermodynamically favored β‐form crystal when the s‐PS is cold‐ or melt‐crystallized. This phenomenon leads to a change in a conventional mechanism of molecular packing for the s‐PS. Evidently, the clay significantly affects the crystallinity and crystallization behavior of the s‐PS. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2492–2501, 2002     

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).     

The atactic polystyrene molecular weight effect on the thermal properties and crystal structure of syndiotactic polystyrene/atactic polystyrene blends

This work examined how the molecular weight of atactic polystyrene (aPS) affects the thermal properties and crystal structure of syndiotactic polystyrene (sPS)/aPS blends using differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction (WAXD) technique. For comparative purposes, the structure and properties of the parent sPS was also investigated. The experimental results indicated that these blends showed single glass transition temperatures (T_gs), implying the miscibility of these blends in the amorphous state regardless of the aPS molecular weight. The non-isothermal and isothermal melt crystallization of sPS were hindered with the incorporation of aPSs. Moreover, aPS with a lower molecular weight caused a further decrease in the crystallization rate of sPS. Complex melting behavior was observed for parent sPS and its blends as well. The melting temperatures of these blends were lower than those of the parent sPS, and they decreased as the molecular weight of aPS decreased. Compared with the results of the WAXD study, the observed complex melting behavior resulted from the mixed polymorphs (i.e. the α and β forms) along with the melting-recrystallization-remelting of the β form crystals during the heating scans. The degree of melting-recrystallization-remelting phenomenon for each specimen was dependent primarily on how fast the sPS crystals were formed instead of the incorporation of aPSs. Furthermore, the existence of aPS in the blends, especially the lower molecular weight aPS, apparently reduced the possibility of forming the less stable α form in the sPS crystals.     

Morphology,structure, and kinetic analysis of nonisothermal cold‐ and melt‐crystallization of syndiotactic polystyrene

Nonisothermal cold‐ and melt‐crystallization of syndiotactic polystyrene (sPS) were carefully carried out by Perkin–Elmer Diamond differential scanning calorimetry, polarized optical microcopy (POM), and wide angle X‐ray diffraction. The experimental data subjected to the two types of processing were thoroughly analyzed on the basis of Avrami, Tobin, Ziabicki, and combination of Avrami and Ozawa models. Avrami, Tobin, and Ziabicki analyses indicate that nonisothermal cold‐crystallization (A) characterizes smaller Avami and Tobin exponent and larger Ziabicki kinetic crystallizability index G than those obtained from nonisothermal melt‐crystallization (B) possibly due to the existence of partially ordered structures in the quenched samples. Kissinger and the differential isoconversional method (DICM) of Friedman's were utilized to obtain effective energy barrier of A, in good agreement with that obtained by using Arrhenius equation to analyze the isothermal cold‐crystallization, indicating that Kissinger and Friedman equations can be applied to obtain activation energy from A of sPS. X‐ray diffraction analysis indicates that cold‐crystallization mainly produces α‐type crystal but for melt‐crystallization the contents of α‐type and β‐type crystals depend on the cooling rates. The POM also indicates the difference of end morphology of the sample between A and B. At the same time, the DICM of Friedman's was applied to analyze experimental data of B, which were divided into two groups with 20 K/min as the threshold, and it was found that the formation of β‐type crystal possesses larger absolute value of effective activation barrier than the formation of α‐type crystal. © 2006Wiley Periodicals, Inc. J Appl Polym Sci 103: 1311–1324, 2007     

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.     

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.     

Syndiotactic polystyrene‐b‐atactic polypropylene block copolymer alloy as a compatibilizer for syndiotactic polystyrene/isotactic polypropylene blends

A polymeric alloy (SP–A) containing syndiotactic polystyrene (sPS), atactic polypropylene (aPP), and about 66 wt % sPS‐b‐aPP diblock copolymer, was prepared by the sequential feed of monomers in the presence of the half‐titanocene Cp*Ti(OBz)₃ (where Cp* is C₅Me₅ and Bz is PhCH₂), modified methylaluminoxane, and external triisobutylaluminum. The effects of the SP–A alloy as a compatibilizer for sPS and isotactic polypropylene (iPP) blends were evaluated. The blending of sPS and iPP, with and without SP–A, was performed in a single‐screw miniextruder with a side channel that allowed the continuous recycling of materials. The influence of SP–A on the mechanical and thermal properties of the immiscible sPS/iPP blends was investigated over a range of composition. The presence of the SP–A alloy resulted in a significant improvement of the impact strength of the blends compared with that of pure sPS and their pure blends. This improvement was particularly obvious in the sPS/iPP (90/10 wt %) blend containing 5 wt % SP–A. Morphological analysis of the impact‐fractured surface of the ternary blends indicated that the sPS‐b‐aPP diblock copolymer contained in the SP–A alloy acted as an efficient compatibilizer by decreasing the dispersed‐phase iPP particle size, improving the interfacial adhesion, and generating a stable microphase‐separated state. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1596–1605, 2003     

Cold‐crystallization kinetics of syndiotactic polystyrene

The kinetics of the isothermal and nonisothermal cold crystallization of syndiotactic polystyrene (s‐PS) were characterized with differential scanning calorimetry. A Johnson–Mehl–Avrami analysis of the isothermal experiments indicated that the cold crystallization of s‐PS at a constant temperature followed a diffusion‐controlled growth mode with a decreasing nucleation rate. Furthermore, the slow nucleation rate was the controlling step of the entire kinetic process. For nonisothermal cold‐crystallization kinetics, we used a simple model based on a combination of the well‐known Avrami and Ozawa models. The analysis revealed that, unlike for melt crystallization, the Avrami and Ozawa exponents were not equal. The activation energies for the isothermal and nonisothermal cold crystallizations of s‐PS were 792.0 and 148.62 kJ mol^?1, respectively, indicating that the smaller motion units in cold crystallization had a weaker temperature dependence than those in melt crystallization. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3464–3470, 2003     

Crystallization behavior of syndiotactic and atactic 1,2‐polybutadiene blends

Two 1,2‐polybutadiene samples, ie syndiotactic 1,2‐polybutadiene (sPB) and atactic 1,2‐polybutadiene (aPB), were synthesized by using the same Iron(III ) catalyst system, although with changing contents of the third catalyst component. Effects of the composition of aPB on the crystallization behavior of sPB have been characterized by wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). Analysis of the results indicates that the intensity of the (010) peak of sPB is influenced much more easily than that of the (200)/(110) peak by the amorphous aPB sample, but the structure of sPB does not change with the changing composition of sPB. aPB is thermodynamically miscible with sPB in the melt, and retards the melt crystallization of sPB. At the same time, non‐isothermal crystallization kinetic studies of neat sPB and 1/2.5 blends were carried out by DSC. The Avrami equations modified by Jeziorny and Privalko, were used to fit the primary stage of the non‐isothermal crystallizations of neat sPB and the 1/2.5 blends. A larger Avrami exponent for neat sPB than for the blends was observed, and possible reasons are discussed. The activation energies (ΔE) were determined to be ?88 and ?106 kJ mol^?1 by the isoconversional Friedman method for neat sPB and sPB in the blends, respectively. Copyright © 2004 Society of Chemical Industry     

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.     

Formation of hollow fibers in the melt‐spinning process

This article reports an investigation of the formation of hollow fibers in a melt‐spinning process. Experimental results indicate that die swelling is largely responsible for a negative effect on hole formation. The factors that positively affect die swelling, including a decrease in temperature, a decrease in capillary length, and an increase in shear rate, are thus not recommended for the spinning of hollow fibers. For vinyl‐type polymers such as polypropylene, in which the apparent elasticity leads to serious die swelling, the formation of hollow fibers is more complex than that of a typical condensation polymer. Our results further demonstrate that when hollow fibers are being made in a variety of shapes (but of the same denier), spinning a polygonal hollow fiber is significantly more unstable than spinning a circular one. Moreover, an asymmetric bridge along the polygonal contour leads to a melt twist and interrupts the entire spinning process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2896–2902, 2001     

Structure formation and characterization of PVDF hollow fiber membranes by melt‐spinning and stretching method

Melt‐spinning and stretching (MS‐S) method was proposed for preparing poly(vinylidene fluoride) (PVDF) hollow fiber membranes with excellent mechanical properties. The morphology and properties of PVDF fibers and membranes were investigated by small angle X‐ray scattering (SAXS), differential scanning calorimeter (DSC), field emission scanning electron microscope, mercury porosimeter, and tensile experiment. SAXS results indicated that the stacked lamellar structure aligned normal to the fiber axis was separated and deformed when the fibers were strained, and the long period of the strained fibers increased accordingly. Factors affecting the membrane properties were mainly spin‐draw ratio, annealing temperature, time, and stretching rate. Experimental results showed that the average pore size, porosity, and N₂ permeation of the membranes all increased with the increasing spin‐draw ratios and annealing temperatures. Annealing the nascent PVDF hollow fibers at 145°C for 12 h was suitable for attaining membranes with good performance. In addition, the amount and size of the micropores of the membrane increased obviously with stretching rate. Tensile experiment indicated PVDF hollow fiber membranes made by MS‐S process had excellent mechanical properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Dynamic mechanical behavior of atactic and high‐impact polystyrene

Results of the dynamic mechanical behavior of atactic polystyrene (PS) and high‐impact polystyrene (HIPS) for temperatures between 300 and 425 K at a frequency of the order of 50 kHz are presented. The storage Young's modulus, (E′), of the HIPS is lower than the PS value, being the relationship between them a function of the rubber phase volume fraction, independent of the measurement frequency. The glass transition temperature (T_g) of HIPS is shifted to lower temperature in respect to the PS. The γ relaxation appears at 308 K in PS at 50 kHz, while it seems to move toward lower temperatures in the HIPS. Both shifts are attributed to the presence of mineral oils in the HIPS. The values of E′, T_g, and the temperature of the γ relaxation at 50 kHz are discussed within the scope of the theory of viscoelasticity. Finally, the effect of thermal treatments, using different annealing times, on the behavior of both materials is shown. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 865–873, 2000     

Pressure and cooling rate‐induced densification of atactic polystyrene

The exact knowledge of postprocessing polymer‐specific volume is often a factor of enormous strategic importance from an industrial point of view. The subject is complicated by the fact that the specific volume of solid polymers at a constant temperature and pressure is not only a function of the current temperature and pressure, but is also a consequence of the whole formation history from the melt. In this work, specific volumes of samples solidified in different conditions are analyzed and related to their formation history. A wide range of cooling rates (from 5 × 10^?3 to 300 K/s) and solidification pressures (from 0.1 to 80 MPa) are examined. The results show a synergic effect of the cooling rate and solidification pressure: Lower cooling rates result in a much higher pressure‐induced densification with respect to higher cooling rates. A simple phenomenological model which essentially links the densification effect to the dependence of the glass transition temperature upon the cooling rate and solidification pressure is adopted to describe the experimental data. Starting from the densification effect, the effect of the pressure and cooling rate on the glass transition temperature is evaluated. Furthermore, some conclusions about the dependence of the volume relaxation time on the temperature and pressure in the glass transition range are achieved. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 184–190, 2003     

Glass transition phenomena in melt‐processed polystyrene/polypropylene blends

Blends of an amorphous and a semi‐crystalline polymer—polystyrene and polypropylene, respectively—were prepared by melt processing in an extruder at 220°C. These polymers are known to be immiscible and the composite morphologies were characterized by electron microscopy and thermal analysis. Fine micron‐scale morphologies, ranging from 0.5 to 20 microns were observed. Thermal analysis and dynamic mechanical analysis showed changes in both the polystyrene and polypropylene glass transition temperatures (T_g) over the composition range. The major effect was a sharp increase in polystyrene T_g with increasing polypropylene content in the blend. A T_g elevation of 5.5°C was observed at 85% polypropylene. The polypropylene T_g also increases with increasing polypropylene content, starting at a depressed value in discrete polypropylene domain environments and approaching the bulk polypropylene value after the phase inversion is crossed. Qualitative structural models are proposed based on spatial and mechanical interactions between the components. POLYM. ENG. SCI., 45:1187–1193, 2005. © 2005 Society of Plastics Engineers     

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.     

Solid‐state structure of thermally crystallized syndiotactic polystyrene

The solid‐state structure of syndiotactic polystyrene (s‐PS) after crystallization from the melt and the glassy state was examined by differential scanning calorimetry (DSC), density, and X‐ray diffraction analysis. It was possible to prepare semicrystalline s‐PS containing either the pure α‐ or the pure β‐crystalline form by melt crystallizing s‐PS from 280 or 330°C. The measurements confirmed the low density of both crystalline forms, which in the case of α‐crystalline form was smaller and in the case of β‐crystalline form was only slightly larger than the density of the glassy amorphous s‐PS. An endeavor to introduce the crystalline phase in s‐PS through cold crystallization at constant temperature above the glass transition resulted in a complex ordered phase. This ordered phase, depending on the crystallization temperature, contained the planar chain mesomorphic phase and the α‐crystalline phase with a low degree of perfection (cold crystallization in the range 120–175°C) or a mixture of the α‐ and β‐crystalline forms with a high degree of perfection (cold crystallization in the range 210–260°C). The combination of DSC and X‐ray measurements enabled us to resolve the complex ordered structure in semicrystalline s‐PS after cold crystallization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2705–2715, 2002