Lamellar morphologies and crystal stability of syndiotactic polystyrene in α-crystalline form

Syndiotactic polystyrene (sPS) samples melt-crystallized into neat α″ hexagonal modifications were prepared at various temperatures thoroughly for the extensive morphological studies. Lamellar morphologies of the as-prepared sPS samples were investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Absence of a discernible scattering peak was found for SAXS conducted at room temperature, resulting from a negligible difference in the electron density between the lamellar and amorphous layers. To enhance the scattering contrast and strength, SAXS was carried out at 180 °C to obtain more reliable morphological parameters. Due to the broad thickness distribution of morphological features as revealed from the TEM observations, a pronounced variation is found for the long periods derived from the Bragg's law, one-dimensional correlation function, and interface distribution function of the SAXS data. In addition, relatively irregular packing of lamellar stacks with short lateral dimensions was detected in the as-prepared α″-form sPS, leading to the absence of spherulitic birefringence under polarized optical microscopy. Based on the interface distribution function analysis of the SAXS intensity profiles, the lamellar thicknesses were estimated. Using the Gibbs-Thomson relation, the ratio of fold surface free energy (σ_e) to the fusion enthalpy for α″-form sPS was successfully deduced to be ca. 0.057 nm, which is lower than that of β′-form sPS, ca. 0.12 nm. On this basis, a comparison of critical lamellar thicknesses for α″- and β′-form sPS at various crystallization temperatures is provided and the crystal stability associated with the lamellar thickness is discussed as well.     

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.     

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.     

Radiation effects and re-crystallization mechanism of syndiotactic polystyrene with β′-crystalline form

The double melting behavior of syndiotactic polystyrene (sPS) with β′-form crystallites was systematically investigated by several analytical techniques, including differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), as well as wide-angle and small-angle X-ray scattering (WAXD, SAXS). For preventing the possible chain re-organization during intermediate melting, a high-energy electron beam (e-beam) radiation was carried out on the melt-crystallized samples to chemically cross-link the amorphous chains between lamellar crystals. The WAXD intensity profiles of the irradiated sPS samples revealed that no crystal transformation took place, and the crystallinity fraction remained unchanged for a received dose up to 2.4 MGy. As the received dose was increased, however, the high melting temperature peak was gradually diminished and finally disappeared after 1.8 MGy e-beam radiation, suggesting that the double melting phenomenon was mainly attributed to the melting/re-crystallization/re-melting behavior. The re-crystallization mechanism of sPS samples was studied using DSC and PLM to reveal the effects of heating rate and annealing temperature on the Avrami exponent and re-crystallization rate constant. In addition, the lamellar morphologies of the re-crystallized samples were also investigated by means of SAXS and TEM. With increasing heating rate or annealing temperature, the derived Avrami exponent was slightly decreased from 1.4 to 1.1; in comparison, the re-crystallization rate showed a shallow maximum at a rate of 10 °C/min, but it became evidently reduced at high annealing temperatures. Based on the morphological observations, we proposed that the re-crystallization of β^′-form sPS crystals involved with the presence of broad lamellar thickness distribution as well as abundant irregular loose folding chains on the lamellar surfaces, which became tightened and crystallized into the un-melted lamellae when the neighboring thinner lamellae trapped in-between were melted. Thus, the high melting temperature is dependent on the average thickness of lamellae consisting of the un-melted lamellae developed initially and thickened ones associated with re-crystallization.     

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.     

Criteria dilemma of phase behavior in ternary blends comprising polystyrene, poly(alpha-methyl styrene), and poly(4-methyl styrene)

A ternary blend system of polystyrene (aPS), poly(α-methyl styrene) (PαMS), and poly(4-methyl styrene) (P4MS) (blends of three styrenic homopolymers of similar molecular structures) was investigated by using differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), scanning electron microscopy (SEM), and solid-state ¹³C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR). This ternary system of aPS/PαMS/P4MS exhibits a small miscibility loop only at relatively high percentages of PαMS (≥80%), while most of the aPS/PαMS/P4MS compositions exhibit two coexisting aPS/PαMS and aPS/P4MS phases in the immiscibility loop. In the immiscible loop, SEM measurements revealed evidence in contrasting with DSC for criteria of miscibility. DSC characterization revealed a single T_g for most ternary blend compositions; however, SEM graphs apparently indicated sub-micron phase domains, except for several PαMS-rich compositions (>80%). The and techniques using NMR were found to resolve the dilemma between the conventional thermal analysis and microscopy results, and indeed supported that the ternary blends at above 80% PαMS are completely homogeneous on the molecular level. Attempts have been made to resolve the seemingly contradictory interpretations on the complex ternary phase structures and domains from the DSC, and microscopy results.     

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.     

Lamellar Thickness of a Syndiotactic Polystyrene Determined from Small-angle X-ray Scattering and Transmission Electron Microscopy

Extensive morphological studies on a syndiotactic polystyrene (sPS) sample prepared from compression molding were carried out using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). SAXS was conducted at 25C as well as at 150C to enhance the scattering contrast in order to obtain more reliable morphological parameters. The compression-molded sample was crystallized into a orthorhombic crystal lattice characterized by wide-angle X-ray diffraction (WAXD). A similar weight fraction of crystallinity, ca. 0.37, was obtained from both WAXD and differential scanning calorimetry measurements.In addition to the scattering peak at a scattering vector of ca. 0.36 nm^–1 attributable to the presence of lamellar/amorphous layers, anomalous scattering at the zero angle was found from the SAXS intensity profiles. Based on the Debye–Bueche theory, the scattering profile of this peculiar zero-angle scattering was deduced and was subtracted from the raw intensity profile to obtain the intensity profile exclusively associated with the lamellar/amorphous structure. A consistent long period was obtained for SAXS measured either at 25 or 150C, provided that the appropriate subtraction of intensities due to the zero-angle scattering was conducted. Moreover, the lamellar thickness deduced from the one-dimensional correlation function was in good agreement with TEM results. A difference scattering pattern derived from SAXS measured at 150 and 25C was obtained and a comparison of the morphological parameters was provided as well.     

Isotactic polypropylene/polystyrene blends: Effects of the addition of a graft copolymer of propylene with styrene

A novel graft copolymer of unsaturated propylene with styrene (uPP-g-PS) was added to binary blends of isotactic polypropylene (iPP) and atactic polystyrene (aPS) with a view to using such a copolymer as compatibilizer for iPP/aPS materials. Differential scanning calorimetry, optical microscopy, scanning electron microscopy (SEM), wide angle X-ray scattering, and small angle X-ray scattering (SAXS) techniques have been carried out to investigate the phase morphology and structure developed in solution-cast samples of iPP/aPS/uPP-g-PS ternary blends. It was found that the uPP-g-PS addition can provide iPP/aPS-compatibilized materials and that the extent of the achieved compatibilization is composition-dependent. Blends of iPP and aPS exhibited a coarse domain morphology that is characteristic of immiscible polymer systems. By adding 2% (wt/wt) of uPP-g-PS copolymer a very broad particle-size distribution was obtained, even though the particles appeared coated by a smooth interfacial layer, as expected according to a core–shell interfacial model. With increasing uPP-g-PS content (5% wt/wt), a finer dispersion degree of particles, together with morphological evidence of interfacial adhesion, was found. With further increase of uPP-g-PS amount (10% wt/wt) the material showed such a homogeneous texture that neither domains of dispersed phase nor holes could be clearly detected by SEM. The type of interface developed in such iPP/aPS/uPP-g-PS blends was accounted for by an interfacial interpenetration model. The iPP crystalline texture, size, neatness, and regularity of iPP spherulites crystallized from iPP/aPS/uPP-g-PS blends were found to decrease when the copolymer content was slightly increased. Assuming, for the iPP spherulite fibrillae, a two-phase model constituted by alternating parallel crystalline lamellae and amorphous layers, it was shown by SAXS that the phase structure generated in iPP/aPS/uPP-g-PS blends is characterized by crystalline lamellar thickness (L_c) and interlamellar amorphous layer thickness (L_a) higher than that shown by plain iPP; the higher the copolymer content, the higher the L_c and L_a. It should be remarked that considerably larger increases have been found in L_a values. Such SAXS results have been accounted for by assuming that a cocrystallization phenomenon between propylenic sequences of the uPP-g-PS copolymer and iPP occurs and that during such a process PS chains grafted into copolymer sequences remain entrapped in iPP interlamellar amorphous layers, where they form their own separate domains. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1539–1553, 1997     

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.     

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.     

Effect of shear memory on the coarsening behaviour of polystyrene and poly(vinyl methyl ether) blend

The effect of shear memory on the coarsening behaviour of polystyrene/poly(vinyl methyl ether) (PS/PVME) blend which shows a typical lower critical solution temperature (LCST)-type phase diagram has been thoroughly investigated for the near critical composition (PS/PVME=30/70) using a time-resolved light scattering technique. The measurements were carried out at 135 °C (20 °C above the quiescent cloud point) at two different directions, parallel and normal to the direction of flow. Different shear memories were generated in the melt using a simple shear apparatus of parallel plate geometry. The coarsening process was influenced to a great extent by the shear history of the blend over the time scale of the measurement. The average domain size of the dispersed particles obtained from the analysis of the light scattering data on the basis of Deby Bueche theory was found to be shear memory dependent. The coarsening process was elevated and suppressed at low and high shear memory, respectively. This behaviour was attributed to the shift of the cloud point observed under same values of shear rates. In addition, the coarsening behaviour of this blend was found to be flow direction independent due to the very high viscosity ratio of the blend, which led to in turn rather circular domains of PS in PVME matrix without any elongation or orientation in the direction of flow. Furthermore, the coarsening process for all the measured samples was followed the general power low, regardless the shear history and the flow direction of the blends. This result indicated that; the shear could only retard or elevate the rate of domain growth without any effect on the coarsening mechanism.     

Carbon nanocapsules-reinforced syndiotactic polystyrene nanocomposites: Crystallization and morphological features

Syndiotactic polystyrene (sPS) composites filled with well-dispersed carbon nanocapsules (CNC) were prepared through solution blending along with ultrasonication. Several analytic techniques, including DSC, FTIR, PLM, WAXD, TEM, and TGA were performed to reveal the CNC effects on the crystallization, morphology and the thermal degradation of the as-prepared sPS/CNC composites. Addition of CNC was found to favor the crystalline modification of β-form sPS and depress the α-form ones. For the dynamic crystallization, a gradual reduction of cold-crystallization temperature of the α-form sPS was observed by increasing the CNC content although the glass transition temperature remained unchanged (∼96 °C). In contrast, the melt-crystallization temperature of the β-form sPS was elevated from 238 °C for the neat sPS to 251 °C for the 99/5 composite in spite of the fact that the equilibrium melting temperature (∼290 °C) determined from the linear Hoffman-Weeks plot was irrelevant with CNC concentrations. The former was attributable to the formation of an effective heat-conduction path to trigger an earlier overall crystallization. On the other hand, the latter resulted from the enhanced nucleation sites due to the presence of uniformly dispersed CNCs. Results of the isothermal crystallization of the β-form sPS concluded that the presence of 1% CNCs led to a significant increase in the crystallization rate as much as an order of magnitude. Moreover, the Avrami exponent changed to ∼2.0 from a value of 2.8 for the neat sPS, suggesting a different crystallization mechanism involved. At a given crystallization temperature, PLM results showed a negligible variation in the crystal growth rates and a decrease in spherulitic sizes, indicating that nucleation played the key role in enhancing the crystallization rate. For samples isothermally crystallized at 260 °C, the lamellar thickness was constant to be ∼7.2 nm regardless of the CNC content. Due to the enhanced nucleation, however, lamellar stacks were more randomly oriented and its lateral dimensions became shorter with increasing CNC contents. For composites with more than 1 wt% CNC, the crystallizability of sPS chains was reduced and the annealing peak located ca. 4 °C higher than the crystallization temperature became more evident, suggesting the plausible formation of micro-crystals in between the lamellar stacks. The TGA results illustrated that a better thermal stability was reached for the CNC-filled sPS composites.     

Morphology,crystallization, and thermal behavior of isotactic polypropylene/polystyrene blends: Effects of the addition of a graft copolymer of propylene with styrene

Optical microscopy, differential scanning calorimetry, and small‐angle X‐ray scattering techniques were used to study the influence of crystallization conditions on the morphology and thermal behavior of samples of ternary blends constituted by isotactic polypropylene (iPP), atactic polystyrene (aPS), and a novel graft copolymer of unsaturated propylene with styrene (uPP‐g‐PS) with the purpose of assessing the uPP‐g‐PS capability to act as a compatibilizer for iPP/aPS materials. It was shown that the presence of the uPP‐g‐PS copolymer affects the interfacial tension between the iPP and aPS phases in the melt state, with the aPS particle size and the particle‐size distribution being, in fact, strongly modified. In samples of iPP/aPS/uPP‐g‐PS blends, isothermally crystallized from the melt at a relatively low undercooling in a range of the crystallization temperature of the iPP phase, the addition of the uPP‐g‐PS copolymer induced a drastic change both in the aPS mode and the state of dispersion and in the iPP spherulitic texture and inner structure of the spherulite fibrils. In particular, the phase structure developed in the iPP/aPS/uPP‐g‐PS materials was characterized by a crystalline lamellar thickness of the iPP phase comparable to that shown by the plain iPP. The extent of the induced modifications, that is, the degree of compatibilization achieved, resulted in a combined effect of composition and undercooling. Also, relevant thermodynamic parameters of the iPP phase, such as the equilibrium melting temperature (T_m) and the folding surface free energy (ς_e) of the lamellar crystals, were found to be influenced by the presence of the uPP‐g‐PS copolymer. A linear decrease of the T_m and ς_e values with increasing uPP‐g‐PS content was, in fact, observed. Such results have been accounted for by an increase of the presence of defects along the iPP crystallizable sequences and by the very irregular and perturbed surface of the crystals with increasing copolymer content. The observed decrease in T_m values revealed, moreover, that, in the iPP/aPS/uPP‐g‐PS blends, the iPP crystal growth occurs under comparatively lower undercooling, in line with higher crystalline lamellar thickness shown by SAXS investigation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1429–1442, 1999     

Toughness of syndiotactic polystyrene/epoxy polymer blends: microstructure and toughening mechanisms

Thermoplastic/epoxy blends were formed using an amine-cured epoxy polymer and a semi-crystalline thermoplastic: syndiotactic polystyrene (sPS). Complete phase-separation of the initially soluble sPS from the epoxy occurred via ‘reaction-induced phase-separation’ (RIPS) or via ‘crystallisation-induced phase-separation’ (CIPS), depending upon the thermal processing history employed. Dynamic mechanical thermal analysis showed that no sPS was retained dissolved in the epoxy polymer. For RIPS, at concentrations of sPS of up to 8 wt%, the sPS is present solely as spherical particles. However, macro phase-separation, giving a co-continuous microstructure, accompanied by local phase-inversion, dominates the RIPS blends containing more than 8 wt% sPS. In the CIPS blends, the sPS is present as spherulitic particles, and this microstructure does not change over the range of sPS concentrations employed, i.e. from 1 to 12 wt% sPS. The pure epoxy polymer was very brittle with a value of fracture energy, G_Ic, of about 175 J/m². However, the addition of the sPS significantly increases the value of G_Ic, though the toughness of the RIPS and CIPS blends differs markedly. For the RIPS blends, there is a steady increase in the toughness with increasing content of sPS and an apparent maximum value of G_Ic of about 810 J/m² is obtained for 8-10 wt% sPS. On the other hand, the measured toughness of the CIPS blends increases relatively slowly with the concentration of sPS, and a maximum plateau value of only about 350 J/m² was measured in the range of 8-12 wt% sPS. The relationships between the microstructure of the RIPS and CIPS sPS/epoxy blends and the measured fracture energies are discussed. Further, from scanning electron microscopy studies of the fracture surfaces and optical microscopy of the damage zone around the crack tip, the nature of the micromechanisms responsible for the increases in toughness of the blends are identified. For the RIPS blends, (i) debonding of the sPS particles, followed by (ii) plastic void growth of the epoxy matrix are the major toughening micromechanisms. The increase in toughness due to such micromechanisms is successfully predicted theoretically using an analytical model. In the case of the CIPS blends, the increase in the value of G_Ic results from (i) crack deflection and (ii) microcracking and crack bifurcation.     

Wall slip and melt-fracture of polystyrene melts in capillary flow

We investigated slip and unstable flow phenomena of polystyrene melts in capillaries from the view of the effects of temperature and molecular weight by using three polystyrene samples with different molecular weights (M_w = 192,000, M_w = 258,000, and M_w = 321,000). The slip velocities are estimated by the Mooney method and the modified Mooney method. We found that the slip velocity increases and the critical slip stress above which a slip starts to occur decreases with the temperature. We also observed the melt-fracture at above a critical melt-fracture stress higher than . We found that the onset of melt fracture is affected by the extensional stress near the entry region to the capillary in the barrel and the melt-fracture tends to easily occur with increase of the molecular weight, but is not sensitive to the temperature.     

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