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.     

Polymorphism and mechanical properties of syndiotactic polystyrene films

The dynamic-mechanical behaviour and the tensile moduli of unstretched and stretched semicrystalline s-PS films, presenting different polymorphic forms (α, γ, δ and clathrate) but similar crystallinity and orientation, have been compared. The main aim is to elucidate the possible influence of different crystalline phases, being largely different in chain conformation and density, on mechanical properties of s-PS semicrystalline samples. For unstretched films presenting a preferential perpendicular orientation of the chain axes, the highest elastic modulus is observed for films with the high density γ phase while for uniaxially oriented films the highest modulus is observed for films with the trans-planar α phase. As for the clathrate films, the guest molecules when only included into the crystalline clathrate phase, have no plasticizing effect.     

Nucleation effect on polymorphism of melt-crystallized syndiotactic polystyrene

The main purpose of this study is to examine the effect of nucleation on the formation of polymorphism in melt-crystallized syndiotactic polystyrene (sPS). By depressing the nucleation barrier (i.e., introducing nucleating agent), the tendency of forming, a metastable crystalline phase, α crystal, significantly increased. The metastability of crystalline phase was dependent upon the nucleation process in which the lower nucleation barrier led to the formation of metastable phase. Consistently, by crystallizing under shearing, the formation of α crystal dominated the crystalline polymorphism. Herein we suggest a practical method to control the polymorphism of melt-crystallized sPS.     

Preparation and characterization of maleic anhydride-functionalized syndiotactic polystyrene

The free radical-induced grafting of maleic anhydride (MA) onto syndiotactic polystyrene (sPS) has been accomplished in the solution process by using 1,1,2-trichloroethane as solvent and dicumyl peroxide as free radical initiator. The amount of MA grafted on sPS was evaluated by a titration method. Grafted products separated from the reaction mixture were purified and analyzed. Fourier transform infrared spectroscopy and ¹H NMR studies indicate that MA attaches to the sPS in the form of single succinic anhydride rings as well as short oligomers. The results obtained by GPC analysis suggest that the degradation and chain extension reaction do not occur under the reaction conditions. Moreover, the crystallization behavior of MA-functionalized sPS was also studied by means of differential scanning calorimetry. It was found that the crystallization behavior of the grafted polymer exhibited somewhat differences in comparison to the neat sPS. The MA-functionalized sPS crystallizes at higher rate than the unmodified polymer, on the other hand, the degree of crystallinity (X_c) are lowered by the presence of the MA grafts.     

Nascent morphology of syndiotactic polystyrene synthesized over silica-supported metallocene catalyst

The nascent morphology of semi-crystalline syndiotactic polystyrene (sPS) polymerized over silica-supported pentamethyl cyclopentadienyl titanium trimethoxide (Cp^∗Ti(OCH₃)₃) catalyst in a liquid slurry polymerization has been investigated under various reaction conditions. The scanning electron microscopic analysis of nascent polymers reveals that sPS molecules grow as long nanofibrils of 30-50 nm diameter and X-ray diffraction analysis shows the co-crystalline phases including both sPS and low molecular weight guest molecules of monomer and diluent. The energy dispersive X-ray spectroscopy also shows that the disintegration of silica primary particles occurs during the polymerization as evidenced by the uniform dispersion of silicon and aluminum in a polymer particle. The fibrous growth of the polymer inside a polymer particle leads to the shape replication of the original silica particles.     

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.     

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.     

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.     

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.     

Gas sorption and transport in syndiotactic polystyrene with nanoporous crystalline phase

In this contribution we analyse sorption and transport of several gases in semicrystalline syndiotactic polystyrene with nanoporous crystalline δ form. Investigation was performed on amorphous samples and on samples characterized by different degrees of crystallinity. Sorption isotherms of carbon dioxide, nitrogen and oxygen in the crystalline phase have been determined starting from experimental results obtained for semicrystalline and amorphous samples. Corresponding isosteric heats of sorption were evaluated for the crystalline and amorphous phase. Permeation tests were also performed to gather information on mass transport properties of semicrystalline samples, evaluating average diffusivities of carbon dioxide and oxygen, in the limit of small concentrations as function of degree of crystallinity.     

Compatibilization effects of block copolymers in high density polyethylene/syndiotactic polystyrene blends

Three triblock copolymers of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) of different molecular weights and one diblock copolymer of poly[styrene-b-(ethylene-co-butylene)] (SEB) were used to compatibilize high density polyethylene/syndiotactic polystyrene (HDPE/sPS, 80/20) blend. Morphology observation showed that phase size of the dispersed sPS particles was significantly reduced on addition of all the four copolymers and the interfacial adhesion between the two phases was dramatically enhanced. Tensile strength of the blends increased at lower copolymer content but decreased with increasing copolymer content. The elongation at break of the blends improved and sharply increased with increments of the copolymers. Drop in modulus of the blend was observed on addition of the rubbery copolymers. The mechanical performance of the modified blends is strikingly dependent not only on the interfacial activity of the copolymers but also on the mechanical properties of the copolymers, particularly at the high copolymer concentration. Addition of compatibilizers to HDPE/sPS blend resulted in a significant reduction in crystallinity of both HDPE and sPS. Measurements of Vicat softening temperature of the HDPE/sPS blends show that heat resistance of HDPE is greatly improved upon incorporation of 20 wt% sPS.     

Annihilation kinetics of color center in irradiated syndiotactic polystyrene at elevated temperatures

In situ absorbance of syndiotactic polystyrene (sPS) at elevated temperatures is investigated. The irradiation environments of air, vacuum and oxygen are considered. The decreasing loss in transmittance after irradiation follows the sequence: oxygen, air and vacuum. The UV cut-off and shoulder wavelengths of sPS irradiated in an air atmosphere are greater than those in vacuum. Two types of color centers are responsible for the reduction of transmittance in the irradiated specimen. They are annealable and permanent. A first order annihilation model is proposed to analyze the annealable color centers. The results show that the annealable color center of sPS irradiated in vacuum required to overcome less energy barrier to annihilate than that in air, but the permanent color centers in the former is less than those in the latter after annealing. No annealable color center is observed in sPS irradiated in an oxygen atmosphere. The EPR spectra and FTIR spectra were also studied to enhance the understanding of kinetics of color center.     

Crystallization and morphological features of syndiotactic polystyrene induced from glassy state

Spherulitic growth rates and microstructure of syndiotactic polystyrene (sPS) cold-crystallized isothermally at various temperatures, T_c (115–240 °C), have been investigated by small-angle light scattering (SALS), optical microscopy and transmission electron microscopy. The derived activation energy for sPS chain mobility at the crystal growing front is 5.4 kJ/mol, which is relatively lower than that of isotactic polystyrene, 6.5 kJ/mol. In addition, the Hv scattering invariant (Q_Hv) measured by SALS on the crystallized sPS samples displays a pronounced minimum at 150 °C. Despite a wide range of T_c used, however, the sample crystallinity estimated by Fourier transformation infrared spectroscopy remains unchanged. Prior to crystallization, the correlation length derived from the Vv patterns on the basis of Debye–Bueche model is ca. 1.13 μm regardless of T_c used. Interconnected domains with a width of ca. 1.8±0.5 μm are readily observed in all the crystallized samples under phase contrast microscopy and the phase-separated structure is conserved within sPS spherulites whose diameters are increased with increasing T_c.

Based on the above facts, we conclude that the presence of a Q_Hv minimum is ascribed to the resultant events of the two competitive transitions i.e. liquid–solid crystallization, and liquid–liquid demixing resulting from the spinodal decomposition (SD). At lower T_c, the unstable SD transition overwhelms the crystallization. Despite the low chain mobility, the coarsening process driven by the interfacial energies has reached a certain level before crystalline nucleation takes place. At higher T_c, on the other hand, cold crystallization becomes the dominant process due to the enhanced chain mobility, leading to the suppression of ongoing SD coarsening process. At an intermediate T_c range, comparable competition of the phase separation and crystallization prohibits the development of ordered symmetry within spherulites, giving the presence of Q_Hv minimum.     

Guest exchange process in syndiotactic polystyrene thin films measured by ATR-FTIR spectroscopy

Guest exchange and desorption processes in syndiotactic polystyrene(sPS)-solvent complex systems (δ form) were studied by means of ATR-FTIR spectroscopy. The intensities of the bands sensitive to the amount of guest molecules and the conformational order of sPS were followed to clarify the mechanism on the guest exchange. Rapid exchange of the guest molecules was observed on the exposure of the δ form sample to toluene and chloroform vapors. The desorption of guest molecules occurs in two stages, which correspond to the rapid desorption mainly from the amorphous region and the slow desorption mainly from the crystalline region, respectively. The diffusion coefficients of the desorbate molecules were evaluated. The time dependence of the intensities of the sPS bands showed that the conformational regularity of sPS changed during the guest exchange process, and the behavior depended on the combination of sorbate and desorbate.     

Analyses of crystal forms in syndiotactic polystyrene intercalated with layered nano-clays

A mixed polymorphic morphology of intercalated/exfoliated structure was observed in syndiotactic polystyrene (sPS)/clay nano-composites, which were successfully prepared by solution intercalation technique using 1,1,2,2-tetrachloroethane (TCE) as a solvent. Furthermore, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses were used to examine the effect of montmorillonite clays (MMT, in pristine or organo-modified forms) in isothermally melt-crystallized sPS at several available crystallization temperatures (T_c) in a competitive environment of coexisting α- and β-crystals. A significant change in polymorphism of sPS was observed by the inclusion of different clays and the temperature regime of the α-crystal formation in sPS was found to increase considerably up to 250 °C by the presence of the organo-clay. Pristine clay (Na-MMT) was found to induce the β-crystal of sPS at all T_c's studied in this work. The overall thermodynamics of crystallization remained unchanged as the β-phases were found in major proportion at higher temperature of crystallization (∼260 °C), irrespective of the nature of the clays. The dispersibility of the clays in sPS matrix is assumed to play the pivotal role in modifying the crystalline structures, which was further corroborated by the polarized optical microscopy (POM). The spherulitic morphology clearly indicates differences in crystallites as affected by the nano-clays. Incorporation of organo-clay with nanoscale dispersibility through the intercalation of sPS molecules into the clay galleries was found to promote rapid formation of α-forms, which develops into spherulites of smaller dimension as compared to those of the β-forms. The alteration in melting behavior of sPS is attributed to the different crystallite structures that lead to formation of different kind of spherulites.     

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.     

Supramolecular transition metal catalysts in two-phase systems

The concept of covalently connecting a catalytically active transition metal center with a water-soluble receptor (host molecule) makes a new type of supramolecular catalysis possible in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of the commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins unusual degrees of substrate selectivity based on the molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (H₂O/organic) hydrogenation of nitro-aromatics also is a smooth process with these supramolecular catalysts. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system.     

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.     

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.     

Synthesis and characterization of benzoylated syndiotactic polystyrene

Benzoylated syndiotactic polystyrene, a functional polymer bearing benzophenone(BP) moiety, has been synthesized in a heterogeneous process by using carbon disulfide as the dispersing medium, and benzoyl chloride and anhydrous aluminum chloride as benzoylating agent and catalyst, respectively. The benzoylation reaction can be well controlled and the resultant polymer was characterized by FTIR and NMR spectroscopy. The incorporation of benzoyl groups into syndiotactic polystyrene was found to have a profound effect on the thermal properties of these new materials. The melting point and the degree of crystallinity decreased by the presence of benzoyl groups, while the glass transition temperature increased.