Structure and properties of the mesophase of syndiotactic polystyrene. VIII. Solvent sorption behavior of syndiotactic polystyrene/p‐chlorotoluene mesophase membranes

A crystalline δ form of a syndiotactic polystyrene (sPS) membrane was prepared from a solution of sPS (1 wt %) and p‐chlorotoluene (p‐CT) by a solution‐casting method. The mesophase (δ empty form) of sPS was obtained by the extraction of the guest solvent from the δ form of sPS by a stepwise solvent‐extraction method. The sPS/p‐CT mesophase membrane [p‐CT (A‐M)] was used for the sorption of 1 mol % p‐CT for different times and for the sorption of different concentrations of p‐CT, chlorobenzene (CB), p‐xylene (p‐X), toluene, and chloroform for 48 h. The presence of solvents in the sPS membrane was confirmed by IR analysis. A thermal study revealed that the sorption amount of 1 mol % p‐CT increased with increasing immersion time, and the sorption amounts of different solvents increased with increasing solvent concentration. Differential scanning calorimetry results showed that the desorption peak temperature increased as the amount of the solvent increased in the clathrated sPS membrane. Wide‐angle X‐ray diffraction results showed that 2θ at 8.25° was slightly shifted toward 8°, and there was no change in the peak position at 10° for p‐CT (A‐M), which was immersed in different solvents (1 mol %); however, the intensity of 2θ at 10° was not similar for all the samples. Among the solvents used for the sorption studies at 1 mol %, p‐CT (A‐M) could sorb more p‐CT and CB than p‐X, toluene, and chloroform. The solvent sorption isotherm was the Langmuir sorption mechanism. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3439–3446, 2004     

Structure and properties of the mesophase of syndiotactic polystyrene. I. Effect of casting conditions on the preparation of the mesophase

A syndiotactic polystyrene–toluene solution was cast under two different casting conditions to obtain the δ form. A systematic study of its conformational transition, thermal behavior, and structural transformation as functions of the annealing temperature and time was performed. Spectroscopic studies revealed the content of its helical conformations and its retention up to 190 °C. Thermal analyses showed a significant difference in the transformation from the γ form to the α form. The retention of the intermediate emptied clathrate form (mesophase) of the conformational order for a longer duration (from 120 to 180 °C) in a syndiotactic polystyrene membrane cast at room temperature was confirmed by X‐ray diffraction analysis. On the basis of the experimental results in this work, the transition mechanism is discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 530–536, 2002; DOI 10.1002/polb.10120     

Structure and properties of the mesophase of syndiotactic polystyrene. II. Effect of stepwise extraction on the preparation of the mesophase

In this study, a novel stepwise extraction method has been examined. The guest molecules housed between the helices of the clathrate δ form of syndiotactic polystyrene can be removed completely with this method. A systematic study of the preparation of a solvent‐free mesophase (emptied clathrate) membrane, its helical and residual solvent contents, and its structural transformations has been performed. In this first attempt, an enhancement in the TTGG helical content has been observed in the extracted membrane, and a conceptual mechanism is proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 269–273, 2003     

Vapor sorption in emptied clathrate samples of syndiotactic polystyrene

The analysis of chloroform vapor sorption at 35°C in semicrystalline syndiotactic polystyrene samples shows remarkably different sorption isotherms, depending on the crystalline form of the samples. In particular, “emptied” clathrate (“emptied” δ form) samples are characterized by higher equilibrium sorption levels and the differences are particularly relevant for low vapor activities. Moreover, sorption kinetics detected at a vapor activity equal to 0.5 show that in the case of “emptied” δ form samples the sorption rate is much higher than for the other semicrystalline samples. The larger sorption equilibrium uptakes and sorption rates of the “emptied” δ form samples are essentially due to their ability to absorb chloroform, already for low activities, by clathration in the crystalline phase. The measured equilibrium uptakes and sorption kinetics suggest that “emptied” δ form samples of syndiotactic polystyrene could be suitable for removing polluting chlorinated compounds from vapor and liquid streams. © 1997 John Wiley & Sons, Inc.     

Study on δ‐form complex in syndiotactic polystyrene–organic molecules systems. IV. Formation of complexes with a mixture of solvents and structural changes during the sorption of solvents by syndiotactic polystyrene mesophase membranes

The syndiotactic polystyrene (sPS) δ form was crystallized from mixtures with different compositions of p‐chlorotoluene–chlorobenzene (p‐CT–CB) and p‐chlorotoluene–1,1,2‐trichloroethane (p‐CT–TCE). The presence of the δ form and TTGG helical conformation was confirmed by Fourier transform infrared and wide‐angle X‐ray diffraction (WAXD) analyses. In sPS/p‐CT–CB and sPS/p‐CT–TCE δ‐form membranes, the relative absorbance (RA) of the p‐chlorotoluene (p‐CT) solvent peak was very high even at very low concentrations of p‐CT in comparison with the RA of higher concentrations of chlorobenzene (CB) or 1,1,2‐trichloroethane (TCE) in the mixtures. However, the RAs of both CB and TCE solvent peaks decreased with decreasing concentrations of CB and TCE in the mixtures. A negligible decrease in the RA of the TTGG helical content was observed with a decreasing concentration of CB or TCE. The WAXD results showed that the 2θ peak positions of the [010] and [ 10] planes of the sPS/p‐CT–CB and sPS/p‐CT–TCE δ‐form membranes appeared in the same position and were almost equal to those of the sPS–p‐CT δ‐form membrane. The mesophases of the sPS–p‐CT [p‐CT (A‐M)] and sPS–TCE [TCE (A‐M)] membranes were used for the sorption studies with different concentrations of various solvents. The sorption amount of aromatic solvents by both p‐CT (A‐M) and TCE (A‐M) mesophase membranes was higher than that of aliphatic solvents, regardless of the size, shape, and nature of the molecular cavity. The 2θ peak of the [010] plane of the sPS membranes slowly approached the original δ‐form value when the sPS mesophase membranes were immersed in various solvents of different concentrations for 48 h. The shifting of the 2θ peak of the [010] plane to the original δ form depended on the nature and concentration of the immersion solvents. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2380–2387, 2005     

Structure and properties of the δ‐form and mesophase of syndiotactic polystyrene membranes prepared from different organic solvents

Syndiotactic polystyrene (sPS) membranes were prepared with different organic solvent systems and compared to get the information about the δ‐form complexing behavior of sPS. Further, the guest molecules included in the clathrate δ form of sPS are removed by stepwise extraction method. The conformational changes during the TTGG helical formation of sPS/organic solvent systems have been identified by FTIR spectroscopy, and it was concluded that the TTGG helices were constructed in regular sequences, which depends on the nature of the respective solvents. The TTGG content in the mesophase is found to be increased by removing the guest molecules. The structural changes of sPS/organic solvent systems have been characterized by WAXD analysis. Moreover, the different clathrate structures were found and showed the different crystalline reflections in the WAXD profiles, which are significantly changed with the kind of guest solvent included in sPS. The content of solvents in the clathrated sPS and the desorption temperatures were determined by thermal analysis. The resulted mesophase of sPS membrane contains the nanoporous molecular cavities that depend on the size of the guest molecule. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1873–1880, 2005     

Chloroform sorption in nanoporous crystalline and amorphous phases of syndiotactic polystyrene

The transport of chloroform in films of atactic polystyrene and of semicrystalline syndiotactic polystyrene in its nanoporous form (δ‐form) has been investigated by gravimetric analysis. Experimental tests have been performed at 35, 49, and 56 °C and at several vapor pressures ranging from 0.5 to 100 Torr. Nonequilibrium lattice fluid prediction of the amorphous sorption behavior was used to enucleate the sorption contribution of the crystalline nanoporous phase from semicrystalline samples. Sorption behavior of the crystalline phase has been interpreted on the basis of Langmuir equation. Moreover, the chloroform sorption at low activities in the crystalline nanoporous phase has been predicted by using Grand Canonical Monte Carlo molecular simulations. Isosteric heats of sorption were also experimentally evaluated for the crystalline phase, and compared with the corresponding prediction of molecular simulation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 8–15, 2008     

Effects of the oriented mesophase on the cold crystallization of syndiotactic polystyrene and its blend with poly(2,6‐dimethyl‐1,4‐phenylene oxide)

The effects of molecular orientation on the crystallization and polymorphic behaviors of syndiotactic polystyrene (sPS) and sPS/poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) blends were studied with wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry. The oriented amorphous films of sPS and sPS/PPO blends were crystallized under constraint at crystallization temperatures ranging from 140 to 240°C. The degree of crystallinity was lower in the cold‐crystallized oriented film than in the cold‐crystallized isotropic film. This was in contrast to the case of the cold crystallization of other polymers such as poly(ethylene terephthalate) and isotactic polystyrene, in which the molecular orientation induced crystallization and accelerated crystal growth. It was thought that the oriented mesophase was obtained in drawn films of sPS and that the crystallization of sPS was suppressed in that phase. The WAXD measurements showed that the crystal phase was more ordered in an sPS/PPO blend than in pure sPS under the same annealing conditions. The crystalline order recovered in the cold‐crystallized sPS/PPO blends in comparison with the cold‐crystallized pure sPS because of the decrease in the mesophase content. The crystal forms depended on the crystallization temperature, blend composition, and molecular orientation. Only the α′‐crystalline form was obtained in cold‐crystallized pure sPS, regardless of molecular orientation, whereas α′, α″, and β′ forms coexisted in the cold‐crystallized sPS/PPO blends prepared at higher crystallization temperatures (200–240°C). The β′‐form content was much lower in the oriented sPS/PPO blend than in the isotropic blend sample at the same temperature and composition. It was concluded that the oriented mesophase suppressed the crystallization of the stable β′ form more than that of the metastable α′ and α″ forms during the cold crystallization of sPS/PPO blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1665–1675, 2003     

Thermal properties and crystalline structure of syndiotactic polystyrene‐based miscible blends

This work examined the effect of the pre‐melting temperature (T_max) on the thermal properties and crystalline structure of four miscible syndiotactic polystyrene (sPS)‐based blends containing 80 wt % sPS. The counterparts for sPS included a high‐molecular‐weight atactic polystyrene [aPS(H)], a medium‐molecular‐weight atactic polystyrene [aPS(M)], a low‐molecular‐weight atactic polystyrene [aPS(L)], and a low‐molecular‐weight poly(styrene‐co‐α‐methyl styrene) [P(S‐co‐αMS)]. According to differential scanning calorimetry measurements, upon nonisothermal melt crystallization, the crystallization of sPS shifted to lower temperatures in the blends, and the shift followed this order of counterpart addition: P(S‐co‐αMS) > aPS(L) > aPS(M) > aPS(H). The change in T_max (from 285 to 315 °C) influenced the crystallization of sPS in the blends to different degrees, depending on the counterpart's molecular weight and cooling rate. The change in T_max also affected the complex melting behaviors of pure sPS and an sPS/aPS(H) blend, but it affected those of the other blends to a lesser extent. Microscopy investigations demonstrated that changing T_max slightly affected the blends' crystalline morphology, but it apparently altered that of pure sPS. Furthermore, the X‐ray diffraction results revealed that the α‐form sPS crystal content in the blends generally decreased with an increase in T_max, and it decreased with a decrease in the cooling rate as well. The blends showed a lower α‐form content than pure sPS; a counterpart of a lower molecular weight more effectively reduced the formation of α‐form crystals. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2798–2810, 2006     

Thermal and mechanical properties of syndiotactic polystyrene/organoclay nanocomposites with different microstructures

The fabrication of syndiotactic polystyrene (sPS)/organoclay nanocomposite was conducted via a stepwise mixing process with poly(styrene‐co‐vinyloxazolin) (OPS), that is, melt intercalation of OPS into organoclay followed by blending with sPS. The microstructure of nanocomposite mainly depended on the arrangement type of the organic modifier in clay gallery. When organoclays that have a lateral bilayer arrangement were used, an exfoliated structure was obtained, whereas an intercalated structure was obtained when organoclay with a paraffinic monolayer arrangement were used. The thermal and mechanical properties of sPS nanocomposites were investigated in relation to their microstructures. From the thermograms of nonisothermal crystallization and melting, nanocomposites exhibited an enhanced overall crystallization rate but had less reduced crystallinity than a matrix polymer. Clay layers dispersed in a matrix polymer may serve as a nucleating agent and hinder the crystal growth of polymer chains. As a comparison of the two nanocomposites with different microstructures, because of the high degree of dispersion of its clay layer the exfoliated nanocomposite exhibited a faster crystallization rate and a lower degree of crystallinity than the intercalated one. Nanocomposites exhibited higher mechanical properties, such as strength and stiffness, than the matrix polymer as observed in the dynamic mechanical analysis and tensile tests. Exfoliated nanocomposites showed more enhanced mechanical properties than intercalated ones because of the uniformly dispersed clay layers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1685–1693, 2004     

End‐functionalization of syndiotactic polystyrene by vinylsilane inducing selective chain transfer reactions

Structurally well‐defined end‐functionalized syndiotactic polystyrene (sPS) can be prepared by conducting a simultaneous selective chain transfer reaction during the syndiospecific polymerization of styrene in the presence of vinylsilanes. The production of vinylsilane end‐capped sPS involves a unique selective chain transfer pathway via the incorporation of a terminal vinylsilane unit at the polymer chain end by 2,1‐insertion. This unusual insertion pattern situates the bulky silyl functional group at a closer β‐position from the active catalyst center, thus deactivating the propagating chain by a steric jam between the vinylsilane end group and the active catalyst. Subsequently, chain releasing by hydrogen addition (in the presence of H₂) or by β‐elimination (in the absence of H₂) can take place, which leads to the production of end‐functionalized sPS with precise controls of stereoregularity and of the location of functionality. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1690–1698, 2010     

Competitive influence of atactic polystyrene and supercritical carbon dioxide on the conformation of syndiotactic polystyrene

The crystallization behavior of miscible syndiotactic polystyrene (sPS) and atactic polystyrene (aPS) blends with different sPS/aPS weight ratios was investigated in supercritical CO₂ by using Fourier‐transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction. Supercritical CO₂ and aPS exhibited different effects on the conformational change of sPS and competed with each other. Increasing the content of amorphous aPS in the blends made its effect on the conformational change of sPS gradually surpass that of supercritical CO₂. Supercritical CO₂ favored the formation of the helical conformation of sPS in lower temperature range and the all trans planar conformation in higher temperature range, instead of forming the latter one only in higher temperature range in ambient atmosphere. However, increasing aPS content in the blends pushed the range for forming the helical conformation to lower temperature and made the all trans planar conformation dominant in aPS/sPS 25/75 blend after treating in supercritical CO₂ above 60 °C. The all trans planar zigzag conformation was more favorable than the helical conformation after mixing aPS in sPS in supercritical CO₂. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1755–1764, 2007     

Diffusion of oxygen and carbon dioxide in thermally crystallized syndiotactic polystyrene

The diffusion, solubility, and permeability behavior of oxygen and carbon dioxide were studied in amorphous and semicrystalline syndiotactic polystyrene (s‐PS). The crystallinity was induced in s‐PS by crystallization from the melt and cold crystallization. Crystalline s‐PS exhibited very different gas permeation behavior depending on the crystallization conditions. The behavior was attributed to the formation of different isomorphic crystalline forms in the solid‐state structure of this polymer. The β crystalline form was virtually impermeable for the transport of oxygen and carbon dioxide. In contrast, the α crystalline form was highly permeable for the transport of oxygen and carbon dioxide. High gas permeability of the α crystals was attributed to the loose crystalline structure of this crystalline form containing nanochannels oriented parallel to the polymer chain direction. A model describing the diffusion and permeability of gas molecules in the composite permeation medium, consisting of the amorphous matrix and the dispersed crystalline phase with nanochannels, was proposed. Cold crystallization of s‐PS led to the formation of a complex ordered phase and resulted in complex permeation behavior. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2519–2538, 2001     

Solidification of syndiotactic polystyrene by a continuous cooling transformation approach

Syndiotactic polystyrene (sPS) was solidified from the melt under drastic conditions according to a continuous cooling transformation methodology developed by the authors, which covered a cooling rate range spanning from approximately 0.03 to 3000 °C/s. The samples produced, structurally homogeneous across both their thickness and surface, were analyzed by macroscopic methods, such as density, wide‐angle X‐ray diffraction (WAXD), and microhardness (MH) measurements. The density was strictly related to the phase content, as confirmed by WAXD deconvolution. The peculiar behavior encountered (the density first decreasing and then increasing with the cooling rate) was attributed to the singularity of the phases formed in sPS; that is, one of the crystalline phases (α) was less dense than the amorphous phase, and the latter, in turn, was less dense than the other crystalline phase (β). With an increasing cooling rate, the thermodynamically stable phase (β) disappeared first, and it was followed by the α phase. On the other hand, the MH values remarkably depended on the amount of the β phase, the α‐phase content influencing the mechanical properties only to a minor extent. The behavior of the crystallization kinetics was described through a modified multiphase Kolmogoroff–Avrami–Evans model for nonisothermal crystallization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2688–2699, 2007     

Crystallization kinetics and crystallization behavior of syndiotactic polystyrene/clay nanocomposites

We investigated the effects of montmorillonite (clay) on the crystallization kinetics of syndiotactic polystyrene (sPS) with isothermal differential scanning calorimetry analyses. The clay was dispersed into the sPS matrix via melt blending on a scale of 1–2 nm or up to about 100 nm, depending on the surfactant treatment. For a crystallization temperature of 240 °C, the isothermal crystallization data were fitted well with the Avrami crystallization equation. Crystallization data on the kinetic parameters (i.e., the crystallization rate constant, Avrami exponent, clay content, and clay/surfactant cation‐exchange ratio) were also investigated. Experimental results indicated that the crystallization rate constant of the sPS nanocomposite increased with increasing clay content. The clay played a vital role in facilitating the formation on the thermodynamically more favorable all‐β‐form crystal when the sPS was melt‐crystallized. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2097–2107, 2001     

Polymorphic crystal forms and morphology of syndiotactic polystyrene in miscible states

X-ray diffraction and optical microscopy characterization were performed to evaluate the phenomenon of alteration of polymorphism of syndiotactic polystyrene (s-PS) in the presence of other blending miscible polymers: poly(2,6-dimethyl-p-phenylene oxide) (PPO) or atactic polystyrene (a-PS). Both α and β crystal forms were observed in the neat s-PS sample, but only β-form crystal was found in miscible blends of s-PS with a-PS or PPO. The order and neighboring chain segments of neat s-PS are different from those of s-PS/PPO or s-PS/a-PS blends; thus, it is plausible that the greater randomness in the melt state of s-PS/a-PS or s-PS/PPO blends might be unfavorable for formation of α-form crystals from melts. The final spherulitic morphology the s-PS/a-PS or s-PS/PPO blends suggests that the amorphous-state miscibility of does not change much the spherulitic structure of s-PS. The radial growth rate is, in general, depressed with the presence of blending miscible polymers in s-PS of equal T_g or PPO of higher T_g. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2725–2735, 1998     

Strong diffuse scattering and lamellar morphologies of syndiotactic polystyrene: Polymorphic effects

Syndiotactic polystyrene (sPS) samples melt‐crystallized into neat α″‐ hexagonal and β′‐orthorhombic modifications were prepared at various temperatures thoroughly for extensive morphological studies. The lamellar morphologies of the as‐prepared sPS samples were investigated with small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). For SAXS measured at 25°C, a barely observed scattering hump was detected for β′‐form sPS, whereas no discernible scattering feature was found for α″‐form sPS because of a small difference in the electron density between the crystalline and amorphous phases. For increased scattering contrast and strength, SAXS was carried out at 150°C so that more reliable morphological parameters would be obtained. In addition to the enhanced scattering peak relevant to the lamellar features, strong diffuse scattering near the beam stop was observed for both α″‐ and β′‐form sPS samples. The contribution of the diffuse scattering at low q regions (where q is the scattering vector) was rather prominent, obscuring the precise position of the scattering peak. On the basis of the Debye–Bueche theory, the strength and inhomogeneity length were derived to render the diffuse scattering. After the subtraction of the diffuse scattering from the observed intensities, scattering intensities exclusively associated with the lamellar features were obtained. Lamellar thicknesses were further derived from the one‐dimensional correlation function of the modified intensities, and a good agreement was reached in comparison with TEM results. From exhaustive TEM observations on the RuO₄‐stained samples, long and parallel lamellae were readily observed in β′‐form sPS. However, 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. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2457–2469, 2003     

Two‐phase crystallization kinetics of syndiotactic polystyrene

In this work, a two phase crystallization model based on the extension of the Kolmogoroff approach was proposed and verified by comparison with experimental isothermal and nonisothermal crystallization data of Syndiotactic Polystyrene (sPS) in a very wide range of cooling rates, up to 600 °C/s. To investigate the effects of high cooling rate on the sPS crystalline structure, a homemade apparatus was adopted. The morphology in solid samples was analyzed by densitometry, IR spectroscopy, and X‐rays diffraction. The coupling of these techniques allows the determination of the fractions of different crystalline phases. In agreement with melt‐crystallization studies of sPS proposed by different authors, either α and β forms could be produced depending on the thermal history of the sample. Results show that the stable β form is favored for specimens solidified at higher temperature or under low cooling rates, whereas α and mesomorphic forms are favoured at low temperature or high cooling rates. The proposed multiphase crystallization kinetics model successfully described all the range of experimental data. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1757–1766, 2010