Effects of blending on the polymorphic behavior of melt-crystallized syndiotactic polystyrene

The complex polymorphic behavior of syndiotactic polystyrene (s-PS) in melt-crystallized samples is altered by blending with poly (2,6-dimethyl-1,4-diphenylene oxide) (PPO). In particular, to render the beta form in these blends, starting with samples containing the α or γ froms, requires much lower temperatures and shorter melting times than for pure s-PS. On the basis of the results, it is suggested that this phenomenon is due to more rapid loss of the memory of the α form, for the same temperature and time, in the melt in the presence of PPO molecules.     

Kinetics of melting and characterization of the thermodynamic and kinetic properties of syndiotactic polystyrene

A study of the crystallization process of syndiotactic polystyrene is carried out, with particular emphasis on the effect of previous melt annealing on the subsequent crystallization kinetics and polymorphism, with a combination of thermal analysis, infrared spectroscopy, and X‐ray diffraction. An effort is made to obtain quantitative results in terms of the relative crystallinity content of the α and β phases after a given thermal treatment. A kinetics of melting for the crystalline memory is proposed, which enables the determination of the time at a given temperature that ensures the complete fusion of the α crystals. This leads to a generalization of the effects of the time–temperature couple on the memory effect. A particular protocol is identified, which upon solidification from the melt induces the development of just one crystalline phase, either α or β. This allows the determination of the enthalpy of crystallization, the maximum attainable crystal volume fraction, the crystallization half‐time as a function of temperature, and the Avrami index for each of the two crystalline phases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 196–207, 2007     

Syntheses and properties of syndiotactic polystyrene

Syndiotactic polystyrene (SPS) is a new crystalline engineering thermoplastic. With a melting point of 270 °C and its crystalline nature, SPS has high heat resistance, excellent chemical resistance and water/steam resistance. Since SPS has excellent dielectric properties, it is useful as a capacitor insulation material. The rate of crystallization is very fast in comparison with isotactic polystyrene (IPS), thus, SPS can be used in a number of forming operations, including injection molding, extrusion and thermoforming. A system composed of a homogeneous titanium compound and methylaluminoxane (MAO) is an effective catalyst for syndiospecific polymerization of styrene. On the other hand heterogeneous titanium compounds containing halogen make a mixture of isotactic and syndiotactic components. The amount of syndiotactic polystyrene obtained is dependent on the mole ratio of Al to Ti. The result of ESR measurement suggests the Ti³⁺ species are important as a highly active site for producing syndiotactic polystyrene. A comparison of the stereoregularities of polypropylene and polystyrene formed by various metallocene catalysts is studied. The (C₆H₆)₂C(η₅-C₅H₄)(η₅-C₉H₆)TiCl₂/MAO system gives a homogeneous catalyst for the polymerization of propylene giving isotactic rich polypropylene and of styrene to give syndiotactic polystyrene.     

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     

Solvent-induced crystallization of syndiotactic polystyrene in toluene

The diffusion of toluene in glassy syndiotactic polystyrene was analyzed for films with different thickness. It was found that the reduced sorption curves superimpose, denoting that the diffusion follows a Fickian behaviour. The intensity of transmitted light in a polarizing microscope was followed during swelling, and it was found that there is development of order and it is strongly diffusion-controlled. Films dried after immersion in toluene show a crystalline wide angle X-ray pattern, while in the swollen state the possibility of a liquid-crystalline-like state can be suggested.     

Neutron scattering studies of syndiotactic polystyrene crystals

Neutron scattering experiments are reported on solution grown crystals of syndiotactic polystyrene (sPS) at small and large q, in order to investigate the chain conformation within the lamellar crystals. From the results in both the Guinier and the intermediate ranges, we come to the following conclusions: The chain folds in one or several sheet depending on the molecular weight, comparison between measured and calculated intensity plots give strong indication that a is the fold direction. The statistical model recently built for the spatial arrangement of stems in a sheet-like structure is successful again, and demonstrate a strong statistical preference for adjacent re-entry.     

Mechanisms of reorganization of lamellae in syndiotactic polystyrene

The multiple melting behavior of syndiotactic polystyrene (sPS) and its possible mechanisms via preexisting lamella types and/or scanning-induced lamellar reorganization were investigated by using X-ray diffraction, DSC, and scanning electron microscopy. Melt-crystallized sPS samples, upon DSC scanning, exhibited three melting peaks (I, II, III). A morphological analysis showed that flat-on lamellae develop first, which yield P-I and P-II melting, and during scanning recrystallize to thickened edge-on lamellae with a P-III melting peak. Upon scanning, melting of P-I (crystal of the lowest melting peak) is followed by repacking into thickened P-III crystal, the lamella of which also reoriented to a perpendicular orientation. The P-II crystal, however, melts at temperatures too close to the melting temperature of P-III; thus, during scanning up, the P-II crystal simply melts without sufficient time to repack into the thickened P-III crystal. In addition to the P-III crystal species that can be added by melting P-I and repacking to P-III, it is believed that preexistence of different lamella crystals was jointly responsible for the multiple melting. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3210–3221, 2000     

Drawing behaviour of syndiotactic polystyrene (sPS)

The drawing behaviour of amorphous and solvated structure of syndiotactic polystyrene has been analysed. By means of X-ray analysis and infra-red spectrometry we have studied the conformational transitions induced by the drawing processing.     

Preparation of syndiotactic polystyrene using the doubly bridged dinuclear titanocenes

As a new kind of dinuclear metallocene four DBDM (doubly bridged dinuclear metallocene) that hold two different bridging units linking two metallocenes have been prepared and their polymerization properties have been pursued. The selected bridging ligands for DBDM were polymethylene and dialkoxy terminated derivatives to bond two cyclopentadienyls and two titanium centers, respectively. The syntheses of new dinuclear metallocenes 5-8 were able to be achieved by the reaction between dinuclear half-titanocenes 1 and 2 and ditrimethylsiloxy terminated derivatives 3 and 4 at −78 °C. The EI mass spectra and ¹H and ¹³C NMR spectral data were very informative to identify their formulations as well as structural features. The polymerization of styrene was conducted by using DBDM 5-8. From the polymerization studies it was found that (i) DBDM 7 and 8 holding 2,2-diethyl-1,3-dipropanoxy (DEP) bridge not only show greater activity but also produce higher syndiotactic polystyrene than DBDM 5 and 6 holding 1,1,4,4-tetramethyl-1,4-dibutanoxy (TMB) bridge between two titanium centers, (ii) activities increase with increase in the polymerization temperature, the amount of cocatalyst and monomer concentration, (iii) the most crucial factor to control the stereoregularity of the resulting polystyrene is the styrene concentration in reaction system, (iv) surprisingly the catalyst structure does not have much effect on molecular weight of the obtained polystyrenes. The most significant feature from this study is that the second bridging dialkoxy terminated ligands connecting two titanium centers likely exert more pronounced influence than the polymethylene bridges between two Cp groups on the activity of the catalyst as well as the stereochemistry of the generated polymers.     

Lamellar morphology and equilibrium melting temperature of syndiotactic polystyrene in β‐crystalline form

Lamellar morphology and thickness of syndiotactic polystyrene (sPS) samples melt‐crystallized at various temperatures were probed using transmission electron microscopy (TEM) and small‐angle X‐ray scattering (SAXS). In addition, the melting temperature and enthalpy of the crystallized samples were characterized with differential scanning calorimetry. Under appropriate thermal treatments, all the samples investigated in this study were crystallized into β′ crystal modification, as revealed by wide‐angle X‐ray diffraction. From the SAXS intensity profiles, a scattering peak (or shoulder) associated with lamellar features as well as the presence of anomalous scattering at the zero‐scattering vector were evidently observed. The peculiar zero‐angle scattering was successfully described by the Debye–Bueche model, and subtraction of its contribution from the raw intensity profiles was carried out to deduce the intensity profile merely associated with the lamellar feature. The lamellar thickness obtained from Lorentz‐corrected intensity profiles in this manner agrees with that measured from the TEM images, provided that the two‐phase model is applied. On the basis of the Gibbs–Thomson equation, the modest estimations of equilibrium melting temperature and the surface free energy of the fold lamellar surface are 292.7 ± 2.7 °C and 20.2 ± 2.6 erg/cm², respectively, when lamellar thicknesses measured by TEM are applied. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1626–1636, 2002     

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.     

Chemical separations by nanoporous crystalline samples of syndiotactic polystyrene

Chemical separations based on absorption phenomena into syndiotactic polystyrene samples including a nanoporous crystalline phase are described. These separation phenomena are due to the presence into the crystalline phase of cavities whose size and shape have been studied through sorption experiments relative to linear hydrocarbons and through calculation procedures relative to the available crystalline structures of different polymorphic and clathrate forms of syndiotactic polystyrene.     

Nonisothermal crystallization behavior of syndiotactic polystyrene/montmorillonite nanocomposites

X‐ray diffraction methods and differential scanning calorimetry were used to investigate the crystalline structure and crystallization kinetics of syndiotactic polystyrene (sPS)/clay nanocomposites. X‐ray diffraction data showed the presence of polymorphism in sPS/montmorillonite (MMT) nanocomposites, which was strongly dependent on the processing conditions (premelting temperature and cooling rate) of the sPS/MMT nanocomposites and on the content of MMT in the sPS/MMT nanocomposites. The α‐crystalline form could be transformed into β‐crystalline forms at higher premelting temperatures. The nonisothermal melt‐crystallization kinetics and melting behavior of the sPS/MMT nanocomposites were also studied at various cooling rates. The correlation of the crystallization kinetics, melting behavior, and crystalline structure of the sPS/MMT nanocomposites was examined. The results indicated that the addition of a small amount of MMT to sPS caused a change in the mechanism of nucleation and the crystal growth of the sPS crystallite. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 560–570, 2003     

Drawing and dynamic-mechanical behavior of syndiotactic polystyrene an introduction

The drawing behavior of syndiotactic polystyrene was analyzed at different temperatures. Amorphous films were used and, depending on drawing temperature, strain-induced or thermal-induced cold crystallization was observed. This phenomenon, when present, greatly affects the drawing behavior. The dynamic-mechanical behavior of drawn samples was analyzed, and the obtained results indicate that the glass transition is affected by drawing, and that the effect depends drastically on the drawing temperature. Particularly interesting is the dynamic behavior at high temperature where the elastic modulus is weakly affected by temperature also near the melting point.     

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     

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