Influence of high pressure on the structure,hardness and brittle-to-ductile transition of NbSi2 ceramics

We apply first-principles calculations to study the influence of pressure on the structure, elastic modulus, hardness, brittle-or-ductile behavior and melting point of NbSi₂. Four NbSi₂ phases: C40, C11_b, C54 and C49 are considered based on the structural feature. The results show that the calculated formation enthalpy of the four NbSi₂ phases is negative within the pressure range of 0–60 GPa, indicating that they are thermodynamically stable in whole pressure. In particular, the calculated formation enthalpy of the C54 NbSi₂ is smaller than the other NbSi₂ phases, indicating that the C54 NbSi₂ is more thermodynamically stable than the other NbSi₂ phases. The calculated elastic modulus and Vickers hardness of NbSi₂ increase with increasing pressure. Note that the pressure results in brittle-to-ductile transition of the C40 NbSi₂, C11_b NbSi₂ and C54 NbSi₂ between 30 GPa and 60 GPa. Naturally, the increasing of mechanical properties is that the pressure enhances the electronic hybridization between Nb and Si, which is demonstrated by the chemical bonding such as Nb–Si bond and Si–Si bond.     

Non-ionic polystyrene latices in aqueous media

Colloidally stable non-ionic polystyrene latices were prepared in aqueous media, with a narrow distribution of particle sizes, using a nonyl phenol polyethylene glycol condensate as the surface-active agent, methoxy polyethylene glycol methacrylate as the comonomer-stabiliser and ascorbic acid-hydrogen peroxide as the initiator system. For comparison, a typical charge-stabilised polystyrene latex was also prepared using an anionic surface-active agent and potassium persulphate as the initiator. These latices were characterised using transmission electron microscopy, conductometric titration and microelectrophoresis. The stability of the latices to the addition of electrolyte was examined and also the stability to freeze-thaw conditions. The glass transition temperatures of the latices were also determined.     

由废弃的聚苯乙烯泡沫塑料制取无毒的粘合剂

简单介绍在实验室用废弃聚苯乙烯泡沫塑料制取无毒粘合剂的几种配方及一些体会。具体配方如下：配方1：聚苯乙烯259环已烷叩ml醋酸乙酯15ml醋酸异戊酯15ml邻苯H甲酸H丁酯sml酚难树脂Zg配方2：聚苯乙烯259醋酸异成酯40ml丙酮30ml邻苯二甲酸二丁酯5ml酚醛树脂Zg配方3：聚苯乙烯259环己烷40ml醋酸乙酯20ml甲基丙烯酸甲酯10ml邻苯二甲酸二丁酯5ml蔬烯树脂Zg配方‘：聚苯乙烯万g丙酮观ml醋酸乙酯30ml邻苯Th田酸H丁酯sml蒲烯树脂Zg配制方法及几点体会：制取粘合剂时，先将聚苯乙烯泡沫塑料弄碎（若太脏，还需洗净、凉干）置烧林内。在常温下，…     

废旧聚苯乙烯泡沫塑料的综合利用

聚苯乙烯泡沫塑料多为一次性使用,大量的废弃物既造成浪费,也污染环境。本文从８个方面简要阐述了其综合利用的途径和方法。     

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.     

聚苯乙烯市场分析及预测

分析了聚苯乙烯国内外的市场行情，对１９９５年聚苯乙烯的市场前景进行了分析和预测。     

The thermal degradation of polystyrene nanocomposite

Nanocomposite formation brings about an enhancement of many properties for a polymer, including enhanced fire retardancy. This study was carried out to determine if the presence of clay causes changes in the degradation pathway of polystyrene. In the case of virgin PS, the degradation pathway is chain scission followed by β-scission (depolymerization), producing styrene monomer, dimer and trimer, through an intra-chain reaction. As the clay loading is increased, the evolved products produced through inter-chain reactions become significant. Due to the barrier effect of the clay layers, the radicals have more opportunity to undergo radical transfer, producing tertiary radicals, and then radical recombination reactions, producing head-to-head structures, and hydrogen abstraction from the condensed phase also occurs. In the presence of clay, the color of solid residues darkens as the clay content increases. It is thought that this color change is caused by the formation of conjugated double bonds in the presence of clay.     

Degradation of polystyrene in supercritical n-Hexane

Degradation of polystyrene was carried out in supercritical n-hexane under reaction temperature ranging from 330 °C to 390 °C, pressure ranging from 30 bar to 70 bar and reaction duration of 90 min. The conversion of polystyrene increased with rising temperature and pressure. The degradation performance was influenced by the temperature rather than applied pressure. Polystyrene rapidly degraded in 30 min after reaching a prescribed temperature ranging from 350 °C to 390 °C. At a prescribed temperature of 390 °C, the degree of degradation was higher than 90%. The degradation reaction was examined experimentally at a relatively low temperature of 330 °C. The degradation of polystyrene by using supercritical n-hexane has been found to be more effective compared to general pyrolysis (thermal degradation). Among the selectivity of liquid products, that of a single aromatic ring group like styrene at 390 °C increased up to 65% in 90 min. It was found from the analysis by a gel permeation Chromatograph (GPC), that high molecular-weight compounds decreased but oligomers increased with rising temperature.     

Pyrolysis of polystyrene in a batch-type stirred vessel

Isothermal pyrolysis of PS was investigated in the relatively low temperature range of 370 to 400 °C, using a stirred tank of batch-type with respect to the liquid phase; the gas phase was operated in an open system. To maintain an isothermal condition, the stirring speed was 80 rpm. At the initial stage of pyrolysis at each temperature, the first-order kinetic law was found to be operative. The activation energy was evaluated to be 224 kJ/mol a value close to the previously reported one. The main liquid products were single and double aromatic ring species: styrene, α-methylstyrene, toluene and 1,3-diphenylpropane, 1,3-diphenylbutene. The yield of styrene was much larger than any other product and amounted to ca. 70 % by weight. The present reactor, operated in an open system with respect to the gas phase, tends to suppress the scission of volatile diphenyl radicals because of short residence times of these radicals.     

Friction of polystyrene: Consequence on nano-wear

Friction is able to induce major consequences on surface polymer properties (wear, scratch, etc.). These problems are crucial in the case of organic coatings (paints, varnish). The aim of this work is to analyse the influence of friction on the nano-wear behaviour of polystyrene. Studies will be focused on the analysis of the transfer layer induced by the friction of a polystyrene cylinder in contact with a flat and smooth substrate. The model substrate is a hydrophilic silicon wafer (hydroxylated by a piranha treatment). Friction experiments are performed with a translation tribometer which measures the tangential force between the polymer and the substrate for controlled normal force and friction speed. The transfer layer is analysed using atomic force microscopy (AFM). Tentative correlations between transfer layer characteristics and polymer properties are proposed.     

Structure of oriented polystyrene monofilaments and its relationship to brittle-to-ductile transition

Oriented atactic polystyrene monofilaments show brittle-to-ductile transition in the vicinity of a degree of birefringence Δn = ?2 × 10^?3 (at a temperature of 20°C and at the stretching rate of 100%/min) independently of the various spinning conditions. The amorphous orientation of a cylindrically symmetric system was investigated by wide-angle x-ray scattering experiments. The orientation of polystyrene monofilaments in real space is denoted by P₂(cos α) within the precision of measurement. The coefficient D₂(r) of the second term in an expansion of cylindrical distribution function in spherical harmonics has two main peaks, near r = 1.6 Å and r = 4 ～ 5 Å. The negative peak near r = 1.6 Å indicates orientation of phenyl groups in a plane perpendicular to the fiber axis. The positive peak near r = 5 Å (brittle region) or near r = 4 Å (ductile region) indicates the piling up of phenyl groups for the direction of the fiber axis. It is most probable that the amorphous state of atactic polystyrene consists of an appropriate cohesion of planar zigzag chains of syndiotactic polystyrene. The lower spacing (r = 4 Å) of the positive second peak in ductile region suggests that there are interchain phenyl groups near r = 5 ～ 6 Å in a plane perpendicular to the fiber axis and that the molecular chains are extended parallel to the direction of the fiber axis. This parallel packing of chain segments along the fiber axis suppresses the formation and growth of cracks of polystyrene monofilaments resulting in ductile fracture.     

MALDI-TOF MS characterization of polystyrene synthesized by ATRP

In this work, we investigated the peculiar peaks found in the MALDI-TOF mass spectra of polystyrenes (PS) with a bromine end prepared by atom transfer radical polymerization when silver trifluoroacetate (AgTFA) and THF were used as cationization agent and solvent, respectively, in the MALDI sample preparation. In the MALDI mass spectrum, PS with a terminal bromine was not detected but the species with a terminal double bond (U series), and −22 m/z (T series) and +18 m/z (W series) peaks relative to U series appeared as major peaks. While the U species was reported as a result of the elimination reaction of HBr, but the origin and the structure of the other species have not been elucidated. We found that the −22 m/z peak of U_n is in fact T_n-2, which is the adduct of THF and the anion of the Ag salt, TFA⁻ (+186 m/z) to U_n-2. The +18 m/z peak was confirmed to be the species with a terminal OH. All these reactions were catalyzed by Ag salt and the T and W series were not found at all when NaTFA was used as a cationization agent.     

Experimental study and PC-SAFT simulations of sorption equilibria in polystyrene

The knowledge of sorption equilibria of blowing agents in polystyrene (PS) is necessary for the optimization of PS foam production. The sorption equilibria were studied experimentally using a gravimetric apparatus and simulated by the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. As chlorinated and fluorinated hydrocarbons are being phased out, alternative blowing agents are important. We place emphasis on n-pentane, as sorption data for the PS+n-pentane system are scarce in the literature. The measured n-pentane and iso-pentane sorption isotherms were used to evaluate the PC-SAFT binary interaction parameters. The sorption of CO₂, N₂ and He in PS was also studied. Cosorption of pentanes in PS was predicted and comparison of the results with our experimental data proved good performance of the PC-SAFT model. The industrially interesting sorption enhancement and inhibition effects were studied using both experimental and simulated ternary data.     

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.     

Transparent,conductive polystyrene in three dimensional configurations

Development of technologies for constructing three-dimensional (i.e. non-planar) transparent conductive electrodes from polymeric materials is a major goal in diverse applications, including optoelectronic devices, flexible electronics, photovoltaics, and others. We present a facile new strategy for creating conductive, transparent gold layering on polystyrene, a widely-used polymer, in different shapes and surface morphologies. The approach is based upon amine functionalization of the polystyrene surface followed by incubation in an aqueous solution of Au(SCN)₄⁻ and brief plasma treatment. We show that this simple deposition process resulted in a homogeneous, transparent, and highly conductive crystalline Au coating. Importantly, electrical conductivity was attained for long distances, even in highly non-planar surfaces containing physical barriers. We further show that the approach can be employed for fabrication of conductive hollow tubes using electrospun polystyrene fibers as templates. The new synthesis scheme might make possible varied applications in polymer-based electronic and photonic devices.     

Adhesion between submicrometer polystyrene spheres

The sphere-substrate contact method was usually used to study adhesion theory because it is rather difficult to make two micrometer or submicrometer spheres contact precisely. Here, we used sphere-sphere contact method by a novel, simple process to investigate deformations of spheres. The polystyrene particles size ranges from 60 nm to 600 nm. We found that the polystyrene particles underwent plastic deformations due to van der Waals interaction. The contact radii were observed by the scanning electron microscope (SEM).     

Friedel-Crafts crosslinking methods for polystyrene modification: 2. Tg′Tll and T′ll transitions of grated polystyrene

Polystyrene was grafted with 1,4-dimethyl-2,5-dichloromethyl benzene in dichloroethane solution at 50°C using SnCl₄ as a catalyst. Thermal analysis of grafted polystyrene samples using differential scanning calorimetry revealed two liquid-liquid transitions, T_ll and T′_ll, in addition to the glass transition, T_g′ and the decomposition temperature, T_d. The effect of the number of junctions per polystyrene chain on T_g′T_ll and T′_ll is examined and analysed.     

EPR kinetics in irradiated syndiotactic polystyrene at elevated temperatures

Free radicals are formed upon irradiation of polymers. The annealing of γ-ray irradiated syndiotactic polystyrene (sPS) with doses 10, 23 and 36 kGy was studied with electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra are possibly attributed to three types of radicals, (a) the benzyl radical Ra, (b) resonance structure of the phenyl radical Rb, and (c) carbon-superoxide-centered radical Rc with three-, four-, and single-line features, respectively. Radical Ra can be easily formed by the removal of the proton from the tertiary carbon; radical Rb is created from delocalization of the spin of the radical Ra onto the benzene ring; and radical Rc may be generated from the interaction of a carbon-centered radical with dioxygen from the air which forms a carbon-superoxide center. By comparing EPR spectra of the radicals with a DPPH standard, the spin numbers of the radicals can be calculated. The spin number of all radicals decreases exponentially with time in the temperature range of 60-90 °C regardless of dose of the irradiation. The annealing of Ra, Rb, and Rc follows first-order kinetics. The activation energies of the annihilation are determined to be 15.8-19.0, 16.0-19.5, and 23.2-26.6 kJ/mol for radicals Ra, Rb, and Rc, respectively. The kinetic study presented herein can serve as a criterion for γ-ray irradiation process in various applications, such as sterilization of polymer materials and devices.     

Simulation of mechanical properties of oriented glassy polystyrene

Mechanical properties of processed polymers depend sensitively on their microstructure. In order to understand how different processing conditions affect the mechanical properties of polymers, one needs a means to describe the process-induced microstructure. Because the characteristic relaxation times of processed polymer chains often span several orders of magnitude, it is commonly the case that partial relaxation of the chains is frozen into the final product. We report results of molecular simulations by the Semi-Grand Canonical Monte Carlo (SGMC) method to study the orientation-dependent elasticity of glassy polystyrene as a function of both the system-average degree of orientation and the degree of relaxation of chain ends at a constant average orientation, in accord with the tube model of Doi and Edwards. Our simulations reproduce quantitatively the experimentally observed trends in the tensile modulus E₁₁ as a function both of the system-average orientation and of the inhomogeneity of the orientation along the chain due to rapid relaxation of chain ends. The results show that the partial relaxation of the polymer chains is sufficient to explain the observed variation of mechanical properties for samples that differ in processing history, yet have the same observed birefringence.     

New methodologies for separating glass transitions from energy of film formation in polystyrene latexes

Polystyrene latexes were synthesized to study water evaporation details, and to separate the energy of film formation from the glass transition temperature. Three experimental techniques were employed in the present work to study the formation of polystyrene latex films: standard differential scanning calorimetry (DSC), modulated DSC, and the Mahr method applied to modulated DSC. High-resolution thermogravimetric analysis (TGA) was used to study the water evaporation. The TGA experiments revealed evaporation curves consistent with the notion of surface tension and tortuosity effects being the controlling factors. The method of Mahr was combined with the new software package for DSC to provide an improved method of separating the glass transition of polystyrene from its surface tension-based free energy of film formation. The result was that the films formed under the extremely mild conditions employed had 30–70% of their surfaces obliterated by interfacial adhesion. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1763–1768, 1999