Prediction of brittle-to-ductile transitions in polystyrene

In this study it is attempted to predict brittle-to-ductile transitions (BDTs) in polystyrene blends, induced either by an increase in temperature or by a decrease in inter-particle distance. A representative, two-dimensional volume element (RVE) of a polystyrene matrix with 20% circular voids, is deformed in tension. During deformation a hydrostatic-stress based craze-nucleation criterion [1] is evaluated. The simulations demonstrate that crazes initiate at low temperatures while a transition from crazing to shear yielding (BDT) is found around 75 °C. The numerical results correlate well with tensile tests on similar heterogeneous polystyrene. The presence of an absolute length, as experimentally found, is more difficult to explain. Near a free surface a T_g-depression is measured for polystyrene and also the resistance to indentation in polystyrene is lower than expected from bulk properties. Both observations are rationalised by an enhanced segmental mobility of chains near a free surface. As a consequence of these findings, an absolute length-scale could be incorporated in the numerical simulations. For simplicity, the length-scale is modelled by taking a temperature gradient over a thin layer near the internal free surfaces of the RVE. Deformation of the RVE with different absolute length-scales shows that indeed also the experimentally found brittle-to-ductile transition can be predicted if the ligament thickness between the inclusions (‘voids’) in polystyrene is below a critical value of ca. 15 nm.     

Solution intercalation of polystyrene and the comparison with poly(ethyl methacrylate)

Polystyrene nanocomposites have been prepared via solution intercalation method. Combination of wide-angle X-ray diffraction and thermogravimetric analysis is used to study the effect of solvents on the morphology of the nanocomposites as a function of the amount of residual solvent. d-Spacing of the polystyrene nanocomposites has a minimum value when the residual solvent concentration is between 2 and 4 wt%. Different interaction level between the solvent molecules and polymer chains is considered to be the reason for this special d-spacing change behavior. By comparing the solution intercalation of polystyrene and poly(ethyl methacrylate) from different solvents, it is concluded that the interactions between polymer-surfactant, solvent-surfactant, and polymer-solvent play important role for the solution intercalation of polymers.     

Low Electrical Percolation Threshold of Silver and Copper Nanowires in Polystyrene Composites

Silver and copper nanowires have been synthesized using a scalable method of AC electrodeposition into porous aluminum oxide templates, which produces gram quantities of metal nanowires ca. 25 nm in diameter and up to 5 and 10 μm in length for Ag and Cu, respectively. The nanowires have been used to prepare polystyrene nanocomposites by solution processing. Electrical resistivity measurements performed on polymer nanocomposites containing different volume fractions of metal indicate that low percolation thresholds of nanowires are attained between compositions of 0.25 and 0.75 vol %.     

Studies of the positron lifetime and Doppler-broadened annihilation radiation of polypropylene-polystyrene alloys

Positron annihilation lifetime spectroscopy and Doppler-broadened annihilation radiation (DBAR) experiments were performed on polypropylene-polystyrene (PP-PS) alloys (0-100% styrene) prepared by in situ polymerisation of styrene in a PP host matrix. The mean size of free-volume holes estimated from the ortho-positronium lifetime τ₃ shows a continuous decrease from 0.119 nm³ in PP to 0.095 nm³ in PS. The intensity of the o-Ps component, I₃, the average positron lifetime τ_av, the curve-shape parameter S and the peak height H of the DBAR spectra increase linearly with the styrene concentration. This is attributed to a linear superposition of the Ps yields of PP and PS in the PP-PS alloys. The DBAR spectra were fitted by a sum of three Gaussians, the narrowest of them is attributed to self-annihilation of para-positronium confined within holes. After its subtraction, a ‘broad component’ is obtained which represents the momentum distribution of electrons bound to molecules. Its normalised peak height does not show any change with the composition which reflects the fact that both constituents of the PP-PS alloys contain only hydrogen and carbon atoms in their chemical units. Immersing of PP into styrene liquid leads to a very pronounced increases in the lifetime parameters which is attributed to the plasticisation of PP.     

Synthesis and properties of polystyrene/graphite nanocomposites

In this paper, graphite/polystyrene nanocomposite is synthesized by in situ polymerization of styrene in a tetrahydrofuran (THF) solution system of potassium (K)-THF-graphite intercalation compound (GIC). K-THF-GIC has proved to initiate polymerization of styrene by the anionic mechanism. Due to the interfacial interaction between the graphite nanolayers and the polymer, the composites exhibit higher glass transition temperature and higher thermal stability when compared to polystyrene. The percolation threshold in the conductivity of the composites is lesser than 8.2 wt% and the dielectric constant can reach as high as 136.     

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.     

消失模精密铸造工艺研究

借助熔模铸造制壳工艺制成消失模铸造用的超薄空腔型壳，以减少消失模铸造在浇注过程中因聚苯乙烯气化而产生的铸件增碳、增氢等缺陷。聚苯乙烯模失模工艺为随炉升温至600℃，保温60min,型壳焙烧工艺为800℃焙烧1h。