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.     

Brittle-to-ductile transition in high temperature pre-stretched polycarbonate

Journal of Polymer Research - There is considerable scientific literature on MOF-based antibacterial textiles, especially with in situ methodologies for their synthesis. On the contrary, the ex...     

The glass transition behaviour and thermodynamic properties of amorphous polystyrene

The effect of molecular weight (MW) and thermal history on the heat capacity (c_p) of amorphous polystyrene over the temperature range from 200–440 K has been measured using differential scanning calorimetry (d.s.c.) General equations are given that show the influence of MW on c_p in both the glassy and liquid states. An enthalpic definition of the glass temperature reflects changes in the glass structure, due to thermal history, that are not found using conventional d.s.c. constructions. Thermodynamic and statistical mechanical theories give reasonable approximations to the observed T_g-MW curve but small systematic deviations are found. The effect of thermal history on T_g can be reproduced using a MW-dependent activation enthalpy.     

Mechanism for thermal shrinkage of oriented polystyrene monofilaments

Thermal shrinkage behaviour of oriented atactic polystyrene monofilaments, which show a brittle-to-ductile transition in the vicinity of Δn = ?2 × 10^?3 (at a temperature of 20°C and a stretching rate of 100%/min), is investigated by thermomechanical analysis (t.m.a.). The maximum contraction ratio $\text{ΔL}{}_1\text{L}{}_0$ at a constant rate of heating decreases linearly with increasing external tensile stress, and the limiting contraction ratio (at zero external load) has a broad maximum near the transition point. The time dependence of the isothermal contraction ratio $\text{[ΔL(∞) ? ΔL(t)]}\text{ΔL(∞)}$ is represented by an exponential function and an Arrhenius type temperature dependence is realized with an activation energy of several tens of kcal/mol. The degree of birefringence after isothermal shrinkage (Δn(∞)) increases linearly with an increase in external tensile load. Degrees of birefringence before isothermal shrinkage (Δn(0)) and Δn(∞) are related to each other almost linearly. The two-state model is effective in understanding these thermal shrinkage behaviours.     

Properties of densified amorphous polystyrene

Atactic polystyrene was subjected to an elevated pressure–temperature cycle with the resulting densification, mechanical properties, and thermal scanning behavior observed. Most densifications were carried out with the PST as a viscous liquid. In this manner, ambient residual compactions greater than 2% were produced. Pressures up to 90,000 psi and temperatures to 320°C were employed. The technique used for vitrification from the high pressure–temperature region was found to drastically affect the mechanical behavior. If the polystyrene was vitrified from the treatment region by lowering the temperature, the material exhibited enhanced yield strength, by up to 40%. If the polystyrene was quenched by raising the pressure, the samples exhibited much lower mechanical strength. While the mechanical behavior of temperature-vitrified samples is enhanced compared to the pressure-vitrified materials, their densities are comparable. The compaction achieved is primarily determined by the pressure applied as the polymer vitrifies. Thermal scanning behavior of the pressure-vitrified materials show endothermic and exothermic responses below T_g, while the temperature-vitrified materials do not. Annealing the compacted polystyrene at room temperature caused little change in density. However, at temperatures above 60°C, the density relaxed rapidly. Samples which had been temperature vitrified and annealed such that the compaction completely relaxed, still maintained the enhanced mechanical properties of the densified materials.     

Diffusivity of alcohols in amorphous polystyrene

Gravimetry is used to study the diffusion of a homologous series of alcohols (C_n, with n = 1, 2, 3, 4, 6, 10, and 16) in amorphous polystyrene at temperatures from 35 to 145°C, that is both below and above the polymer glass transition temperature of 100°C. All the mass uptake results are well described by a simple Fickian model (for t < t_1/2) and are used to calculate the corresponding diffusion coefficients using the thin‐film approximation. At any particular temperature, the alcohols C_n with n = 3, 4, and 6 possess very similar diffusivities despite the fact that the chain length doubles from C₃ to C₆. This peculiar diffusivity behavior is explained based on the variation of the polymer–solvent chemical affinity along the homologous series. As expected, at any particular temperature, the diffusivity decreases considerably from C₆ to C₁₀ and from C₁₀ to C₁₆. For each alcohol, its log(D) increases linearly with the decrease in liquid viscosity associated with an increase in temperature. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Molecular behavior of amorphous polystyrene under tensile stress

Summary Molecular level changes occurring due to external stress application have been studied by the dynamic infrared spectroscopic technique using Fourier Transform Infrared Spectrometer. The subtraction spectrum (stressed — 88% unstressed) of polystyrene has shown even minute changes in 540, 903, 1154, 1310, 1376, and 1602 cm¹ infrared bands assigned to various vibrational modes of phenyl side groups. Both infrared and x-ray results have indicated certain conformational rearrangements (under stress) of these bulky phenyl side groups present in the PS chains.     

Glass transition of polystyrene model molecules

Sumnary The glass transition temperatures of 2,4-diphenylpentane, 2,4,6triphenylheptane and 2,4,6,8-tetraphenylnonane are shown to follow the relation Tg = 325 – 2.64 × 10⁴/M_n where M_n is the molecular weight. 2,4-diphenylpentane-2,4,6-triphenylheptane mixtures exhibit only one glass transition temperature which follow the relation Tg = (1 – ₂) Tg₁ + ₂ Tg₂ where ₂ is the weight % of 2,4-diphenylpentane. The configurational structure of these polystyrene model molecules is shown to slightly influence the Tg's values.     

Viscosity of polystyrene near the glass transition

A quantitative explanation is given for the apparent viscosity increase with increasing capillary shear rate for polystyrene at temperatures approaching the glass transition, T_g. Possible shifts in T_g as a function of the parameters shear rate, frequency, and pressure are interrelated to viscosity changes. Experimentally, the Instron capillary rheometer and the Weissenberg rheogoniometer provided a means for uncoupling the variables for individual consideration. Calculated and experimental data for the apparent viscosity as a function of the given parameters are presented and discussed. The explanation of the apparent viscosity increase in capillary flow can be quantitatively explained through the pressure dependence of T_g. Brief mention is made of the pressure effects on the Bagley and Rabinowitsch corrections normally made in capillary measurements.     

Glass transition elevation of polystyrene by crosslinks

Analysis of data by Ueberreiter and Kanig on the influence of crosslinking on the elevation of the glass transition temperature (ΔT) of polystyrene gives a plot which departs from a straight line in a way which is suggestive of an entanglement effect. Therefore this work was reinvestigated by differential scanning calorimetric studies of networks prepared by copolymerization of styrene and p-divinyl benzene (DVB). ΔT was found to be simply proportional to the feed of DVB. A case is made that the earlier results differ because of an artifact caused by residual monomer and it is concluded that in crosslinked polystyrene, there is no evidence that entanglements contribute to an elevation of T_g. Similar conclusions are drawn from previously reported data on natural rubber and polymethyl methacrylate.     

Change in the optical properties of amorphous films of transition metal oxides upon formation of the nanocrystalline phase

Weakly absorbing films of tantalum oxide Ta₂O₅ with the crystallization kinetics typical of amorphous films have been investigated. The films have been prepared by reactive magnetron sputtering on substrates of different natures (titanium and optical silica glass). The crystallization heat treatment of the films has been performed at temperatures ranging from 500 to 700 °C. Changes in the optical properties of the films in the visible range have been studied at the initial stage of the formation of nanosized nuclei. Investigations have been carried out using X-ray powder diffraction, atomic-force microscopy, and optical measurements. It has been demonstrated that a new reflecting layer with the refractive index smaller than that of the film is formed on polycrystalline substrates due to the heterogeneous nucleation of the crystalline phase at the film-substrate interface. The crystallization of the films on amorphous substrates occurs through the homogeneous mechanism and leads to a change in the transmission coefficient of the film as a result of light scattering from the nanocrystalline phase.     

Biaxial orientation behavior of polystyrene: Orientation and properties

Square sheets of extrusion grade polystyrene (PS) were biaxially stretched using a laboratory biaxial stretcher. The effects of process parameters such as stretch ratio, drawing sequence, drawing speed, and temperature were studied. Birefringence, mechanical properties, and thermal shrinkage of the stretched sheets were the focus of this study. A high orientation was achieved at high stretch ratio, and orientations from uniaxial to equi‐biaxial were obtained by controlling the relative magnitude of stretch ratio in machine (MD) and transverse (TD) directions. Stretching increased tensile strength and elongation at break significantly, which indicated an improvement in the toughness of the oriented PS sheets. Those properties were correlated with biaxial orientation factors: a rapid increase was observed for both tensile strength and elongation at break for birefringence levels above ?0.005, and below, a plateau was observed. The shrinkage strain and stress were found to correlate well with the biaxial orientation factors. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 487–496, 2003     

Vapour induced ordering phenomena in syndiotactic polystyrene amorphous films

Summary Films of amorphous syndiotactic polystyrene were exposed to dichloromethane vapour at varying activity, up to a=1. Infrared analysis and wide angle X-ray scattering were used to follow the development of ordered structures, induced by the vapour. It has been found that characteristic bands in the infrared spectrum appear after activity a=0.45, whereas X-rays show that the sample is still amorphous. The appearance of the crystalline reflections occurs at higher activities and a continuous improvement up to a=1 in the X-ray pattern is observed.     

Recent transition metal catalysts for syndiotactic polystyrene

Syndiotactic polystyrene has attracted much interest in scientific and industrial research after its first synthesis in 1985 and has led to a fast commercialization of this polymer. The catalyst systems used for this coordination polymerization of styrene are a key point in this development to provide high polymerization activities and syndiotacticities of the polymers obtained.This literature review gives a comprehensive overview on the recent transition metal catalysts comprising the literature since about 2000 and especially on the transition metal complexes investigated in the syndiospecific homopolymerization of styrene. It includes the polymerization activity of the catalysts, the syndiotacticity of the polymers received as well as the discussion of the relationships between catalyst structure and polymerization activity. The complex-coordination mechanism of the syndiospecific polymerization of styrene is summarized in general at the beginning.The review of the recent transition metal catalysts for the syndiospecific styrene polymerization includes transition metal complexes, cocatalysts (methylaluminoxanes and boron compounds), activators and chain transfer agents, and supported and heterogenized catalysts. Transition metal complexes contain group 4 transition metal complexes (mono- and bis-cyclopentadienyl complexes, metal complexes of other ring systems such as indenyl, fluorenyl and other complexes, di- and multi-nuclear complexes, and non-metallocene complexes), and metal complexes of other transition metals (groups 8–10, rare earth metals, and others). The chapter on mono-cyclopentadienyl complexes demonstrates an overview on the investigations using unsubstituted cyclopentadienyl and pentamethylcyclopentadienyl complexes, on the influence of the variation of the structure of the cyclopentadienyl ligand, and on the effect of the variation of ancillary complex ligands besides cyclopentadienyl.This summary also considers recent developments in the preparation of new transition metal complexes based on the synthesis of completely novel π-ligands of the half-metallocenes, the success in attaining high syndiospecificities with transition metal complexes based on rare earth metals, the coordination polymerization in aqueous systems, the syndiospecific living polymerization, and new activators for the catalysts, with regard to syndiotactic polystyrenes.     

Ultradrawing of amorphous polymers by coextrusion illustrated with atactic polystyrene

Films of pure and high-impact atactic polystyrene were prepared by the recently developed technique of solid-state coextrusion. The films were produced at extrusion rates ≥4 cm/min at 126°C with a maximum extrusion draw ratio (EDR) of 11.6. These ultradrawn films are fibrous, have a high birefringence of ?2.24 × 10^?2, and exhibit a 72% elastic recovery. The material has a tensile modulus of ～4–5 GPa and a tensile strength to break of 85 MPa. Thermal analysis suggests a constant T_g.     

Orientation in amorphous polymers II. Characterization of orientation

A method based on network theory is developed for characterizing molecular orientation in amorphous polymers. The proposed procedure gives not only the orientation distribution function for the chain segments in the polymer network (and hence the average orientation) but also a quantitative measure of how this orientation is distributed among the various types of entanglement junctions. The orientation of chain segments can be characterized by two parameters, one which gives the average orientation and another which reflects how much orientation is concentrated in long time entanglement junctions. The new method of characterizing orientation is used to interpret tensile strength data for both brittle and ductile failures.     

Specification of biaxial orientation in amorphous and crystalline polymers

The fundamental concepts for specifying orientation in amorphous and crystalline polymers are reviewed. A new set of orientation factors is proposed to represent the second moments of biaxial orientation. The factors are defined both for the chain axis and for three crystallographic axes of an orthorhombic (or pseudo-orthorhombic) crystal structure. The orientation factors are defined in terms of the angles between the crystallographic axes and Cartesian coordinate reference axes defining the machine, transverse and thickness directions of films. This makes the orientation factors symmetric with respect to the machine and transverse directions unlike the Stein-Nomura-Kawai orientation factors which are defined in terms of Euler's angles. A graphical procedure for representing the state of orientation as a point inside an isoceles triangle is described. Methods of measuring the orientation factors are also reviewed. The paper concludes with examples of the application of these concepts to orientation in amorphous polystyrene films fabricated in our laboratories and to crystalline polyethylene samples discussed in the literature.