The rate analysis of steady plastic flow in glassy polymers has verified that the structure of the polymers is changed into a liquid-like structure during the yield process. This result motivated us to develop a mechanical model which predicts the stress-strain curve for glassy polymers. The model is composed of a dashpot and two elastic springs, where the viscosity of dashpot is a single parameter quantifying the structural transition and is approximated by a simple mathematical equation decreasing with increasing strain. Using this model, stress-strain relations were simulated for poly (methyl methacrylate) and poly (vinyl chloride) under uniaxial constant-rate tension and compression. The model reproduced successfully the experimental stress-strain curves over a wide range of temperature and strain rate for each case. This led to a conclusion that at a uniaxial constant deformation rate the structural change in the glasses is mainly governed by strain. Moreover, the optimum equation for the internal viscosity determined by the simulation indicated that the degree of structural change is closely related to both the yield strain and the strain range in the lower yield state.
The effects of degree of polymerization and temperature on flow properties of PVC melts have been studied by using a Koka Flow Tester. Each flow curve is expressed by two straight lines of different slopes which meet at a breaking point. The critical shear rate γ′wc at the breaking point increases as the degree of polymerization decreases or the temperature increases. The critical shear stress τc at γ′wc increases as the degree of polymerization decreases, being almost independent of the temperature. The end correction coefficient ν shows a minimum or begins to increase around γ′wc. The coefficient ν increases as the degree of polymerization increases or the temperature decreases. The extrudate is soomth on the surface at low shear rates, shows a small wavy roughness (shark skin) at shear rates just below γ′wc, becomes fairly smooth again beyond γ′wc, and then shows intensive irregular roughness (melt fracture) at higher shear rates. The die swell becomes notable as degree of ploymerization decreases or the temperature increases. The die swell takes a maximum at a shear rate just below γ′wc, where a shark skin appears on the extrudate. Differing from usual thermpolastic resin melts, PVC melts shows a negative correlation between the die swell ratio and the end correction coefficient. This seems to be due to the following reason: PVC melts have high viscosities and have long relaxation times for strain recovery, and thus the extrudates solidify without being fully relaxed. This tendency of solidification without relaxation is more noticeable as the degree of polymerization increases or the temperature decreases.
The high-density polyethylene (HDPE) films have been prepared either through melt quenching or isothermal crystallization. The fatigue behavior of those HDPE films under constant strain amplitude has been investigated based on dynamic viscoelastic and small angle light scattering (SALS) measurements during the fatigue process. Non-linear viscoelastic behavior under cyclic fatigue has been observed for both melt quenched and isothermally crystallized HDPE's. Melt quenched HDPE showed greater fatigue strength compared with isothermally crystallized HDPE. The SALS measurement under cyclic fatigue can reveal the deformation process of spherulitic structure during the fatigue process. Melt quenched HDPE showed elongation of spherulitic structure under cyclic fatigue. On the other hand, isothermally crystallized HDPE didn't show any elongation until the onset of fatigue fracture. The short fatigue lifetime and small deformation of spherulitic structure for isothermally crystallized HDPE can be ascribed to the sharp boundary between spherulitis.
Stress relaxation and creep behavior of 16 commercial propylene-ethylene block copolymers was measured in a molten state. The relaxation moduli of samples with monomodal molecular weight distribution decrease monotonously with time, while those with bimodal molecular weight distribution show plateau or shoulder at long times. The bimodality of molecular weight distribution in the latter samples is due to the ethylene-propylene copolymer component with high molecular weight. The plateau at long times is more noticeable as the bimodality is stronger. The height and width of the plateau is independent of the ethylene content. A PP-HDPE-EFR blend which was made as a model material of propylene-ethylene block copolymer does not show the plateau. The relaxation modulus at high temperature is always lower than that at low temperature, the time-temperature superposition can be pefrormed well in all the measured range, and the shift factor obeys the Arrhenius equation. It was assumed from the above experimental results that appearance of the plateau at long times originates mainly from the bimodality of molecular weight distribution, not from the dispersion state of the ethylene-propylene copolymer particles.
We developed an unique device for measuring the viscosity of the liquids, using the mechanism of the tuning-folk vibration. This device has a pair of plate springs with a circular plate mounted on the end. When a pair of circular plate is operated in a sample by a resonance vibration in a reverse phase at a constant frequency, its amplitude is inversely proportional to the viscosity of the sample. So the viscosity can be obtained by measuring the amplitude. This device uses the standard liquid for calibration, and stores the calibration curve of the relationship between the amplitude value and the viscosity of standard sample. The rheologycal complicated materials can also be characterized by using this device.
The change of optical property on transverse drawing of a uniaxally oriented PET film was investigated. The birefringence in the plane of film decreased in proportion to transverse draw ratio, and come to zero in the optically balanced condition. The optical axis of uniaxally oriented film separated into two optical axes in the plane perpendicular to the transverse draw direction. At the optically balanced condition, the two separated optical axes joined into one optical axis perpendicular to the plane of films. These observations could be described well with a model based on the assumption that a refractive index in each principal direction changes in proportion to draw ratio.
An apparatus was constructed for simultaneous measurements of the dynamic birefringence and the complex Young's modulus. The frequency range was 1-200 Hz. The instrument was calibrated with a polybutadiene film, for which the stress optical law was expected to hold well. The stress optical coefficient estimated by dynamic birefringence measurement was consistent with the result of the static measurement. The stress-optical law did not hold valid for a polystyrene film containing 5 wt% dibutyl phtalate in the glass-to-rubber transition region.
A simple relationship between the steady-shear polymer viscosity and the zero-frequency storage modulus for dilute polymer solutions is developed. It combines the Oldroyd-B model with the Zimm theory yielding a result that enables the prediction of elastic properties from a given ηp at zero deformation conditions. A correlation established from a large number of experimental data obtained for various solutions of polyacrylamide (Separan) and a polyisobutylene (PIB) agrees well with the theoretical expression. The relationship is also found to be applicable to other polymeric systems such as dilute solutions of xanthan gum and Pusher 700. Based on the simple fluid theory, the correlation may be used to predict the first normal stress coefficient at zero sear rate condition.