Kenzo Okamoto

Observation of deformation and recovery of poly(isobutylene) droplet in a poly(isobutylene)/poly(dimethyl siloxane) blend after application of step shear strain

The deformation and recovery of a poly(isobutylene) droplet with a lower viscosity embedded in a poly(dimethyl siloxane) matrix are directly observed from two directions after application of a large step shear strain. The droplet shape and recovery time strongly depended on the magnitude of the applied strain. Just after application of a large strain, a droplet deforms almost affinely to a flat ellipsoid. It then changes into a rodlike shape, a dumbbell, and to an ellipsoid of revolution, and finally to a sphere. Model calculations show that the primary driving force for the shape recovery is the interfacial energy in order to reduce the surface area of the deformed droplet.

Shape recovery of a dispersed droplet phase and stress relaxation after application of step shear strains in a polystyrene/polycarbonate blend melt

We observed the stress relaxation and shape recovery of a dispersed droplet phase after application of step shear strains in a polystyrene/polycarbonate blend melt. A polystyrene makes a droplet phase in a polycarbonate matrix of higher viscosity. The orientation angle of the droplet is independent of the initial radius. The angle does not change during stress relaxation and is nearly equal to the angle given by the affine deformation. The shape recovery of the droplets leads to the decay of the relaxation modulus at long times. The stress relaxation slows down at long times for large strains, reflecting the retarded shape recovery of the droplets. Calculated time dependences of the relaxation modulus based on the rate equations by Doi and Ohta [J. Chem. Phys. 95, 1242–1248 (1991)] do not agree with the observed slowing down of the stress relaxation. A force balance equation developed by Cohen and Carriere [Rheol. Acta 28, 223–232 (1989)] explains the retarded shape recovery of the droplet from a prolonged ellipsoid of revolution to a sphere.We observed the stress relaxation and shape recovery of a dispersed droplet phase after application of step shear strains in a polystyrene/polycarbonate blend melt. A polystyrene makes a droplet phase in a polycarbonate matrix of higher viscosity. The orientation angle of the droplet is independent of the initial radius. The angle does not change during stress relaxation and is nearly equal to the angle given by the affine deformation. The shape recovery of the droplets leads to the decay of the relaxation modulus at long times. The stress relaxation slows down at long times for large strains, reflecting the retarded shape recovery of the droplets. Calculated time dependences of the relaxation modulus based on the rate equations by Doi and Ohta [J. Chem. Phys. 95, 1242–1248 (1991)] do not agree with the observed slowing down of the stress relaxation. A force balance equation developed by Cohen and Carriere [Rheol. Acta 28, 223–232 (1989)] explains the retarded shape recovery of the droplet from a prolonge...

Bulk properties of multibranched polystyrenes from polystyrene macromonomers : rheological behavior I

Abstract Dynamic shear moduli of multibranched polystyrenes were measured as functions of frequency and temperature using a parallel-plate rheometer. The multibranched polystyrenes are poly(macromonomer)s of ω-methacryloyloxyethyl polystyrene macromonomers (MA-PSt)s and statistical copolymers of the MA-PSt with methyl methacrylate (MMA) monomer. The master curve of the storage dynamic shear modulus G′ for the poly(macromonomer)s did not show the so-called plateau region and the G′ gradually decreased from the edge of the glass transition region to the terminal zone and the loss modulus G″ was always larger than G′. The plateau region became clear in the copolymers with less branch density. These results indicate that the intermolecular chain entanglement might be strongly restricted in the poly(macromonomer) systems due to the multibranched structure of high branch density, which also explains the brittle property of the poly(macromonomer) films.

Shear Modification and Elongational Behavior of Two Types of Low‐Density Polyethylene Melts with Different Long Chain Branching

Transient biaxial elongational viscosity was measured by the lubricated squeezing flow method in constant volume condition for two kinds of low density polyethylenes (LDPE) with different long chain branches. Significant strain‐hardening was observed at low strain rates for both LDPEs. In addition, strain‐hardening of both LDPEs was comparable although strain‐hardening in uniaxial elongation exhibits great difference. The results may be explained by effect of flow history on elastic properties of LDPEs.