The strain energy density function of isoprene rubber vulcanizate has been revealed recently by the experiments using biaxial extension method. The functional form of the strain energy density function, W, obtained is
W=cT(I₁-3)+β(I₁, I₂)
where c : constant, T : temperature, I₁ and I₂: invariants of deformation tensor, and β: function of I₁ and I₂. The first term of the right hand side of the equation is presumed to be the term based on entropy elasticity and the second term is on energetic elasticity of chain network on the basis of a thermodynamic analysis for the biaxial deformation. A concentrated discussion is made in this paper on the characteristics of the β function to clarify its origin based on some experimental results obtained recently. The value of the β function decreases with increasing degree of swelling of rubber and increases with approaching to the glassy state from the rubbery state. These results lead us to consideration of the effect of inter-molecular force on strain energy density function. A simple network model is presented to interpret the β function, where the three forces are considered i.e., attractive and repulsive forces between molecules and the forces of entropy elasticity acting at crosslinked points.
The derived form of the strain energy density function is similar to the form of the functional form observed by experiments. It is concluded that the β function may be related with the inter-molecular forces between network chains.

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A brief review on the rheology of particulate filled plastics in relation to their processability was given, and then the blow molding of particulate filled polypropylene was discussed.
The rheological properties of polypropylene melt filled with various fine particles were studied. The fillers were white carbons, magnesium carbonate, calcium carbonate, talc, and glass beads. It was found that the shear viscosity at low shear rate increased and the steady-state elasticity decreased by the filling. At the same time, the die swell and the melt fracture were suppressed. The most noticeable effects were observed with white carbons. It was inferred that by the filling the parison draw-down should decrease and the high-speed moldability should improve in the blow molding.
To confirm the prediction, the blow moldability of white carbon filled polypropylene was evaluated by use of an actual blow molding machine. With filling of white carbons, the critical temperature, below which the skin roughness of the parison occurred, was reduced and hence the high-speed moldability was improved. The die swell and the draw-down decreased, and hence the parison approached an ideal form.

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The toughening mechanism of rubber modified plastics has been explained by the crack theory of Merz on the first stage. Newman has then proposed the local yielding mechanism of the matrix near rubber particles. More recently, the craze conception proposed by Bucknall et al. has been widely supported by experiments.
This paper describes the recent topics on fracture of ABS resins, mainly works done by Gaggar and Newmann. Gaggar studied the impact strength of prestrained tensile bars beyond the yield conditions. The craze initiation step is extremely important and the contribution of energy absorbed during the initiation step for the fracture is not very significant. Newmann investigated the fracture mechanism of ABS resins with notched specimens and concluded that the craze growth and the crack initiation at the crack tip are both geometry dependent, and the crack growth at maximum load is a function of loading process.

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Rheopexy of disperse systems consisting of silica gel particles in a silicone oil was examined in relation with the state of dispersion of the particles. The suspensions were subjected to a double-step shearing test. Particularly, we investigated the effects of particle concentration and of shear rate on rheopectic processes, and obtained following results: (1) The suspensions with high particle concentration exhibit a unique stress overshoot phenomenon in the rheopectic processes.(2) The initial process of aggregate formation obeys a second-order kinetics.(3) The rate of aggregate formation strongly depends on the surface condition of flocs especially on their surface potential.
An equation for aggregate formation and rupture is proposed. The critical condition for aggregate rupture may be expressed as a function of the shear stress and the average degree of aggregation. The condition is independent of particle concentration.

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Stress-strain behavior of degraded natural rubber vulcanizates was analyzed by using the Mooney-Rivlin type plot to clarify the relation between degraded network structure and property. Mooney-Rivlin plots exhibited a good linear relation even for degraded samples. However, the dependence of C₂ coefficient on the network chain densities for degraded samples was utterly different from that for undegraded samples. It is suggested from the differences in C₂ behavior that the physical flow which occurs in degraded network may influence chemical stress relaxation. A model was presented to explain the effect of the physical flow on chemical relaxation, and the difference between samples vulcanized in dry state and in swollen state was discussed.

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