A molecular theory is developed to describe quantitatively the permanent set taking place in thin samples of vulcanized natural and synthetic rubbers held at constant extension at elevated temperatures. Permanent set: is considered to be the result of the formation, through the action of molecular scission and cross-linking reactions, of a dual molecular network in the rubber sample, in which the network chains are of two types: chains which are at equilibrium when the sample is at its unstretched length, and chains which are at equilibrium when the sample is at its stretched length. According to the theory the amount of permanent set in a rubber sample is a function of only two quantities: the relative ratio of the number of chains of the two types, and the elongation at which the sample was held. Experimental data on permanent set for various rubber types and under different conditions are presented and are shown to be in good agreement with the theory.

In theories of the minor phase (domain) formation in polyblends rendered as emulsions it is usually assumed that the size and shape of the domains are the result of melt viscosity effects (Taylor, Wu) or viscoelasticity effects (VanOene, Elmendorp) being balanced by interfacial tension. This assumption would predict a monotonic decrease of the domain size to a final limiting size with increasing energy of mixing. However, a systematic study of the dependence of domain morphology on industrial mixing processes which was carried out on a "model" LDPE/PS (2/1) mixture and the related polyalloy (i.e., the same mixture with a corresponding block copolymer as compatibilizer) does not support this expectation. Domain size was found to go through a minimum a s mixing energy was increased. A similar minimum was seen in data on specific volume of the melt vs. mixing energy, which indicates a correlation between melt specific volume and domain size. Calculation of the approximate surface area of the domains using a simple model of domain shape indicated that total interfacial energy in the polyblend and/or polyalloy is a trivial part of the mixing energy introduced. These calculations also indicated that if compatibilizer was located entirely at the interface, the surface layer would have a thickness of about 90 nm. Some micrographs seem to show such a surface layer. We propose that a n abrasion mechanism is responsible for the early stage of the dispersion process, and that the final domain size may be controlled by a dispersion-coalescence equilibrium. This is compared with the theories of final particle size proposed by VanOene and Wu. A. P. Plochocki, S. S. Dagli, a n d R. D. A n d r e w s groups the larger domains in the vicinity of the extrudate courtesy of M . J . Doyle, Exxon Corp. center (right hand side of the photograph) (cf. 31 b).

A yield criterion is constructed for glassy polymers from results of constant strain‐rate tests in different stress fields. A significant effect of hydrostatic pressure (or volume change) on the yield stress of polystyrene was found. Direct dilatometric measurements were made on polystyrene, poly(methyl methacrylate), polycarbonate, and poly(vinyl formal) samples subjected to uniaxial compression. Both a volume contraction proportional to stress (Poisson's ratio effect) and a volume expansion, which depends on the extent and the history of plastic yielding, were observed. These results are discussed qualitatively in terms of structural models for the glassy state of amorphous polymers.

Actual substances exhibit a very complicated behavior under mechanical stresses which cannot be described by classical elasticity theory nor by the classical theory of the hydrodynamics of viscous fluids. A general molecular theory describing the behavior of matter under stress is discussed and related to previous investigations and to experimental observations. Particular attention is devoted to rubberlike substances for which the classical theories are definitely inadequate. Experimental results on relaxation and creep of rubbers are interpreted in terms of modern structural concepts. I t is found that these substances exhibit three regions of stress-temperature-time dependence. At intermediate temperatures there exists a region of relative stability in which the statistical-thermodynamic theory of rubber elasticity is valid. At elevated temperatures relaxation and creep are caused by chemical changes involving the rupture and formation of primary valence bonds. These chemical changes, which are responsible for the aging of rubber, can be isolated and studi~d by appropriate experimental techniques. At low temperatures relaxation and creep are caused by the slipping of secondary interchain bonds which are breaking and reforming in times comparable to experimental times of measurement. Theories are advanced to explain .the observed stress-temperature-time behavior of rubbers over the entire temperature range studied.

The deformation of glassy polystyrene under a fixed tensile dead load can take place in three different modes: (a) drawing by formation and propagation of a neck, (b) drawing by proliferation of deformation bands without necking, and (c) homogeneous creep. Which mode is observed depends primarily on stress level and temperature. In homogeneous creep, deformation bands are not found; however, they are always observed in connection with the two types of drawing. At any given temperature a boundary stress is observed above which drawing takes place and below which only creep is observed. This critical stress decreases with increasing temperature. It is believed that this is the stress necessary either for the formation or propagation of deformation bands. It seems likely that drawing by shear band proliferation is the ideal mode which would be obtained in a perfectly homogeneous and uniform sample; however, drawing usually goes over into the necking mode because of nonuniformities in specimen geometry or uneven temperature. Neck initiation is delayed for a certain time interval after application of the dead load. The logarithm of the delay time is found to be a linear function of stress and, also, an approximately linear function of temperature.