Expressions are derived from free volume considerations which predict the concentration dependence of viscosity and glass temperature for polymer‐diluent systems. Experimental data for several systems were examined and general agreement with theory was found over broad ranges of concentration. Particular emphasis is placed on the limited amount of information required for the application of these expressions to describe the viscous properties of polymer‐diluent systems.

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A molecular theory for the softening of filled rubbers which is caused by prestressing is presented. It is based upon the assumption that the centers of the filler particles are displaced in an affine manner during deformation of the rubber. Those network chains which are fastened at both ends to filler particles will break when the filler particles have separated enough to stretch the chains to near full elongation. The loss of these chains causes a prestressed rubber to exhibit a much lower modulus than did the original rubber. Equations are derived to describe this phenomena and are tested by comparing with data for black filled synthetic rubber. Good agreement is found if the filler surface area per chain attachment is taken as 44 A.2 and if the strength of the chain is 2 × 10−4 dynes. It is shown that the chains break loose from the filler particle rather than breaking at C C bonds along the chain. The theory appears to offer a convenient tool for systematic studies of rubber–filler interactions.

A previously reported theory for the Mullins softening effect has been used to interpret various new data for the behavior of filled SBR rubber under tensile load. The strength of the filler–rubber bond, the filler surface area per polymer molecule attachment, and the average filler surface separation have been determined for two fillers, HAF black and Hi Sil‐233 (a silica). A styrene–butadiene type filler (Pliolite S‐6) has also been investigated. The temperature dependence of the filler–rubber bond has been measured; results lead to the conclusion that the bonds to carbon black and silica are high energy bonds, probably chemical in nature. It is shown that the recovery of hardness in prestretched, filled SBR is a rate process having an activation energy of about 22 kcal./mole. It is inferred from this and from permanent set data that the recovery is the result of the chemical breaking and reforming of the rubber chain network at the higher temperatures where recovery occurs. Silica‐filled rubbers are shown to possess a pseudoyield stress which gives rise to an anomalous shape for the stress–strain curve of this material when it is stretched for the first time. A prestretched, silica‐filled rubber recovers its hardness when left at 115°C. for 20 hr., but the anomalous portion of the curve is replaced by more normal behavior. Possible interpretations of the observed results are given.

A theory for the shear degradation of high polymers is presented. It assumes the molecules to exist in the rubbery or liquid state and does not include the effects of chemical reactions which might occur subsequent to chain rupture. The computation shows that the entanglements along the chains play a major part in the rupture process. Expressions for the rate of chain rupture are obtained in terms of the melt viscosity, shear rate, and molecular weight. Other factors enter also, but the sensitive variables are those listed. The variation of molecular weight distribution with shear degradation is considered. It is pointed out why oxidative scission during shear is not a random process. In general, the theoretical result agrees with the limited experimental data available.