Modulus of Plasticized Ionic Rubbers

The rubbery modulus of a crosslinked ionizable rubber is explored in the absence of the crosslinking effect of ions, that is, only through the effect of ions on conformation. The crosslinking effect is eliminated by plasticizing the rubber with a liquid of high dielectric constant. Experimentally, a rubber is prepared by mixing poly (acrylic acid) and poly (vinyl alcohol) and cross-linking thermally. The relationship between the rubbery moduli of the unionized and the ionized forms of the plasticized material is studied. A theory for the modulus of such a two-component rubber is derived and the predictions are compared with experiment. It is shown that the predicted ratio of the moduli in the acidic and ionic forms is in the same direction as determined experimentally, but that the calculation underestimates the magnitude of the effect. Several reasons for this discrepancy are given.

Recent Advances in Glass Transitions in Polymers

Since the publication of our review “Glass Transition in Polymers” in 1966, a number of interesting advances have been made in this field. The present article is intended to be an addendum to this review, reporting the progress that has been made by various workers during the past three years. In addition, a number of topics were not covered in the previous article due to the relatively incomplete understanding at that time. These will now be discussed. This is not because these topics are at present fully understood. Rather, we hope it will serve as a progress report to stimulate further interest in areas where further work is needed. The numbering systems and notations in this paper will follow those in the previous article for the sake of continuity.

Glass Transitions in Polymers

We hope that the preceding presentation has made it clear that the glass transition is not a simple phenomenon, but a very complicated process which is still not understood in all of its ramifications. The nature of the glass transition temperature (Tg), to begin with, has been the source cf considerable controversy for many years. The observation of its rate effects, on the other hand, and its similarity to second-order transition, on the other, has caused much confusion. It is not until recently that there is promise for reconciliation. It appears at present that Tg is what one experimentally observes when the material is on its “slow boat” to T2 In other words, Tg is a kinetic manifestation of a thermodynamic process (T2). There is, in fact, no inherent controversy between these two views, since each explains one aspect of the “many-faced” transition. The solution of this problem was actually alluded to in the classical paper of Kauzmann, although it was not recognized as such. The credit of explicit clarification goes to Gibbs and DiMarzio, who first argued the case in quantitative terms.