Pulsed deuteron NMR line shapes have been analysed to characterize type and time scale of the phenylene group motion in glassy bisphenol-A polycarbonate. The motional mechanism involves ~z-flips about the CIC4 axis augmented by small angle fulctuations about the same axis, reaching arms amplitude of _+ 35 ~ at 380 K. The distribution of correlation times for the n-flips is heterogeneous in nature and can be described either by a log-Gaussian or an asymmetric distribution with a more rapid decay at high correlation times comparable to the Williams-Watts distribution. From both distributions essentailly the same mean activation energy of 37 kJ/mol is obtained, whereas the temperature dependent width of the highly asymmetric distribution is somewhat smaller compared to the log-Gaussian distribution. Time scale and activation energy of the n-flip motion are correlated to secondary mechanical relaxations. Low molecular mass additives, which suppress the mechanical relaxation, also hinder the phenylene motion for a substantial fraction of phenylene groups. The effect of additives is not only to shift the mean value of the distribution of correlation times to higher values but also to increase drastically the width of the distribution. The results of this work strongly suggest that the secondary mechanical relaxation and the large amplitude motions of the phenylene groups in polycarbonate are related.

Two effects are of outstanding importance for the mechanical behaviour of polymer-additive mixtures, (a) the plasticizer effect and (b) an effect, which we shall refer to as the "D-suppression"effect: Additives can suppress the secondary (0-) relaxations of the polymer. The O-suppression effect is studied on a number of polymers and additives {dynamic mechanical and dielect r i c measurements), the molecular motions involved are characterized ( H-NMR spectroscopy) and the thermal density fluctuations are measured (SAXS experiments). A model t o explain the effect in terms of the free volume fluctuations i s presented. 2 1 . INTRODUCTION Mixtures of polymers w i t h low molar mass additives have been investigated extensively i n the solution range where the polymer i s the minor component. Much less attention has been devoted t o the concentration range where the polymer i s the major component. Mixtures of the l a t t e r type may be called "plasticized polymers" because of the well known "plasticizer" effect whereby the glass transition temperature of the polymer i s reduced. Nevertheless, the mixtures are quite often glassy a t room temperature, so t h a t , other than with solutions, b o t h t h e i r liquid and t h e i r glassy statesare to be considered.Our attention was focussed on the mechanical In the l i q u i d range, the plasticizer effect l P dominates the mechanical behaviour. I t i s best defined as an acceleration of the cooperative a-relaxation processes of the polymer. The effect has been satisfactorily explained on the basis of conventional free volume theories '). The situation i s entirely different in the glassy s t a t e well below T the glassy mixtures are not a t a l l "plasticized" b u t rather found t o be harder and more b r i t t l e t h a n the parent polymers; they are "antiplasticized"' '3). roperties.Here g '