RELAXATION DYNAMICS IN POLY(METHYLPHENYLSILOXANE), 1,1-BIS(P-METHOXYPHENYL)CYCLOHEXANE, AND THEIR MIXTURES

Mixtures of the small-molecule glass former 1,1-bis(p-methoxyphenyl)cyclohexane (BMC) with poly(methylphenylsiloxane) (PMPS) are especially interesting because the glass transition temperatures of the components are nearly equal.1 Moreover, these T(g) differences can be systematically varied by varying the molecular weight of the polymer. Dynamic mechanical measurements of the glass transition dispersion have been carried out on the neat liquids and their mixtures. The former exhibit normal segmental relaxation behavior, including a correlation of time and temperature dependences. While the behavior of the polymer-rich mixtures is unexceptional, the BMC dynamics are modified in a very interesting way by the presence of the polymer. In particular, the introduction of a small quantity of higher T(g) PMPS is found to reduce the reorientational relaxation time of the BMC; that is, the relaxation speeds up. A similar anomaly has been observed in polychlorinated biphenyl containing a few percent of poly(vinylethylene).2,3 This behavior is at odds with a simple free volume approach to relaxation in the vicinity of T(g); in fact, the excess volume is negative for BMC mixed with 10% PMPS. An interpretation for these results based on the coupling model of relaxation is suggested.