Relaxation in poly(alkyl methacrylate)s: Crossover region and nanophase separation

Relaxations in a poly(alkyl methacrylate) series are systematically influen ced by chemical modifications like the variation of side-chain length, rand om copolymerization, or molecular weight. Recent results concerning the inf luence of chemical modification on special parts of the relaxation chart ar e reviewed. The discussion is focused on two points: (i) The influence of c hemical modifications on the crossover region of dynamic glass transition, where the relaxation time of alpha relaxation and Johari Goldstein mode bet a approach each other, is discussed. A general crossover scenario with a se parate onset of cooperative alpha relaxation is observed for all lower memb ers of this series. High temperature process a above and cooperative alpha relaxation below the crossover are shown to be distinct processes. Chemical modifications related to an increase in free volume shift this scenario ma inly to lower frequency and temperature. Further details depend on the spec ific modification. (ii) The nanophase separation of incompatible main- and side-chain parts in all higher members of the poly(alkylmethacrylate) serie s is discussed. This effect is concluded from the coexistence of two dynami c glass transitions in these homopolyers, the conventional a (or alpha) pro cess and an additional low temperature glass transition alpha (PE). It is s hown that the low T-g process is related to cooperative motions in the poly ethlene-like side-chain parts. The existence of static nanodomains in the r ange of 0.5 to 1.5 nm is confirmed by means of wide and small angle X-ray s cattering data. The estimated nanodomain size is compared with the size of dynamic heterogeneities estimated independently from calorimetric data for the polyethylene-like glass transition using the fluctuation approach.