Restricted Segmental Mobility in Side-Chain Crystalline Comblike Polymers, Studied by Dielectric Relaxation Measurements

Abstract

The dynamics of comblike polymers with a methacrylic backbone and flexible side chains were investigated by frequency and temperature dependent dielectric measurements. The cooperative motions of the main-chain segments and the side chains were first characterized in an amorphous polymer, i.e., the homopolymer of oleoyl methacrylate (POMA). Then the chemically similar homopolymers of stearyl (PSMA) and perfluorododecyl methacrylate (PFMA) were studied where the side chains can crystallize, forming lamellae. Above the melting point, the segmental relaxations of PSMA and PFMA are similar to those of the amorphous POMA. Below the melting point, however, the mobility of the methacrylic main chains is almost entirely suppressed, although the chain backbones are not built into the crystalline lamellae of the side chains. As a particularly interesting case, the random copolymer (SMA−FMA) of stearyl (SMA) and perfluorododecyl (FMA) methacrylate units in equal shares was studied. In SMA−FMA, the SMA and FMA side chains crystallize independently in a bilayer structure. The FMA side chains melt at a higher temperature than the SMA side chains. Between the two melting points, the copolymer SMA−FMA consists alternatingly of molten SMA lamellae and crystalline FMA lamellae. In this situation, the methacrylic main chains are expected to be only partly liberated. In fact, the relaxation strength was found to increase stepwise at the melting points but also continuously between the melting points. The latter phenomenon indicates that, in the restricted bilayer structure of a comblike copolymer where molten and crystalline side-chain layers alternate, the crystalline layers lose, at increasing temperatures, successively their immobilizing effect on the main chain.