Properties of random and block copolymers of butadiene and styrene. I. Dynamic properties and glassy transition temperatures

The effect of monomer sequence on physical properties was investigated for butadienestyrene solution copolymers made by organolithium initiation. The polymers varied from random copolymers of uniform composition along the polymer chain to ideal block polymers of specific block sequence arrangement and included rubbers of intermediate degrees of randomness. Uniform composition random copolymers exhibit a single glass transition temperature and a very narrow dynamic loss peak corresponding to this transition. The glass transition can be predicted from the styrene content and the microstructure of the butadiene portion of the rubber. Random copolymers in which composition varies along the polymer chain, and to some extent between molecules, exhibit a single glass transition, but the dynamic loss peak is broadened. The extent of this broadening is shown to be compatible with the sequence distribution, polymer segments of various compositions losing mobility at different temperatures. This indicates a tendency for association between segments of different temperatures. This indicates a tendency for association between segments of different chains which are similar in composition. Block copolymers display two transitions, corresponding to T_g for each type of block. The position and width of the dynamic loss peaks are related to block length and compositional purity of the blocks.