The Role of Interfacial Fluctuations and Self-Concentration on the Segmental Dynamics of Block Copolymer Melts

Block copolymers spontaneously self-assemble into well defined structures with variable geometry (spherical, cylindrical, lamellar etc) depending on the chemical details (molecular structure, composition etc) and external conditions such as temperature and pressure. It is of significant interest to understand how this structure formation, typical in the range of some tenths to hundreds of nanometer, affects the polymer chain dynamics. For this purpose, poly(isoprene)-poly(dimethyl siloxane) block copolymer melts are ideal as a model system as the both polymers exhibit a low glass transition temperature, strong mutual repulsion (χ ~ 0.11) and well-separated dynamics. Consequently PI-PDMS spontaneously self-assemble to well defined equilibrium structures. Most importantly in the context of dynamics, cis-PI exhibits a net dipole both parallel and perpendicular to its backbone allowing a simultaneous observation of both the global normal mode relaxation as well as the local alpha relaxation using dielectric relaxation spectroscopy. By comparing the dynamics of the corresponding pure PI homopolymers, the effect of confinement on the dynamics can be directly deduced. We will present first results of a study of various PI-PDMS block copolymer melts where the PI part varies between 5 000 to 10 000 g/mole. Using small angle x-ray scattering (SAXS), we deduce the detailed structure and the ordering behaviour (crystal structure) of the system. Dielectric spectroscopic measurements show that the confinement induces a faster and broadened normal as well as local relaxation process having an extra fast relaxing component in the high frequency side of the dielectric loss peak.