Effect of cross-linking on the structure and thermodynamics of lamellar block copolymers

Small-angle X-ray scattering, depolarized light scattering, and transmission electron microscopy were used to study the dependence of structure and thermodynamics on cross-linking of a symmetric poly(styrene-block-isoprene) copolymer. The polyisoprene block was cross-linked well above the order-disorder transition (ODT) temperature of the un-cross-linked block copolymer. A mean-field theory based on a coarse grained free energy and an RPA-like formulation was developed to predict the dependence of the ODT temperature on cross-linking density. Our theory is limited to cross-linking densities below the gel point. In both experiments and theory the ODT temperature increases as the number of junctions is increased for cross-linking densities below the gelation point. We show that reversible order-disorder transitions are obtained in these materials even when the cross-linking density is larger than that required to obtain a gel. At cross-linking densities near the gel point, the depolarized light scattering signal from the ordered state suddenly increases by orders of magnitude due to the presence of extremely long and thin grains. At sufficiently high cross-linking densities, the disordered state is trapped, and order-disorder transitions are no longer obtained.