TRANSITION MECHANISMS FOR COMPLEX ORDERED PHASES IN BLOCK-COPOLYMER MELTS

We describe microstructural aspects of phase transitions between the l amellar (L), perforated layer (PL), and gyroid (G) morphologies in dib lock copolymer melts. Using small-angle scattering, dynamic mechanical spectroscopy, and transmission electron microscopy, we show that thes e transformations proceed through the nucleation and growth of the fin al phase, in contrast to recent calculations that assume evolution fro m a thermodynamically unstable initial state. Direct L --> G transitio ns are suppressed by the high surface tension associated with L-G grai n boundaries; the formation of the metastable PL structure under such conditions reflects the ease with which the L --> PL transition can oc cur, compared to L --> G. Similar effects dominate the G --> L transit ion. Mismatches in spacings between epitaxially related lattice planes also influence relaxation kinetics; the P --> LG transition rate depe nds strongly on the relative spacings of the PL [10] and G [211] plane s, and the considerable discrepancy between the G [211] and L [10] spa cings at the L-G boundary may further retard that transformation. Simi lar factors have been shown to govern the evolution of amphiphilic sys tems, supporting geometrically inspired attempts to understand this ph ase behavior.