STABILITY OF THE PERFORATED LAYER (PL) PHASE IN DIBLOCK COPOLYMER MELTS

We reexamine the stability of hexagonally modulated layer (HML) and he xagonally perforated layer (HPL) morphologies in a number of block cop olymer systems of low to moderate molecular weight. Using small-angle X-ray scattering and dynamic mechanical spectroscopy, we show that the se structures are long-lived nonequilibrium states which convert to th e bicontinuous gyroid (G) morphology upon isothermal annealing. Compar ison of phase transition kinetics across chemically distinct systems s panning a wide range of molecular weights and monomeric friction coeff icients reveals a composition dependence to these dynamics. This sugge sts effects associated with the mobility of individual chains are of l esser importance in explaining the apparent metastability of the HML a nd HPL structures: instead, the composition dependence of the transiti on mechanism appears to dominate the observed behavior. The revised ph ase behavior for these materials is in excellent agreement with mean-f ield predictions for diblock copolymer melts.