KINETICS OF PHASE-TRANSITIONS IN WEAKLY SEGREGATED BLOCK-COPOLYMERS -PSEUDOSTABLE AND TRANSIENT STATES

We study the kinetics of order-disorder and order-order transitions in weakly segregated diblock copolymers using a time-dependent Ginzburg- Landau (TDGL) approach. In particular, we investigate the microstructu ral change as well as the order-parameter evolution after a sudden tem perature jump from one phase to another. Direct numerical simulation o f the TDGL equations shows that depending on the extent of the tempera ture jump, these transitions often occur in several stages and can inv olve nontrivial intermediate states. For example, we find that transit ion from the lamellar phase to the hexagonal cylinder phase goes throu gh a perforated lamellar state within a certain temperature range. The numerical results are elucidated by a multimode analysis under the si ngle-wave-number approximation. The analysis reveals that the geometri c characteristics of the free energy surface, particularly saddle poin ts and ridgelike features, are responsible for the nontrivial intermed iate states on the kinetic pathways. On the basis of this analysis, a generalized kinetic ''phase diagram'' is constructed, which is able to account for all the different scenarios observed in the numerical sim ulation. Our results are discussed in connection with available experi mental observations. In particular, we suggest the possibility that th e perforated-modulated lamellar structures obtained by Bales and co-wo rkers [I. W. Hamley, K. A. Koppi, J. H. Rosedale, F. S. Bates, K. Almd al, and K. Mortensen, Macromolecules 26, 5959 (1993); S. Forster, A. K . Khandpur, J. Zhao, F. S. Bates, I. W. Hamley, A. J. Ryan, and W. Bra s, Macromolecules 27, 6922 (1994)] may be kinetic, intermediate states rather than new equilibrium phases.