CRITICAL-DYNAMICS OF RANDOM COPOLYMER MELTS

The transport properties of an A-B random copolymer melt near the orde r-disorder phase transition are examined by using a mode coupling appr oach. The key parameters of the model are the volume fraction of type A monomers, f, the Flory chi parameter, and a parameter lambda that ch aracterizes the monomer sequence distribution. The equilibrium phase d iagram for this model of random copolymers contains a Lifshitz point a t lambda = lambda(L). For lambda < lambda(L), CoMposition fluctuations first become unstable at finite wavelength and the homogeneous phase separates into microphases, while for lambda > lambda(L) the fluctuati ons first become unstable at infinite wavelength giving a binary fluid type transition. Our analysis shows that for lambda < lambda(L) the O nsager coefficient remains finite as the phase transition is approache d, while the viscosity diverges proportional to epsilon-3/2, where eps ilon = (T - T(c))/T(c), is the dimensionless difference between the te mperature and the critical temperature. For lambda > lambda(L), there is a very weak divergence in the viscosity, but the Onsager coefficien t diverges proportional to epsilon-1/2 for epsilon-1(lambda - lambda(L )) much greater than 1, and proportional to epsilon-3/4 for epsilon-1( lambda - lambda(L)) much less than 1. The dependence of the Onsager co efficient and viscosity on chi, f and lambda are explicitly determined .