Macromolecules 1997,30, 4756-4765

Andrey v. Dobrynin*

Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27599-3290

Ludwik Leibler

Unité Mixte de Recherches 167 CNRS-ELF Atochem, B.P. 108,95 rue Danton, 92303 Levallois-Perret, France

Received April 5, 1996; Revised Manuscript Received May 5, 199~

ABSTRACT: We study the s~lf-organization of a polydisperse A-B multiblock copolymer melt with repulsive interactions betwee~ the different monomers. ln the framework of the one-loop (Hartree) approximation we consider the influence of the polydispersity parameter u, which is the ratio of the first two moments of the ~)ock length distribution u = "12)/(1)2) -1, on the condition for the order-disorder transition. At lowpolydispersity (u < 0.3) the system undergoes the ordinary block copolymer microphase separation transîtion. The value of the Flory parameter X on the line of the order-disorder transition increases with increasing polydispersity of the system. The transition to macrophase separation at the classical Lifshitz point u = 0.5 is precluded by a fluctuation-induced instability of the homogeneous state with respect to finite-wavelength composition fluctuations. The characteristic length scale L of these critical fluctuations above the microphase separation transition and the period of the domain structure below changes weakly with temperature as "T -T.)/T.)-1/6. ln the case of an exponential block length distribution (u ~ 1), the period of the domain structure exhibits stronger temperature dependence L ~ "T -T .)/T .)-1/4 than that derived for this system near the Lifshitz point due to strong coupling between composition fluctuations. For values of the polydispersity parameter u > 0.4 the value of the Flory parameter on the line of the first-order order-disorder phase transition decreases with increasing polydispersity of the system.