Microphase Separatxon of Charged Diblock Copolymers
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MXCROPHASE SEPARATXON OF CHARGED DIBLOCK CO]POLYMERS J. F. MARKO* AND Y. RABINt The James Franck Institute, The University of Chicago, 5640 Ellis Avenue, Chicago, Illinois 60637 ABSTRACT The microphase segregation properties of diblock copolymers composed of a neutral polymer joined to a polyelectrolyte are described. For weakly charged diblocks under melt conditions, we have constructed a theory of weak concentration fluctuations and have used it to study the spinodal instabiity to microphase separation that is driven by incompatibility of the polyion and neutral blocks. As in polyelectrolyte melts, the charging has a strong compatibilizing effect because of the large counterion entropy cost of phase separation. The electrostatic interactions also introduce a new length scale, the Debye screening length, which competes with the free chain radius to determine the microphase wavelength. A scaling theory of the strongly-segregated phase indicates that the salt-free microphases cannot have sharp interfaces beyond a threshold number of charges per chain. INTRODUCTION AND MODEL Diblock copolymers composed of a neutral block joined to a polyelectrolyte combine the celebrated microphase segregation properties of uncharged diblocks[l] with those of charged polymers. Experiments on such diblocks are now in progress(2]. Previously, Khokhlov and co-workers have developed a theory of blends of charged and neutral chains[3]. Their study, which focused on the weak-segregation limit, indicated that increasing the charge of the polyelectrolyte (a) stabilizes the mixed phase (moves the spinodal point to lower temperatures) and (b) changes the character of the transition from macro- to microphase separation at a characteristic length dependent on the polyion charging and the amount of added salt. Similar effects were noted by Joanny and Leibler in their study of polyelectrolytes in poor solvent[4]. A central difference between these earlier studies[3,4] and our investigations[5,6) is that in our case of diblock copolymers, bulk phase separation is prohibited: we anticipate that the microphase properties may be tuned over a wider range than in a system where bulk segregation may occur. We consider copolymers composed of a charged polymer A of length fNa joined to a neutral polymer B of length (1 - f)Na, where N is the total chemical length, a is the monomer size, and f is the fraction of monomers in the polyion block. Each A block contains n
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