Collapse of random copolymers

Abstract

We propose a theoretical approach to the description of the coil–globule transition of random copolymers having a fixed sequence of units. For simplicity, we consider copolymers formed by two different units only, although the generalization to any other number is straightforward. The theory is based on self-consistent minimization of the intramolecular free energy, which includes two-body attractive interactions among units of a given type, two- and three-body repulsive interactions among all the units, and configurational entropy. Chain connectivity is accounted for throughout. Considering copolymers with 20%–60% mutually attractive units, we predict in all cases a first-order coil–globule transition, unlike the analogous homopolymer. The monomolecular micelle formed by the collapsed copolymer consists of two basic conformations: (a) stable compact globules, having the mutually attractive units clustered in a dense core, wherefrom the other units are expelled; (b) metastable open globules, where most attractive units are still within the core, but a few of them are outside, interspersed with the other units. Possible connections with ionomer behavior in apolar solvents and with current results on globular proteins are also discussed.