On the Collapse of Random Copolymers

We present simulations of a random copolymer model introduced by Kantor and Kardar ( Europhys. Lett. , 28 (1994) 169). In this model there are two types A, B of monomers. Neighbouring monomers interact with a potential + v 0 if they are of the same type, and with − v 0 else. When N A ≈ N B , we find a θ-transition at T θ close to that found by Kantor and Kardar. But this θ-collapse disappears for smaller values of x = | N A - N B | / ( N A + N B ) than found by these authors, and we argue that it will be indeed difficult to show convincingly that there exists a finite T θ for any x ≠ 0. The reason for this difficulty is that the polymer first seems to collapse as N = N A + N B increases, but this collapse stops for large N . This suggests that long chains with x ≠ 0 at low temperatures have a structure different from that proposed for copolymers with long-range interactions.