Interfacial Strengthening with Random Copolymers

Abstract

Recent experiments show increased interfacial strength of interfaces between immiscible A and B homopolymers when A/B random copolymers (RCs) are added at the interface. These results have been interpreted in terms of the work of Garel et al., which predicts that RCs at such A-B interfaces will adopt conformations in which long loops (of order 1/χ² monomers) slightly rich in A (B) monomers will be found on the A (B) side of the interface. We reexamine these results in light of a simple Flory-Huggins description, which shows that such RCs would be immiscible with either A or B homopolymer with an effective χ parameter χ eff=χ/4 for symmetric RCs, and would tend to collapse at the A-B interface. Thus, the RC-homopolymer interface would then be only as strong as an interface between homopolymers with χ=χeff, which would be strong only if the A-B interface is nearly broad enough to be self-entangled already. Alternatively, if the RCs have a broad composition distribution, they can segregate by composition in strata, producing a “graded” interface. We derive criteria for the RC composition distribution and layer thickness in order that the resulting interface be strong.