Polymer Phase Behavior

Predicting the Microphase Behaviour of Gradient Copolymers

Gradient copolymers are a relatively new class of copolymers where the comonomer composition is tapered linearly from one side of the chain to the other (in contrast to, e.g., a diblock copolymer where there is a discrete junction point between the “A” and “B” comonomer blocks of the chain). Like diblock copolymers, gradient copolymers self-assemble into microphases at sufficiently low temperature or large A/B immiscibility, forming A-rich and B-rich domains at the nanoscopic scale. But due to their tapered comonomer construction, the interfaces between the A- and B-phases in gradient copolymer melts are broad and highly mixed.

We have studied the self-assembly of gradient copolymer melts into lamellar microphases using self-consistent field theory (SCFT), gaining insight into their unique phase behaviour by examining the “strong-stretching” limit where A/B immiscibility grows towards infinity.
PhiPlots-OurWThe image at right shows composition profiles for gradient copolymer lamellae as A/B comonomer immiscibility grows large (light gray to black). In the limit of infinite comonomer immiscibility, the lamellar composition profile becomes “saw teeth”, in contrast to the square wave profile for strongly-segregated diblock copolymer lamellae. This leads gradient copolymers to exhibit a wide diversity of local molecular environments—each with distinct local glass transition temperature—leading such copolymers to have at times a very broad experimental glass transition. To view our publication on this work, click here.

FigDWe have also studied the conformations of diblock and gradient copolymers in homopolymer melts, showing how gradient copolymers adopt a “yarn ball” conformation (in contrast to the “tadpole” conformation of a diblock copolymer). The image at left is a cartoon of radial distribution functions for monomers belonging to these two conformations, where red is A-monomer concentration and blue is that for B monomers. (Both chains are in a B homopolymer melt.) To view our publication on this work, click here.