Diffusion of block copolymers in liquid CO2:: Evidence of self-assembly from pulsed field gradient NMR

A study of self-assembly of the block copolymers poly(vinyl acetate)-b-poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) and polystyrene-b-PFOA in liquid CO2 is conducted using pulsed field gradient (FPG) NMR. In the temperature/density regime where micelles do not form, a narrow distribution of diffusion coefficients is found for the copolymers. However, when self-assembly occurs, the PFG-NMR data become strongly dependent on the diffusion time are consistent with a two-site exchange model involving fast and slow diffusive modes. At the shortest diffusion times two diffusion rates are resolved, while in the long diffusion time limit only an average diffusion rate can be measured. Thus, the signature for self-assembly in these systems is the exchange of polymers between relatively free and associated states on a time scale accessible to PFG-NMR.