Effect of composition fluctuations on tracer diffusion in symmetric diblock copolymers

We report forced Rayleigh scattering measurements of the tracer diffusion of poly(ethylenepropylene) (PEP) homopolymers in nearly symmetric poly(ethylenepropylene)-b-poly(ethylethylene) (PEP-PEE) block copolymer matrices, both above and below the order-disorder transition. Measurements of PEP-PEE copolymer diffusion in the same matrices have been reported previously. The results indicate that fluctuations suppress diffusion of both homopolymer and copolymer tracers, as predicted by recent theories. However, in the quenched ordered state, diffusion suppression is considerably stronger for block copolymer tracers than for homopolymer tracers, which appears to be counter to theoretical expectation. Varying the tracer molecular weight has little effect on the suppression of homopolymer diffusion, whereas it has a substantial effect on copolymer tracer diffusion. These differences are attributed to differential chain localization or confinement: an entangled copolymer tracer tends to be pinned near the interface between microdomains, and has to pay a significant enthalpic penalty for diffusion in any direction, as the reptating chain has to drag one block through the other microdomain. On the other hand, although a homopolymer tracer tends to be located within one microdomain, it can presumably diffuse across several grains without penalty, by reptating predominantly within a single lamella.