Diffusion of Triblock Copolymers in a Spherical Domain Structure

Abstract

Self-diffusion and tracer diffusion (diffusion in a fixed diblock copolymer matrix) of ABA triblock copolymers of styrene and 2-vinylpyridine (PVP-PS-PVP) have been measured in a spherically ordered domain structure and compared to that of diblock copolymers. In most of the cases, the self-diffusion and tracer diffusion of the triblock copolymer is controlled by a walking diffusion mechanism in which one PVP end block is activated at a time, giving rise to an exponential decrease in D/D 0 as exp(- X N PVP ), where D is the diffusion coefficient of the triblock copolymer, D 0 is the diffusion coefficient of the triblock copolymer in a (hypothetical) disordered phase, X is the interaction parameter, and N PVP is the number of segments in each PVP end block. The normalized diffusion coefficients of the triblock copolymers are not so different from those of diblock copolymers with the same X N PVP , Nevertheless, if one compares the diffusion of triblock and diblock copolymers with the same total molecular weight, the triblock copolymer diffuses much faster. However, when XN PVP < 6 and the spacing of the matrix domains is much larger than the natural domain spacing of the triblock copolymer domains, the normalized tracer diffusion coefficient D/D 0 decreases exponentially as exp(-2XN PVP ). In this case we propose that the diffusion occurs by both end blocks being activated at the same time, a mechanism we call double activation.