Diffusion of triblock copolymers in a spherical domain structure

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 compa red to that of diblock copolymers. In most of the cases, the selfdiffusion 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, giv ing rise to an exponential decrease in D/D-0 as exp(-chi N-PVP), where D is the diffusion coefficient of the triblock copolymer, Do is the diffusion c oefficient of the triblock copolymer in a (hypothetical) disordered phase, chi is the interaction parameter, and N-PVP is the number of segments in ea ch PVP end block. The normalized diffusion coefficients of the triblock cop olymers are not so different from those of diblock copolymers with the same chi N-PVP. Nevertheless, if one compares the diffusion of triblock and dib lock copolymers with the same total molecular, weight, the triblock copolym er diffuses much faster. However, when chi N-PVP < 6 and the spacing of the matrix domains is much larger than the natural domain spacing of the tribl ock copolymer domains, the normalized tracer diffusion coefficient D/D-0, d ecreases exponentially as exp(-2 chi N-PVP). In this case we propose that t he diffusion occurs by both end blocks being activated at the same time, a mechanism we call "double activation".