The effect of molecular architecture on the grain growth kinetics of AnBn star block copolymers

To investigate the effect of molecular architecture on the grain growth kinetics of star block copolymers, a series of AnBn miktoarm star block copolymers with different numbers of arms (n = 1, 2, 4 and 16) was studied. Across this entire series of materials, all the A arms are polystyrene (PS) blocks from the same anionically synthesized batch, and thus all the A arms are identical. Likewise, all the B arms are polyisoprene (PI) blocks from the same anionically synthesized batch, and thus all the B arms are identical. All the stars employed in this study are therefore composed of the same A and B arms liked together in symmetric numbers. The coarsening kinetics of grain growth was monitored in real space by transmission electron microscopy (TEM), followed by subsequent micrograph image analysis. It was found that the molecular architecture influenced the grain growth kinetics of these AnBn star copolymers dramatically. The grain coarsening kinetics was found to follow a scaling law as V approximately t(beta), where V is the characteristic grain volume and t is time. The exponent, beta, was found to be about 0.2 for the diblock copolymer (n = 1) and 0.4 for all three of the star block copolymers (n = 2, 4 and 16) in the series. It is postulated that the difference in grain growth rate between the diblock and the various stars is due to a reduction in molecular entanglements resulting from chain stretching near the junction points in the stars.