Consequences of block number on the order-disorder transition and viscoelastic properties of linear (AB)n multiblock copolymers

The effect of block number on the order-disorder transition (ODT) and viscoelastic properties were studied for linear (AB)(n) multiblock copolymers. A series of symmetric poly(styrene-b-isoprene)(n) multiblocks ((SI)(n), n = 1-10) were synthesized by anionic polymerization, and their order-disorder transition temperatures (T-ODT) were located using dynamic mechanical spectroscopy. As n increases, T-ODT approaches an asymptotic value, consistent with random phase approximation calculations. A systematic difference between the experimental and theoretical results is attributable to the effects of fluctuations, independent of the number of blocks. Addition of up to 20 vol % of a nonselective solvent depresses T-ODT, independent of n. The interaction parameter at the transition, chi(ODT), decreases with polymer volume fraction phi as chi(ODT) similar to phi(-1.3), consistent with previous reports for diblocks (n = 1). In contrast, the viscoelasticity of (AB)n block copolymers depends strongly on block number. A crossover frequency, w(x), demarcating the transition from block/chain- and domain-dominated relaxation, scales as n(-7.5), which is much stronger than the molecular weight dependence of the longest relaxation time for entangled homopolymers. Coherent lamellar grains were imaged for quenched and annealed (SI)n, n = 1, 6, and 10, by transmission electron microscopy. Ellipsoidal lamellar grains, with aspect ratios of 2-3, were recorded, independent of block number, but the grain size decreased with increasing n. These results establish the criteria for designing multiblock copolymers based on chi, N, n, and concentration.