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, _ODT, decreases with polymer volume fraction φ as χ _ODT ∼ φ _-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, ω _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 χ, N, n, and concentration.