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

Lifeng Wu, Eric W. Cochran, Timothy P. Lodge, Frank S. Bates

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)3360-3368
Number of pages9
JournalMacromolecules
Volume37
Issue number9
DOIs
StatePublished - May 4 2004

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