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Dresden 2009 – scientific programme

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DY: Fachverband Dynamik und Statistische Physik

DY 24: Pattern formation in colloidal and granular systems I

DY 24.7: Talk

Thursday, March 26, 2009, 16:15–16:30, ZEU 255

Fractionation in random symmetric A-B block copolymers — •Alice von der Heydt and Annette Zippelius — Institut für Theoretische Physik, Göttingen, Germany

Starting from a mean-field free energy based on a microscopic model, we investigate the coexistence of homogeneous and structured phases of a triblock copolymer melt with an incompatibility χ∝ 1/T between A- and B-blocks. The natural random block sequence distribution is generated by a Markov process with an overall A-probability p=1/2 and a correlation parameter λ. The latter determines the structure of the ordered phases which the initially disordered melt forms on increasing χ: lamellae with wave number k(λ) appear for λ < λc, two homogeneous A- and B-rich phases for λ > λc [1].

An explicit fractionation ansatz takes into account the exchange of individual sequences between coexisting lamellar and homogeneous phases: higher χ give rise to two additional homogenous phases for λ < λc, and to a third lamellar phase with an increased content of alternating sequences for λ > λc. The new phases emerge with zero volume fraction, but with a finite deviation from the natural sequence distribution, and, for the fractionated lamellae, with finite k. Reliable results can be obtained especially in the vicinity of the tricritical point (λc, χc), where other methods encounter difficulties.

[1] G. H. Fredrickson, S. T. Milner, and L. Leibler, Multicritical Phenomena and Microphase Ordering in Random Block Copolymer Melts, Macromol. 25 (1992), 6341

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