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RESEARCH PRODUCT

Chiral helical nanostructures based on amorphous abc-triblock copolymers

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description

So far all lattices of regular microdomain structures reported in the literature, either based on binary AB, ABA or ternary ABC block copolymers, can be described using simple symmetry elements like mirror planes, symmetry centers and rotation axes. The microphase separation in a linear ternary block copolymer consisting of three immiscible components namely polystyrene, polybutadiene and polymethyl methacrylate gives rise to the formation of a chiral supramolecular assembly in which chirality is not defined on a molecular (or macromolecular) level but only by the assembly of the linear triblock copolymer molecules. The polybutadiene (7, 12 wt.-%) phase forms helical strands surrounding the polystyrene cylinders (25, 26 wt.-%). This chiral mesoscopic assembly can be recognized unambiguously by transmission electron microscopy.1 Both left- and right-handed helical structures are formed, either on different cylinders but also along the same cylinder. In the latter case helix reversals are observed. The helical morphology has been obtained by casting the block copolymers from various solvents, with different solubilities for the three components. It also does not change with annealing time. This is a strong indication that the helical morphology is the equilibrium structure of the system. The formation of the helical structure in the present polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock copolymers can be explained on the basis of the theory of block copolymers in the strong segregation limit which was originally developed by Alexander2 and De Gennes3, 4 for lamellar interfaces and extended by Semenov5 to curved (micellar) systems. This approach was successful for the description of symmetric ABC block copolymer systems. The details of the calculations for the helical morphology will be published elsewhere.6