Mechanically anisotropic materials with a zero Poisson's ratio : Unidirectional alignment of block copolymer nanostructures in bulk films using the shear-rolling process.

Block copolymers(BCP) are well-known materials for realizing various nanostructures, and various directed self-assembly methods have been suggested to arrange them to desired patterns. In particular, we recently developed the shear-rolling process which can unidirectionally align sub-10 nm BCP line patterns with high uniformity at extremely high temperatures in a second at the 4-inch wafer.

We applied the shear-rolling process to bulk BCP film to create mechanically anisotropic materials. Elastomers, potentially can be used as substrates of stretchable display, have a Poisson’s ratio of ~0.5, which causes image distortion during stretching. Approaches to achieve a zero Poisson’s ratio include special structures like auxetic structure or mechanical metamaterials, but these approaches typically result in reduced transparency and thickness-direction undulation under deformation due to micrometer-sized structures or refractive index mismatches in composites. In this study, we executed the shear-rolling process to bulk BCP polystyrene-b-polyisobutylene-b-polystyrene(PS-b-PIB-b-PS) films of 0.7 mm thickness, successfully unidirectionally aligning glassy cylindrical PS in rubbery PIB matrix. The aligned BCP film exhibited notable (over 4 times) mechanical anisotropy and Poisson’s ratio less than 0.07. In particular, the alignment of nanostructures much smaller than the size of visible light showed high transparency and also the same monotonic strain distribution at all positions.