Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2020 / Paper 2 / Section B / 22 / iv

Choose an asymmetric diblock copolymer in a cylinder-forming regime: minority cylinders lie in a continuous majority matrix. Form and anneal a film to develop microphase separation, control substrate and top-surface preferences or apply a suitable field to orient cylinders normal to the film, then selectively remove the minority phase. The retained majority matrix becomes the porous support. Infiltrate the channels with liquid crystal. The majority phase forms the channel walls; removing that continuous matrix would not generally leave a continuous nanoporous membrane.

With homeotropic anchoring, the director is radial at a cylindrical side wall. A strictly transverse radial texture has a line core, but an escaped radial texture can avoid it:The director turns axially at the center and is normal to the wall at radius $a$. The original axial-section sketch shows this three-dimensional escape. End conditions, channel dimensions and anchoring strength determine whether escape or a defect-containing profile is favored.

With planar anchoring on the cylindrical side wall, uniform axial alignment $\mathbf{n}=\hat{\mathbf{z}}$ is a simple defect-free profile. The axial direction is tangent at every wall point, so it satisfies the lateral boundary everywhere without elastic distortion.

This assumes degenerate planar anchoring, or an azimuthal preference compatible with axial alignment. End constraints, selected circumferential anchoring or imposed twist can favor another state. planar anchoring alone does not require a circumferential vortex. The original axial-section sketch shows the uniform axial option.