Page 100 - Nanotechnology an introduction

[155]: structures such as antisymmetric surface-parallel lamellae, antisymmetric hybrid structures (cf. Stranski–Krastanov film growth, Section
8.1.2), and surface-perpendicular lamellae or columns are typically formed. There is at present considerable interest in such processes for
fabricating photolithography masks in the nanoscale range more conveniently than by electron beam writing. Reticulated structures seem to have
been investigated the most extensively: block copolymer micelles can be formed by dissolving the polymer in a fluid that is a solvent for only one of
the components, and then used to coat surfaces, yielding a more or less regular array. This process has attracted interest as a route to making
nanoporous membranes. For example, polystyrene–polymethylmethacrylate (PMMA) copolymers, prepared with a volume fraction, such that there
are cylindrical microdomains of the PMMA in the bulk, can be coated on a suitable substratum (silicon or silica) such that the cylinders are oriented
normal to the surface. Exposure to ultraviolet light crosslinks the polystyrene but degrades the PMMA, which can then be selectively dissolved out of
the film, leaving a nanoporous polystyrene membrane with a pore size controllable by varying the molecular weight of the copolymer [169].
One advantage of these polymer-based processes is the tremendous variety of starting materials available (by the same token, the systematic
experimental investigation of the effects of compositional variation across the whole range of possibilities represents a huge undertaking). As well
as changing the chemical nature of the monomers and the degrees of polymerization, the block copolymers have also been mixed with
homopolymers as a way of modifying the characteristic scale of the texture [69].