Photocontrol over the ordering transitions in block copolymer thin films.

摘要

One of the current challenges in materials science is establishing a simple way to generate an ultradense arrays of addressable nanoscopic elements on macroscopic scales. The addressability of nanomaterials is essential for many applications, ranging from high-density magnetic storage to high-density, ultrahigh resolution displays to photovoltaics. Among the strategies available, "photocombing" has been proposed as a promising route to create long-range ordered nanostructures in self-assembled block copolymers (BCPs) over macroscopic distances through a photocontrollable ordering transition. In this process, bands of light act as a "comb" to sweep across BCP thin films unidirectionally, reversibly bringing the BCPs through an ordering transition, like the disorder-to-order transition (DOT) and the order-to-order transition (OOT). Thus, defects are "combed" out, forming arrays of highly ordered BCP microdomains on a macroscopic length scale. It is similar in principle to the classic zone refining method, which is used to produce large single crystals of metals and semiconductors.;In this dissertation, I will focus on three systems to investigate photocombing. System I is the supramolecular assembly of poly(2-vinylpyridine)-block-poly(n-butyl methacrylate) and polystyrene-block-poly(2-vinylpyridine) di-BCPs with azobenzene-containing 2-(4-hydroxyphenylazo)benzoic acid chromophores. In these systems, an ordering transition from lamellae to hexagonally packed cylinders was observed after one hour of UV radiation at 150 °C. System II is the deuterated polystyrene-block-poly(n-butyl methacrylate) BCPs with photoisomerizable azobenzene functionalities. They exhibit an entropy-driven lower DOT, the characteristic of "compressibility", similar to their parent BCPs. System III is anthracene-functionalized tri-BCPs containing deuterated polystyrene (d8-PS) and poly(methyl methacrylate) (PMMA) blocks, as well as a small middle block of poly(2-hydroxyethyl methacrylates) that is randomly functionalized by anthracene. Under UV exposure, the junction between d8-PS and PMMA blocks in the tri-BCPs is joined together through anthracene photodimers, thereby resulting in a significantly increase in the total molecular weight of the tri-BCPs. As a consequence, the tri-BCPs undergo an ordering transition from a disordered state to an ordered state, when it is phase-mixed but close to the boundary of the ordering transition.