Nanostructured thin films of degradable block copolymers and their applications

Abstract

Nanopatterning using self-assembled thin films of degradable block copolymers (BCPs) has attracted considerable attention and inspired a variety of appealing applications in different research areas. To create useful nanopatterns for practical uses, controlling the orientation of the BCP microdomains is essential. The generation of oriented microdomains in BCP thin films will be discussed in this article. This discussion is followed by a review of methods that use polylactide-containing BCP (that is, polystyrene-b-poly(L-lactide) (PS-PLLA)) thin films to generate nanoporous PS after hydrolysis, which can be used for templated synthesis to create functional nanohybrids. The nanoporous PS thin films with well-oriented cylinder nanochannels can be used for pore-filling various ingredients to create specific drug delivery systems and optoelectronic devices. Moreover, by utilizing templated synthesis, nanoporous ceramics with a high-specific surface area and high porosity can be fabricated for optical applications using hydrolyzed gyroid-forming PS-PLLA as a template for the sol–gel reaction. In addition, the nanolithography applications using silicon-containing BCP (that is, polystyrene-b-polydimethylsiloxane) thin films, which can form inorganic nanoporous templates after oxygen plasma treatment, will be discussed in this review. Polymers that consist of two or more different components organized in distinct blocks give rise to intriguing and useful processes. By virtue of their similar properties, blocks of the same kind tend to aggregate. This ‘microphase separation’ phenomenon in turn organizes the bulk material in regular patterns. Subsequently removing one of the blocks selectively, by wet or dry etching, produces materials featuring well-organized, regular pores — sought after for a variety of applications ranging from drug delivery systems to optoelectronic devices. Rong-Ming Ho and co-workers from the National Tsing Hua University in Taiwan review how the blocks’ orientation can be controlled, focusing on materials made of polystyrene and either organic poly(L-lactide) (PS–PLLA) or inorganic, silicon-containing poly(dimethyl-siloxane) (PS–PDMS). For example, removing PLLA segments from the former by hydrolysis produces a nanoporous thin film that serves as a template for sol–gel reactions, whereas removing PS blocks from the latter gives a robust matrix that can be exploited as a mask in nanolithography. In this review, nanoporous thin-film template was obtained from the self-assembly of block copolymer PS-PLLA, at which the PLLA block can be hydrolyzed to form the nanopatterns. Nanoporous PS thin films with well-oriented cylinder nanochannels can be used for pore-filling with various ingredients to create specific drug delivery systems and optoelectronic devices. Moreover, nanoporous ceramics with high-specific surface area and high porosity can be fabricated for optical applications using hydrolyzed gyroid-forming PS-PLLA for templated sol–gel reaction. In addition, the formation of inorganic nanoporous templates from self-assembled PS-PDMS after oxygen plasma treatment and its corresponding applications in nanolithography will be discussed.