Nanoporous polymers with gyroid nanochannels can be fabricated from the self-assembly of degradable block copolymer, polystyrene-b-poly(l-lactide) (PS-PLLA), followed by the hydrolysis of PLLA blocks. A well-defined nanohybrid material with SiO2 gyroid nanostructure in a PS matrix can be obtained using the nanoporous PS as a template for sol-gel reaction. After subsequent UV degradation of the PS matrix, a highly porous inorganic gyroid network remains, yielding a single-component material with an exceptionally low refractive index (as low as 1.1).

This work presents a new method for forming well-defined nanostructured thin films from self-assembled polystyrene-block-poly(l-lactide) (PS-PLLA) on Si wafers with a functionalized SiO2 surface. Large, well-ordered, perpendicular PLLA cylinders in PS-PLLA thin films can be formed using the functionalized substrate. In contrast to random copolymers, a neutral substrate for the PS and PLLA blocks is formed by functionalizing a substrate with hydroxyl-terminated PS (PS-OH) followed by hydroxyl-terminated PLLA (PLLA-OH). The heterogeneous grafting of PS-OH and PLLA-OH can be substantially alleviated using this two-step functionalization. Accordingly, the surface properties can be fine-tuned by controlling the ratio of grafted PS-OH to PLLA-OH to control the orientation of the PLLA cylinders on the functionalized SiO2. Nevertheless, the orientation that is driven by the neutral substrate is surprisingly limited in that the effective length of orienting cylinders is less than twice the interdomain spacing. Thermal annealing at high temperature can yield a neutral air surface, rendering perpendicular PLLA cylinders that stand sub-micrometers from the air surface. Consequently, the neutral substrate can be used to enable truly film-spanning perpendicular cylinders in films to be fabricated using the high-temperature thermal treatment. In addition, the perpendicular cylinders can be laterally ordered by further increasing the annealing temperature. The ability to create these film-spanning perpendicular cylinders in films with a well-ordered texture and sub-micrometer thickness opens up possible applications in nanotechnology.

This work presents an approach to achieve controlled ordering of polystyrene-block-poly(l-lactide) (PS–PLLA) gyroid thin films on a neutral substrate using solvent annealing. Interesting morphological evolution from gyroid to cylinder can be found while using a partially selective solvent for the PS block to anneal the PS–PLLA thin film. To acquire a thin-film sample with thermodynamically stable gyroid morphology, a nonpreferential solvent should be used for solvent annealing to enable controlled ordering of gyroid thin film with the (211)G plane parallel to the air surface and also the functionalized substrate. By taking advantage of degradable character of the PLLA block, nanoporous PS with well-defined texture can be fabricated by hydrolysis and used as a template for synthesis of various nanohybrids and nanoporous materials.

In this study, we aim to examine the morphological evolution of block copolymer (BCP) nanostructured thin films through solvent evaporation at different rates for solvent swollen polystyrene-block-poly(l-lactide) (PS-PLLA). Interesting phase transitions from disorder to perpendicular cylinder and then gyroid can be found while using a partially selective solvent for PS to swell PS-PLLA thin film followed by solvent evaporation. During the transitions, gyroid-forming BCP thin film with characteristic crystallographic planes of (111)G, (110)G, and (211)G parallel to air surface can be observed, and will gradually transform into coexisting (110)G and (211)G planes, and finally transforms to (211)G plane due to the preferential segregation of constituted block to the surface (i.e., the thermodynamic origin for self-assembly) that affects the relative amount of each component at the air surface. With the decrease on the evaporation rate, the disorder phase will transform to parallel cylinder and then directly to (211)G without transition to perpendicular cylinder phase. Most importantly, the morphological evolution of PS-PLLA thin films is strongly dependent upon the solvent removal rate only in the initial stage of the evaporation process due to the anisotropy of cylinder structure. Once the morphology is transformed back to the isotropic gyroid structure after long evaporation, the morphological evolution will only relate to the variation of the surface composition. Similar phase transitions at the substrate can also be obtained by controlling the ratio of PLLA-OH to PS-OH homopolymers to functionalize the substrate. As a result, the fabrication of well-defined nanostructured thin films with controlled orientation can be achieved by simple swelling and deswelling with controlled evaporation rate.

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. In recent decades, block copolymers (BCPs) have been extensively investigated because of their ability to self-assemble into various ordered nanostructures, such as spheres, cylinders, gyroids and lamellae, owing to the incompatibility of their constituent blocks and the corresponding chemical junction. 1,2 Well-defined nanostructured phases can be tailored by the molecular engineering of synthetic BCPs to obtain promising features for nanotechnological applications. For such nanostructured materials to prove useful in thin-film applications, thin-film samples with oriented periodic arrays over a large area must be formed to generate a uniform thickness and sufficiently thin films with well-ordered BCP microdomains. Nanoporous materials from BCP thin films have been attracted considerable attention owing to their appealing applications in different research areas. The etching techniques used to generate nanoporous materials can be classified as wet and dry processes depending on the etching methods. 3 The wet etching process is a common approach for fabricating nanoporous materials. By utilizing the degradable character of polymer chains, nanoporous polymers can be fabricated using self-assembled nanostructures through the selective degeneration of one of the constituent blocks. For instance, polylactide-containing BCPs can be used to fabricate nanoporous polymeric materials because of the unstable character of the ester group, which can be hydrolytically decomposed. 4 After hydrolysis of the polylactide segments, a nanoporous thin film with a uniform thickness and domain size can be successfully prepared on different substrates. Consequently, the nanoporous thin films can be used as nanor...

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