Highly ordered nanoporous thin films from cleavable polystyrene-block-poly (ethylene oxide)

Nanoporous polymer thin films produced from ordered block copolymer precursors have received considerable attention, because they can be used in many applications including templating, separation, catalysis, and sensors. Since 1988, various block copolymers have been used to prepare nanoporous thin films via the selective removal of degradable minor components from self-assembled block copolymers. For cylinder-forming block copolymers, selective etching of the cylindrical microdomain in an ordered block copolymer results in the formation of nanoscopic channels. Specially, if the etchable cylinders are orientated normal to the film surface, removal of the cylinders will lead to the nanolithographic masks used in block-copolymer lithography. Currently, the generation of nanoporous materials from ordered block copolymers relies on a selective etching protocol that does not compromise the integrity of the matrix material. Ozonolysis, UV degradation, reactive ion etching (RIE), and chemical etching are the methods that are commonly used for the removal of minor components to create nanopores. However, the number of polymers that can be degraded under mild reaction conditions in the thin-film state is limited. The preparation of nanoporous thin films from block copolymers without a degradable component requires a rational design of the polymer. In principle, to generate nanopores from self-assembled block copolymers, it is not necessary to degrade the backbone bonds in the minor component; rather, a cleavage of the juncture between two immiscible blocks is sufficient. Therefore, if one could put a cleavable juncture into a block copolymer; the cleavage will not depend on the chemical nature of the minor component and should be applicable to a much larger variety of block copolymers. Cleavage of the juncture point of block copolymer in the ordered state followed by removal of the minor component via selective solvents is therefore a very promising strategy for the generation of nanoporous structures. For most of the nanoporous polymer thin films prepared to date, one of the major problems is the lack of long-range order in the nanopore array, which prevents them from being used in applications where addressability is required (e.g., magnetic storage devices). Long-range order in thin films of the precursor block copolymers is a prerequisite for obtaining nanoporous materials with well-ordered pores. We have recently demonstrated that the solvent-annealing of thin films of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers leads to highly ordered arrays of PEO cylinders oriented normal to the film surface. The long-range lateral order and the orientation in the annealed PS-b-PEO thin films arise from the strong nonfavorable interactions between PS and PEO, enhanced mobility caused by the presence of solvent, as well as the directionality of solvent evaporation. The subsequent removal of the PEO domain will lead to nanoporous films with identical long-range order. However, the poor degradability of PEO prevents the direct generation of nanopores from ordinary PS-b-PEO. Mao and Hillmyer recently reported the successful use of hydroiodic acid to degrade PEO in the bulk sample of PS-b-PEO, but the harsh reaction conditions they used are not applicable to thin films. To impart the degradability to the PS-b-PEO system, we recently synthesized a triblock copolymer, polystyrene-bpoly(methyl methacrylate)-b-poly(ethylene oxide) (PS-bPMMA-b-PEO), where PS is the major component and PMMA and PEO are minor components. By taking the advantage of the photodegradability of the PMMA mid-block, nanoporous thin films with excellent long-range order suitable for block copolymer lithography were prepared. In this method, the degradation efficiency depends critically on the molecular weight and phase behavior of the PMMA block, and only when PMMA formed a distinct microdomain did efficient degradation occur. In this paper, we show that by placing a cleavable juncture, a triphenylmethyl (trityl) ether linkage, between PS and PEO, C O M M U N IC A IO N