Mesoporous Polystyrenes via Lamellar Block Copolymer Assembly

Abstract

Block copolymers (BCPs) are well-known to self-assemble into a variety of nanostructures, such as spherical, cylindrical, gyroid-like, and lamellar morphologies, depending on the volume fraction of the block and/or the chain architecture. Organic mesoporous materials applicable to separations, sensors, etc., could be one of the interesting fields using block copolymers. To prepare organic mesoporous materials, block copolymers should consist of chemically inert and labile blocks. A polystyrene-block-polylactide (PS-b-PLA) system is a representative example. The PS block is mechanically and chemically robust, while the PLA block is easily hydrolyzed under basic conditions. In a given morphology, PLA domain structures determine the resulting pore morphology. Among possible pore architectures, one-dimensional cylindrical pores have been intensively studied, perhaps because the morphology exists over a wide range of volume fraction, and also because it consists of confined pores. Recently, a three-dimensionally connected mesoporous structure from a gyroid morphology was also reported. On the other hand, due to its wide volume fraction range near 0.5, a lamellar morphology can be obtained as readily as a cylindrical morphology. Nevertheless, a lamellar morphology has rarely been utilized for preparing organic mesoporous materials. With this in mind, it would be interesting to prepare lamellar mesoporous materials using a lamellar BCP. In this paper, we report a mesoporous material, consisting of PS layers, by a chemical etching of sacrificial PLA layers in a lamellar structure. To obtain the porous material, we first synthesized a block copolymer (PS-b-PLA) using a click reaction of individually prepared PS and PLA blocks. The PS was prepared by the anionic polymerization of styrene. The PS end was functionalized with a hydroxyl group, which was converted into an alkynyl group for the next click coupling (Scheme S1(b)). From the H NMR data, the number average molecular weight (Mn) of the obtained PS was determined to be 14,400 g/mol. The other PLA block was synthesized by a ring opening polymerization (ROP) of D,L-lactide using triethyl aluminum as the catalyst (Scheme S1(c)). For the click coupling with the PS block, the ROP initiator contained an azide end. From the H NMR analysis, the Mn was estimated to be 10,500 g/mol, and the GPC data showed the polydispersity (Mw/Mn) to be 1.06. The click coupling was carried out using Cu(I)Br/PMDETA as the catalyst (Scheme 1). The obtained block copolymer showed a triazolyl hydrogen at 7.39 ppm in the H NMR spectrum (Figure S1), and the polydispersity was 1.04, indicative of its narrow molecular weight distribution (Figure S2). The volume fraction ( f ) of the PS block was calculated to be 0.60. The differential scanning calorimetry (DSC) data of the block copolymer displayed two glass transitions at 51.4 C