Block copolymer is capable of organizing various selfassembly structures on a nanometer scale. This unique characteristic has been utilized for preparation of the materials composed of nanometer pores. 1-11 However, such an application of block copolymer is often limited to ultrathin films. Its restricted longitudinal space induces the periodic arrangement of the phase separations, which spreads over a lateral direction parallel to the surface. 1, 3, 7, 8, 11 In this work, we demonstrate an easy method of preparing a nanoporous polyethylene film having submillimeter thickness from a block copolymer precursor. The block copolymer exhibits various types of microphase separations, including spherical, cylindrical, and lamellar structures, depending on the component ratio of the different blocks. Among these morphologies, the sphere is most widely used as a precursor of nanoporous materials. Recently, Yokoyama et al. 9 developed a pore preparation method with supercritical CO2, which remains in the spheres and expands, giving numerous nanopores within the sample. A cylinder can also be converted to the continuous pore structure when it can be removed. Hillmyer et al. 4, 5, 10 prepared nanoporous materials from polystyrene (PS)/poly (lactic acid)(PLA), poly (cyclohexyl ethylene)/PLA, or PS/poly (ethylene oxide)(PEO) diblock copolymers by selective removal of PLA or PEO cylinders using alkali or acid solvents. Here, an orientation of the original cylinders perpendicular to the etched surface is required to improve the resultant pore continuity. Therefore, an extrusion technique with a channel die was used.

In contrast, a lamellar structure is not used for preparing nanoporous material, but the gyroid structure, 12 which is one type of diagonals between lamella and cylinder, is used. This structure is composed of a set of two networks having different chemical structures. Selective removal of one network produces the continuous nanopores without any additional processing. Thomas et al. 2 successfully prepared nanoporous ceramic films from triblock copolymer precursors having the double-gyroid structure. A combination of ozonolysis and ultraviolet irradiation causes the selective removal of the hydrocarbon block and the conversion of the silicon-containing block to a ceramic structure. The calculated interfacial area of this nanoporous ceramic film approached 40 m2/g due to the unique gyroid interconnections of both blocks within the precursor. These phase separations of block copolymer are dominated by the component ratios of the precursor blocks, which are determined by the chemical synthesis of the starting material. In contrast, we tried to control the phase separation by inducing another self-assembly ability of “crystallization” in the blocks. If one or more blocks are semicrystalline, the two self-assembly effects of the usual phase separation induced by chemical difference and crystalline/amorphous separation will overlap. This suggests the possibility that the phase separation of block copolymer is controllable by crystallization procedure without any chemical change.