Preparation Of Porous Polyimide Microspheres By Thermal Degradation Of Block Copolymers

The porous polyimide materials, combining both unique properties such as good thermal, excellent mechanical properties of polyimide and the advantages of porous structure such as low density, large specific surface and feasible functionalization, have widely used as low-k dielectrics devices, gas separation and storage films, drug carriers and so on. In order to expand the application of porous polyimide materials in catalyst carrier, column packing, insulating materials, it is desired to prepare porous polyimide microspheres.Current researches about preparation porous polyimide films and nanofoams have made significant progress, but there are only a few studies concern on synthesis porous polyimide microspheres. Our group has developed non-aqueous emulsion system to prepare polyimide microspheres with good reproducibility. We also tried to prepare porous polyimide microspheres by using inorganic small molecules and oligomers as the porogen and etching SiO2. Unfortunately, the final pores on the microspheres were uperficial and shallow. Moreover the reproducibility of these experimental procedures was poor. This paper is, combining the characteristics of the non-aqueous emulsion, to prepare porous polyimide microspheres by thermal decomposition of block copolymers.Thermal labile oligomers were block into polyamide acid chain by covalent bond; a microphase separation structure was formed after two-step imidization. The pores remained by thermal decomposition the thermal labile block. After that, a systematic study was carried out on each of the main factors in the preparation process including the mechanism of microphase separation and formation of porous structure. The main contents and conclusions of this paper were as follow:(1) Synthesis of copolymer microspheres of polyimideIn non-aqueous emulsion system, we prepared triblock copolymers microspheres by polycondensation of the thermal labile blocks, J-1000/J-2070 with the content of 20-40wt%, and monomers (pyromellitic dianhydrede and 4,4’-oxydianiline). The J-1000/J-2070 was blocked into the chain of polyamide acid with covalent bond. Two-step imidization was subsequently performed the copolymer microspheres of polyimide with microphase separated structure. The result showed that, J-1000/J-2070 could only be added in the system before the formation of polyamide acid; J-2070 with the molecular weight more than 2000 was the suitable thermal labile block; the suitable content of J-2070 was 20-30wt%; the actural block rate calculated by TGA curves was 65-70%.(2) Preparation of porous polyimide microspheresThe porous polyimide microspheres were generated by thermal degradation the copolymer microspheres under vacuum. The porous structure was consisted of micro-pore on the surface of the porous microspheres and the macro-pores within the porous microspheres. The conditions of thermal treatment were optimized. The result showed that, the optimal condition of thermal imidization was 100℃×1 h,150℃×1 h,200℃×1 h,250℃×10 h, 300℃×1 h under N2; the optimal condition of thermal degradation was 260℃×10 h under vacuum of -0.03～-0.06 MPa. We could not obtain the porous polyimide microspheres when the copolymer microspheres were thermal degradation under ambient pressure. The pore size were increasing with the vacuum degree increasing, the macroporous structure with the pore size of 300-500nm were produced in -0.06 MPa.