Polyimide/polystyrene nanocomposite films as membranes for gas separation and precursors for polyimide nanofoams

A method for the preparation of polyimide (PI) nanofoams with high thermal stability, which can be used as dielectric layers in microelectronics, has been presented in this paper. Polystyrene (PS) and poly[styrene-co-(4-vinylpyridine)] (PSVP) nanospheres were prepared by emulsion polymerization. The diameter of these particles was 30 to 40 nm. PMDA-ODA PI nanocomposite films with various contents of PS and PSVP were obtained by in-situ condensation polymerization in the presence of nanospheres. The distribution of the nanospheres in the films and the morphology of the films were characterized by TEM. Relatively homogeneous distributions were observed for the PI/PSVP nanocomposite films, which can be ascribed to the formation of poly(amic acid) amine salts between pyridinyl and carboxyl moieties. The transport properties of CO2, O-2 N-2, and CH4 gases through these nanocomposite films were measured. For the PI/PS-nanosphere composite films, with the increase of PS nanospheres in the films, the gas permeabilities increased and selectivities decreased: these were due to the increase in gas diffusion coefficients and the decrease in diffusion selectivities. For PI/PSVP-nanosphere composite films, increases in both gas permeabilities and selectivities were observed when the PSVP-nanospheres increased from 10 to 20 wt% in the films, while both the diffusion coefficients and diffusion selectivities decreased. It was found that the membrane containing 20 wt% PSVP-nanospheres showed both high gas permeability and high permselectivity toward CO2. These results can be ascribed to the high solubility of CO2 and 0, in the pyridine-containing polymers and the relatively homogenous distribution of PSVP-nanospheres in the PI matrix. Upon thermal treatment, the thermally unstable nanospheres undergo thermolysis, leaving pores with the size and shape dictated by the initial nanosphere morphology. The resulted foams were characterized by TEM, DSC, TG, density measurements and dielectric constant measurements. The results revealed that nanofoams with high thermal stability and low dielectric constant approaching 2.0 could be prepared using PI nanocomposite approach.