Preparation and properties of porous polyimide films

Porous polyimide films were prepared in two steps. The first step was the p reparation of poly(urethane-imide) films by a reaction between a phenol-ter minated polyurethane prepolymer and poly(amide acid) obtained from pyromell itic dianhydride (PMDA) and oxydianiline (ODA) followed by thermal treatmen t at 200 degrees C. The second step was the pyrolysis of the poly(urethanei mide) films. Upon thermal treatment of the poly(urethane-imide) films at 30 0 to 400 degrees C, the thermally less stable urethane domains decomposed, leaving porous polyimide films. The presence of pores was confirmed from th e scanning electron microscope (SEM) picture of the surface and the cross s ection of the films. The size distribution of the pores was narrow. With in creasing urethane content, the apparent density decreased but the size of t he pore did not increase so much. Instead, the number of pores increased. T ensile measurements revealed that, with increasing urethane component the t ensile modulus did not vary very much, but the elongation and thus the tens ile strength decreased. Viscoelastic analyses showed that all the porous po lyimide films had a high glass transition temperature, above 400 degrees C, and storage moduli were maintained up to 400 degrees C.