David F. Sanders

Synthesis and characterization of Thermally Rearranged (TR) polymers: influence of ortho-positioned functional groups of polyimide precursors on TR process and gas transport properties

Aromatic polyimides bearing various ortho-functional groups (i.e., acetate group and pivalic acetate group) were prepared via acetylation of a poly(hydroxyimide) containing ortho-positioned hydroxy groups using acetic anhydride or pivalic anhydride. The completeness of acetylation was confirmed by 1H NMR and FTIR. Chemically derivatized polyimides were used as precursors for an imide-to-benzoxazole thermal rearrangement (TR) process. The influence of various ortho-functionalities on the TR process and gas transport properties of the resulting TR polymers was studied. Differing from the –OH groups in a poly(hydroxyimide), the acetate groups of acetylated polyimide precursors degrade at elevated temperatures, and the degradation process interplays with imide-to-benzoxazole conversion. The acidic degradation product, as detected by 1H NMR, is suspected to have some catalytic effect on the TR process, which along with the protecting function of the acetate groups, resulted in a lower onset TR conversion temperature, the ability to conduct the TR process in air, and a higher TR conversion level. Gas permeation properties greatly depend on the ortho-functionality of polyimide precursors as well. The precursor films containing larger functional groups are much more permeable with comparable gas selectivities. Similarly, the resulting TR polymers formed from polyimides with larger leaving groups also showed much higher gas permeabilities despite similar degrees of TR conversion. The incorporation of bulkier functional groups in the TR precursors provides an effective way to significantly improve the gas transport performance, particularly the gas permeabilities of both the polyimide precursors and the resulting TR polymers.

Synthesis and characterization of thermally rearranged (TR) polymers: effect of glass transition temperature of aromatic poly(hydroxyimide) precursors on TR process and gas permeation properties

Soluble aromatic polyimides containing ortho-positioned hydroxy groups were synthesized as precursors for thermal rearrangement (TR) to polybenzoxazoles (PBOs). Fully imidized polyimides with high-molecular-weights were afforded via a ‘one-pot’ solution imidization technique (i.e., ester-acid method). The poly(hydroxyimide)s were designed to vary in their glass transition temperatures (Tg) by carefully selecting dianhydride–bisaminophenol combinations to introduce various levels of chain rigidity. TR conversion (imide-to-benzoxazole conversion) occurred in solid-state films only under inert atmosphere and over a temperature range of 300–450 °C, depending on the chemical structure (chain rigidity) of precursors. The effect of the precursor Tg on TR conversion was studied using TGA, DSC, FTIR and gel fraction measurements. The TR conversion temperature of imide-to-benzoxazole rearrangement strongly depended on the precursor Tg. Thus, for example, the feasible TR temperature was successfully reduced by ∼100 °C by lowering the precursor Tg by using a bisphenol A type dianhydride in the polymer synthesis. Gas permeation properties of representative TR systems are also reported. The TR process significantly increased gas permeabilities while maintaining good selectivities. By correlating the TR conversion degree with gas transport properties, there appears to be an optimal TR conversion degree that can maximize both gas permeability and selectivity. Systematic studies on TR polymers derived from low Tg precursors were suggested to further explore this correlation.