CHANGE IN GAS TRANSPORT PROPERTIES OF HYDROXY-CONTAINING POLYIMIDE (HPI) AND THERMALLY REARRANGED POLYBENZOXAZOLE (TR-PBO) BY THERMAL TREATMENT

Recently, polymer materials having high free volume have drawn many researchers' attentions for the application of gas transport/separation membranes, since diffusivity and solubility of gas molecules in polymer membranes have a strong relationship with the amount of the free volume. Among them, thermally rearranged polybenzoxazole (TR-PBO) has been showing not only outstanding mechanical and chemical stability but also high permeability that surpass the limits of conventional polymers. In particular, TR-PBO is prepared from aromatic polymide containing ortho-positioned hydroxyl groups by thermal treatment and resulting rearrangement in the solid state structure, which can generate unusual microstructure and high free volume. In this presentation, we will report on the change in gas transport properties of polyimide precursors (HPIs) and the resulting thermally rearranged polybenzoxazole (TR-PBO) membranes before and after thermal treatment using molecular dynamics (MD) simulation. Computer simulations including MD simulation are a powerful tool to enhance the understanding about invisible microscale or nanoscale systems and to predict the properties of the systems such as molecules or fluids. Among them, MD simulation focuses on the atom-level systems from a few small molecules to macromolecules such as polymers. First, we generated the 3D model structures of hydroxy-containing polyimides before thermal treatment and those of the thermally rearranged polybenzoxazoles after the thermal treatment, according to their temperature conditions. As expected, HPIs and TR-PBOs shows very different free volume morphologies; HPIs have a unimodal distribution of free volume areas, which partially coalesce in larger areas having, however, a relatively narrow size. On the contrary, TR-PBO shows a bimodal cavity distribution and, after thermal treatment and TR reaction, the free volume structures in TR-PBO are maintained. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. As a result, HPI and TR-PBO polymers show different transport properties from the simulation results of gas molecules in the both models, respectively, which was well in consistent with the chracteristics of free volume morphology of each model.