Polymerization mechanism for radiation-induced grafting of styrene into alicyclic polyimide films for preparation of polymer electrolyte membranes

Alicyclic polyimides (APIs) derived from 1, 2, 4, 5-pyromellitic dianhydride (PMDA) bis(4-aminocyclohexyl)methane (DCHM) were successfully applied to radiation-induced graft polymerization for developing polymer electrolyte membranes for fuel cells. Pyromellitic-type CoPIs were prepared from PMDA and a mixture of aromatic and alicyclic diamines with certain molar ratios, and the radiation-induced grafting of styrene was then examined using pre-irradiation with a dose of 220 kGy, followed by graft polymerization in a bulk styrene solution at 60 °C. The grafting into fully aromatized polyimide barely proceeded (degree of grafting (DG) of less than 5%), whereas that of styrene into the API films proceeded with styrene DGs of up to 70%. Electron spin resonance measurements clearly showed that the irradiated API with the highest alicyclic component possessed the largest amount of radicals (up to 1.13 × 1018 spins/g). Furthermore, in combination with ultraviolet–visible (UV–vis) spectroscopy, the radical species was identified as a long-lived intermediate with a spectrum containing λmax = 420 and 600 nm, and 10% of the radicals were consumed as grafting initiators. Note that the alicyclic structure of the API films is advantageous for radiation-induced graft polymerization because of the generation of high-concentration initiation radicals. The moderate reaction conditions allowed for selective sulfonation on the polystyrene grafts, and not on the API substrates, to give API-based polymer electrolyte membranes (PEMs) with ion exchange capacities (IECs) of 1.7–2.8 mmol/g. The PEMs exhibited appropriate proton conductivity (0.06 ≤ σ ≤ 0.34 S/cm) and low water uptake (≤100%), together with excellent mechanical properties (tensile strength, 90 MPa), compared with conventional PEMs such as Nafion®.