The degradation manner of graft-type polymer electrolyte membranes consisting of hydrophilic poly(styrenesulfonic acid) (PSSA) graft polymers (grafts) and thermally and mechanically stable hydrophobic polymer substrates such as crosslinked poly(tetrafluoroethylene) (cPTFE), poly(ethylene-co-tetrafluoroethylene) (ETFE), and poly(ether ether ketone) (PEEK) was examined in water at 85 and 95 °C. Severe weight loss was observed in PSSA-grafted cPTFE and ETFE films but not in a PSSA-grafted PEEK film. The decomposed extracts in water were characterized as PSSA during the whole course of degradation, clearly showing that the PSSA grafts detached from the fluorinated substrates without decomposition or scission of graft polymer chains. This is quite a new degradation manner for graft-type polymer electrolyte membranes. PSSA-grafted PEEK films showed severe weight loss in aqueous propanol solutions making PSSA grafts swell; clear correlation was observed between the degree of swelling in PSSA grafts and the film weight loss. The solubility parameters between grafts and the polymer substrates and small angle neutron scattering (SANS) data implied that the grafted films possessing a clear boundary at the interface resulted in significant weight loss. From the above observation, it should be concluded that hydrophilic PSSA grafts detach from hydrophobic polymer substrates due to swelling-induced stress at the interfacial boundary in the grafted films.

Poly(ether ether ketone)-based graft-type polymer electrolyte membranes (PEEK-PEMs) with wide ion exchange capacity (IEC) ranges were prepared by radiation-induced graft polymerization for investigating the relative humidity (RH) dependence of their electrochemical and mechanical properties at 80 °C for fuel cell applications. The proton conductivity ranges of 1.73–3.08 mmol g⁻¹ IECs at 30% and 95% RH were 0.001–0.009 and 0.108–0.431 S cm⁻¹, respectively. Unlike aromatic hydrocarbon-type PEMs, PEEK-PEMs' conductivity exhibited less dependence on RH. PEEK-PEMs with IECs > 3.08 mmol g⁻¹ exhibited a similar conductivity (0.009 S cm⁻¹) under 30% RH and showed 1.4 times higher tensile strength (14 MPa) under 100% RH at 80 °C in comparison to Nafion 212. The MEA fabricated by the PEEK-PEM with IEC = 2.45 mmol g⁻¹ showed the maximum power densities (E_max) of 860 and 826 mW cm⁻² at 2140 and 2180 mA cm⁻² under 100% and 30% RH, respectively. In particular, PEEK-PEM showed low RH dependence for E_max, which was 2.5 times higher than that of Nafion 212 at 30% RH. The X-ray scattering analyses revealed that the crystallinity of approximately 28–32% was maintained and propagated during the graft polymerization of graft-type PEEK-PEMs. The abovementioned unique structures were the origins of higher conductivity and tensile strengths compared with conventional PEMs.

We investigated the morphology and swelling behavior of a new graft-type of anion exchange membrane (AEM) containing 2-methylimidazolium groups by using a contrast variation small angle neutron scattering (SANS) technique. These AEMs were prepared by radiation-induced grafting of 2-methyl-1-vinylimidazole and styrene into poly(ethylene-co-tetrafluoroethylene) (ETFE) films and subsequent N-alkylation with methyliodide, and possessed both high alkaline durability and high conductivity. Our results showed that the crystalline lamellar and crystallite structures originating from the pristine ETFE films were more or less conserved in these AEMs, but the lamellar d-spacing in both dry and wet membranes was enlarged, indicating an expansion of the amorphous lamellae due to the graft chains introduced in the grafting process and the water incorporated in the swelling process. For the first time, the swelling behavior of the AEMs was studied quantitatively in various water mixtures of water and deuterated water with different volume ratios (contrast variation method), and the morphology of these membranes was elucidated by three phases: phase (1) crystalline ETFE domains, which offer good mechanical properties; phase (2) hydrophobic amorphous domains, which are made up of amorphous ETFE chains and offer a matrix to create conducting regions; phase (3) interconnected hydrated domains, which are composed of the entire graft chains and water and play a key role in promoting the conductivity.

A simplified radiation-induced emulsion graft polymerization (SREG) method is proposed. This method involves a convenient and easy degassing process of a monomer solution using a commercially available sealed glass jar. A loaded weight on the lid of the jar was used to control the jar’s internal pressure as the degassing of the monomer solution took place using a vacuum pump. The degassing method was highly reproducible, resulting from no bumping of the monomer solution. The initial grafting velocity was proportional to the absorbed doses of pre-irradiation between 5 and 20 kGy. This result indicates that dissolved oxygen was sufficiently eliminated from the monomer solution at such a level where the remaining oxygen had little effect on the grafting reaction at a dose of 5 kGy. The method was then applied to the fabrication of a heavy metal adsorbent that possessed a sufficient adsorption capacity of Co(II) ions. The SREG method is applicable to the fabrication of a wide variety of functional graft polymers because high-dose-rate gamma-ray radiation and expensive experimental equipment are not necessary.