Studying the Diffusion of Ethanol in Crosslinked Carboxylated Nitrile Butadiene Rubber Using Positron Annihilation Lifetime Spectroscopy

ABSTRACT The diffusion coefficients of ethanol through carboxylated nitrile–butadiene rubber (NBR/XNBR) thin films were measured using penetration tests and the free‐volume characteristics were analyzed via positron annihilation lifetime spectroscopy (PALS). The results demonstrate that both hydrogen bonding (H‐bonding) and the free‐volume structure influence the diffusion process, with H‐bond interactions exerting a stronger effect. The activation energy associated with H‐bonding was determined as approximately 14 kJ/mol. PALS offers superior insight into the microstructural details of free‐volume holes: average hole size, size distribution, and relative hole quantity, rather than only the overall specific free volume. The results show that while the average free volume hole size remains nearly constant, the relative hole quantity decreases linearly with increasing crosslink density ( X c ). These findings are consistent with the widely applied Vrentas–Duda free volume theory for molecular diffusion in polymer matrices. By fitting the PALS data to the Vrentas–Duda model, the overlap factor between a free volume hole and an ethanol molecule was determined to be 0.28—lower than the commonly reported range of 0.50–1.0 obtained from conventional temperature‐dependent diffusion measurements. Thus, PALS provides an alternative means to estimate the overlap factor, offering valuable cross‐verification with traditional diffusion—temperature dependence experiments.