Comparative Study on the Transport Characteristics and Electrolyte Resistance of Brominated Butyl Rubber‐ and Natural Rubber‐Based Rubber Films Using the Mass Uptake Measurement

ABSTRACT This study evaluates brominated butyl rubber (BIIR) and natural rubber (NR) as sealing materials for lithium‐ion batteries by investigating their electrolyte resistance. BIIR, synthesized through bromination of butyl rubber (IIR), demonstrates enhanced chemical stability and aging resistance due to its polar CBr bonds and dense molecular architecture. NR, composed primarily of cis‐1,4‐polyisoprene, offers superior elasticity and processability. Vulcanized samples of BIIR and NR are immersed in pure and mixed solutions of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) at 45°C, 60°C, and 80°C, respectively. Swelling behavior and transport properties are analyzed by calculating diffusion ( D ), solubility ( S ), and permeability ( P ) coefficients. Results reveal that BIIR exhibits 42%–58% lower swelling ratios than NR under high‐temperature (80°C) and polar electrolyte (EC‐rich) conditions due to its compact molecular structure. NR displays selective adsorption behavior, with transient mass increases of 15%–20% during initial solvent exposure. Temperature elevation accelerates transport rates, reducing equilibrium swelling time by 65%–72% for BIIR compared to NR. These findings establish BIIR as a promising candidate for high‐temperature battery sealing applications, while the cost‐effectiveness and resilience of NR, on the other hand, remain advantageous under moderate conditions.