Synthesis of a photocrosslinkable polyimide with a chalcone side chain and its properties as a coating material for a secondary battery separator

A carboxyl-containing polyimide (PI) was synthesized with a flexible main chain. Then, a polymer reaction was conducted to introduce a chalcone side chain. The polymer reaction occurred stoichiometrically, and a high yield of the resulting photocrosslinkable polyimide (PI-Ch) polymer was obtained (91 %) along with a high molecular weight of 9.4 × 10⁴ g/mol. The PI-Ch had a density of 1.15 g/cm³, a fractional free volume of 0.20, and a solubility parameter of 18.8 MPa^1/2. Due to its low polarity, the PI-Ch was very soluble in a moderately polar, low-boiling-point solvent of THF. Moreover, the temperature required to promote 5 % weight loss was quite high at 383 °C, while the glass transition temperature was relatively low at 146 °C. The THF solution was considered to be optimal for the dip-coating method and was applied to PE separator coating. The ATR-mode FTIR analysis of the separator coated with a 1 wt% PI-Ch solution showed that the PI-Ch was coated extensively on the surface of the PE separator. Further, the photodimerization reaction of the chalcone side chain via UV irradiation was successful. The PE separator had a thermal shrinkage of 46 % before coating, which decreased to 29 % after coating and UV treatment. When coated with a 3 wt% PI-Ch solution, some of the pores in the separator were blocked, while the air permeability and average pore size also decreased. All the pores appeared blocked after coating with the 5 wt% PI-Ch solution.