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

초록

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 x 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 MPa1/2. 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 degrees C, while the glass transition temperature was relatively low at 146 degrees 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.

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