Effect of phase morphology on water diffusion in phosphorus-containing thermotropic liquid crystal copolyester film

In this paper, effect of phase morphology on water diffusion in phosphorus-containing aromatic liquid crystalline copolyester (P-TLCP) named as poly(-hydroxybenzate-co-DOPO-benzenediol dihydrodiphenyl ether terephthalate) (PHDDT) was investigated by two-dimensional correlation infrared (2DIR) spectroscopy, in order to understand well the relationship between structure and properties of P-TLCP. The phase morphologies of the PHDDT films were observed by polarized light microscope. The experimental results showed that the clear nematic texture, which was observed for PHDDT film treated at 250 °C for 3 min under a nitrogen atmosphere. However, lots of bright spots were observed for untreated PHDDT film due to weak crystallization of PHDDT molecules. Moreover, the density of the untreated PHDDT film (1.1631 ± 0.0257 g/cm−3) was lower than that of the treated one (1.2969 ± 0.0134 g/cm−3), indicating that the arrangement of the molecules in treated PHDDT film was compact in comparison with that in untreated one. Therefore, the average diffusion coefficient of water in treated PHDDT film was lower than that in untreated one. The mechanisms of water diffusion into PHDDT films with different phase morphologies can be obtained through 2DIR analysis in OH stretching and bending bands. It was found that water diffused into the treated PHDDT film by forming moderate hydrogen bonds prior to forming strong and weak hydrogen bonds, while diffused into the untreated one by forming strong and weak hydrogen bonds prior to forming moderate hydrogen bonds. It was also found that the spectral intensity of PO varied prior to that of CO during water diffusion into untreated PHDDT film, which was reversed for treated PHDDT film.