Effect of the Sequence Distribution and Internal Structure on the Hydrophilicity of Poly(ethylene terephthalate-co-aliphatic amide) Copolymers

Improving the hygroscopicity of poly(ethylene terephthalate) (PET) is a desired goal, and aliphatic amides with hydrophilic groups are ideal modified monomers. In this paper, two aliphatic amides (adipate pentamethylenediamines (APs) and phosphorylacetic acid (APA)) with different segment lengths were prepared by the reactions between 1,5-pentane diamine and adipic acid, which were copolymerized with purified terephthalic acid (PTA) and ethylene glycol (EG) to synthesize the copolymers PET/APs and PET/APA, respectively. The chemical structures of the modified monomers and copolymers were confirmed by Fourier transform infrared (FTIR) and NMR spectroscopy. In addition, the copolymers exhibited excellent moisture regain (MR) and water absorption capability (WAC). When the molar content of modified monomers was 12%, the MR and WAC of PET/APs increased by 200 and 420% and the MR and WAC of PET/APA increased by 218 and 475%, respectively. Moreover, PET/APA exhibited better hydrophilicity than PET/APs due to the effect of the microsequence structure. Based on the analysis of the sequence distribution and isothermal crystallization kinetics, PET/APA possessed a more random sequence distribution and larger amorphous region due to the effect of the long flexible segments of APA, promoting the diffusion of water molecules and binding to hydrophilic groups. In addition, PET/APA possessed a more qualified yellow index (b value) than that of PET/APs. Thus, it provided a synthesis technique and broad application prospects.