Synthesis and characterization of poly(butylene terephthalate)-co- poly(butylene succinate)-b-poly(ethylene glycol)block copolymers with different soft segment length

The poly (butylene terephthalate)-co-poly (butylene succinate)-b-poly (ethylene glycol) block copolymers (PTSG), employing PEG homopolymers with different number-averaged molecular weight from 400 to 2000 as precursors, with PBS molar fraction (M_PBS) varying from 10% to 30% were synthesized by melt polycondensation technique and characterized by means of GPC, NMR, DSC and mechanical testing etc. The result of GPC measurements shows single-modal curves with a polydispersity below 2.0 for the copolymers. Analysis on the sequence distribution of the two hard segments in the copolymer chains reveals that the average sequence length of PBT hard segments decreases while that of PBS increases with the increase of M_PBS, and the random distribution of the two hard segments is also validated by the degree of randomness around 1.1. Microphase-separation structure is demonstrated by the appearance of two glass transition temperatures (T_g,s, T_g,h) and two melting points (T_m,s, T_m,h) in DSC curves of most samples. Especially, the decrease of the average block length of PBT and the co-crystallization between the two hard segments may cause the depression of the melting point and crystallinity of the hard segments, when M_PBS changes from 10% to 30% under constant soft segment length. The increase of elongation at break and equilibrium water up-take with the increase of PEG molecular weight and its weight ratio in the copolymer, as well as the decease of crystallinity of hard segments provide a promising method of modulating the flexibility and hydrophilicity of copolymer chains.