The influence of soft segment length on the properties of poly(butylene terephthalate- co-succinate)- b-poly(ethylene glycol) segmented random copolymers

Abstract

Three series of poly(butylene terephthalate- co-succinate)- b-poly(ethylene glycol) segmented random copolymers with starting PEG number-average molecular weight ( M n(PEG)) at 600, 1000 and 2000, respectively, as well as hard segment poly(butylene succinate) (PBS) molar fraction ( M PBS) increasing from 10% to 30% were synthesized through a transesterification/polycondensation process and characterized by means of GPC, NMR, DSC, WAXD and mechanical testing etc. The investigations were mainly focused on the influence of M n(PEG) on the properties of resulting copolymers bearing two sorts of hard segments. It is revealed that all the samples show a relatively symmetrical GPC curves with the number-average molecular weight more than 4 × 10 4, while the polydispersity decreases from 1.9 to 1.4 as the increasing M n(PEG) because of the prolonged time for polycondensation and the faster exclusion of small molecules by-product with the decreased molten viscosity. The sequence distribution analysis shows that the average sequence length of hard segment PBT decreases while that of PBS increases with the increasing M PBS and are independent of the soft segment length. The approximate unit degree of randomness as well as the soft segment length turns out that the segments take a statistically random distribution along the backbone. Micro-phase separation structure is verified for the appearance of two glass transition temperatures and two melting points, respectively, in DSC thermograms of most samples. The depression of melting points and the reduction of crystallinity of hard segments with increasing M PBS are related to the crystal lattice transition from α-PBT to PBS and discussed in the viewpoint of cohensive energy. Mechanical testing results demonstrate that the increase of amorphous domains the increase of M PBS as well as M n(PEG) will provide high elongation and good flexibility of copolymer chain. The in vitro degradation experiments show that the partial substitution of aromatic segment PBT with aliphatic PBS will substantially accelerate the degradation rate with enhanced safety of degradation by-products and while changing M n(PEG) broaden the spectrum to tailor the properties.