Study On Pseudoeutectic Phenomenon Of Random Copolyesters

Aliphatic polyesters have attracted much attention due to their great degradability and biocompatibility.Regulations of crystallization behavior and crystal structure are important approaches to adjust the properties and performance of polymer,and copolymerization,the most commom regulation method for polymer crystallization,can help fabricate continuously changing properties of polymers,which also results in complicated and interesting crystallization behaviors.Herein,the crystallization behaviors of random bipolyester-poly(decamethylene succinate-ran-decamethylene fumarate)(PDSF)and terpolyesters-poly(butylene succinate-ran-butylene adipate-ran-butylene fumarate)(PBSAF)have been systematically studied,which would enrich understanding of the crystallization mechanism of random copolyesters.The main results are described as follows.The PDSF copolyesters displayed clear melting point and high crystallinity in the whole composition range,and showed a special pseudoeutectic phenomenon with a wide composition region.X-ray diffraction measurements confirm in the pseudoeutectic region two type of crystals both exhibited partial diffraction peaks.The PDSF37(with37mol% DF)and PDSF57(with 57mol% DF)in the pseudoeutectic region were in detail studied.Epitaxial and confined crystallization simultaneously occurs between PDS-rich and PDF-rich crystals in PDSF57 and PDSF37.PDSF57 has the similar crystallization behavior as commonly observed isodimorphic random copolyesters showing two crystals of temperature dependent formation.Yet,PDSF37 exhibited a particular crystallization process,during which the content of two types of crystals were not affected by varying temperature.And the PDF-rich crystal can exist under overheating conditions due to the PDS-rich confined it,leading to an abnormally high melting point.In PBSAF terpolyesters two types of crystal structures,named PBSF-rich and PBA-rich crystals,appeared when the incorporated BF units was 0?9 mol%.The formation content of PBA-rich crystal decreased with the increase of BF unit content,and disappeared when BF unit content was higher than 9 mol%.By using temperature-dependent FTIR,we have clarified that the BF units were confined not being included in the PBSF-rich lattice,but not the PBA-rich one.The reasons for BF units hindering PBA-rich crystallization can be ascribed to two aspects depending on cooling rate.During slowly-cooling process(< 20 ?/min),the incorporated BF units promoted the crystallization ability of PBSF-rich phase,resulting in hard confinement that made on PBA-rich phase crystallize more difficultly.During fast-cooling process(?20 ?/min),through PBSF-rich did not crystallized prorior to PBA-rich phase,the weak hydrogen-bonding network fabricated among the BF units decreased mobility polyester chains,and thus suppressed the cold-crystallization of PBA-rich from amorphous when heated.Furthermore,the phase diagram of crystalline phase of PBSAF tercopolyesters was proposed and presented.The crystallization behavior of PBSAF changing from isodimorphism to isomorphism with increase of BF units can be clearly explained by using the phase diagram.