The Relationship Between Hierarchical Structure And Properties Of Polybutylene Terephthalate-b-polytetrahydrofuran

Poly(butylene terephthalate-b-tetrahydrofuran)or(PBT-b-PTHF),as a successfully commercialized thermoplastic polyester elastomer(TPEE),has been used in electronic appliances,kitchen supplies,medical equipment and other fields due to its outstanding elasticity and flexibility.PBT-b-PTHF is composed of polybutylene terephthalate(PBT)as hard segment of polyester and polytetrahydrofuran(PTHF)as soft segment of polyether.By adjusting the ratio of hard segment and soft segment,the products can obtain diametrically different mechanical properties,which endows PBTb-PTHF with great potential in the application of shape memory materials and biodegradable materials.In addition,stress-induced phase transition occurs in the hard segment of PBT by applying external force.Therefore,exploring hierarchical structural changes of PBT-b-PTHF during the strain process is of great significance for the further development of PBT-b-PTHF.In this thesis,it is expected to reveal hierarchical structural evolution with different ratios between soft and hard segments,and also establish the relationship between the structure and macroscopic properties under stress by means of IR,WAXD,SAXS and other instruments.The research contents of this thesis mainly include:1.The effect of the ratio of soft and hard segments of PBT-b-PTHF on performance.Preparation of PBT-b-PTHF thin films by melt quenching.And the mechanical properties and crystallization ability of block copolymers with different proportions were characterized by means of DSC,extensometer and other instruments.The experimental results show that the PBT-b-PTHF samples with higher proportion of soft segment have higher elongation at break and greater shrinkage.At the same time,the worse the crystallization ability of the sample,the lower the melting temperature and the crystallization temperature with the increase of the soft segment ratio.2.The structural evolution and mechanical properties of PBT-b-PTHF in different proportions during the phase transition process.High crystallinity samples can be prepared using melt-quenched film samples that are stretched by a factor of four and then annealed under stress.The relationship between material properties and hierarchical structure during the orientation process of the phase transition between PBT alpha and beta forms was characterized by the Linkam extensometer combined with WAXD or SAXS.The experimental results show that the phase transition between the alpha form and the beta form of the samples with different ratios is reversible,which is only related to the stress,and has nothing to do with the ratio of the soft and hard segments of the material.When the stress of the material reaches the critical stress of 50 MPa,the alpha form transforms into the beta form,and when the stress is lower than 50 MPa,the beta form transforms into the alpha form.Due to the uneven distribution of internal stress caused by different ratios of soft and hard segments,the material will undergo phase transformation within a stress range near the critical stress,and the residual stress inside the material will lead to a certain hysteresis in the transformation of beta form to alpha form.A plateau appeared in the stress-strain curves of different ratios of PBT-b-PTHF,and the plateau corresponds to the phase transition process of the sample crystal.And the length of the plateau is related to the crystallinity of the sample,the higher the crystallinity,the more obvious the plateau area.The SAXS data show that the lamellae length period is positively correlated with the strain during the tensile-retraction process of PBT-b-PTHF samples with different ratios.When the strain increases,the lamellar long period increases,and when the strain decreases,the lamellar long period decreases.The samples shrunk in different proportions showed the characteristics of lamella tilt,especially the 5# sample with the highest content of soft segment showed more complex structure characteristics.The lamellae are more inclined along the orientation direction.The formation of larger periodic scattering patterns in its interior may be related to the crystallization of the PTHF.