Deformation of Polyether-Polyester Thermoelastoplastics: Mechanothermal and Structural Characterisation

Abstract

The complex investigation of copoly(ether-ester) based on poly(butylene terephthalate) (PBT) and poly(tetramethylene oxide) (PTMO) reveals its microphase separated nanodomain structure. Initial morphology includes the stacks of crystalline blocks of α-PBT embedded in amorphous matrix with the different degree of continuity of the crystalline network. Two types of amorphous regions can be distinguished, the PTMO-rich phase and the other one containing the PTMO and PBT segments. Reduction of PBT content and proper decrease of its fragments length results in dramatic change in crystalline ordering, the crystallites became smaller and distorted, and more and more PBT segments are included in the amorphous phase. The initial reversible stage of deformation is controlled mostly by elastic deformation of amorphous phase, highly constrained by the network of crystallites. The further stretching results in plastic deformation and reorganisation of the crystalline blocks of PBT and a new crystalline morphology arises. Moreover, at large deformations the soft blocks of PTMO can crystallise and form very distorted paracrystalline regions. Finally, at high enough stress (≈25–30 MPa) the transition from α to β crystalline form in PBT crystal lattice occurs due to conformational changes in the tetramethylene segments. After large deformation, both the morphology and the polymer conformations are far from the equilibrium state. Annealing of the stretched samples at high temperature results in partial recovery of material properties, however the morphology is still far from the initial one even after such annealing.