Stress-induced solid-state transition has been found to occur in the multi block copolymers of poly(tetramethylene terephthalate)[PTMT] and poly(tetramethylene oxide) [PTMO]. In the crystalline regions of PTMT hard segments, the reversible structural change has been observed when the uniaxially-oriented sample is stretched along the draw direction. This is essentially the same phenomenon as that observed in PTMT homopolymer, although the transition in the block-copolymer requires appreciably larger tensile strain than the homopolymer case because of the coexistence of the PTMO soft segments with very low Young's modulus. In the PTMO soft segments, which are in the amorphous state at room temperature under free tension, there has been found to occur the stress-induced crystallization into the fully extended trans-zigzag conformation. At lower temperature below the glass transition point, these transitions in the hard and soft segments can be observed more clearly and sharply. The transitional behaviors have been compared among a series of samples with different molar fractions of PTMT hard segments and interpreted semiquantitatively based on a simple mechanical series model coupled with the crystalline phase transition in the PTMT hard segment. By measuring the temperature dependence of the critical stress at which the phase transition occurs abruptly, the transitional enthalpy and entropy have been evaluated.

The stress-induced solid-state phase transition in the uniaxially-oriented poly(tetramethylene terephthalate)-poly(tetramethylene oxide) block copolymers has been investigated at room temperature by X-ray diffraction and infrared and Raman spectroscopy. The phase transition in the hard crystalline region has been found to occur broadly over wide stress and strain region. The small-angle X-ray scattering measurement showed that, as the sample is stretched, a new meridional scattering is observed in addition to the original one, indicating a very heterogeneous structural deformation within the sample. The phenomenon has been considered to originate from the broad distribution .of the hard segmental lengths in the multi-block copolymer samples .

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