Crystal Transition Mechanisms in Poly(tetramethylene succinate)

Abstract

Poly(tetramethylene succinate) (PTMS) showed a crystal transition between the α (T7GT G̅) and β (T10) form under the strain and relaxation conditions, where T, G, and G̅ denoted trans, gauche, and minus gauche, respectively. We have investigated the mechanisms of this crystal transition by FT-IR and X-ray diffraction. In the FT-IR, the absorbance peaks at 920 cm−1 and 955 cm−1, corresponding to the α form, started decreasing at strain of ε∼8%, while the absorbance at 977 cm−1, corresponding to the β form, appeared at ε∼8%, then increased with strain. In addition, the isobestic point was observed at 965 cm−1, indicating that the crystal transition occurred only between the α and β form, where no amorphous part contributed. In the X-ray diffraction, the meridional reflection of α (at 2θ=25.1°) started decreasing at ε∼8%. In addition, the reflection of β (at 2θ=22.5°) appeared at ε∼8%, then increased with ε. These FT-IR and X-ray results were thus consistent with each other. The molar fraction of the β form, χβ, was determined as a function of stress, σ, by X-ray. The χβ showed a drastic increase at a critical value of σ=140 MPa. It was hence concluded that the thermodynamic first-order phase transition was the operative mechanism of the transition. Such a crystal transition mechanism had been also reported in poly(butylene terephthalate) (PBT). The free energy difference between the α and β form, ΔG, was determined to be ΔG∼1.6 (kJ mol−1 of monomer unit), being close to the reported value of ΔG∼1.4 (kJ mol−1 of monomer unit) for the crystal transition in PBT. The stress–strain curve was measured. The σ increased with ε when ε<8%, then remained approximately constant up to ε∼16%, followed by the second increase for ε>16%. Such a stress–strain characteristics could be explained in terms of the crystal transition.