Abstract
Poly(ethylene succinate) [PES] showed a crystal transition between the α (T 3GT 3Ḡ) and β (T 8) form, where T, G and Ḡ denoted trans, gauche and minus gauche, respectively. Here, the β form appeared only under the strain. We have investigated the mechanisms of this crystal transition by FT-IR and X-ray diffraction. In the FT-IR spectrum, the absorbance peaks at 841 and 873 cm −1, corresponding to the α form, started decreasing at strain of ϵ∼20%, while the absorbance at 805 cm −1, corresponding to the β form, appeared at ϵ∼20%, then increased with strain. In addition, the isosbestic point was observed at 820 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, streaks appeared in layer lines in the β form, indicating that the β form had some disorder along the fiber axis. The equatorial reflection of α (at 2 θ=20.4°) started decreasing at ϵ∼15%. On the other hand, the reflection of β (at 2 θ=21.9°) appeared at ϵ∼15%, then increased with ϵ, consistent with FT-IR results. The molar fraction of the β form, χ β, was determined as a function of stress, σ, by X-ray, where χ β showed a drastic increase at a critical value of σ ∗=190 MPa. The thermodynamic first-order phase transition was hence the operative mechanism of the transition as similar to poly(butylene terephthalate) [PBT] and poly(tetramethylene succinate) [PTMS]. The free energy difference between the α and β form, Δ G, was determined to be Δ G∼2.3 (kJ/mol of monomer unit), being larger than the reported value of PBT (Δ G∼1.4) and PTMS (Δ G∼1.6). This difference would arise from the locus of conformational change upon the transition. In the case of PES, the conformational change was observed in both alcohol and acid units, while in the cases of PTMS and PBT, it was observed in the alcohol unit.
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