Poly(ethylne succinate) [PES] showed a crystal transition between the alpha (T(3)GT(3)(G) over bar) and beta (T-8) form, where T, G and (G) over bar denoted trans, gauche and minus gauche, respectively. Here, the beta 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 a form, started decreasing at strain of epsilon similar to 20%, while the absorbance at 805 cm(-1), corresponding to the beta form, appeared at epsilon similar to 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 alpha and beta form, where no amorphous part contributed. In the X-ray diffraction, streaks appeared in layer lines in the beta form, indicating that the beta form had some disorder along the fiber axis. The equatorial reflection of a (at 2 theta = 20.4 degrees) started decreasing at epsilon similar to 15%. On the other hand, the reflection of beta2 (at 2 theta = 21.9 degrees) appeared at epsilon similar to 15%, then increased with epsilon, consistent with FT-IR results. The molar fraction of the beta form, chi (beta), was determined as a function of stress, sigma, by X-ray, where chi (beta) showed a drastic increase at a critical value of sigma* = 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 alpha and beta form, DeltaG, was determined to be DeltaG similar to 2.3 (kJ/mol of monomer unit), being larger than the reported value of PET (DeltaG similar to 1.4) and PTMS (DeltaG similar to 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 PET, it was observed in the alcohol unit. (C) 2001 Elsevier Science Ltd. All rights reserved.