The detailed structure and crystallization behavior of poly(butylene succinate) (PBS) have been investigated by Fourier transform infrared (FTIR) and other methods systematically. For the first time, we confirmed that the C═O stretching modes of PBS can respond to three distinguish absorption bands in the FTIR spectrum, at around 1736, 1720, and 1714 cm respectively. The 1736 cm band is adopted as the stretching mode of C═O groups in free amorphous fraction (MAF); the 1714 cm band which is relevant to more stable structure, displays more anisotropic in polarized FTIR spectra, and has been confirmed as stretching vibrations of hydrogen-bonded C═O groups in the crystalline phase. The 1720 cm band is linked to crystallization but comes from less ordered structure. Moreover, the 1720 cm band can be destroyed prior to 1714 cm band during heating and constructed behind 1714 cm band during cooling. Thus, the 1720 cm band is reasonably ascribed to the C═O groups in rigid amorphous fraction (RAF) or intermediate phase which locates between MAF and crystalline phase. The corresponding investigation by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) further supports that the three particular C═O absorption bands indeed reveal the typical three-phase structure for PBS. More important, the FTIR spectrum of PBS is very sensitive to sample preparation process and measurement mode. The relative content of each band depends on the crystallization temperature (T) and measured thickness. The higher T, the more RAF content appears when measured at room temperature; the thinner penetration thickness of FTIR measurement, the less RAF content can be detected, and the penetration thickness-dependent behavior is suggested as the result of higher mobility of chains in the air/bulk surface. Additionally, the particular three absorption bands of C═O groups in PBS force us to carefully reconsider previous reports on structure and interaction state obtained by FTIR spectroscopy in PBS and its composites.

Inspired by the epitaxial crystallization theory and isomorphism phenomenon, poly(ω-pentadecalactone) (PPDL) and the random copolyesters of ω-pentadecalactone and ε-caprolactone Poly(PDL-co-CL)s have been synthesized and tested as polymeric nucleating agents for commercial poly(ε-caprolactone) (PCL). Crystallization behavior and spherulite structure of the neat PCL and nucleated PCL specimens have been investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and atomic force microscopy (AFM). The PPDL and poly(PDL-co-CL)s are found to provide massive heterogeneous nuclei, significantly increase the crystallization temperature to 314.2 K at a cooling rate of 10 K/min, and exhibit a supernucleation role in the PCL matrix. Moreover, investigation on crystallization kinetics demonstrates that the greatly enhanced nucleation dominates the overall kinetics. The enhanced crystallization ability of PCL and the polymeric nature of additive PPDL lead to a 12.4% improvement in tensile strength, without any loss of elongation at break. Three characteristics of perfect lattice matching, good surface wettability, and strong crystallization ability are suggested to guarantee the supernucleation effect of PPDL in the PCL matrix.

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