Nucleation Efficiencies of Calcium Hexahydrophthalic\nAcid for Poly(ε-caprolactone) Crystallization

Poly­(ε-caprolactone)\n(PCL) has received widespread attention\ndue to its biocompatibility and biodegradability. Introduction of\nthe nucleating agent (NA) is an effective method to control the crystallization\nbehavior of semicrystalline polymer and regulate their properties.\nUnfortunately, only a few NAs for PCL have been reported up to now.\nIn this work, considering the structure similarity between PCL and\npolyethylene (PE), we tried to synthesize the calcium hexahydrophthalic\nacid (Ca-HHPA), the main ingredient of a special NA for PE, by neutralization\nreaction in deionized water and investigated its possible nucleating\neffect for PCL. It was interesting to find that the as-synthesized\nproduct undergoing different thermal treatments exhibited obviously\ndifferent nucleation effects. The Ca-HHPA80, obtained by drying at\n80 °C for 24 h, only slightly increased the crystallization peak\ntemperature (Tc) of PCL by no more than\n1.7 °C with the loadings investigated, while Ca-HHPA80-200, obtained\nby further annealing Ca-HHPA80 at 200 °C for 5 min, could notably\nincrease the Tc by 5.2 °C in similar\ndosages. The results of structural characterization indicated that\nthe Ca-HHPA80 and Ca-HHPA80-200 were crystalline monohydrated and\ndehydrated carboxylate, respectively. The investigation of structural\nevolution behavior during the heating process revealed that the monohydrated\ncarboxylate lost its crystal water and gradually transformed to dehydrated\none at 140–190 °C accompanied with the conformational\ntransformation of the methylene from gauche to trans conformation,\nwhich led to a great variation of their alkyl chain packing in crystal\nlattice. Based on the epitaxial crystallization theory, the possible\nreasons for the obviously improved nucleating efficiency of Ca-HHPA80-200\nwere discussed. The alkyl chain packing of Ca-HHPA80-200 leading to\nthe lower mismatching with the PCL crystals may be accountable for\nits relatively higher nucleation efficiency.