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

Poly(ε-caprolactone) (PCL) has received widespread attention due to its biocompatibility and biodegradability. Introduction of the nucleating agent (NA) is an effective method to control the crystallization behavior of semicrystalline polymer and regulate their properties. Unfortunately, only a few NAs for PCL have been reported up to now. In this work, considering the structure similarity between PCL and polyethylene (PE), we tried to synthesize the calcium hexahydrophthalic acid (Ca-HHPA), the main ingredient of a special NA for PE, by neutralization reaction in deionized water and investigated its possible nucleating effect for PCL. It was interesting to find that the as-synthesized product undergoing different thermal treatments exhibited obviously different nucleation effects. The Ca-HHPA80, obtained by drying at 80 °C for 24 h, only slightly increased the crystallization peak temperature (Tc) of PCL by no more than 1.7 °C with the loadings investigated, while Ca-HHPA80-200, obtained by further annealing Ca-HHPA80 at 200 °C for 5 min, could notably increase the Tc by 5.2 °C in similar dosages. The results of structural characterization indicated that the Ca-HHPA80 and Ca-HHPA80-200 were crystalline monohydrated and dehydrated carboxylate, respectively. The investigation of structural evolution behavior during the heating process revealed that the monohydrated carboxylate lost its crystal water and gradually transformed to dehydrated one at 140–190 °C accompanied with the conformational transformation of the methylene from gauche to trans conformation, which led to a great variation of their alkyl chain packing in crystal lattice. Based on the epitaxial crystallization theory, the possible reasons for the obviously improved nucleating efficiency of Ca-HHPA80-200 were discussed. The alkyl chain packing of Ca-HHPA80-200 leading to the lower mismatching with the PCL crystals may be accountable for its relatively higher nucleation efficiency.