The effect of structure evolution upon heat treatment on the beta-nucleating ability of calcium pimelate in isotactic polypropylene

Abstract

Calcium pimelate (CaPim) is known as one of the most stable and effective β-nucleating agents for isotactic polypropylene (iPP). To establish the structure-properties relationships of CaPim, the structure and corresponding β-nucleating ability of as-synthesized CaPim and CaPim annealed at temperature (Ta) varying from 130 to 330 °C were investigated. Our results revealed that as-synthesized CaPim and CaPim annealed at a relatively lower Ta were crystalline hydrates. However, the highly ordered crystalline structure would irreversibly destroyed and transformed to a weakly ordered form when treated at about 180 °C, accompanied by the complete deprivation of crystal water. The weakly ordered CaPim still maintained relatively high ordered degree at Ta range of 180–290 °C, but a further increase of Ta leads to the conversion of CaPim to disordered amorphous form. The measurements of relative β-form content (kβ) of iPP samples containing 0.05 wt% of various annealed CaPim suggested that the β-nucleating selectivity was closely correlated to the ordered degree of CaPim. As expected, the crystalline CaPim could induce high kβ values of 0.82–0.91. Interestingly, the weakly ordered CaPim with residual ordered structure still showed a rather high β-selectivity with kβ values of 0.53–0.68, while the amorphous CaPim almost lost its β-nucleating ability. Combined with the epitaxial crystallization theory, a probable mechanism on the correlation between structure and β-nucleating selectivity of CaPim was proposed.