How does a polymerized compounding affect the nucleation effect of a sorbitol derivative nucleating agent in isotactic polypropylene melt crystallization?

摘要

In this article, we investigate the effect of a new polymerized compounding method on the nucleation behavior of a sorbitol derivative nucleating agent for isotactic polypropylene (iPP). This method uses a sorbitol derivative nucleating agent, for example, 1,3: 2,4-dimethylbenzylidene sorbitol (DMBS), which displays itself as aggregated fibrillar crystals, as a support for C _2-symmetric ansa-metallocenes best represented by rac-Me _2Si[2-Me-4-Naph-Ind] _2ZrCl _2. Activating this DMBS-supported metallocene catalyst with methylaluminoxane, a following in situ propylene polymerization results in the formation of iPP/DMBS composition polymers having controllable DMBS concentrations (by virtue of polymerization productivity control), with the DMBS dispersion driven by catalyst fragmentation. Three iPP/DMBS composition polymers having DMBS concentrations of 0.22, 031, and 0.52 wt % were prepared by this means, which, along with their analog melt-mixed counterparts, were subjected to melt crystallization kinetics and crystal morphology studies using differential scanning calorimetry, polarized optical microscopy, and UV-vis spectroscopy. With systematically higher nucleation efficiencies and lower crystallization activation energies, the polymerized iPP/DMBS composition polymers exhibited generally stronger DMBS nucleation effects when compared with their melt-mixed counterparts. Such a benefit was accounted for by a more homogeneous dispersion and better wettability by iPP matrix of DMBS that led to the formation of more nucleation sites and faster crystallization.