Thermoreversible DMDBS phase separation in iPP: The effects of flow on the morphology

The crystallization rate of isotactic polypropylene (iPP) is often enhanced with sorbitol-based nucleating agents like 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol or DMDBS. In the concentration range from the eutectic point (∼0.1 wt %) to 1 wt %, DMDBS crystallizes before the polymer, building a network of nanofibrils. The surface of this network hosts a large number of tailored nucleation sites that promotes the epitaxial nucleation of iPP. The formation of DMDBS fibrils corresponds to a liquid-solid phase separation that occurs on cooling, at a temperature TcDMDBS. On heating, DMDBS fibrils dissolve in iPP at a temperature TmDMDBS > TcDMDBS. For DMDBS contents less than 1 wt %, both T cDMDBS and TmDMDBS depend on the composition. We show that in the temperature window Tc DMDBS < T < TmDMDBS a percolated network of DMDBS fibrils is retained. This network can be broken with a shear flow that aligns the fibrils. At these high temperatures, iPP does not crystallize. Upon cooling, in the early stages of nucleation, the orientation of the fibrils is transferred to the lamellae because of the epitaxial relation between iPP and DMDBS. With this templating mechanism, we obtained well oriented crystalline morphologies of iPP after a shear pulse (shear rate of 60 s-1 applied for 3 s) at 185°C for 0.3 wt %, at 195°C for 0.7 wt % and at 210°C for 1 wt % of DMDBS. © 2008 American Chemical Society.