(576f) Mechanical and Structural Analyses of Toughened Syndiotactic Polypropylene Gels: Effects of Gel-Preparation Temperature

Physical gels, with physical crosslinkings such as hydrogen bonds or polymer crystals, could be readily re-molded due to their reversible crosslinking points. However, from the viewpoint of mechanical properties, physical gels are generally inferior to chemical gels that have strong crosslinking points made of chemical bonds. Our recent studies revealed that the mechanically-tough syndiotactic polypropylene (sPP) gel (one of the physical gels with crosslinking points made of sPP crystals) could be obtained by quenching the sPP/decahydronaphthalene (decalin) solution with liquid nitrogen (LN). In this study, the effects of the gel-preparation temperature on the mechanical properties of the sPP/decalin gels were investigated and the structural analyses of the sPP gels were also conducted by the polarized optical microscopy (POM), the scanning electron microscopy (SEM), and the small-angle neutron scattering (SANS) measurements to discuss the relationship between the microstructures and the physical properties of the sPP gels. The Young’s modulus and the fracture stress of the 10 wt% sPP/decalin gel prepared by the LN quenching (Gel_LN) were 12 times and 96 times, respectively, higher than those of the 10 wt% sPP/decalin gel prepared at room temperature (Gel_RT). sPP spherulites with the diameter of ~15 μm were observed by POM and by SEM micrographs for Gel_RT. For Gel_LN, however, no spherulites but much smaller sphere-like structures with the diameter of 100 nm were observed, assembling to form a flat plate. The SANS analyses showed that the structure in Gel_LN was smaller than that in Gel_RT. These results suggested that the quenching caused the formation of dense and smaller structures of the sPP gels, resulting in the relatively high toughness of the sPP gels.