Effect of Mono-Substituents in Heptaisobutyl-Substituted Polyhedral Octasilsesquioxanes on Orientationally Disordered Phase Transition

A series of mono-functionalized polyhedral octasilsesquioxane (POSS) derivatives (R-POSSs) were prepared by corner capping reaction of heptaisobutyl incompletely condensed POSS and their orientationally disordered (OD) phase transition properties studied by DSC, XRD, and solid state NMR analyses. According to DSC analysis, octaisobutyl-POSS (iBu-POSS) showed the highest OD phase transition temperature among R-POSSs, whereas the OD phase transition temperatures decreased with replacing the mono-substituents smaller and larger than the isobutyl unit. The solid state NMR analysis suggests that the OD phase transition was caused by increasing the mobility of the substituent not by rotating the POSS cage. The composite films were prepared by drop-casting of a chloroform solution of PMMA with heptaisobutyl-butyl-POSS (Bu-POSS). No appearance change of films was observed despite the heating and cooling of the films on the hotplate due to the absence of drastic density difference below and above the OD phase transition.