Properties and self-assembled packing morphology of long alkyl-chained substituted polyhedral oligomeric silsesquioxanes (POSS) cages

Abstract

Polyhedral oligomeric silsesquioxane (POSS) cubic cage systems (octa-n-octadecyloctasilsesquioxane, (T₈C₁₈) and octakis(n-octadecyldimethylsiloxy)octasilsesquioxane, (Q₈C₁₈)) were synthesised with eight long n-alkyl chain (R = C₁₈H₃₇) substituent arms, as model nano-functionalized compounds. The crystalline packing morphology of the cages was studied using time-resolved Small- and Wide-angle X-ray scattering (SAXS/WAXS), thermal and optical techniques. From thermal analysis the melting and crystallization temperatures of the Q₈ cage were significantly less than those for the T₈ cage. X-ray scattering showed that both cage systems have long-range crystalline ordering where the alkyl chains align in a parallel axial disposition from the POSS core giving a ‘rod-like’ self-assembled packing morphology. The packing length-scale can be directly related to the overall dimensions of the POSS molecules. Compared to the T₈ cages, the Q₈ cages pack more efficiently allowing the interdigitation of the alkyl chain arms. Different packing modes and thermal behaviour observed for the T₈ and Q₈ cages is directly attributed to their structural chemistry. For the Q₈ cage, the presence of the OSiMe₂ spacer groups which tether the alkyl chain arms to the cage (absent in the T₈ cages) allows greater flexibility of the arms letting them interdigitate with each other when packing which is not observed for the analogous T₈ cages.