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.

Morphology and crystallization kinetics of polyethylene/long alkyl-chain substituted Polyhedral Oligomeric Silsesquioxanes (POSS) nanocomposite blends: A SAXS/WAXS study

“…Firstly, looking in detail at the SAXS data which gives information of the long-range ordering and molecular length-scale; Figures 3A and 3B show the 1D SAXS data for the pure POSS materials and the pure PE and PE-POSS blends respectively, where the d-spacings of the peaks are labelled in Angstroms. As previously reported [29,30], the SAXS data for the pure POSS material show very strong 1 st order peaks which have been related directly to the length-scale of the POSS molecule, where the alkylchains point away from the POSS cage core, giving an overall rod-like structure. The POSS molecules pack in an efficient manner in a highly crystalline layered structure, where the alkyl-chains do not interdigitate with each other.…”

“…Moreover, we have reported that these particular POSS systems, with long linear alkylchains, pack and crystallize in a lamellar fashion analogous to that of bulk n-alkanes [28][29][30].…”

“…The decrease in L p , indicates increased crystallinity and shorter average lamellar and amorphous stacking units, often from insertion and subsequent growth of other lamellae into the amorphous component [11,29,45]. This decrease is seen as a function of alkyl-chain length and may indicate that the alkyl-chain compatibility and subsequent crystallization in the PE matrix is permissible.…”

“…Recently, we have reported on the thermal behaviour and crystalline packing morphology of these pure POSS cages, which have shown a distinctive self-assembled packing morphology [28][29][30]. The long alkyl-chain groups were seen to align in an axial disposition from the POSS core giving a 'rod-like' self assembled packing morphology, which has a length-scale correlating exactly to the overall length of the molecule.…”

“…Following this T 8 (n-alkyl) 8 cage systems were synthesised by a well established route involving the direct hydrosilylation [32][33][34] of T 8 H 8 with the appropriate 1-alkene using a platinum (Speier's) catalyst which gave a highly pure product with excellent final yields [34]. Specific synthesis and characterisation of the T 8 (n-alkyl) 8 cage systems are detailed elsewhere [28][29][30]. T 8 (nalkyl) 8 cage systems were synthesized where the length of the alkyl-chain group is C n H 2n+1 , and n = 8, 12 and 18.…”