Linear Long Alkyl Chain Substituted POSS Cages: The Effect of Alkyl Chain Length on the Self-Assembled Packing Morphology

Abstract

The self-assembled long-range ordering, packing morphology and thermal behavior of a series of polyhedral oligomeric silsesquioxanes (POSS) T 8 cage systems with long n-alkyl chain pendant groups is presented. State-of-the-art timeresolved small-and wide-angle X-ray scattering (SAXS/WAXS), thermal and optical techniques were used to elucidate their morphology. X-ray scattering showed long-range crystalline ordering where the alkyl chains align in an axial disposition from the POSS core giving a "rod-like" self-assembled packing morphology with a length-scale correlating to the overall molecular dimensions. Phase transitions observed by thermal methods were comparable to those seen in the crystallization of bulk nalkanes. Optical measurements and crystallization kinetics confirmed a lamellar macromorphology again being similar to the crystal structure of bulk n-alkanes. This study reveals for the first time, the direct effect that the alkyl chain length has on the selfassembled crystalline packing morphology of this novel and potentially commercially relevant, series of POSS systems.

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.…”

“…The POSS molecules pack in an efficient manner in a highly crystalline layered structure, where the alkyl-chains do not interdigitate with each other. In fact, the 2 nd and 3 rd order peaks can also be observed for the T 8 C 18 POSS cage system (and 2 nd order for T 8 C 12 ); this regular repeat distance is characteristic of a lamellar or bilayer type of packing [30]. In Figure 3B SAXS data for pure PE and the blends is given.…”

“…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].…”

“…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.…”