X-ray Crystal Structures, Packing Behavior, and Thermal Stability Studies of a Homologous Series of n-Alkyl-Substituted Polyhedral Oligomeric Silsesquioxanes

Abstract

The homologous series of octa-n-alkylsilsesquioxanes (nalkylPOSS) was prepared by hydrosilylation of octahydrido-octasilsesquioxane H 8 T 8 with terminal n-alkenes in good yields (n = 2−18). The compounds were characterized by NMR spectroscopy ( 1 H, 13 C, 29 Si), MALDI−TOF and ESI MS. Most of them have also been characterized using X-ray crystallography, including some polymorphs. The packing systems of the octa-n-alkylsilsesquioxanes are reported. Two types of packing have been observed, rod-like and disk-like (interdigitated). Some n-alkylPOSS crystals were formed in which the chains are tilted away from the axes of the cubic core. It was also observed that as alkylchains are flexible they can take up a variety of conformations. Melting points of the homologous series showed an odd−even alternating effect for alkyl chains longer than four CH 2 units. The TGA of n-alkylPOSS under nitrogen showed that the thermal stability temperatures are between 200 and 400°C. Pyrolysis-gas chromatography/mass spectrometry has been used to elucidate the thermal degradation process of T 8 (n-C n H 2n+1 ) 8 (n = 11 and 18). Thermal cracking along the whole alkyl chain was observed rather than simple loss of the complete arm.

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

“…From Figure 2A, during the heating of the PE-C 8 blend a small but relatively sharp melting peak at ~50 °C is observed. This corresponds directly to the melting temperature of the pure POSS T 8 C 8 cage system [28]. Hence, from this separate melting peak for PE-C 8 POSS blend it would be reasonable to suggest that the POSS is actually aggregated in crystals rather than well dispersed in the PE matrix.…”

“…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].…”

“…In contrast, there is no evidence of the melting peaks of the other pure POSS cages (T 8 C 12 at 74 °C and T 8 C 18 at 85°C) occurring in the heating thermogram for the blends, indicating that the POSS is dispersed in the PE matrix of these two blends. The melting points of the pure POSS cages [28] are all lower than pure PE, thus it can be assumed that the POSS components of the blends are therefore in their melt state and dispersed in the melted PE matrix above 109 °C.…”

“…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.…”