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.
Ray scattering (SAXS).
AbstractThe in-line development of crystalline morphology and orientation during melt extrusion of low density polyethylene (LDPE) tape at nil and low haul-off speeds has been investigated using Small-Angle X-Ray Scattering (SAXS). The processing parameters, namely haul-off speed and distance down the tape-line have been varied and the resulting crystalline morphology is described from detailed analysis of the SAXS data. Increasing haul-off speed increased orientation in the polymer tape and the resulting morphology could be described in terms of regular lamellar stacking perpendicular to the elongation direction. In contrast, under nil haul-off conditions the tape still showed some orientation down the tape-line, but a shish-kebab structure prevails. The final lamellae thickness (~50 Å) and bulk crystallinity (~20%), were low for all processing conditions investigated, which is attributed to the significant shortchain branching in the polymer acting as point defects limiting lamellae crystal growth.2
The dispersal, quiescent crystallization kinetics and morphology of a series of unique polyethylene -polyhedral oligomeric silsesquioxanes (PE-POSS) nanocomposite blends is presented. POSS molecules with long linear alkyl-chain substituents were blended at one composition into a commercial low density polyethylene. Time-resolved Small-and WideAngle X-Ray scattering (SAXS/WAXS) and thermal techniques were used to elucidate the affect that POSS and its substituent groups have on the dispersal and crystallization kinetics of the host polymer. The miscibility and dispersal of the POSS molecules was seen to increase with the increasing alkyl-chain length substituents suggesting increased compatibility and interaction with the host polymer chains. The POSS molecules act as nucleating agents increasing the crystallinity, crystallization kinetics and influencing the final lamellar morphology. Thus, these unique POSS compounds show great potential as nancomposite filler particles in polyolefins where the alkyl-chain substituent plays a vital role in its compatibility and subsequent improvement of physical properties in the host polymer.
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.
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