The Stability of Amino-Functionalized Polyhedral Oligomeric Silsesquioxanes in Water

Abstract

Octa(aminopropylsilsesquioxane) Si8O12[(CH2)3NH2]8 is a very important precursor for many other hybrid organic/inorganic polyhedral oligomeric silsesquioxanes (POSS) because of the reactivity of its primary amine groups. Unfortunately, it is unstable in water, which can lead to the cleavage of its siloxane cage. In the present work, such a degradation was confirmed using solid-state (29)Si NMR spectroscopy, and the molecular features at the basis of this instability were studied using molecular dynamics simulations (MD). It was also investigated whether replacing the primary amine end groups by secondary amines or by amides with long aliphatic chains could lead to an improvement in the water stability of the Si/O framework. In the pure bulk models, all POSS interdigitate with their pendant organic arms intertwined. Upon insertion of isolated molecules into water, the dimensions of the primary amine POSS remain close to those of the bulk, while the secondary amine and the amide POSS favor conformations that optimize the intramolecular chain-chain interactions. When there are several POSS molecules in water, they cluster with each other through both intra- and intermolecular chain-chain interactions. This tendency for the organic chains to intertwine whenever possible provides some protection to the siloxane cages from water, but also leaves some of the siloxane O exposed. As such, the latter are accessible to form transient hydrogen bonds with the water molecules, which could be a precursor step to hydrolysis and thus cage breakage. In the molecular models, a better protection was obtained in the amide POSS for two reasons: its chains tended to wrap efficiently around its cage, and its ketone O kept water from getting close to the siloxanes. The molecular modeling characterizations were found to agree very well with experimental evidence.

High-temperature molecular screening of hybrid polyOAPS-imide networks based on octa(aminophenyl)silsesquioxane for increased thermomechanical resistance

“…Since these hyper-cross-linked networks are aimed at preventing excessive volume dilation when subjected to high loadings of plasticizing gases, 18 the on-diagonal components of their required pressure tensor were first reduced linearly at a rate of À1 bar ps À1 up to 2500 ps in order to assess their resistance to isotropic volume dilation. It has been shown previously 71 that, although model mechanical properties of polymers are known to be slightly dependent on both the thermal history and the rates of deformation, [73][74][75] these effects remains minor in the networks under study. Indeed, the main point is that these systems are being compared under identical conditions.…”

“…Three 216-molecule metaOAPS, paraOAPS and orthoOAPS and three larger B4000-molecule PMDA, 6FDA and ODPA bulk boxes were prepared under NpT conditions at 22 1C according to the procedure described previously. 29,71 While the PMDA and 6FDA models have already been validated, 29,30 this remained to be done for OAPS and ODPA. Table 1 displays their average equilibrium bulk densities r bulk , relaxed box lengths L and molecular dimensions.…”

“…There again, tests made using 0.1 bar ps À1 did not make much difference. 71 The stress-strain curves for the PMDA-based systems at 300 1C are displayed in Fig. 11a.…”

The influence of the dianhydride precursor in hyper-cross-linked hybrid polyPOSS-imide networks

“…It has also been suggested using infrared spectroscopy on POSS-based systems 1,2,13 that some siloxane cages can be partially hydrolysed in the alkaline aqueous solution, but that the opposite silanol condensation reactions occur afterwards during the heat treatment procedure. [1][2][3][4] Since the details of cage degradation are very difficult to assess unambiguously, 53 the integrity of the POSS cages was maintained in the models.…”

Layer-Structured POSS-Modified Fe-Aminoclay/Carboxymethyl Cellulose Composite as a Superior Adsorbent for the Removal of Radioactive Cesium and Cationic Dyes

“…The modification expands the interlayer space of the clays which facilitates the entry of polymer molecules, dyes, heavy metals, and radionuclides. Polyhedral oligomeric silsesquioxane (POSS) is one such nanostructured inorganic–organic molecules comprised of an inorganic silica-like core (Si 8 O 12 ) surrounded by eight corner groups, which can be amines, esters, alcohols, isocyanates, and silanes. − Cubic-structured POSS is biocompatible, thermally stable (>400 °C), and has excellent recyclability. Thus, the POSS-modified clay–polymer composite is expected to have excellent adsorption capacity due to (i) the increase in interlamellar spaces which allows more Cs + ions and dye molecules to be encapsulated and (ii) increase of surface area by good dispersion of clay particles.…”