Synthesis and characterization of polycyanurate/montmorillonite nanocomposites

The advantages of cyanate esters (CEs) versus competitor systems such as epoxies and polyimides, as well as the great reinforcing potential of organoclays properly dispersed into a polymeric matrix, have been examined in a series of polycyanurate (PCN)/montmorillonite (MMT) nanocomposites prepared under appropriate polymerization conditions. The curing schedule applied resulted in gradual propagation of polymerization. Through this procedure, the intragallery curing rate becomes comparable to the extragallery one, allowing intercalation before gelation. Systems with clay loadings from 1 to 3% per weight were synthesized, and their morphology and mechanical properties were studied by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), wide angle X‐ray scattering (WAXS), dynamic mechanical analysis (DMA), and tensile tests. Microscopy investigations revealed better dispersion for the 3 wt % system compared to smaller concentrations, in which aggregation and, in some cases, agglomeration were the conspicuous features. Roughness and area analyses revealed more homogeneous dispersion for this nanocomposite. Topology and 3D‐phase images further suggested considerable reduction of the average particle diameters. WAXS analysis showed that the interlayer spacing of nanocomposites was increased compared to pristine MMT, indicating the formation of intercalated structures. On the other hand, tensile strength and elongation at break values displayed abrupt diminution with MMT addition, while Young's modulus exhibited a slight but systematic increment with MMT content. The decreasing glass transition tendency observed for small clay loadings was reversed in the case of 3 wt %, while secondary transitions were practically unaffected by the presence of MMT. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1036–1049, 2008     

The effect of terminal groups on the structure and properties of oligosilsesquioxanes

The interaction of oligomeric silsesquioxanes, whose organic moieties contain tertiary amino groups and primary and secondary hydroxyl groups, with monoisocyanates and phthalic and acetic anhydrides is employed to synthesize a number of their N-alkyl(aryl)urethane-, carboxyl- and acetyl-terminated derivatives with different degrees of substitution and combinations of the substituents. The obtained derivatives are studied by functional analysis, IR spectroscopy, GPC, DSC, TGA, and wide-angle X-ray scattering. The chemical nature of the incorporated functional groups is found to markedly influence the structure and properties of the synthesized compounds. Depending on the type of substituents and their combination, either the complete amorphization or ordering of the structure of the oligosilsesquioxanes is observed with corresponding changes in their properties.     

Organic-Inorganic Insulating Coatings Based on Sol-Gel Technology

The approach for improving bending strength of temperature-resistant flexible ceramic electric insulating coatings prepared by sol-gel processing is considered. Starting suspensions, based on both tetraethoxysilane (TEOS)-derived modified sols and highly dispersed fillers such as Cr₂O₃, are used to deposit the coatings on metals and alloys. Some polymers and organic substances are tested as additives for improving flexibility of the ceramic coatings. The results of study of both bending strength and electrical strength of flexible hybrid organic-inorganic insulation formed on a nichrome wire are discussed. The data obtained on the chemical and phase structure of the coatings, which have been heat treated at temperatures from 95 to 1100°C, using DSC as well as X-ray scattering techniques are presented.     

Sequential semi-interpenetrating polymer networks based on polyurethane and polystyrene

The formation of microheterogeneous sequential semi-interpenetrating polymer networks based on network polyurethanes with different molecular masses of chain segments between crosslinks and a linear polystyrene has been studied by DSC and small-angle X-ray scattering. It has been shown that variation in the molecular mass of polymer segments between polyurethane network junctions affects the formation of the linear component of semi-interpenetrating polymer networks. As a result, the material structure may change in a wide range from a nearly single-phase system to a two-phase one. SAXS measurements indicate that there is a cymbate dependence between the degree of segregation of components of sequential semi-interpenetrating polymer networks and their microheterogeneous structure on the internetwork space. Two hierarchical heterogeneity levels are found to exist in polymer networks, and the features of each of these levels are analyzed.     

Highly functionalized bridged silsesquioxanes

The objective of this work was to synthesize functionalized mesoporous silsesquioxanes with high concentrations of amine groups. During typical sol–gel syntheses, these materials are obtained by co-condensation of organic precursors with suitable linkers, such as tetraethoxysilane, necessary to prevent the mesoporous structure from collapsing. Thus, concentrations of amine groups in organosilicas usually do not exceed 2.7–3.4 mmol g⁻¹. The use of bridged bis-trimethoxysilanes, however, allowed formation of mesoporous materials with no linker. Polycondensation of bis-trimethoxysilanes containing amine groups was conducted in acidic, neutral and basic media, resulting in high yields of solid bridged silsesquioxanes. Gelation occurred quickly if no acid or base was added to the reaction mixture. The hybrid organic/inorganic nature of obtained materials was confirmed by FT-IR and MAS CP NMR spectroscopy. Elemental analysis showed that amino group concentration in the products was 3.3–4.1 mmol g⁻¹. Measurement of particle size distribution confirmed that choice of reaction media significantly affects particle sizes and agglomeration degrees, with the largest agglomerates (up to 50 μm) formed in basic media. A morphology study, using small-angle X-Ray scattering, displayed two-level fractal structures composed of aggregated 6.5–10.5 nm particles. Reactions in the presence of a surfactant resulted in formation of mesoporous structures. Furthermore, the obtained bridged silsesquioxanes were thermally stable down to 260 °C, but could reversibly absorb water and CO₂ at temperatures below 120 °C. Thus, condensation of the bridged precursor without a linker resulted in formation of a highly functionalized mesoporous material.     

The Interrelation Between the Microphase Structure of Sequential Interpenetrating Polymer Networks and the Crosslinking Density of Penetrating Networks

The microphase structure of single polyurethane (PU) and acrylate networks as well as sequential interpenetrating polymer networks (IPNs), produced by the forming of a PU network in the presence of monomers of a penetrating network, was studied by small- and wide-angle x-ray analysis. It was established that each network component was of a two-phase structure consisting of disordered phase-separated microregions. the higher crosslink density of the acrylate network results in its higher heterogeneity. in IPNs, phase separation of a complex nature is realized: the PU matrix preserves some features of a single network structure, and the second component forms microregions 5-10 nm in size while retaining a certain level of interpenetra-tion of both network components. the microphase structure parameters of such systems are greatly dependent on the crosslink density of the penetrating network. This suggests the influence of a three-dimensional network of chemical bonds on the interdiffusion of branched fragments of the penetrating network and molecular chains of the matrix, one leading to the retardation of phase separation.     

Acoustic relaxation and ionic conductivity of oxyethylene aliphatic polyionenes

Polyionenes composed of oxyethylene and aliphatic hydrocarbon units were studied by differential scanning calorimetry, wide-and small-angle X-ray scattering, and dielectric spectroscopy. These polymers are characterized by the variation of T _g with the concentration of ionic centers. Polyionenes with short ethylene oxide segments are amorphous; however, as the average number of monomer units in the oxyethylene segments increases to 20, a crystalline structure typical of poly(ethylene oxide) is formed. Oxyethylene-aliphatic polyionenes are microphase-separated systems. Polyionenes from this series are characterized by a high ionic conductivity, which increases with an increase in the concentration of ionic sites—the conductivity at room temperature is 10^?5–10^?4 Ω^?1 cm^?1. By means of acoustic spectroscopy, it was found that the isotherms of the ultrasound absorption (frequency domain) and ultrasound speed had two dispersion regions. The mechanism of the dispersions was associated with the softening of the quasi-lattice produced by cationic sites and with the motion of chain segments connecting these sites, The speed of sound in polyionene is abnormally high (1800–2100 m/s) for polymers, a result which is due to a high level of intermolecular interactions.     

Synthesis,structure, and properties of anhydrous organic-inorganic proton-exchange membranes based on sulfonated derivatives of octahedral oligosilsesquioxanes and α,ω-di(triethoxysilyl) oligo(oxyethylene urethane urea)

A method of obtaining new organic-inorganic nanostructured proton-exchange membranes operating via the anhydrous proton-conduction mechanism is proposed. An oligo(ethylene oxide) component serves as a proton-conducting phase in these membranes, and the sulfo derivatives of octahedral oligosilsesquioxanes of the acidic and acid-base types are used as proton-donor dopants. These compounds are synthesized via the reaction of octaaminopropyl oligosilsesquioxane with the cyclic anhydride of 2-sulfobenzoic acid at various ratios and contain sulfo groups solely or sulfo and amine groups in the organic frame. The combination of these compounds taken at concentrations of 20 and 50 wt % with α,ω-di(triethoxysilyl) oligo(oxyethylene urethane urea) and phenyltriethoxysilane via the sol-gel method gives rise to hybrid organic-inorganic proton-exchange membranes. The synthesized dopants are distributed in the oligoether component, but the nature of dopant distribution depends on their structure and concentration and has a significant impact on the structure of the resulting amorphous membranes (according to DSC, SAXS, and AFM data). The synthesized membranes are thermally stable up to 219°C. Their conductivity is provided by the segmental mobility of oligooxyethylene fragments (the Grotthuss mechanism) and, regardless of the dopant structure, is primarily determined by the number of charge carriers and the membrane structure. The temperature dependence of the conductivity is described by the Vogel-Fulcher-Tammann equation. The maximum values of the ionic conductivity are attained at 120°C under anhydrous conditions and dopant concentration of 50%: 1.03 × 10^?4 for ampholytic oligosilsesquioxane and 7.43 ×10^?5 S/cm for fully sulfonated oligosilsesquioxane as a dopant.