Structural characterization and sequence distributions of polysiloxanes using pyrolysis MS/MS

Novel polysiloxanes, with 4-(dialkylamino)pyridine substituents, are characterized by pyrolysis tandem mass spectrometry. These polymers form abundant cyclic oligomeric ions under both desorption electron ionization (DEI) and desorption chemical ionization (DCI) conditions. Product MS/MS spectra of the cyclic ions reveal characteristic fragmentations under low energy collision activated dissociation. Protonated cyclic oligomers higher than the pentamer are mainly due to the proton bound dimers of lower oligomeric units. The cyclic oligomers are shown to have proton affinities greater than 1000 kJ/mole. It is proposed that thermal depolymerization occurs through an intramolecular siloxane bond rearrangement, which is in agreement with a previously proposed "loop mechanism". Markovian statistical calculations are applied to the DCI mass spectral data in order to determine the sequence distribution of siloxane copolymers. Application of this method show that the monomers in the copolymers examined are non-randomly distributed.     

Reactions of titanium alkoxide with SiH containing polymers as a route to titanium/siloxane hybrid materials with enhanced refractive index

Reactions of titanium tetrabutoxide with polyhydromethylsiloxane (PHMS) and with some hydromethylsiloxane copolymers were used for synthesis of light transparent materials having enhanced refractive index. The FTIR, ¹H NMR and ²⁹Si NMR spectroscopies were used to study the reaction between PHMS and titanium tetrabutoxide in toluene. Reaction of SiH functional siloxane polymers with titanium tetrabutoxide results in a titanium-containing heterosiloxane prepolymers that are stable under anhydrous anaerobic conditions. Exposure of the resulting materials to humid air transforms them into light transparent titanium/siloxane nanocomposites with enhanced refractive index. The films of these composites formed during the spin-coating process were characterized by UV–Vis, ellipsometry, IR and ²⁹Si NMR spectroscopies and transmission electron microscopy (TEM). Titanium/siloxane hybrid films obtained on the basis of poly(phenylmethylsiloxane-co-hydromethylsiloxane) copolymer are free of cracks and have a refractive index of up to 1.60.     

Gauge-invariant perfect-fluid Robertson-Walker perturbations

In the preceding paper, a complete set of basic gauge-invariant variables was defined that uniquely characterizes cosmological perturbations in homogeneous, isotropic, ideal-fluid universe models. The calculations were presented in some detail for the case of a general perfect fluid with two essential thermodynamic variables. Among other things, it was demonstrated that the aforementioned set consists of 17 linearly independent, not identically vanishing gauge-invariant variables. One can think of these basic variables as having two aspects. First, their definitions are such that they provide a unique representation of the physical perturbation. (By way of digression, inspection shows that such perturbations can be regarded as being the elements of a certain quotient space.) Second, any complicated gauge-invariant quantity is obtainable directly from the basic variables through purely algebraic and differential operations. The object here is the systematic derivation of the linear propagation equations governing the evolution of these basic variables. To make clear the relation of the present formalism to a series of standard results in the literature, this paper also points out how general propagation equations can be adapted to situations where the pressure vanishes in the background. Finally, the physical interpretation of basic variables and comparison with other gauge-invariant approaches are briefly presented.     

B(C6F5)3 catalyzed dehydrocarbon polycondensation of PhSiH3 with (MeO)4Si as model polyfunctional comonomers in new route to hydrophobic silicone TQ resins

Highly branched methoxy substituted phenylsilsesquioxane-silicate copolymers were obtained by dehydrocarbon polycondensation of phenylsilane (PhSiH₃) with tetramethoxysilane (TMOS) in toluene solution catalyzed by tris(pentafluorophenyl)borane [B(C₆F₅)₃]. This model reaction represents a new non-hydrolytic route to TQ silicone resins, which do not contain reactive and hydrophilic silanol groups and therefore, should be more stable and have better miscibility with some organic polymers. The soluble siloxane copolymers, which do not contain volatile oligomers, were obtained using the PhSiH₃:TMOS molar ratios below 0.9. The dehydrocarbon condensation is accompanied by metathetic exchange of hydrosilyl and methoxysilyl functions, which is responsible for the formation of links between units originating from the same comonomer. Competition between the dehydrocarbon condensation and metathesis is consistent with a mechanism involving the transient formation of a tertiary oxonium ion intermediate. ²⁹Si NMR analysis of the copolymer product proves that extensive cyclization occurs during the polycondensation, most likely leading to a significant proportion of ladder and cage like structures.     

Characterization of a single sample of an unknown polymer by size exclusion chromatography

The relationship between viscosity constants, k', a and K_η from the equations of Huggins and Mark-Kuhn-Houwink has been considered. It is shown, theoretically, that the sum of k' and a must be constant for all flexible-chain macromolecules irrespective of the solvent used. On this basis, a combination of chromatography and viscometry measurements can be used to characterize a new species. The method has been applied to the new polymer, poly[methyl(pyridin-3-yl) siloxane] ( 1 ) where no suitable calibration standards are available. The value of a, k', and K_η for 1 has been calculated. The calculated constants enabled an estimation of different average molecular weights (M_nM_wM_z) and polydispersity (M_w/M_n) from a minimum of experimental data. The new method is general and can be applied to any homogeneous linear flexible-chain nondraining macromolecule.     

High‐Throughput Fabrication,Performance Testing,and Characterization of One‐Dimensional Libraries of Polymeric Compositions

A new general approach for rapid performance‐screening of polymer compositions is provided. Multiple compositions are generated as one‐dimensional libraries in a microextruder with step‐ or gradient‐composition changes in 2–10 g of polymer in < 1 min. To accelerate testing, environmental stress is applied to only local regions, followed by high‐sensitivity spatially resolved characterization. We applied our methodology for weathering of arrays of polymeric compositions and provided ranking of polymer/UV absorber compositions equivalent to traditional weathering data while achieved 20 times faster.

Sampling and measurement strategies for HT analysis of combinatorial 1D polymeric libraries: (1) spectroscopic monitoring of chemical properties, (2) in‐situ monitoring of mass and viscoelastic properties, (3) scanning spectroscopic analysis, and (4) spectroscopic imaging of a coiled polymeric array.     

A note on the existence of heteroclinic connecting orbits in the Belousov-Zhabotinskii system

We prove for a concrete system the occurence of heteroclinic orbits via the Conley index. Because this system is bistable, we have two different connecting orbits. If we include the parameter dependence of the reaction term, there exists two families of connecting orbits. At the ends of the parameter interval, where bistability occurs, we have bifurcation points.