Thermal redistribution reactions in crosslinked polysiloxanes

The thermolysis under argon of various polysiloxane resins containing D, T, D^H, or T^H units was investigated using thermogravimetric analysis combined with mass spectroscopy (TG/MS analysis) and solid-state ²⁹Si-NMR. Redistribution reactions involving the exchange of Si? C/Si? O bonds or Si? H/Si? O bonds were evidenced in addition to the exchange of Si? O/Si? O bonds reported to date. These reactions significantly modify the initial siloxane units and lead to an escape of volatile silanes or siloxanes. The exchange of Si? H/Si? O bonds takes place at lower temperatures (300°C) than the exchange of Si? C/Si? O bonds (500°C).     

Some rheological properties of highly crosslinked polysiloxanes

Seven crosslinked polydimethylsiloxanes were prepared from carboxyl-terminated siloxane oligomers of different average lengths and a trifunctional imine. The 10-sec torsion modulus was measured as a function of temperature, and compression-deflection data were obtained on swollen specimens. Polymer-solvent interaction parameters were calculated from equilibrium swelling ratios and modulus data. The plots of modulus versus temperature show two distinct transitions for all the crosslinked polymers. The low-temperature transition corresponds to the siloxane chains, whereas the high-temperature transition should be attributed to the portion of the network chain derived from the crosslinking agent and the organic part of the oligosiloxane. On shortening the siloxane blocks, both transitions shift to successively higher temperatures and higher modulus levels. However, the siloxane transition temperature shows only a limited rise even when the average block length becomes quite small. The low siloxane polymers seem to exhibit two swelling maxima, each corresponding to one of the constituents. The studied materials from nonrandom copolymeric structures typical of block polymers. Such microphase systems can probably be expected in crosslinked polymers whenever the crosslinking agent participates significantly in the network chains and its compatibility with the flexible chains species is poor.     

Aminolysis of crosslinked polysiloxanes: Tautomeric catalysis by 2-pyridone

Results are presented which extend earlier findings on the transformation of insoluble crosslinked polysiloxanes into soluble linear polymers by selective aminolysis at crosslink sites. The present study provides evidence of a substantial steric effect and catalysis by 2-pyridone. The results with 2-pyridone are attributed to tautomeric catalysis which, to our knowledge, had not been observed before in substitution reactions at silicon. Evidence is also presented that unhindered, difunctional primary amines effect aminolysis with both amine groups to produce insoluble crosslinked polymer, presumably with silylamine functionality at crosslink sites.     

Amphiphilic sorbents based on polysiloxanes crosslinked by an N,N′‐heterocycle

New amphiphilic sorbents have been synthesized by crosslinking of the chloromethyl side‐functionalized polysiloxanes with piperazine. Three structures, differing by the crosslinking degree, have been synthesized as potential sensitive materials for humidity sensors able to work at high humidity or as metal sorbents. Correlations between structure and morphology of the polymeric matrix, as well as their swelling capacity in solvents with different polarities, are discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Cyclic polysiloxanes

The preparation, fractionation and characterisation of synthetic cyclic polysiloxanes (R(CH₃)SiO)_x are reviewed. Particular emphasis is given to the cyclic poly(dimethylsiloxanes) (PDMS) (R is CH₃) cyclic poly(phenylmethylsiloxanes)(PPMS) (R is C₆H₅) and cyclic poly(vinylmethylsiloxanes) (PVMS) (R is CH₂=CH). The application of high performance liquid chromatography (HPLC) and analytical and preparative gel permeation chromatography (GPC) to the preparation and characterisation of cyclic polysiloxanes is discussed. The methods include preparation of sharp fractions of relatively high molar mass cyclic polymers (with heterogeneity indices typically ca. 1.05) and comparison of the GPC and retention volumes of cyclics with the corresponding linear polymers. Investigations of the cyclic polymers prepared have been made using a number of experimental methods and techniques. These include dilute solution viscometry and low angle neutron scattering. Comparisons are drawn between cyclic PDMS and other cyclic polysiloxanes and also between cyclic polysiloxanes and the corresponding commercially-important linear polymers.     

Nematic-nematic modification in side-on-fixed polysiloxanes

Abstract

A series of side-chain liquid crystal polymers has been synthesized with mesogenic groups laterally branched to a polysiloxane backbone (the so-called side-on-fixed). An anomaly of the diamagnetic anisotropy in the nematic state was shown to be very sensitive to a number of structural parameters such as the length of the aliphatic tails, the length of the spacer and the fixation rate of the mesogenic groups on the backbone. On the basis of accurate density measurements, microscopic observations and X-ray diffraction experiments on aligned samples, the occurrence of a nematic-nematic transition connected to this anomaly is questionned.     

Nematic-nematic modification in side-on-fixed polysiloxanes

A series of side-chain liquid crystal polymers has been synthesized with mesogenic groups laterally branched to a polysiloxane backbone (the so-called side-on-fixed). An anomaly of the diamagnetic anisotropy in the nematic state was shown to be very sensitive to a number of structural parameters such as the length of the aliphatic tails, the length of the spacer and the fixation rate of the mesogenic groups on the backbone. On the basis of accurate density measurements, microscopic observations and X-ray diffraction experiments on aligned samples, the occurrence of a nematic-nematic transition connected to this anomaly is questionned.     

Microheterogeneity of orosomucoid in pathological conditions

Studies of orosomucoid (alpha 1-acid glycoprotein) in human serum have revealed that orosomucoid is a mixture of molecules with differences in the glycan chains. This microheterogeneity has been studied using crossed affinoimmuno-electrophoresis with the lectin concanavalin A which binds to biantennary glycans. The relative proportions of the three orosomucoid subtypes are altered in various pathological conditions independently of the total serum orosomucoid concentration. There are reproducible differences in microheterogeneity patterns between some pathological conditions: Acute tissue injury or inflammation results in a high proportion of orosomucoid with biantennary glycans. Conditions with increased estrogen levels are associated with a high proportion of orosomucoid with tri- or tetraantennary glycans and a low total serum orosomucoid concentration. Chronic inflammation also seems to be associated with a high proportion of orosomucoid with tri- or tetraantennary glycans but with a high total serum concentration of orosomucoid. Other diseases, such as cancer, can not be associated with any specific microheterogeneity pattern. The microheterogeneity pattern in these conditions seems to be determined by disease activity and unspecific inflammation in surrounding tissues.     

Thermotropic mesophases in linear polysiloxanes

Flexible linear polysiloxanes with inorganic backbones such as poly(diethylsiloxane) and poly-(dipropylsiloxane) contain no traditional mesogenic groups neither in the main chain nor in side chains and, nevertheless, they are able to form thermotropic mesophases. Recent developments in the study of thermodynamics, kinetics, structure and morphology of these mesophase polymers are considered. The temperature interval of existence of the mesophase is strongly dependent on the length of side radicals and molecular weight. These dependences are examined in detail. X-ray structure and the optical textures seen in the polarizing microscope are discussed. It is shown that large mesophase lamellae represent a very characteristic feature of the morphology of poly(diethylsiloxane). The linear growth rate of the lamellae and the overall calorimetric kinetics of the mesophase formation have been studied at various temperatures and the results obtained have been analyzed using the Avrami equation and the kinetic theory of the linear growth. As a result of the kinetic analysis two-dimensional growth has been suggested in accordance with the microscopic observation. The formation of the mesophase in slightly crosslinked poly(diethylsiloxane) samples can be initiated by stretching. The stress-strain and thermomechanic (deformation calorimetry data) behaviour of such samples is briefly discussed.     

Well-defined side-chain liquid-crystalline polysiloxanes

A route to well-defined side-chain liquid-crystalline polysiloxanes (ratio of weight-to number-average molar masses M̄_w/M̄_n < 1.2 is reported. Anionic ring-opening polymerization of pentamethylvinylcyclotrisiloxane yielded a poly(dimethylsiloxane-co-methylvinylsiloxane) backbone. A flexible disiloxane spacer was used to connect 4-(ω-alkenyloxy)-4′-cyanobiphenyl mesogenic molecules to the vinyl groups which belong to the backbone, leading to a side-chain liquid-crystalline polysiloxane (SCLCP) which has its mesogens distributed regularly along the main chain. Preliminary measurements indicate an electro-optic switching time τ_s = 1 min at 20°C and 7 s at 32°C (dc, 5 V/μm)).     

Ultrasonic and hypersonic dispersion in polysiloxanes

We report ultrasonic attenuation and velocity measurements on poly(dimethylsiloxane) (PDMS), poly(phenylmethyl siloxane) (PPMS), and copolymer poly(dimethyl phenylmethyl siloxane) in the temperature range of 10–50°C and frequency 0.3–45 MHz. The present data complement previously reported Brillouin spectra at hypersonic frequencies. Whereas the ultrasonic velocity u₀ is virtually independent of frequency, the ultrasonic absorption exhibits strong dispersion which can be ascribed to the viscoelastic normal mode relaxation. The ultrasonic attenuation data for PPMS at low temperatures display an additional relaxation process related to localized segmental motion. This mode is also responsible for the relatively large dispersion of the sound velocity and attenuation in the gigahertz frequency range accessible to the Brillouin scattering experiment. The extended information, which can be extracted by studying hypersonic dispersion, is discussed in detail.     

Strong Microheterogeneity in Novel Deep Eutectic Solvents

With the increasing application of template assisted syntheses in deep eutectic solvents and successful application of hydrophobic deep eutectic solvents in extraction processes, where microheterogeneity plays a major role, suggestions for novel deep eutectic solvents which exhibit strong microheterogeneity are desirable. Therefore, classical molecular dynamics simulations were carried out on deep eutectic solvent systems constructed of choline chloride and some of its derivatives mixed with ethylene glycol in a molar composition of 1 : 2 since this is the optimal parent composition. The derivatives consisted of a series of elongated alkyl side chains and elongated alcohol side chains. Of these series only choline chloride ethylene glycol has been investigated experimentally, the other systems are suggested and theoretically investigated as possible target for synthesis. Our domain analysis supported by the clear distinction of polar and nonpolar parts from the electrostatic potentials reveals that strong microheterogeneity within these novel hypothetical deep eutectic solvents exists. Rather stretched than crumbled side chains maximize possible interaction sites for both polar and nonpolar parts which make the suggested compounds valuable objectives for experiments in order to exploit the microheterogeneity in deep eutectic solvents.     

Irradiation of some polysiloxanes in various gases

The changes produced by γ-irradiations on two polysiloxanes in several gaseous atmospheres were studied by both sequential and parallel exposures with an in-situ vibrating reed technique. Irradiation in atmospheres of N₂O, He, air, and a mixture of 20% NO₂ and 80% N₂, caused reduction in the crosslinking rates of the polymers as compared to vacuum. Marked discolorations of the samples occurred when irradiations were conducted in air, N₂O, and 20% NO₂–80% N₂. Similarities in crosslinking rates and color changes were obtained when the polymers were irradiated in either air or the gas mixture.     

Rotational viscosity of ferroelectric liquid-crystalline polysiloxanes

Abstract

Some physical parameters of comb-shaped ferroelectric liquid-crystalline polymers were measured. Their rotational viscosities are two or three orders of magnitude larger than that of low molecular weight ferroelectric liquid crystals. Furthermore, they are found generally to be proportional to the second power of the weight-average molecular weight. Spontaneous polarization of the ferroelectric liquid-crystalline polymers has little dependence on molecular weight.     

Rotational viscosity of ferroelectric liquid-crystalline polysiloxanes

Some physical parameters of comb-shaped ferroelectric liquid-crystalline polymers were measured. Their rotational viscosities are two or three orders of magnitude larger than that of low molecular weight ferroelectric liquid crystals. Furthermore, they are found generally to be proportional to the second power of the weight-average molecular weight. Spontaneous polarization of the ferroelectric liquid-crystalline polymers has little dependence on molecular weight.     

Structure-property relationships of 'diluted' ferroelectric polysiloxanes

A series of new ferroelectric copolysiloxanes with systematically varied comonomer content ('dilution') has been synthesized. Good planar alignment could be achieved for all copolysiloxanes and they were studied with respect to their mesomorphic and ferroelectric properties. Broad enantiotropic S*_C phases and spontaneous polarizations up to 286 nC cm^-2 are found. X-ray diffraction experiments show a linear increase of the smectic layer spacing by 'dilution'. This points to a microphase separated structure of mesogenic groups and siloxane chains. It is found that the fixation of a mesogen to homopolysiloxane leads to an increase of P_s, whereas the 'dilution' of the mesogens with dimethylsiloxane units decreases P_s again. Based on the microphase separated model, it can be shown that the decrease of P_s is not only due to the decrease of the vol % of mesogenic groups. The coupling between different mesogens mediated by the polymer chain, has additionally to be taken into consideration. A remarkable drop in the response times τ with decreasing mesogen content is confirmed and switching times less than 1 ms were measured.     

Density measurements on liquid-crystalline side-chain polysiloxanes

We have studied the behaviour of the density, ϱ, as a function of temperature for a number of side-chain liquid-crystalline polymers. The polymers have a polysiloxane backbone and a benzoate ester side group with different tails or flexible spacers. From the values of ϱ in the smectic A phase and from the layer thickness obtained from X-ray experiments we have determined a molecular area in the plane of the layers. The evolution of this parameter shows clearly the asymmetrical role of the tail and the spacer.     

Antifoam effect of highly fluorinated polysiloxanes

Aqueous foaming solutions of permethylsiloxane surfactants and fluorocarbon surfactants are very resistant to antifoaming. Newly developed polysiloxane fluids having highly fluorinated chains showed very low surface tensions and were examined as antifoaming compounds to test against these two classes of surfactant systems. The fluorinated polysiloxanes evaluated were poly-[(3,3,4,4,5,5,6,6,6 - nonafluorohexyl)methylsiloxane] (PNFHMS), and 1H,1H,2H,2H-perfluorodecyl-terminated polydimethylsiloxane (Def-DMS). Poly(dimethylsiloxane) (PDMS) was also studied as a standard siloxane polymer fluid for comparison. The antifoams composed of PNFHMS and Def-DMS polymer fluids incorporated with silica showed effective antifoaming against the low surface tension aqueous solutions of representative permethylsiloxane and fluorocarbon surfactants.