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.     

Microheterogeneity in crosslinked polysiloxanes

Previously reported studies on crosslinked castable poly(dimethylsiloxanes) have been extended. A series of dimethylsiloxane difumarates were copolymerized with styrene, and some of their rheological properties were determined. Investigation of these and previously prepared polymers has shown that segregation on a microscale can take place in two-component crosslinked systems with relatively short network chains. This may occur even if one of the components is a nonpolymeric species involved in crosslinking. Microheterogeneity was observed in systems made up of prepolymer chains of a single type, when these were short enough to expose a specific structure of the crosslinking site. It seems that microheterogeneity can be expected in crosslinked polymers when the size of the crosslinking site is significant in comparison with the network chain length and when its cohesive energy density differs considerably from that of the main network chains. Additional factors affecting microphase separation include the existence of physical interactions between chemical groups, steric positioning of interacting groups during the process of crosslinking, and physical effects of the degree of crosslinking leading to local differences in the density of the system.     

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.     

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.     

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).     

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)).     

Synthesis and radiation behavior of perfluoroaromatic polysiloxanes

In a study aimed at the development of a polymeric material stable to the space radiation environment, the synthesis and properties of two new perfluoroaromatic polysiloxanes have been investigated. Di(pentafluorophenyl)diethoxysilane and pentafluorophenyltriethoxysilane have been prepared via Grignard synthesis from pentafluorobromobenzene and tetraethoxysilane. By hydrolytic polymerization of these intermediates, polydi(pentafluorophenyl)siloxane and polypentafluorophenylsilsesquioxane have been synthesized. The effect of ultraviolet and low-energy proton (solar wind) irradiation in high vacuum has been determined in a space-environment-simulation facility. These polymers possess excellent resistance to low-energy proton irradiation but are less stable to ultraviolet irradiation. It is suggested that the enhanced polarization of the carbon–silicon bond due to the strong inductive effect of the pentafluorophenyl groups is responsible for the relatively low ultraviolet and thermal stability of the perfluoroaromatic polysiloxanes.     

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.     

Cyclolinear polysiloxanes. I. Synthesis and characterization

The synthesis and characterization of two novel cyclic siloxanes, diacetoxydiethyltetramethylcyclotetrasiloxane and diacetoxytriethylpentamethylcyclopentasiloxane, and cyclolinear polymers synthesized from these monomers are presented. The cyclic siloxanes were synthesized from tetramethylcyclotetrasiloxane and pentamethylcyclopentasiloxane, respectively, by acetylation followed by ethylation. The cyclic monomers were characterized with ¹H NMR spectroscopy. Subsequently, the cyclic siloxanes were self‐condensed into cyclolinear polysiloxanes and cocondensed (extended) with silanol‐terminated polydimethylsiloxane into high‐molecular‐weight polymers containing cyclic units withlinearpolydimethylsiloxane spacers (extended cyclolinear polysiloxanes). The molecular weights of both the cyclolinear polysiloxanes and extended cyclolinear polysiloxanes were determined. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4039?4052, 2006     

Liquid-crystalline polysiloxanes with fluoro-substituted side chains

Sixteen new side chain liquid polysiloxanes, either homopolymers derived from poly(hydrogenmethyl)siloxane or copolymers derived from poly(hydrogenmethyl-dimethylsiloxane), are reported with side chains carrying fluoro-substituents. The known effects of fluoro-substitution in low molar mass liquid crystals are strongly echoed in the polymeric analogues and interesting comparisons are made between homo- and co-polymers carrying the same fluorinated side chains.     

Mean-square radius of gyration of polysiloxanes

The mean-square radius of gyration for polysiloxanes has been derived according to the exact definition. Taking account of the examples of symmetrically substituted poly(dimethylsiloxane) and unsymmetrically substituted poly(methylphenylsiloxane), we find that the dependence of 〈S²〉 on the molecular weight follows the general formula 〈S²〉 = aM^b with b = 1 ± 0.016, which is analogous to the theoretical outcomes for vinyl or vinylidene polymers even though the skeletal bone of polysiloxanes consists of alternating heteroatoms. A numerical comparison of the rigorous expression of the mean-square radius of gyration given in this paper with that reported by Flory shows that the difference is obvious for low-molecular-weight polymer and it gradually declines with increasing degree of polymerization.