Siloxane Chain Research Articles

Dynamics in Polydimethylsiloxane: The Effect of Solute Polarity

The temperature dependent dynamics of polydimethylsiloxane (PDMS) melts are investigated by measuring orientational relaxation of a dissolved probe molecule, 2-naphthyltriethoxysilane (NTES) using time resolved fluorescence depolarization. The temperature dependent viscosity of PDMS is also reported for two molecular weights. The measurements of nonpolar NTES probe dynamics are compared to previous measurements on the polar probe, N-(triethoxysilylpropy1)dansylamide. The activation energies for the orientational relaxation of the two probes arevery different. This is discussed in terms of the influence of the polarity of the solutes on the local structure in the melts. The results have implications for possible modifications of the physical properties of PDMS materials by using solutes or side groups of varying polarity. The synthesis of the NTES probe, which can also be used as a cross-linking reagent for PDMS, is also described. Silicone polymers are specialty materials used in a wide variety of commercial products including lubricants, cosmetics, con- struction sealants, oxygen-permeable membranes, drug delivery systems, and biomedical implants.'-' Much of this multibillion dollar industry is based on polydimethylsiloxane (PDMS) and its copolymers, either as melts or cross-linked rubbers. The increasing demands for siloxane polymers are fueled by the desirable properties of their inorganic backbone. Siloxane oils enjoy high thermal stability, a low melting point, a weakviscosity dependence on temperature, high hydrophobicity, and relatively good bio- compatibility. Most of these properties are macroscopically observable, yet they are ultimately derived from the characteristics of the silicon-xygen bond, and the interactions between siloxane chain segments. This study concentrates on the relationship between local siloxane chain dynamics, over a distance scale of a few monomer lengths, and the long range interactions measurable by viscometry. The main experimental technique employed is the measurement of time resolved fluorescence polarization anisotropy of a chromophore attached to a probe molecule that is dissolved in the bulk polymer. The specific systems studied are melts of polydimethylsiloxane above the polymer glass transition tem- perature ( Tg - 152 K) that contain a low concentration of the

Effect of siloxane chain lengths of monomers on characteristics of pervaporation membranes prepared by plasma polymerization

AbstractPlasma‐polymerized membranes were prepared from a series of monomers with the different siloxane chain lenghts, i. e., hexamethyldisiloxane, octamethyltrisiloxane, and decamethyltetrasiloxane, and effects of the starting monomers on both the structures of the plasma polymers and the pervaporation characteristics of ethanol/water mixture were investigated. By IR and XPS measurements, it was found that with increase of the siloxane chain lenghts of the monomer the siloxane chains in the plasma polymers were linked more continuously and smaller amounts of additional oxygen were newly incorporated into the polymers. All membranes obtained from the three monomers were found to be ethanol‐permselective. From the standpoint of obtaining both high selectivity and high permeation rate, the monomer with the longer siloxane chain was profitable. The relations between the results of the analysis of the plasma polymers by IR and XPS measurements and the pervaporation characteristics obtained are discussed. © 1994 John Wiley & Sons, Inc.

Structural evolution during pyrolysis of sol-gel derived hybrid materials

Siloxane-titania materials have been prepared starting from diethoxydimethylsilane and titanium isopropoxide. The transparent starting gels can be described as nanocomposites formed with siloxane chains and TiO2-based particles. The pyrolysis process under argon atmosphere up to 1600°C has been followed by various techniques such as infrared and solid state NMR spectroscopies, X-ray diffraction and Ti K-edge X-ray Absorption. Si-C bond cleavage begins at low temperature leading to redistribution reactions with the formation of new Si-O bonds. Crystallization of TiO2 at low temperature is prevented and titanium carbide starts to form around 800°C. Samples pyrolyzed at 1600°C show the presence of SiO2 and TiC crystalline phases.

Influence of the External (Electric or Mechanic) Field on the Dielectric Relaxation in Liquid Crystalline Side Chain Polymers

Abstract The effect of orienting fields on the relaxation of dipole polarization in siloxane and methacrylate liquid crystalline polymers with cyanobiphenyl groups in side chains has been investigated. The temperature-frequency dependences of dielectric losses of comb-like polymers with the main siloxane chain were studied in the mesomorphic state (in bulk). Two relaxation processes, a and δ, were established. They are related to the reorientation of the transverse and longitudinal components of the dipole moment of the mesogenic group. Similarity of the temperatures of the α- and δ-processes and of the glass transition are due to the dependence of the main chain segment motion on the interaction between side mesogenes, with the segmental mobility occurring only when the mesogenes move. The effect of the external orienting field leading to homeotropic orientation of mesogenic fragments results in the increasing intensity of δ-transition and the suppression of α-transition. The relaxation times and the act...

Microstructure of puffed silica xerogel

The structural evolution that takes place during sol-to-gel and gel-to-glass transformations has fascinated both experimentalists and theoreticians [1-5]. In situ techniques [6] such as nuclear magnetic resonance, Raman and infrared spectroscopy, small-angle X-ray, neutron and light scattering, and studies including gas adsorption-desorption analyses [6], mercury porosimetry [1-3] and photoprobe investigations [7] have contributed to the development of theoretical models to explain growth in acidand base-catalysed gels, and their subsequent densification due to viscous sintering at elevated temperatures. The structure of silica gel obtained from acid hydrolysis of tetraethyl orthosilicate (TEOS) under excess-water conditions has been known to be one of extensive cross-limking of siloxane chains in all three directions. Such gels possess a texture that is distinctly different from that of base-catalysed gels of equivalent composition. Base-catalysed gels consist of dense agglomerates of particles with closed pores, whereas acid-catalysed gels are formed of lattices of particles with open pores and resemble the "soots" in optical fibre technology [1-3]. Scherer [1-3] modified the phenomenological theory of sintering by Mackenzie and Shuttleworth, originally used :for the calculation of the rate of densification of a viscous body containing closed pores, to explain the sintering behaviour of a glass containing open pores and non-spherical particles. He described the rate at which a cubic array of cylinders densified by viscous flow driven by surface energy reduction. The cubic array was proposed as a model for the microstructure of low-density openpore materials such as gels and the preform making "soots" as described above. Although Scherer's model of a cubic array of cylinders agreed reasonably well in explaining the changes in physical parameters such as the pore size and its distribution, linear shrinkage, etc., during sintering of low-density glasses, such a model is too simplistic and does not conform with the FloryStockmayer (FS) theory, which is essentially a theory of dendritic polymerization. Furthermore, a silica gel is an amorphous structure, a not-toodistant relative of fused silica, the only difference being that in the former not all oxygen atoms are bridging (Si-O-Si) but many are dead-ended in hydroxyl ions (Si-OH) or functional alkyl groups (Si-OR). The FS theory assumes that all molecules created by the reaction of monomers have a tree-like form of the Bethe lattice type. Fig. 1 shows the skeleton of a possible molecule

Polymeric light stabilizers based on siloxanes

Polymeric light stabilizers were synthesized by hydrosilylation of unsaturated 2-hydroxybenzophenone derivatives or pentamethylpiperidine derivatives with different hydridosiloxanes. Investigations of the hydrosilylation reaction by NMR and HPLC show that the stabilizer groups are mainly bound to the siloxane chain by non-hydrolysable SiC bonds. When allyl ethers are used as starting material, side reactions such as ether cleavage, hydrogenation and isomerization take place. To avoid these side reactions, unsaturated ethers or esters with long methylene chains between the double bond and the oxygen were used. The polymeric light stabilizers were tested as stabilizers of polypropylene and compared with TINUVIN 770. The activity of polymeric light stabilizers decrease, with increasing molecular weight. Extraction experiments demonstrate that the polymeric stabilizers are resistant against extraction by organic solvents while TINUVIN 770 is extracted under the same conditions. Thus, our polymeric HALS show a higher effectiveness than TINUVIN 770. The polymeric HALS derivatives with the best activity have molecular weights ( M n) between 2000 and 4000.

Vulcanized siloxane chains swollen by polymer chains: NMR investigations into free-chain dynamics

The dynamics of low molecular weight siloxane chains moving freely in siloxane gels was studied from NMR of protons attached to the free chains. The variation of the relaxation rates of the free chains upon variation of the gel fraction was interpreted as a free volume decrease upon crosslinking. The volume fractions of free volume in the gels were deduced from this dependence. This parameter was related to equilibrium swelling degrees and it was found that it was a decreasing function of the crosslink density.

Synthesis and characterization of siloxane-titania materials

Diethoxydimethylsilane and titanium isopropoxide have been used for preparation, via the sol-gel process, of mixed siloxane-titania materials. Depending on the Ti/Si molar ratio, flexible to brittle gels can be obtained. The rheological properties of the sols can be adjusted in order to obtain films and transparent monoliths. Due to the high amount of hydrophobic methyl groups in these materials, thick films with an average thickness of 20 μm can be obtained. These films have been used for laser dye matrices. A characterization of the various steps of the hydrolysis-condensation process as well as of the structure of the final solid gel determined by liquid and solid state NMR and TiK edge X-ray absorption spectroscopy are presented. A structural model for the gel is proposed mainly based on siloxane chains and TiO 2 based particles. Optical properties of Coumarin 4 and Rhodamin 6G incorporated in such films are discussed.

Polyimides synthesized from siloxane‐equilibrated 1,3‐bis (3,4‐dicarboxyphenyl)‐1,1,3,3‐tetramethyldisiloxane dianhydride

AbstractSilicone polyimides were made with siloxane‐equilibrated 1,3‐bis (3,4‐dicarboxyphenyl) ‐ 1,1,3,3‐tetramethyldisiloxane dianhydride (PADS) as the soft portion and [(1‐methylethylidene) bis (4,1‐phenyleneoxy)] bis‐1,3‐isobenzofurandione (BPADA) as the hard portion in combination with either para or meta‐phenylene diamine. The silicone polyimides were made with varying siloxane chain lengths and with varying overall weight percent silicone. There were linear correlations between these variables and flexural modulus, percent elongation at break, and cut‐through temperature. Glass transition temperatures were evaluated by dynamic mechanical analysis and the thermal stabilities were determined to be greater than 400°C by thermal gravimetric analysis. Melt viscosities were determined on the various copolymers and demonstrated a correlation between type of amine used, siloxane chain length, and overall weight percent silicone.

Synthese d'oligomeres de polydimethylsiloxane reticulables—partie I. introduction de groupements acryliques sur des polydimethylsiloxanes possedant des groupements 3-hydroxypropyle

We describe the synthesis of polydimethylsiloxane with 3-hydroxypropyl groups in the chain or at the ends by copolycondensation of 3-acetoxypropyl methyl dichlorosilane with various α, ω-disilanols. The length of the chain was controlled by addition of vinyl or 3-acetoxy propyl monochloro dimethyl silane. Methacryoyl chloride was then grafted to give a polyunsaturated product. These oligomers were crosslinked by u.v. with reactive diluents and photoinitiators to give products with very good “release properties” especially with the longer siloxane chains.

Surface characteristics of fluoroalkylsilicone—poly(methyl methacrylate) block copolymers and their PMMA blends

AbstractSurface properties of fluoroalkylsilicone—poly(methyl methacrylate) block copolymers and their poly(methyl methacrylate) (PMMA) blends have been intensively studied to draw a relationship between surface segregation behavior and adhesion performance. The fluoroalkylsilicone—PMMA block copolymers containing thre kinds of fluoroalkyl side chains (i.e., 3,3,3‐trifluoropropyl) (CF3CH2CH2−, abbreviated as TFP), tridecafluoro‐1,1,2,2‐tetrahydrooctyl [F3C(CF2)5CH2CH2−, TFO], and heptadecafluoro‐1,1,2,2‐tetrahydrodecyl groups [F3C(CF2)7CH2CH2−, HFD] on the siloxane chains were prepared according to the macroazoinitiator method. Contact angle measurement and X‐ray photoelectron spectroscopy (or ESCA) were used to obtain structural information on the surface of the solution‐cast films. A significant effect of fluroalkyl groups on the surface accumulation not only in silicon but also fluorine atoms were observed for relatively long fluoroalkyl side chains (i.e., TFO and HFD). On the contrary, TFP group was found to be effective to orient to the hydrophilic surfaces (i.e. glass‐side surfaces). The surface accumulation of fluoroalkyl groups in the films was also reflected by the 180° peel strength against pressure‐sensitive adhesives as a measure of the adhesion performance. These results indicated that incorporation of relatively long fluoroalkyl groups much enchanced the surface hydrophobicity (and/or oleophobicity) of solution‐cast films in addition to that of the siloxane‐enriched surfaces. The composition and orientation of the functional groups on the surface were also speculated based on ESCA data.

Copolymers with 1,3-diorgano-1,3-disila-2-oxaindane fragments in the dimethyl siloxane chain

Siloxane copolymers with disilaindane cycles in the main chain have been synthetized by heterofunctional polycondensation of 1,3-dichloro-1,3-diorgano-1,3-disilia-2-oxaindanes with α-ω-dihydroxydimethl-siloxanes in presence of pyridine and their thermomechanical, thermogravimetric and radiographic investigations undertaken.

Surface characteristics of polydimethylsiloxane—poly(methyl methacrylate) block copolymers and their PMMA blends

AbstractTo draw a relationship between surface concentration of the siloxane segment and adhesion performance, surface properties of the polydimethylsiloxane—poly(methyl methacrylate) block copolymers(PDMS‐b‐PMMA) prepared via poly(azo‐containing siloxaneamide)s and their PMMA blends have been studied by measurements of FT‐IR spectra, water contact angle, ESCA spectra and 180° peel strength toward pressure‐sensitive adhesive tape. The water contact angles of the chloroform‐cast blend films increased abruptly with siloxane bulk concentrations, or siloxane contents, particularly, on the air‐side surfaces to reach almost 100° in low siloxane content. A marked increase of the contact angle was observed in the blends containing siloxane chain length (SCL) of longer than 2000. ESCA data evidently confirmed for these blend systems that the siloxane segments with low surface energy were accumulated or enriched mainly on the air‐side surface, and that, on the other hand, polar PMMA segments with high surface energy were oriented to the glass‐side surface and the inside of the films. This surface accumulation behavior of the siloxane segments reflected the 180° peel strength, as a measure of adhesion performance. The water contact angle and 180° peel strength were unequivocally correlated to the siloxane surface concentration estimated from ESCA data. Conversely, in the compression‐molded blend films made by a hydraulic press between a Teflon and a stainless steel plate, the extent of surface accumulation of the PDMS segment was lower than that of the chloroform‐cast films, suggesting lower degree of segment migration in hot‐press films, probably due to substrate surface energy and lower relaxation in the blend melts.

Preparation of Silica Glass Fibers from Tetraethylorthosilicate and Hexamethyldisiloxane

Silica glass fibers were prepared from mixed solutions of tetraethylorthosilicate (TEOS) and hexamethyldisiloxane (HMDS). An addition of HMDS improves spinnability since the increase in the viscosity of spinnable solutions becomes moderate and the range of the amount of added water for the occurrence of spinnability is widened. Spinnable solutions are obtained when the ratio of silicon in HMDS to total silicon, r, ranges from 0 to 0.6. It is assumed that the fibers drawn from this system consist of linear polymeric particles with double or triple siloxane chains capped with trimethylsilyl groups. Gel fibers from the solutions of r≤0.5 were converted to visually colorless and transparent silica glass fibers by rapidly heating them at 600°C.

Hydroxyl or methyl terminated poly(dimethylsiloxane) chains: kinetics of adsorption on silica in mechanical mixtures

The kinetics of adsorption of siloxane chains on silica aggregates was observed in mechanically induced mixtures. The amount of polymer Q r( r) bound, to silica per unit mass, was found to obey the time dependence: Q 1 r − Q r (t) = (Q 1 r − Q r (0)) exp (− √ t τ ) ; Q 1 r is the limit amount of adsorbed polymer and Q r(0) represents the initial adsorption resulting from the mechanical mixing. Two polymer species were structured: hydroxyl terminated chains M n = 2.4 × 10 4 ) or methyl terminated ones ( M n = 2.9 × 10 5 ). Values of the characteristic time τ were 7.1 × 10 7 s and 6.2 × 10 8 s, respectively. The silica concentration was 0.29 (w/w) for both species.

Cyclic metallasiloxane compounds of TiIV and ZrIV. X-Ray crystal structures of a cis-bis(pyridine) bis(cyclosiloxy)titanate and a pyridinelithiated tris(cyclosiloxy)zirconate

The reaction between Si2Ph4O(OLi)2(1) and TiCl4in 2 : 1 or 3 : 1 molar ratio gave on addition of pyridine (py) the titanatrisiloxane compound [[graphic omitted]}2(py)2]·2C6H5Me (2). Treatment of ZrCl4 with (1) in 1 : 2 molar ratio gave the zirconadisiloxane system Li2Z[graphic omitted])3]·3py· 1.5C6H5Me (3). Compound (2) was also isolated from reactions employing the diol Si2Ph4O(OH)2 and TiCl4in 2 : 1 molar ratio in the presence of pyridine. Siloxane chain expansion was also observed in the product from reaction of (1) with SiMe2Cl2 but not from reaction with SiMe3Cl. The formation of the cyclic titanatrisiloxane and the cyclic tetrasiloxane is presumably a consequence of the ring strain in the titanadisiloxane and trisiloxane systems respectively, rather than a tendency of the siloxanediolate species (1) to oligomerise in the presence of base or metal halides. Compounds (2) and (3) have been studied by X-ray crystallography.

Spectral manifestations of partial silicon-oxygen double bond character in oligophenylsiloxanes

1. We have considered the UV and IR spectra of a number of oligophenylsiloxanes with different structures. 2. The data obtained have been discussed in light of the concept of partial double bond character for Si and O atoms in the siloxane chain.

Estimation of formation mechanism of spherical fine ZrO2-SiO2 particles by ultrasonic spray pyrolysis

Spherical fine ZrO2-SiO2 (1∶1) particles were synthesized from a hydrolysed mix-solution of Si(OC2H5)4 and ZrOCI2 · 8H2O by ultrasonic spray pyrolysis, and the formation mechanism of the particles discussed. All of the resultant particles identified as t-ZrO2 and amorphous SiO2 with an atomic order dispersion were spherical, and mainly of diameter 0.8 to 1.0μm. It was estimated that the three-dimensional ladder siloxane chains containing an equimolar Zr4+ homogeneously dispersed were formed by hydrolysis, and an atomized droplet itself converted into an isolated ZrO2-SiO2 (1∶1) particle without aggregation.

Block copolymers derived from azobiscyanopentanoic acid. XI. Properties of silicone–PMMA block copolymers prepared via polysiloxane(azobiscyanopentanamide)s

AbstractMechanical, thermal, and surface properties of poly(dimethylsiloxane)–poly(methyl methacrylate) block copolymers (PDMS‐b‐PMMA) prepared by the use of polysiloxane(azobiscyanopentanamide)s were intensively investigated. The mechanical strength of block copolymers was found to decrease with an increase of siloxane contents. Dynamic mechanical analysis (DMA) of block copolymers having long siloxane chain length (SCL) and high siloxane content revealed the existence of two glass transitions attributable to microphase separation of two segments. Differential scanning calorimetry (DSC) also gave some evidence of microphase separation supporting the DMA results. It was observed that the incorporation of PDMS segments in block copolymers improved thermal stability of PMMA, as confirmed by thermogravimetric analysis. Surface analysis of the block copolymers films cast from several solutions indicated surface accumulation of PDMS segments, as revealed by water contact angle and ESCA measurements.

Block copolymers derived from azobiscyanopentanoic acid. X. Synthesis of silicone–vinyl block copolymers via polysiloxane(azobiscyanopentanamide)s

AbstractThe synthesis of silicone–vinyl block copolymers has been studied by the use of poly(azo‐containing siloxaneamide)s (abbreviated as PASAs), i.e., polysiloxane (azobiscyanopentanamide)s as macroazoinitiators. PASAs with number‐average molecular weight of 12000–31000 and with siloxane chain lengths of 250–9800 were prepared by the condensation of azobiscyanopentanoyl chloride and α,ω‐bis(3‐aminopropyldimethyl)polysiloxanes in equimolar feeds. Several kinds of silicone–vinyl block copolymers were synthesized by radical polymerization of vinyl monomers such as methyl methacrylate, styrene, and vinyl acetate, in the presence of PASA in homogeneous media. The block copolymers with siloxane contents up to 30 mol % were then characterized on the basis of infrared absorption, proton NMR spectra, and gel permeation chromatography.