Piezoelectric and dielectric properties of siloxane elastomers filled with bariumtitanate

Piezoelectric elastomers were prepared from suspensions of bariumtitanate (BaTiO₃) particles in a telechelic polydimethylsiloxane (t-PDMS) by crosslinking the t-PDMS under an electric field. Crosslinking reaction was monitored by measurement of complex dielectric constant ε′ − iε″. Dielectric constant ε′ increased with increasing BaTiO₃ content, and agreed approximately with the theoretical ε′ calculated with the Maxwell–Wagner theory. Piezoelectric constant d₃₃ of the poled elastomers was measured by application of compressions in the direction of the poling field. It was found that d₃₃ was of the order of 10⁻¹¹ C/N and increased steeply with increasing content of BaTiO₃ but became almost independent of composition in the range of BaTiO₃ content from 3 to 14 vol %. To examine the effect of electric field on the aggregation structure of the particles, we observed light scattering of the suspension under an electric field, and found that the scattering pattern became anisotropic. This indicated that the particles are connected like a pearl necklace and are stretched in the direction of the poling field. The dependence of d₃₃ on the volume fraction of BaTiO₃ was explained by a model proposed by Banno. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3065–3070, 1999     

Morphological hysteresis in immiscible PIB/PDMS blends filled with fumed silica nanoparticles

The morphological hysteresis behavior of immiscible polymer blend reflects the dependence of their steady-state morphology on the shear protocol applied. In this work, the influences of hydrophobic and hydrophilic fumed silica nanoparticles on the morphology hysteresis behavior of immiscible polyisobutylene (PIB)/polydimethylsiloxane (PDMS) (10/90) blends under simple shear flow were investigated by using optical shear technique. Compared with particle-free blend, the morphology hysteresis zone of filled blends was found to be expanded by the addition of hydrophobic or hydrophilic fumed silica nanoparticles. It was found that the expansion of the morphology hysteresis zone in hydrophobic nanoparticle-filled blend stemmed from the suppression of droplet coalescence. However, the expansion in the morphological hysteresis zone for hydrophilic nanoparticle-filled blend, which was less noticeable, might originate from the more difficult breakup of PIB droplets upon the addition of nanoparticles.     

Molecular mobility of ultrasonically devulcanized silica‐filled poly(dimethyl siloxane)

We used NMR relaxation and pulsed‐gradient diffusion measurements at 70 °C in precipitated silica‐filled poly(dimethylsiloxane) (PDMS; silicone rubber) after crosslinking, after subsequent devulcanization by intense ultrasound, and after subsequent revulcanization. As in unfilled PDMS, transverse relaxation displays three distinct rate components attributed to an entangled and crosslinked network (similar in T₂), light sol plus dangling network fragments, and unreactive trace oligomers. Ultrasound produces copious amounts of extractable sol. The T₂ relaxation times decreased modestly with increasing filler content, but they and the components' proportions correlated mainly with the sol fraction, that is, the network degradation. In rupturing the network, devulcanization produces large diffusing fragments and dangling ends; revulcanization largely reverses these effects. The rates and amplitudes of the bimodal diffusivity distribution confirmed this conclusion. The weakness of the effects of filler suggests that ultrasound devulcanization is easily adaptable to the recycling of the preponderantly particulate‐filled industrial rubbers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 454–465, 2003     

Cyclics in styrene-dimethyl siloxane block copolymers

Cyclics (with polystyrene backbones linked by short polydimethylsiloxane segments) at concentrations dependent on the conformations of the polystyrene block, have been found in solution equilibrates of poly(dimethylsiloxane-b-styrene-b-dimethylsiloxane). The concentrations of small siloxane cyclics agreed well with those reported for solution equilibrates of homopolydimethylsiloxane, indicating that the dimensions of short siloxane chains are unaffected by the presence of a polystyrene block.     

Novel nonionic siloxane surfactants

A new method for ethoxylation without application of pressure is described. Butynediololigo(oxyethylene) [H(OCH₂CH₂)_n? OCH₂? C?C? CH₂O(CH₂CH₂O)_nH with n=1–16] has been prepared in the presence of an electrophilic catalyst in a specially developed reciruculating apparatus. The products have been characterized by NMR and IR spectroscopy. New nonionic silicone surfactants have been synthesized by hydrosilylation of these butynediololigo(oxyethylenes) with defined siloxanes and polysiloxanes. Protection of the hydroxyl group before hydrosilylation was not necessary. Hydrosilylation was carried out in the presence of a solvent. It has been possible to obtain surfactants with a surface tension of about 21-22 mN m^?1 and an interfacial tension of 2 mN m^?1.     

A novel cyclic dithiocarbonate having siloxane moiety; Approach to curable siloxane oligomer

A novel 5‐membered cyclic dithiocarbonate (DTC) having a siloxane moiety, 5‐(3‐trimethoxysilylpropyloxymethyl)‐1,3‐oxathiolane‐2‐thione, was synthesized from the corresponding epoxide precursor by its cycloaddition with carbon disulfide. The siloxane group underwent condensation reaction by the treatment with water and a basic or acidic catalyst, to afford the corresponding oligomer having siloxane main chain and DTC pendant. The resulting oligomer was liquid and soluble in organic solvents such as THF and chloroform. Treatment of the oligomer with amines resulted in selective ring‐opening reaction of the DTC group, generating a thiol group, which underwent oxidative coupling reaction to make the oligomer successfully cured. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4422–4430, 2005     

Transitions in poly(diethyl siloxane)

An adiabatic heat capacity study of poly(diethylsiloxane) confirms that it has a single glass transition occurring at 130°K, the lowest glass transition reported to date for a high molecular weight polymer. The two previously reported glass transitions are first-order thermodynamic peaks whose location is dependent upon prior thermal history. Combination of these data with low-temperature x-ray diffraction indicates that the transitions in this temperature range are related to a crystal–crystal transformation. A crystal melting transition is observed near 270°K. In addition an anomalous rise in heat capacity near 60°K suggests a sub-glass transition of unknown origin.     

Viscous Sublayer

Russian Journal of Electrochemistry - The eddy viscosity and the eddy diffusivity are calculated for the viscous sublayer in turbulent flow near a solid surface by using Fourier transforms of a...     

Stepwise synthesis of siloxane chains

Siloxane chains of designated lengths can be synthesized with high yields by reacting tris(tert-butoxy)silanol alternately with dichlorosilane and silanediol.     

Permeation hysteresis in PdCu membranes

H 2 permeation hysteresis has been observed during cycling of a 3 mum thick supported PdCu membrane with approximately 50 atom % Pd through the fcc/bcc (face-centered cubic/body-centered cubic) miscibility gap between 723 and 873 K. Structural investigations after annealing of membrane fragments under H 2 at 823 K reveal retardation of the fcc(H) --> bcc(H) transition, which is attributed to the occurrence of metastable hydrogenated fcc PdCu(H) phases. The H(2) flux at 0.1 MPa H(2) pressure difference in the well-annealed bcc single phase regime below 723 K can be described by J(H2) = (1.3 +/- 0.2) mol.m (-2).s (-1) exp[(-11.1 +/- 0.6) kJ.mol (-1)/( RT)] and that in the fcc single phase regime above 873 K by J(H2) = (7 +/- 2) mol.m (-2).s (-1) exp[(-30.3 +/- 2.5) kJ.mol (-1)/( RT)].     

Adsorption-desorption hysteresis in porous networks

As a result of computer experiments with model porous networks, the factors (other than pore shape and size distribution) determining the form of the adsorption-desorption hysteresis loop have been elucidated.

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Contact angle hysteresis

In thermodynamic equilibrium, the contact angle is related by Young's equation to the interfacial energies. Unfortunately, it is practically impossible to measure the equilibrium contact angle. When for example placing a drop on a surface its contact angle can assume any value between the advancing Θ_a and receding Θ_r contact angles, depending on how the drop is placed. Θ_a − Θ_r is called contact angle hysteresis. Contact angle hysteresis is essential for our daily life because it provides friction to drops. Many applications, such as coating, painting, flotation, would not be possible without contact angle hysteresis. Contact angle hysteresis is caused by the nanoscopic structure of the surfaces. Here, we review our current understanding of contact angle hysteresis with a focus on water as the liquid. We describe appropriate methods to measure it, discuss the causes of contact angle hysteresis, and describe the preparation of surfaces with low contact angle hysteresis.     

Endgroup-assisted siloxane bond cleavage in the gas phase

Unimolecular dissociation of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) generates SiC(5)H(16)NO(+) and SiC(5)H(14)N(+). The formation of SiC(5)H(16)NO(+) involves dissociation of a Si[bond]O bond and formation of an O[bond]H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D(2)N(CH(2))(3)SiOSi(CH(2))(3)ND(3)(+) provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages.     

Capillary hydrodynamics of oligomer binders

In this work, we consider wetting and flow of oligomer binders for tissue fibers made of carbon and organic fibers. A correlation between the specific surface of fibers and strength of the boundary layers is established. The effect of fibers on structure formation of binders in the boundary layers is explained via the surface density of reticulation of fiber. The fibers with minimum surface reticulation exert a pronounced influence on the structure of binders, forming thicker and strong boundary layers.     

Viscous fingering in polymer solutions

Characteristics of viscous fingering patterns in polymer solutions were investigated by radially pushing air in a radial Hele‐Shaw cell containing hydroxypropyl methyl cellulose (HPMC) solutions. Low and high molecular weight HPMC samples were used. An increase in molecular weight easily yielded chain entanglements, which led to a strong shear thinning and an increase in elasticity. For the low molecular weight HPMC solutions, only the dense‐branching patterns were observed over the entire injection pressure range. When isopropyl alcohol was added into the HPMC solutions, leading to less elasticity, the pattern was tip‐splitting one, which is observed for Newtonian fluids. On the other hand, for the high molecular weight HPMC solutions, the resulting patterns showed a systematic change from dense‐branching to skewering patterns through tip‐splitting patterns as the injection pressure increased. The measured tip velocity was compared with the modified Darcy's law, but the coincidence is poor. The velocity corrected by the displaced area ratio provided a good linear relation.     

Formation of aromatic siloxane self-assembled monolayers

We describe reproducible protocols for the chemisorption of self-assembled monolayers (SAMs), useful as imaging layers for nanolithography applications, from p-chloromethylphenyltrichlorosilane (CMPS) and 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane on native oxide Si wafers. Film chemisorption was monitored and characterized using water contact angle, X-ray photoelectron spectroscopy, and ellipsometry measurements. Atomic force microscopy was used to monitor the onset of multilayer deposition for CMPS films, ultimately allowing film macroscopic properties to be correlated with their surface coverage and nanoscale morphologies. Although our results indicate the deposition of moderate coverage, disordered SAMs under our conditions, their quality is sufficient for the fabrication of sub-100-nm-resolution metal features. The significance of our observations on the design of future imaging layers capable of molecular scale resolution in nanolithography applications is briefly discussed.     

Carbosilane dendrimers based on siloxane polymer

A method for synthesizing growth ring‐type dendritic macromolecules starting from siloxane polymer with Si H bonds [Me₃SiO(MeSiOH)_nSiMe₃] as the core and dichlorovinylsilane, dichloromethylsilane, lithium phenylacethylide, and allylmagnesium bromide as the building blocks is described. The siloxane polymer was produced to the second dendritic generation by the repetition of hydrosilation and alkenylation as well as by an alkynylation process. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 724–729, 2000     

Gelation of siloxane cross-linked organic molecules

We have studied by ²⁹Si NMR and small angle neutron scattering the cross-linking by formation of siloxane bridges of two amino-epoxy-silane molecules at different concentration in acidic water. We relate our observations to the fact that one of the studied systems never gels, while the second one displays a concentration threshold for gelation that is accounted for by percolation theory.     

The thermal properties of polysiloxanes poly(dimethyl siloxane) and poly(diethyl siloxane)

New measurements and literature data on polysiloxanes covering heat capacities, transition parameters, enthalpies, entropies and Gibbs energies are presented and critically reviewed. TheATHAS computation method is used to bring heat capacities into agreement with an approximate frequency spectrum. The various crystal and mesophases are discussed. TheATHAS (1990) recommended data are as follows: For poly(dimethyl siloxane) the glass transition is at 146 K with an increase in heat capacity of 29.24 J/(K mol). The completely crystalline sample melts at about 219 K with a heat of fusion of 2.75 kJ/mol. For poly(diethyl siloxane) the glass transition is at 135 K with an increase in heat capacity of 34.48 J/(K mol). The completely crystalline sample changes to a condis crystal at 206.7 K with a heat of disordering of 2.72 kJ/mol. The transition to a poorly characterized viscous crystal with thermodynamic properties close to the melt occurs at 282.7 K with an enthalpy of transition of 1.84 kJ/mol. Final fusion occurs at 308.5 K and a small endotherm of about 231 J/mol. Tables of heat capacities, enthalpies, entropies and Gibbs energies are given from 0 K to 550 K.

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Zusammenfassung Neue Messungen und Literaturangaben von Polysiloxanen über Wärmekapazität, Umwandlungsparameter, Enthalpien, Entropien und Gibbssche Energien werden vorgestellt und kritisch betrachtet. Das Rechenverfahren ATHAS wurde benutzt, um die Wärmekapazitäten mit einem annähernden Frequenzspektrum in Einklang zu bringen. Es wurden die verschiedenen Kristall- und Mesophasen diskutiert. Die von ATHAS (1990) empfohlenen Werte sind wie folgt: Für Poly(dimethylsiloxan) beträgt der Glasumwandlungspunkt 146 K bei Zunahme der Wärmekapazität um 29.24 J/(K.mol). Die vollständing kristalline Probe schmilzt bei etwa 219 K mit einer Schmelzwärme von 2.75 kJ/mol. Für Poly(diethylsiloxan) beträgt der Glasumwandlungspunkt 135 K bei Zunahme der Wärmekapazität um 34.48 J/(K.mol). Die vollständig kristalline Probe wandelt sich bei 206.7 K um, die Fehlordnungswärme beträgt 2.72 kJ/mol. Die Umwandlung in einen wenig verstandenen viskosen Kristall, dessen thermodynamische Eigenschaften denen der Schmelze gleichen, erfolgt bei 282.7 K mit einer Umwandlungsenthalpie von 1.84 kJ/mol. Letztendlich verläuft das Schmelzen bei 308.5 K mit einem kleinen endothermen Effekt von etwa 231 J/mol. Wärmekapazitäten, Enthalpien, Entropien und Gibbssche Energien sind für den Bereich 0 K–550 K tabellarisch angegeben.

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, , , , . ATHAS , - . . ATHAS (1990) : 146 29,24 / ·. 219 2,75 /. 135 34,48 /·. 206,7 2,72 /. « » 282,7 1,84 /. 308,5 231 /. , , 0–550 .

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This work was supported by the National Science Foundation, Polymers Program, Grant #DMR 83-17097 and early work of J.P.W. was supported by the Am. Chem. Soc. Petroleum Research Found, Grant 12431-AC7. In addition at Oak Ridge National Laboratory the work was sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems Inc.