An Investigation of Structure–Property Relationships in Silicone‐Based Dielectric Electroactive Elastomers by Varying Stoichiometric Imbalance of the Network

Silicone‐based elastomers are promising materials for future dielectric elastomer actuators. To ensure optimum performance and the long‐term reliability of the actuators, it is essential to gain a fundamental understanding of the correlation between the elastomer's network structure and the mechanical and electrical responses of the material. For this purpose, mechanical and electrical tests are performed on a series of silicone elastomer films with different crosslinking densities, which are prepared by changing the stoichiometric imbalance of the network. It is determined that higher cross‐linking density leads to a higher elastic modulus and a longer fatigue lifetime, whereas reduced permittivity is observed because of lower chain mobility. Dielectric breakdown strength is also observed to increase in line with increasing cross‐linking density, and the variations in relation to the measured elastic modulus and permittivity agree well with the Stark–Garton model based on electromechanical instability.

     

Development and characterization of hydrocarbon polyol polyurethane and silicone magnetorheological polymeric gels

Magnetorheological polymeric gels (MRPG) have been developed for use in semi‐active magnetorheological fluid (MRF) dampers and other magnetorheological (MR) devices. The novel MRPGs are prepared by suspending iron particles in polymeric gels. Off‐state (i.e, no applied magnetic field) viscosity and settling behavior can be controlled through the selection of polymeric gels. In this study, tunable rheological properties were investigated with a piston‐driven flow type rheometer with a shear rate varying from 20 s^?1 to 6,000 s^?1. Silicone MRPG (with 84.5 wt % iron particles) has controllable viscosity and a high shear yield stress over a wide range of shear rates. Silicone MRPG (79.5 wt % iron particles) has the lowest viscosity of those studied. Polyurethane MRPG has the lowest settling rate. The order of addition of magnetic particles and polymer during the polymerization process affects the MRPG final off‐state apparent viscosity (80% increase in apparent viscosity for silicone MRPG polymerized after adding iron particles). This indicates that polymer gels modify the surface properties of the magnetic particles, causing interaction among particles. The dynamic shear yield stress is higher for fluids with better dispersion stability. Polyurethane MRPG, which has the lowest settling rate, has a high dynamic yield stress (23 kPa at 350 mT). Both dynamic and static shear stress values of the MRPGs were found to be similar in magnitude (5–8 kPa at 120 mT for silicone MRPG with 84.5 wt % iron particles and polyurethane MRPG), indicating that MRPGs can provide consistent performance in devices. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1176–1182, 2004     

DC current voltage characteristics of silicone rubber filled with conductive carbon black

Direct current (DC) current‐voltage (I‐V) characteristics of silicone rubber filled with conductive carbon black (CB) were studied at room temperature in the voltage range of 1–46 V. The current‐voltage relationship can be expressed as I = AV^B, where A and B are constants that show capability and property of electrical conduction, respectively. The I‐V curve can be divided into ohmic and nonohmic regions. In nonohmic region, B < 1, and the resistance increases with the rise of voltage. Higher CB loading leads to lower transforming voltage from ohmic to nonohmic region and much deviation from Ohm's law. The reason for this deviation is the unbalance between the heat generated and the heat loss of conductive silicone rubber during the measurement. When the heat effect is eliminated completely, the electrical conduction is ohmic. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 587–592, 2004     

Silicone‐Organic Resin Hybrid Matrix Composites

Summary: Glass fabric reinforced hybrid matrix composites of a toughened silicone resin and a vinyl ester resin were fabricated and their properties investigated. The hybrid composites consisted of multi‐layers of fiber reinforced silicone resins and vinyl ester resins. The toughened silicone resin, a crosslinkable phenylsilsesquioxane resin with high thermal and thermal oxidation resistance but relatively low T_g, was chosen to be the outer layers. The vinyl ester resin, with better strength, toughness and a much higher T_g than the toughened silicone resin, was used as the inner layers. A co‐cure process proved to establish a strong interface between the two in a hybrid composite. The hybrid composites had better flammability properties and much lower short term moisture absorption than the vinyl ester composites. The strength and modulus retention of the hybrid composites at elevated temperatures was higher than the composites using any single resin as the matrix. For example, when tested at 150 °C the flexural modulus and strength values of a twelve layer composite, with eight inner vinyl ester resin layers and four silicone outer layers, were almost an order of magnitude higher than the composite using the silicone resin alone, and were significantly higher than the one using vinyl ester resin alone. The room temperature short beam shear strength of the hybrid composites was also higher. DMA revealed that the inter‐diffusion of reactive components between the two resins was probably responsible for this synergistic effect, resulting in an α transition temperature of 182 °C for the hybrid composite, higher than that of either the silicone resin (85 °C) or the vinyl ester resin (162 °C).

     

Poly(dimethylsiloxane) coatings for controlled drug release. I. Preparation and characterization of pharmaceutically acceptable materials

The basic technology for forming crosslinked elastomers by the end linking of low molecular weight hydroxyl‐terminated poly(dimethylsiloxane) (PDMS) with tetraethoxysilane (TEOS) was modified to make it suitable for preparing coatings for controlled drug release. The requirements for this application included establishing conditions for the end linking that did not require the usual toxic tin or platinum catalysts; preparation of emulsions of small PDMS network particles that had extensive shelf‐lives and that could be coalesced into cohesive thin films or coatings; and the determination of the conditions to make such coatings pinhole free, mechanically robust, and of sufficient thermal stability for coating procedures at elevated temperatures. The approach taken consisted of preparing water‐based emulsions of the PDMS with sodium lauryl sulfate and restriction of the pH of the system to the acidic range with HCl. Evaporation of water from the emulsions resulted in elastomeric free‐standing films of the PDMS that were characterized using stress–strain isotherms in elongation and equilibrium swelling in toluene, both at room temperature. The mechanical properties of the films were found to improve with an increasing molar ratio of HCl/TEOS and an increase in the amount of TEOS (giving increased degrees of crosslinking). Conditions for the optimization of the thermal stabilities as gauged by thermogravimetic analysis were established, including beneficial effects from the introduction of the crosslinks. Scanning electron microscopy showed that the predominant morphology was void‐free films with very small, homogeneously dispersed silica particles from the hydrolysis of some of the TEOS. Additional work should be facilitated by the mechanisms for the crosslinking suggested by the experimental results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 658–666, 2003     

UV‐curable,oxetane‐toughened epoxy‐siloxane coatings for marine fouling‐release coating applications

BACKGROUND: UV‐curable coatings are promising candidates for environmentally friendly marine fouling‐release coatings. Cationic UV‐curable epoxy‐siloxane release coatings show good release performance but suffer from poor coating mechanical properties. A difunctional oxetane monomer, DOX, was co‐photopolymerized with an epoxy‐siloxane oligomer at loading levels from 10 to 40 wt% to obtain toughened fouling‐release coatings. RESULTS: The DOX‐toughened coatings showed enhanced cationic photopolymerization activity, solvent resistance and modulus. DOX‐toughened coatings (10 and 20 wt%) exhibited higher impact resistance. The DOX‐toughened coatings showed no leachate toxicity and the coatings were hydrophobic and non‐toxic to biofilm growth when analyzed with marine bacteria and algae. In general, 10 and 20 wt% DOX‐toughened coatings exhibited better marine bacteria and algae fouling‐release performance among the DOX‐toughened coatings. Pseudo‐barnacle shear release stress for the DOX‐toughened coatings increased with increasing DOX content. Live barnacle reattachment assay showed that 10 and 20 wt% DOX‐toughened coatings had comparable barnacle removal stress to commercial silicone reference coatings. CONCLUSIONS: DOX‐toughened (10 and 20 wt%) UV‐curable epoxy‐siloxane coatings exhibited enhanced mechanical properties and better overall marine fouling‐release performance among the toughened UV‐curable release coatings studied. Copyright © 2008 Society of Chemical Industry     

Self‐Repairable,High Permittivity Dielectric Elastomers with Large Actuation Strains at Low Electric Fields

A one‐step process for the synthesis of elastomers with high permittivity, excellent mechanical properties and increased electromechanical sensitivity is presented. It starts from a high molecular weight polymethylvinylsiloxane, P1 , whose vinyl groups serve two functions: the introduction of polar nitrile moieties by reacting P1 with 3‐mercaptopropionitrile ( 1 ) and the introduction of cross‐links to fine tune mechanical properties by reacting P1 with 2,2′‐(ethylenedioxy)diethanethiol ( 2 ). This twofold chemical modification furnished a material, C2 , with a powerful combination of properties: permittivity of up to 10.1 at 10⁴ Hz, elastic modulus Y_10% = 154 kPa, and strain at break of 260%. Actuators made of C2 show lateral actuation strains of 20.5% at an electric field as low as 10.8 V μm^–1. Additionally, such actuators can self‐repair after a breakdown, which is essential for an improved device lifetime and an attractive reliability. The actuators can be operated repeatedly and reversibly at voltages below the first breakdown. Due to the low actuation voltage and the large actuation strain applications of this material in commercial products might become reality.     

Dynamic properties of rubber vibration isolators and antivibration performance of a nanoclay‐modified silicone/poly(propylene oxide)–poly(ethylene oxide) copolymer with 20 wt % LiClO4 blend system

This research explores the interlayer effect, hysteresis behavior, and dynamic antivibration properties of a poly(propylene oxide)–poly(ethylene oxide) copolymer with 20 wt % LiClO₄ (PEL) for modified silicone [silicone/polymer electrolyte (SP)] blends with clay and organoclay in various amounts. The results show that the polymer chains of PEL expand the clay gallery distance from 1.21 to 1.84 nm. Clay, after it has been organically modified, is added to SP blends, and its gallery spacing shortens from 2 to 1.7 nm. From the compression hysteresis results, along with the increased content of unmodified clay, the antivibration performance of the blends is elevated. In the dynamic antivibration testing results, along with the addition of clay and organoclay, the dynamic ratio of the blends is decreased; thus, in the vibration isolation performance, there is evidence of an elevated effect. On the other hand, after the clay is modified with ammonium salt, its vibration isolation effect is not better than that of the unmodified clay. In terms of the formulation of the nanocomposites, when the concentration of the clay is less than 4 wt %, it has a better effect on the vibration isolation and antivibration effect of SP blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3713–3720, 2006     

Poly(dimethylsiloxane) coatings for controlled drug release. III. Drug release profiles and swelling properties of the free‐standing films

Previous reports in this series have described the preparation of stable poly(dimethylsiloxane) (PDMS) latices suitable for spray‐coating of drug tablets, as well as the mechanism of associated crosslinking reactions in PDMS emulsions. In the present investigation, in vitro evaluations were performed to study the effects of the amount of channeling agents, the addition of colloidal silica, and the pH of the dissolution media used. The study involved hydrochlorothiazide (as a marker drug) released from compressed tablets, which had been spray coated using PDMS latices with various polyethylene glycol (PEG) loadings as channeling agents. The dissolution results showed that coated tablets containing up to 25% (w/w dps) PEG could have constant release rates. Higher amounts of PEG resulted in nonlinear release patterns. The addition of colloidal silica decreased the rates of drug release. The pH of dissolution media affected the structures of the exposed PDMS films. Swelling tests were carried out to determine water uptake. Scanning electron microscopy and density measurements showed that the films obtained after soaking in higher‐pH media were more condensed, with corresponding changes in drug‐release rates. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 494–501, 2005     

The Science of Adhesive Bonding and its Transfer to Technology

Author's Note

This article is based, in part, on a lecture given by the author on the occasion of his receipt of The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3M at the Society's 14th Annual Meeting in Clearwater, Florida, U.S.A., February 1991. The author takes this opportunity to thank the Adhesion Society for the award and the 3M Corporation for its sponsorship. It is an honor to receive such a tribute from a society and a corporation that have done so much to advance the science and technology of adhesive bonding.