Silica polymers reinforced with

The focus of this review is the properties of polymers reinforced with silica, however, the synthetic approaches of incorporating silica into polymers will also be briefly discussed. 

Modification of the polymer backbone by the incorporation of trifluoropropyl groups leads to substantial decreases in swelling. In vulcanized systems reinforced with hydrophilic silica (30 phr) the swelling decreased with increasing CH2CH2CF3 content as shown in Table III. 

The dynamic response of polydimethylsiloxane (PDMS) reinforced with fused silica with and without surface treatment has been discussed in terms of interactions between the filler and polymer [54]. Since bound rubber measurements showed that PDMS chains were strongly attached to the silica surface, agglomeration due to direct contact between silica aggregates was considered an unlikely explanation for the marked increase in storage modulus seen with increasing filler content at low strains. Instead three types of flller-polymer-flller association were proposed which would cause agglomeration, as depicted in Fig. 15. 

Improvement of the mechanical properties of elastomers is usually reached by their reinforcement with fillers. Traditionally, carbon black, silica, metal oxides, some salts and rigid polymers are used. The elastic modulus, tensile strength, and swelling resistence are well increased by such reinforcement. A new approach is based on block copolymerization yielding thermoelastoplastics, i.e. block copolymers with soft (rubbery) and hard (plastic) blocks. The mutual feature of filled rubbers and the thermoelastoplastics is their heterogeneous structure u0). 

Historically, polysiloxane elastomers have been reinforced with micron scale particles such as amorphous inorganic silica to form polysiloxane microcomposites. However, with the continued growth of new fields such as soft nanolithography, flexible polymer electronics and biomedical implant technology, there is an ever increasing demand for polysiloxane materials with better defined, improved and novel physical, chemical and mechanical properties. In line with these trends, researchers have turned towards the development of polysiloxane nanocomposites systems which incorporate a heterogeneous second phase on the nanometer scale. Over the last decade, there has been much interest in polymeric nanocomposite materials and the reader is directed towards the reviews by Alexandre and Dubois (4) or Joshi and Bhupendra (5) on the subject. 

Organic matrices are divided into thermosets and thermoplastics. The main thermoset matrices are polyesters, epoxies, phenolics, and polyimides, polyesters being the most widely used in commercial applications (3,4). Epoxy and polyimide resins are applied in advanced composites for structural aerospace applications (1,5). Thermoplastics Uke polyolefins, nylons, and polyesters are reinforced with short fibers (3). They are known as traditional polymeric matrices. Advanced thermoplastic polymeric matrices like poly(ether ketones) and polysulfones have a higher service temperature than the traditional ones (1,6). They have service properties similar to those of thermoset matrices and are reinforced with continuous fibers. Of course, composites reinforced with discontinuous fibers have weaker mechanical properties than those with continuous fibers. Elastomers are generally reinforced by the addition of carbon black or silica. Although they are reinforced polymers, traditionally they are studied separately due to their singular properties (see Chap. 3). 

Experiments were performed on DC745U and M97 silica-filled silicone polymers as described elsewhere [1-5]. The gum stocks for all formulations were co-block polymers of dimethylsiloxane, diphenylsiloxane, methylphenylsiloxane, and/or methylvinyl siloxane. The gum stock was reinforced with high surface area silica filler and crosslinked with peroxide curing agents. These materials were tested in both new as well as service return conditions. 

Many of the silicone elastomers that are used in biomedical applications are produced by Dow Chemical Corp., under the trade name SILASTIC . For example, a typical medical-grade silicone (like SILASTIC MDX4-4210 Medical grade elastomer) contains, after curing, cross-linked drmethylsiloxane polymer and silica for reinforcement. Silcones are also reinforced with PET (Dacron) fiber meshes for certain biomedical applications. For implantable medical devices, it is important to realize that the cured polymer contains residual catalysts and silicone cross-linkers, which are necessary for the polymerization. 

A new approach to combating iodine deficiency through controlled release of iodide by a silicone elastomer has been reported (Fisch et ai, 1993). Silicone elastomers or silicone rubbers made from cross linked polymers are reinforced with silica. Silicones, being chemically inert, are safe and stable over a wide range of temperatures and have a high permeability to small molecules. Sodium iodide, being a small molecule, can be intercalated into the silicone matrix from where iodine is released in a controlled manner when water is passed through it. 

Dynamic mechanical thermal analysis of several of the norbornene functional organic resins and the silicone resins gave relatively unremarkable results. The maximum for tan 5 peaks were in good agreement with Tg detamiined by DSC. The silicone elastomer (with 35% fumed silica as reinforcing filler) exhibited a Tg of ca. -90°C and a Tm at ca. -30 C which is typical for this type of polymer. 

Polymers II a-f were found by X-ray diffraction to be noncrystalline amorphous materials. Similar structured polymers were prepared for free radical vulcanization by the introduction of vinyl crosslinking sites. The polymers were formulated Into high consistency elastomers reinforced with silica and were free radical vulcanized. The properties for only lib and Ild are shown In Table I with a commercial elastomer prepared from polymethyl (3,3,3-tr1flu-oropropyl)s11oxane (LS) shown for comparison. Also included Is an elastomer prepared from the following copolymer (III),... 

Since it is silica-based, glass fiber for polymer reinforcement could be thought of as a cousin to mineral fillers. But glass fiber is more carefully produced in controlled, uniform, and symmetrical shapes with extremely high aspea ratios, with particle dimensions that are (usually) visible to the human eye. Glass-fiber reinforcement is probably the most cost-effective and most proven way of reinforcing polymers to inaease tensile and flexural modulus and strength. 

Experimental evidences indicate that the extent of the reinforcement depends strongly on the particle size. The maximum reinforcement is obtained for particles with diameters ranging from 10 to 100 nm. Although polymers filled with such nanoscaled silica fillers, i.e., polymer-silica nano-... 

Non-linear viscoelastic properties were observed for fumed silica-poly(vinyl acetate) (PVAc) composites, with varying PVAc molar mass and including a PVAc copolymer with vinyl alcohol. Dynamic mechanical moduli were measured at low strains and found to decrease with strain depending on surface treatment of the silica. The loss modulus decreased significantly with filler surface treatment and more so with lower molar mass polymer. Copolymers with vinyl alcohol presumably increased interactions with silica and decreased non-linearity. Percolation network formation or agglomeration by silica were less important than silica-polymer interactions. Silica-polymer interactions were proposed to form trapped entanglements. The reinforcement and nonlinear viscoelastic characteristics of PVAc and its vinyl alcohol copolymer were similar to observations of the Payne effect in filled elastomers, characteristic of conformations and constraints of macromolecules. ... 

Carbon fiber has been found to be an effective thermistor [192-194], such as a cement paste reinforced with chopped carbon fiber (about 5 mm long) with silica fume (15 wt% cement). Its electrical resistivity decreased reversibly with increasing temperature (1-45°C), with activation energy of electrical conduction (electron hopping) of 0.4 eV. This value is comparable to semiconductors (typical thermistor materials) and is higher than that of carbon fiber polymer matrix composites. The current-voltage characteristics of carbon fiber reinforced silica fume cement paste were linear up to 8 V at 20°C. 

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