Sohd polymers

Environmental Aging. AH ceUular polymers are subject to a deterioration of properties under the combined effects of light or heat and oxygen. The response of ceUular materials to the action of light and oxygen is governed almost entirely by the composition and state of the polymer phase (22). Expansion of a polymer into a ceUular state increases the surface area reactions of the foam with vapors and Hquids are correspondingly faster than those of soHd polymer. 

Electrical Insulation. The substitution of a gas for part of a soHd polymer usuaUy results in large changes in the electrical properties of the resulting material. The dielectric constant, dissipation factor, and dielectric strength are aU generaUy lowered in amounts roughly proportional to the amount of gas in the foam. 

The successfiil synthesis of a transparent soHd polymer electrolyte (SPE) based on PEO and alkoxysilanes has been reported (41). The material possessed good mechanical properties and high electrical conductivity (around 1.8 x 10 S/cm at 25°C) dependent on the organic—inorganic ratio and PEO chain length. 

Three types of electrochemical water-spHtting processes have been employed (/) an aqueous alkaline system (2) a soHd polymer electrolyte (SPE) and (J) high (700—1000°C) temperature steam electrolysis. The first two systems are used commercially the last is under development. 

Solid Polymer E,kctroljte. The electrolyte in soHd polymer electrolyte (SPE) units is Nafion, a soHd polymer developed by Du Pont, which has sulfonic acid groups attached to the polymer backbone. Electrodes are deposited on each side of the polymer sheet. H" ions produced at the anode move across the polymer to the cathode, and produce hydrogen. The OH ions at the anode produce oxygen. These units have relatively low internal resistances and can operate at higher temperatures than conventional alkaline electrolysis units. SPE units are now offered commercially. 

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

Inversion ofMon cjueous Polymers. Many polymers such as polyurethanes, polyesters, polypropylene, epoxy resins (qv), and siHcones that cannot be made via emulsion polymerization are converted into latices. Such polymers are dissolved in solvent and inverted via emulsification, foUowed by solvent stripping (80). SoHd polymers are milled with long-chain fatty acids and diluted in weak alkaH solutions until dispersion occurs (81). Such latices usually have lower polymer concentrations after the solvent has been removed. For commercial uses the latex soHds are increased by techniques such as creaming. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

The polymer is then dried thoroughly and stored for subsequent processing. Whenever a polyester is made by melt polycondensation, a small amount of cycHc oligomer is formed which is in equiHbrium with the polymer. This can be extracted with solvents from soHd polymer but when the... 

Formaldehyde. Pure formaldehyde, CH2O, is a colorless, pungent smelling reactive gas (see Formaldehyde). The commercial product is handled either as soHd polymer, paraformaldehyde (13), or in aqueous or alcohoHc solutions. Marketed under the trade name Formcel, solutions in methanol, / -butanol, and isobutyl alcohol, made by Hoechst-Celanese, are widely used for making alcohol-modified urea and melamine resins for surface coatings and treating textiles. 

Even in the presence of considerable moisture, soHd polymer never forms above 30°C (23). Below 30°C, Hquid stabiUty decreases with increasing moisture and decreasing temperature (23). The actual formation of soHd polymer has been hypothesized to involve the formation in the Hquid of high molecular weight polysulfuric acids, followed by precipitation. 

A second class of important electrolytes for rechargeable lithium batteries are soHd electrolytes. Of particular importance is the class known as soHd polymer electrolytes (SPEs). SPEs are polymers capable of forming complexes with lithium salts to yield ionic conductivity. The best known of the SPEs are the lithium salt complexes of poly(ethylene oxide) [25322-68-3] (PEO), —(CH2CH20) —, and poly(propylene oxide) [25322-69-4] (PPO) (11—13). Whereas a number of experimental battery systems have been constmcted using PEO and PPO electrolytes, these systems have not exhibited suitable conductivities at or near room temperature. Advances in the 1980s included a new class of SPE based on polyphosphazene complexes suggesting that room temperature SPE batteries may be achievable (14,15). 

Fig. 1. Configuration for a soHd polymer electrolyte rechargeable lithium cell where the total thickness is 100 pm.

Diborane reacts with ethylene oxide at —80° C to form diethoxyborane and a soHd polymer containing approximately eight ethylene oxide units per molecule (88). Potassium thiocyanate or thiourea react ia aqueous solution with ethylene oxide to give ethylene sulfide (89). 

K. O Connor, T. McLeish. Entangled dynamics of heahng end-grafted chains at sohd/polymer interface. Faraday Discuss Chem Sci 95 67-78, 1994. 

Another factor also contributed to the appearance of new concepts in electrochemistry in the second half of the twentieth century The development and broad apphca-tion of hthium batteries was a stimulus for numerous investigations of dilferent types of nonaqueous electrolytes (in particular, of sohd polymer electrolytes). These batteries also initiated investigations in the held of electrochemical intercalation processes. 

Ward IM (1983) Mechanical properties of sohd polymers, 2nd edn. John WHey, New York... 

The recent development of using polymerizable surfactants in microemulsion polymerizations has enabled the production of transparent sohd polymers with some nanostructure. Randomly distributed bicontinuous nanostructures of water channels and polymer domains in sohd polymers can be readily obtained from the polymerization of bicontinuous microemulsions consisting of various types of vinyl monomers and polymerizable surfactants with no allyhc hydrogen. 

The craze front velocity v can be governed by one of two distinct mechanisms of craze matter production. As Argon and Salama have discussed in detail, under the usual levels of service stresses or stresses under which most experiments are carried out, craze matter in single phase homopolymer is produced by the convolution of the free surface of the sohd polymer at the craze tip. This occurs by a fundamental interface instability present in the flow or deformation of all inelastic media when a concave, meniscus-like surface of the medium is being advanced locally by a suction gradient. This is the preferred mechanism of craze advance in homopolymers. In block copolymers with uniform distributions of compliant phases of a very small size, and often weaker interfaces than either of the two phases in bulk, craze advance can also occur by cavitation at such interfaces to produce craze matter as has been discussed by Argon et al. Both of these mechanisms of craze advance lead to very similar dependences of the craze front velocity on apphed stress and temperature that is of the basic form... 

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