Polyphenylenes, stability

Polymers with exceptional heat stability which require special fabrication techniques such as the polyphenylenes. These materials form part of a group of exceptionally heat-stable materials which will be considered further in Chapter 29. 

Polyphenylene sulfide PPS is able to resist 450°F (232° C), and has good low temperature strength as well. It has low warpage, good dimensional stability, low mold shrinkage. Use includes hair dryers, cooking appliances, and critical under-the-hood automotive and military parts. 

Color Urea, melamine, polycarbonate, polyphenylene oxide, polysulfone, polypropylene, diallyl phthalate, and phenolic are examples of what is needed in the temperature range above 200°F (94°C) for good color stability. Most TPs will be suitable below this range. 

Dimensional stability There is plastics with very good dimensional stability, and they are suitable where some age and environmental dimensional changes are permissible. These materials include polyphenylene oxide, polysulfone, phenoxy, mineral-filled phenolic, diallyl phthalate, epoxy, rigid vinyl, styrene, and various RPs. Such products will gain from an after-bake for dimensional stabilization. Glass fillers will improve the dimensional stability of all plastics. 

Polyphenylene oxide modified Very fough engineering plastic, superior dimensional stability, low moisture absorption, excellent chemical resistance Injection molding... 

Modified-polyphenylene oxide (or ether) is a blend of high impact polystyrene (PS) and polyphenylene oxide (PPO), plus thermal stabilizers and a triarylphosphate flame retardant. Studies of the mechanism of the flame retardant in modified-polyphenylene oxide have shown some evidence for both solid phase and vapor phase inhibition (4). Indeed, one is always interested to know whether flame retardant action is on the solid or vapor phase. 

An initial experiment involved determination of Arapahoe Smoke Chamber results for samples with and without the zinc coating present. Data are presented in Table II. Depending upon orientation of the sample, an increase in char occurred for some samples with zinc present, while no change in smoke formation was seen. Initial pyrolysis GC/mass spectroscopy results at 90CPC in helium showed no difference in volatiles formed with or without zinc. These results suggested enhanced char formation as the origin of the Radiant Panel results for zinc on modified-polyphenylene oxide (m-PPO). Zinc oxide is a known, effective thermal stabilizer in the alloy. The next work then focused on DSC/TGA studies. 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene... 

The thermal stability of hyperbranched polymers is related to the chemical structure in the same manner as for linear polymers for example, aromatic esters are more stable than aliphatic ones. In one case, the addition of a small amount of a hyperbranched polyphenylene to polystyrene was found to improve the thermal stability of the blend as compared to the pure polystyrene [31]. 

Since the Diels-Alder reaction takes place at high temperatures only, an important requirement of the functional groups is their thermal stability, which in some cases necessitates protection. So far we were able to decorate our polyphenylene dendrimers via this method with various functional groups including methoxy-, amino-, cyano-, halogen-, thiomethyl-, perylenemonoimidyl-, and thiophenyl. 

The attachment of a distinct number of fluorescent dyes at the periphery of a stiff three-dimensional nanoparticle is of fundamental interest to study interactions between single chromophores in close vicinity to each other. We chose as a dye one of the rylene series, perylenemonoimide, for the decoration of our polyphenylene dendrimers due to their outstanding properties. Rylene dyes show an exceptional chemical and photochemical stability and therefore are well suited for single molecule spectroscopy (SMS) [661. 

A key feature of our polyphenylene dendrimers is that they can be planarized and thus reduced in dimensionality by intramolecular dehydrogenation [29,35]. This results in large, fused polycyclic aromatic hydrocarbons (PAHs). PAHs serve as structurally distinct, two-dimensional subunits of graphite and show attractive properties such as high charge carrier mobility, liquid crystallinity, and a high thermal stability, which qualifies these materials as vectorial charge transport layers [81]. 

Polymerization Solvent. Sulfolane can be used alone or in combination with a cosolvent as a polymerization solvent for polyureas, polysulfones, polysiloxanes, polyether polyols, polybenzimidazoles, polyphenylene ethers, poly(l,4-benzamide) (poly(imino-l,4-phenylenecarbonyl)), silylated poly(amides), poly(arylene ether ketones), polythioamides, and poly(vinylnaphthalene/fumaronitrile) initiated by laser (134—144). Advantages of using sulfolane as a polymerization solvent include increased polymerization rate, ease of polymer purification, better solubilizing characteristics, and improved thermal stability. The increased polymerization rate has been attributed not only to an increase in the reaction temperature because of the higher boiling point of sulfolane, but also to a decrease in the activation energy of polymerization as a result of the contribution from the sulfonic group of the solvent. 

As already shown, the sensitized spectra follow the dye absorption. Most of the dyes have sufficiently narrow absorption bands. This does not permit us to obtain the panchromatic sensitivity in the sufficiently broad spectral range. It was proposed to use the polymers with conjugated bonds as sensitizers [21]. The broad diffuse absorption spectra are inherent to such compounds. One can expect higher thermal stability from such sensitizers. In addition the application of binder may be omitted from the preparation of the photosensitive layers, for example, in electrophotography. Polymers with triple bonds, polyphenylenes and polyoxiphenylenes were used as sensitizers [10, 14, 278-280]. The typical results are shown in Fig. 47. The main rules for photoconductivity sensitized by polymers were the same as for the dyes. Optimum sensitization was obtained at the concentration of the sensitizer of 10 1-10-2 g/cm3 relative to the polymeric photoconductor weight. 

Since the thermal degradation in phenylated polyphenyls of the type XXa is caused by the loss of pendant phenyl groups, and since the reported (4, 7, 9, 10, 11, 14, 15) properties of p-polyphenylenes are quite different from those of XXa, the synthesis of an unphenylated polyphenylene by this pathway was of considerable significance. Only a few results employing this reaction have thus far been obtained. [The p-poly-phenylenes reported are black or brown, insoluble, crystalline materials of lower thermal stability than XXa.]... 

This crystalline aromatic nylon, combines the high strength and stiffness of nylon with the thermal stability of polyphenylene sulfide. Molding characteristics are similar to nylon 6/6, with similar or better chemical resistance, but its 24 h water absorption is only 0.2 versus 0.7% for nylon 6/6. A key behavior is high heat resistance. 

The most popular materials are styrenics and olefins, and engineering plastics such as modified polyphenylene ether or polycarbonate (Chapter 2). Fillers for enhanced physical properties, UV stabilizers, and flame retardants are common additives. 

MABS polymers (methyl methacrylate-acrylonitrile-butadiene-styrene) together with blends composed of polyphenylene ether and impact-resistant polystyrene (PPE/PS-I) also form part of the styrenic copolymer product range. Figure 2.1 provides an overview of the different classes of products and trade names. A characteristic property is their amorphous nature, i.e. high dimensional stability and largely constant mechanical properties to just below the glass transition temperature, Tg. 

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