Stiffening degradation

Construction. Polymeric materials such as adhesives, sealants, and composites have been used considerably in the last several decades for the construction, repair, and rehabilitation of our transportation infrastructures. Even though most processes were experimental until recently, they have evolved to the point where many are now standardized and well accepted. Table 1.6 hsts several common applications for advanced polymeric materials (as well as the polymeric resins that are most commonly employed). In the construction or repair of roads and bridges, epoxy adhesives have primarily been used for bonding concrete and for bonding stiffening members or repair structures to degrading concrete facilities. 

Draining of Consolidants Effect of Concentration. As noted in the introduction, one objective of the studies described here is to examine techniques for applying consolidants under conditions that will reduce the physical hazards to which the fabric substrate is exposed. Since the acrylic resins examined in Part II of this series (I) have outstanding resistance to degradation from UV light and heat and normally do not yellow or stiffen under adverse conditions, some of these resins were re-examined using a much less severe application technique. Milder curing procedures also were utilized. 

Reversion and ring formation have been partially overcome through placement of chain-stiffening units in the silicone-backbone. Thus linear D2-m-carborane-siloxanes (11—14) with one to three trifluoropropyl moieties per repeating unit) exhibit better thermal and oxidative stability than silicones and fluorosilicones (J ). Initial degradation occurs in air about 300 to 350 C, almost 100 above that typically experienced for siloxanes and fluorosilicones. The carborane-siloxanes exhibit T s from -50 to 0 C ( 1 ). The carborane moiety also acts to inhibit formation of six-membered rings because of its size. 

Triple helix formation first of all stiffens the duplex, thus promoting duplex degradation by UV-rays, radicals or metal chelates. Impeded methylation of a DNA sequence by triple helix formation renders the non-methylated DNA sequence a good substrate for endonucleases. It... 

The glass-transition temperature depends on the mobility of the chain segments and can therefore be raised by stiffening the chain (see Section 10.5.3). Thus, a-methyl styrene forms a polymer that, in contrast to poly-(styrene), does not deform at lOC C, because of a glass-transition temperature of 170°C. However, since the thermodynamic ceiling temperature for for the polymerization/depolymerization equilibrium is also simultaneously lowered (see Section 16.3), poly(a-methyl styrene) degrades more easily than poly(styrene), so that it is not so easy to work by injection molding. 

This issue of specific surface area hints at how one might change the nature of reinforcement. In typical micro- and macrocomposites, the properties are dictated by the bulk properties of both the matrix and the flUer. This relationship between the properties of the composite and the properties of the filler is what leads to the stiffening and degraded elongation mentioned earlier. In the case of nanocomposites, the properties of the material are instead tied to the interface. Terms like bound polymer, bound rubber, and interphase have been used to describe the polymer at or near the interface, where significant deviations from bulk structure and properties are known to occur (Fig. 6.2). 

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