Crosslink/Crosslinking influence

Grafting and crosslinking influence the transition temperature of polymers. As an example, it was observed that for cellulose triacetate or secondary cellulose acetate the glass transition increased with increasing molecular mass of the poly(methylmethacrylate) or poly(methacrylic add) grafted chain [42]. 

Vijayabaskar, V, Tikku, V. K., Bhowmick, A. K. (2006). Electron beam modification and crosslinking Influence of nitrile and carboxyl contents and level of unsaturation on structure and properties of nitrile rubber. Radiat. Phys. Chem., 75(7), 779-792. 

In the last three chapters we have examined the mechanical properties of bulk polymers. Although the structure of individual molecules has not been our primary concern, we have sought to understand the influence of molecular properties on the mechanical behavior of polymeric materials. We have seen, for example, how the viscosity of a liquid polymer depends on the substituents along the chain backbone, how the elasticity depends on crosslinking, and how the crystallinity depends on the stereoregularity of the polymer. In the preceding chapters we took the existence of these polymers for granted and focused attention on their bulk behavior. In the next three chapters these priorities are reversed Our main concern is some of the reactions which produce polymers and the structures of the products formed. 

It was concluded that the crosslinking reaction proceeded through initial alkylation of N-7 of adenine by the aziridine ring, followed by guanine N-7 alkylation by the epoxide (Figure 11.12). This seems inconsistent with Armstrong s earlier conclusions, but the very short oligonucleotides used by Saito et al. may influence the DNA alkylation chemistry. 

Crosslinked polymers are rather peculiar materials in that they never melt and they exhibit entropic elasticity at elevated temperatures. The present review on the influence of crosslink density is structured around model polymers of uniform composition but with widely varying numbers of crosslinks. The degree of crosslinking in the polymers was verified by use of the theory of rubber elasticity. 

The free volume model seems to be more adequate to describe the plasticization behaviour of the systems of lower amine content. According to Eq. (5), the higher is the change of the expansion coefficient the lower is the influence of the diluent volume fraction. The three TGDDM-DDS mixtures cured with 20, 30 and 50 PHR of hardener were characterized l2) by changes of the expansion coefficient at the glass transition, respectively, of 0.63, 1.08 and 2.94x 10 3 °C l. The more dense and stiffer resin crosslinked with 50 PHR of DDS should be, in principle, the less... 

Covalent chemical bonds that occur between macromolecules are known as crosslinks. Their presence and density have a profound influence on both the chemical and mechanical properties of the materials in which they occur. 

As we have seen previously the presence of crosslinks between macromolecules influences the way in which these materials respond to heat. Uncrosslinked polymers will generally melt and flow at sufficiently high temperatures they are usually thermoplastic. By contrast, crosslinked polymers cannot melt because of the constraints on molecular motion introduced by the crosslinks. Instead, at temperatures well above those at which thermoplastics typically melt, they begin to undergo irreversible degradation. 

Crosslinking is not the only feature that may influence solubility. Such features as crystallinity, hydrogen bonding, or the absence of chain branching may all increase the resistance of a given specimen of polymer to dissolve. Some of these features are discussed later in the chapter. 

This means that, as a result of the urea formation, higher functional crosslink molecules are formed, which may influence the physical properties of the paint during curing. 

There are over 400 different commercial alkyd resin formulations based on phthalic anhydride used in the coatings business. Alkyd resins for paints are made by reacting phthalic anhydride with a poly-alcohol (usually from naturally occurring sources rather than synthetic) that contains unreacted double bonds. The paint dries by the resin crosslinking through reaction of the double bonds under the influence of oxygen in the air. 

Allcock, H. R., Gebura, M., Kwon, S., and Neenan, T. X., Amphiphilic polyphosphazenes as membrane materials Influence of side group on radiation crosslinking, semipermeability, and surface morphology. Biomaterials. 19. 500, 1988. 

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