Physical Terms Links

Elasticity measurements can serve as a measure of the degree of interconnection in gels. Covalently cross-linked networks can be distinguished from physically cross-linked networks by the use of a technique termed mechanical spectroscopy [333]. Compression of gels has also been used to assess the physical structure [28,168,303]. 

Note 2 Physical aging, crystallization, physical cross-linking, and post-polymerization reactions are sometimes referred to as curing . Use of the term curing in these cases is discouraged. 

Put very simply, copal is young version of amber. There is no definite age at which copal turns into amber, as the process is continuous firom the moment the resin appears on the tree and begins to solidify. In physical terms, when the resin is sufficiently cross-linked and polymerised it becomes amber (see Chapter 13, Plastics ). In other words, the resin has dried out and hardened. This process takes thousands if not millions of years, and not all copal becomes amber as much of it disintegrates with time. Furthermore, as the process is such a long one it is not possible for us to follow it or to replicate it in a laboratory, so there is still much that is speculation. We know, however, that there are some instances of copal that have begun to look like, and take on, the properties of amber. 

Yeh and Tsai 104), fox example, present a comprehensive analytical solution for diffusion in an oscillating flow. They drop the v and w terms and use the average longitudinal velocity for u. This means that all the physical behavior linked with variability of these velocities must now be lumped into the equation coefficients. 

The ideal, classical, rabbeiy properties are displayed by pofymers cross-linked by valence bonds (main-chain bonds or sulphur bridges, see Introduction, Figure 0.1) these are termed chemical cross-links. Physical cross-links are also important in many useful rubbery materials. In pl ical cross-linking the chains are not chemically attached one to another, but are effectively pinned together in one of three ways ... 

The above represents the classical definition of an IPN. The term interpenetrating polymer network was coined before the extent or conseqnences of phase separation were fully realized. This article covers sequential and simultaneous types of IPNs made in bulk and also includes such materials as IPNs based on latexes and suspension-sized particles thermoplastic IPNs, which contain physical cross-links in one or both polymers, and hence may be (partly) soluble and a number of other closely related materials. 

For a perfect phantom network of functionality/, the front factor contains the term (f - 2)1 f, leading to equation (9.71), which considers chemical crosslinks of arbitrary functionality, but no physical cross-links. 

To explain the temperature dependence of the moduli, Gibson et al have considered that the crystalline sequences linking two or more crystal blocks act like short fibres in an aligned short fibre composite. Quantitative predictions of the temperature dependence of the axial modulus were obtained, which were in good agree.-nent with experimental data. In physical terms, the a - relaxation is considered to be a crystalline slip process and the 3 relaxation relates primarily to mobility of the non-crystalline regions. 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

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