Rubber-like elasticity cross-links

It is also possible to estimate the cross-link density from the stress-strain data, using the statistical theory of rubber-like elasticity [47,58]. For a swollen rubber the relationship is... 

The long chains of the rubber molecules become cross linked by reaction with the vulcanizing agent to form a three-dimensional molecular structure. This reaction transforms the soft and weak plastic-like rubber material into a strong elastic resilient product. 

Poly-(L-lactide-co-e-caprolactone) (PLCL) copolymer, was also synthesized by the ring-opening polymerization of L-lactide and e-caprolactone in the presence of Sn(Oct)2 (Fig. 3.5). PLCL is composed of a soft matrix of e -caprolactone moieties and hard domains of L-lactide units. The monomers used in this system differ greatly in mechanical properties and time to reach complete mass loss however, once physically cross-linked in specific monomer ratios, the copolymer system exhibits a rubber-like elasticity. The structure of PLCL was identified by NMR spectra in CDCI3 (Fig. 3.5). The methine protons of the lactide unit appear as two singlets at d 5.1-5.2 (a, a ) as a result of the sequence distribution of the lactyl and caproyl units, while the methylene protons of the caproyl unit adjacent to the ester group appear at d 4.0-4.2 (c, c ) and d 2.3-2.S (g, g ). 

As previously mentioned, further irradiation of polyethylene leads to cross-linking, which eliminates the normal melting point, and, above a temperature of 113°C, the material exhibits a rubber-like elasticity and, as a result, does not flow. The irradiated polethylene still possesses excellent electrical properties and is therefore capable of functioning as a dielectric between the normal operating temperature of conventional... 

Di-tert-alkyl and di-tert-aryl peroxides (e.g., di-tert-butyl peroxide and dicumyl peroxide) find some use in the production of heat-stable vulcanizates of natural rubber. However, the particular significance of these reagents is that their mode of reaction is rather straightforward and it has been possible to establish a direct quantitative relationship between the number of cross-links in a vulcanizate and its elastic modulus. This correlation is important to the theory of rubber-like elasticity. 

Figure 27.8 Desmosine and isodesmosine cross-links in elastin which connect four chains together to give a rubber-like elasticity...

One of the simplest ways to introduce the cross-links required for rubber-like elasticity is to carry out a copolymerization in which one of the comonomers has a functionality of three or higher [9, 17]. This method, however, has been used primarily to prepare materials so heavily cross-linked that they are in the category of relatively hard thermosets rather than elastomeric materials [18]. 

The initial modulus is determined in the limit of small strain. The initial portion of the force-length curve is usually reversible. The deformation of the disordered interlamellar region is involved and the lamellar structure remains essentially intact. Interpreting the modulus, in terms of the basic structural and molecular parameters that define a semicrystalline polymer, is complex. In this region of very small strain, the primary effect is a rubber-like elastic deformation, whereby chain entanglements and other topological features act as effective cross-links. The total system is constrained by the bounding lamellae and their broad basal planes. 

Since the epoxy resin binder is a rigid three-dimensicHial polymer, the findings should be interpreted from the point of view of a change in the density of the network cross-links. With this aim we carried out measurements of the equilibrium modulus of rubber-like elasticity of filled spedn ns. It was shown that the network density with the polymeric filler concentrations reduces. 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

Dispersion of an elastomer with extremely high stability to solvents, self-cross-linking, very elastic rubber-like film, compatible with most fillers. 

Another vivid example of the exceptional role of network topology is the unexpectedly high deformation abUity of hypercrosslinked polystyrenes under loading, which is usuaUy characteristic of conventional slightly cross-linked networks or linear polymers in the rubber elasticity state. Hypercrosslinked polymers, however, differ from the latter in that they retain their mobUity even at very low temperatures. In fact, hypercrosslinked materials do not exhibit typical features of polymeric glasses, nor are they typical elastomers. Their physical state thus cannot be described in terms of generaUy accepted notions. More likely, the hypercrosslinked networks demonstrate distinctly different, unique deformation and relaxation properties. 

The materials are tough and elastic below the glass-transition temperature of the outer blocks, the glassy domains acting as both cross-links and fillers for the rubbery matrix, which would otherwise behave like an unvulcanised rubber. SBS compositions usually have tensile strengths in the... 

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