Elasticity poly , cross-linked

The Nature of the Elasticity of Cross-linked Poly(GVGVP)... 

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

Figure 5.18 Cross-linking profile of poly-CA using the triazine-based cross-linking agent. Filled symbols denote the elastic modulus (G ) whereas empty symbols denote the loss modulus (G") at a strain value of 0.1. The percentage of the triazine-based cross-linking agent is based on the cyanuric acid groups attached to the polymer.

According to the statistical-mechanical theory of rubber elasticity, it is possible to obtain the temperature coefficient of the unperturbed dimensions, d InsjdT, from measurements of elastic moduli as a function of temperature for lightly cross-linked amorphous networks [Volken-stein and Ptitsyn (258 ) Flory, Hoeve and Ciferri (103a)]. This possibility, which rests on the reasonable assumption that the chains in undiluted amorphous polymer have essentially their unperturbed mean dimensions [see Flory (5)j, has been realized experimentally for polyethylene, polyisobutylene, natural rubber and poly(dimethylsiloxane) [Ciferri, Hoeve and Flory (66") and Ciferri (66 )] and the results have been confirmed by observations of intrinsic viscosities in athermal (but not theta ) solvents for polyethylene and poly(dimethylsiloxane). In all these cases, the derivative d In sjdT is no greater than about 10-3 per degree, and is actually positive for natural rubber and for the siloxane polymer. 

The protection of electronic devices has been a key application for specialty silicones, and this application continues to keep pace with the rate of device development (5). Silicones are used in various ways, ranging from resinous circuit board coatings to encapsulants, with the silicone gels representing a unique solution to a diflScult problem, stress relief These dielectric gels are prepared by hydrosilation and are lightly cross-linked poly(dimethylsiloxane)s. Their modulus is extremely low, but they are elastic in their behavior. They have the stress-relief characteristic of a liquid but the nonflow property of an elastomer. These jellylike materials maintain their physical profile over the broad temperature range of-80 to 200 °C. 

Figure 9 shows the temperature dependence of the elastic part of the complex Young s modulus E a>) for various poly(vinyl methyl ether) hydrogel samples in water at a selected frequency of 20.1 rad/s. The polymer was cross-linked by electron beam irradiation (see Sect. 2.4, chapter Synthesis of hydrogels ). These data were compared with the temperature shrinking behaviour of sample PVME 20/80... 

Elastin is an elastic insoluble protein in connective tissue, with cross-linked tropoelastin as a major component. It is responsible for the contraction of skin, lung, and vascular tissues in our body [42]. The insolubility and enhanced immune response [43] of native elastin limits its biomedical application and demands new materials for tissues. The ability of elastin to maintain minimum platelet interaction makes it a suitable material for making biological coatings for synthetic vascular grafts [44,45]. Currently used synthetic vascular biomaterials like poly(ethylene tere-phthalate) (Dacron) and poly(tetrafluoroethylene) show... 

SMP based on miscible blends of semicrystalline polymer/amorphous polymer was reported by the Mather research group, which included semicrystalline polymer/amorphous polymer such as polylactide (PLA)/poly vinylacetate (PVAc) blend [21,22], poly(vinylidene fluoride) (PVDF)/PVAc blend [23], and PVDF/polymethyl methacrylate (PMMA) blend [23]. These polymer blends are completely miscible at all compositions with a single, sharp glass transition temperature, while crystallization of PLA or PVDF is partially maintained and the degree of crystallinity, which controls the rubbery stiffness and the elasticity, can be tuned by the blend ratios. Tg of the blends are the critical temperatures for triggering shape recovery, while the crystalline phase of the semicrystalline PLA and PVDF serves well as a physical cross-linking site for elastic deformation above Tg, while still below T ,. 

A hydrophilic polymer (especially the aoss-linked form) may transition from hard and rigid to soft and elastic when immersed in aqueous media. A good example of this is cross-linked poly(2-hydroxyethyl methacrylate) (pHEMA), the original soft contact lens polymer. When dehydrated, pHEMA is a hard, hrittle polymer. When hydrated, it is a soft elastomer. The hydrated (swollen) form of cross-linked pHEMA contains about 40% by weight of water. Polymers that swell to an equiUhrium level in aqueous solutions are referred to as hydrogels. 

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