Networks, Elastomeric

The above-mentioned general features of elastomeric materials have long been known and, in fact, the area of rubber-like elasticity has had one of the longest and most distinguished histories in all of polymer science (1,2,16). Forex-ample, quantitative measurements of the mechanical and thermodynamic properties of natural rubber and other elastomers go back to 1805, and some of the earliest studies have been carried out by such luminaries as Joule and Maxwell. Also, the earliest molecular theories for polymer properties of any kind were, in fact, addressed to the phenomenon of rubber-like elasticity. 

Some polymers are not elastomeric under normal conditions but can be made so by raising the temperature or adding a diluent ( plasticizer ). Polyethylene is in this category because of its high degree of crystallinity. Polystyrene, poly(vinyl chloride), and the biopolymer elastin are also of this type, but because of their relatively high glass transition temperatures also require plasticization to be elastomeric (13). 

A final class of polymers is inherently nonelastomeric. Examples are polymeric sulfur, because of its chains are too unstable, poly(p-phenylene) because its chains are too rigid, and thermosetting resins because their chains are too short (13). 

There is much current interest in hydrogels, particularly those robust enough to be used in biomedical applications. 

Random Chemical Cross-Linking. In random cross-linking, chemical reactions are used that attack a pair of chains, at essentially random locations (1,13). Examples are the addition of sulfur atoms to the double bonds of diene elastomers and the attack of free radicals from peroxide thermolyses or high-energy radiation on side chains (frequently unsaturated) or on the chain backbone itself. 

---

Sharaf, M. A. Mark, J. E. Hosani, Z. Y. A., Regular Bimodal Polydimethylsiloxane Networks. Elastomeric Properties of the Tetrafunctional Networks. 

BirGfring6nC6 of Polymer Networks. Elastomeric polymer networks deserve special mention because their cross-linked structure gives them unique physical properties, unlike those of other polymers. Covalently bonded networks are insoluble in any solvent, even in those that dissolve their precursor polymers. Optical techniques such as strain-induced birefringence allow the development of structure-property relationships as well as the study of their optical properties. Here we review some of the classical theories as well as some of the latest developments in the field. 

Sharaf MA, Mark JE, Hosani ZYA. Regular bimodal polydimethylsiloxane networks. Elastomeric properties of the tetrafunctional networks. Eur Polym J 1993 29 809-17. 

---