The rubber elastic state

Natural rubber is obtained as a latex from a tree called Hevea Braziliensis. It consists predominantly of cis-lA-polyisopropene (Fig. 3.1). The word rubber is derived from the ability of this material to remove marks from paper, which was noted by Priestley in 1770. Rubber materials are not, however, restricted to natural rubber. They include a great variety of synthetic polymers of similar properties. An elastomer is a polymer which exhibits rubber elastic properties, i.e. a material which can be stretched to several times its original length without breaking and which, on release of the stress, immediately returns to its original length. That is to say, its deformation is reversible. 

A very illuminating experiment is to subject a rubber band to about 100% strain by hanging a dead load on to it and then heat the rubber band. The elongation of the rubber band will suddenly decrease when it is heated. This may first seem anomalous, but after reading this chapter you will understand the reason. 

From a historical perspective, the accomplishment by Charles Goodyear in 1839 of a method to vulcanize natural rubber with sulphur was a crucial breakthrough. Sulphur links attached to the ns-1,4-polyisoprene molecules formed the network structure which is a prerequisite for obtaining elastic properties (Fig. 3.5). 

Later development of vulcanization technology has involved peroxide crosslinking and thermoplastic elastomers. The latter consist of block copolymers with hard segments (physical crosslinks) and flexible segments (Fig. 3.6). The crosslink domains are either glassy amorphous or crystalline. These materials can be processed by conventional thermoplastic process- 

---

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 flexibilizer markedly modifies the relaxation behaviour of the epoxy resin systems shown in Figs. 20 and 21. Its incorporation results in crosslinked two-phase systems. But the two phases are compatible and therefore not clearly separate. The Tg of the resin-hardener matrix (a relaxation) is smoothly passing into the Tg of the incorporated oligoester (a2 relaxation). The maximum of the relaxation shifts to lower temperatures as the flexibilizer content of the system is increased. The QL2 relaxation always occurs at nearly the same characteristic temperature, as is evident from the modulus decay at about 240 K in Figs. 20 and 21. The reduction in modulus observed in the rubber-elastic state shows the decrease in crosslinking density caused by an increase in flexibilizer content. ... 

Scanning tunneling microscopy Light-scattering Ion beam analysis (forward recoil spectrometry) Chaudhury-Whitesides apparatus Atomic resolution, relatively inexpensive Determines interior surface areas Surface composition and diffusion coefficients can be determined Measures adhesion forces nondestructively Resolution limited with insulating materials Secondary scattering must be eliminated Length scale resolution 150 A at best Polymer must be in the rubber-elastic state... 

---