Natural rubber structure thermoplastic elastomers

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Chattopadhyay S., Chaki T.K., and Bhowmick A.K., New thermoplastic elastomers from poly(ethyle-neoctene) (engage), poly(ethylene-vinyl acetate) and low-density polyethylene by electron beam technology structural characterization and mechanical properties. Rubber Chem. TechnoL, 74, 815, 2001. Roy Choudhury N. and Dutta N.K., Thermoplastic elastomeric natural rubber-polypropylene blends with reference to interaction between the components. Advances in Polymer Blends and Alloys Technology, Vol. 5 (K. Finlayson, ed.), Technomic Publishers, Pensylvania, 1994, 161. 

In the same manner as blends of thermoplastics or thermoplastic/ elastomer, irradiated blends of elastomers can undergo chain scission due to degradation or cross-linking, depending especially on the dose range. For example, Zurina et al. i measured tanS versus temperature for 50/50 epoxidized natural rubber (ENR-50)-EVA blends by DMA. At 60 kGy the irradiation-induced cross-link enhanced the Tg of the blend, whereas at higher dose (100 kGy), the Tg decreased due to the occurrence of oxidative degradation that broke the cross-link structure. 

The thermoplastic elastomers (TPE) are a new class of the polymeric materials, which combine the properties of the chemically cross-linked rubbers and easiness of processing and recycling of the thermoplastics [1-8], The characteristics of the TPE are phase micrononuniformity and specific domain morphology. Their properties are intermediate and are in the range between those, which characterize the polymers, which produce the rigid and elastic phase. These properties of TPE, regardless of its type and structure, are a function of its type, structure and content of both phases, nature and value of interphase actions and manner the phases are linked in the system. 

The structure of thermoplastic rubber is unique. It does not behave like conventional elastomers such as natural rubber or styrene-butadiene rubber (SBR) in a number of important respects. The next several sections explain the fundamental principles of its behavior. 

Yamauchi, K., Akasaka, S., Hasegawa, H., Koizumi, S., Deepraserlkul, C., Laokijcharoen, P. et al. (2005) Structural study of natural rubber thermoplastic elastomers and their composites with carbon black by smaH-angle neutron scattering and transmission electron microscopy. Composites Part A Applied Science and Manufacturing, 36, 423—429. 

Control of enantiomorphic selectivity in polymerization of the substituted oxiranes can lead to controlled-structure polymers, the properties of which will range from crystalline thermoplastics to amorphous elastomer precursors such as are used as soft segments in polyurethanes. Crystallizable sequence distributions in highly controlled-structure polymers can lead to thermoplastic elastomers and/or to elastomers that will stress-crystallize that is, crystallize on stretching as does natural rubber (79). 

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