Polymer such as rubber, suberin, melanin and lignin. Naturally occurring polymers such as cotton, starch and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on the market. Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of sulfur. Ways in which polymers can be modified include oxidation, cross-linking and endcapping. Especially in the production of polymers the gas separation by membranes has acquired increasing
Rubber toughening is relieved and voiding occurs. The energy absorption due to this increase in volume is theoretically negligible. Instead, it is the consequent shear band formation that accounts for increased toughness. Before debonding, as the strain increases, the rubber phases is forced to stretch further strengthening the matrix. Debonding between the matrix and the rubber reduces the toughness, creating the need for strong adhesion between the polymer and rubber phases. The damage competition theory models the relative contributions of shear yielding and craze failure, when both are present. there are two main assumptions: crazing, microcracks, and cavitation dominate in brittle systems,