ABSTRACT
Crystallization is a process in which regular molecular structures (crystallites) are generated from amorphous polymer chains. There are two types of crystallization in natural rubber: cold crystallization and Strain-Induced Crystallization (SIC). In cold crystallization, after long exposure to low temperatures, close polymer chains in the rubber network align together so that the unstrained rubber partially crystallizes. The rate of cold crystallization in natural rubbers is maximal at −26°C (Bekkedahl & Wood 1941). The high regularity of the side group orientations in polymer chains allows natural rubbers to crystallize when undergoing large deformation. At room temperature, fibrillar crystals form due to the alignment of some segments of polymer chains along the stretching direction (Che, Burger, Toki, Rong, Hsiao, Amnuaypornsri, & Sakdapipanich 2013). The number of crystallites increases with the extension. It leads to a greater network density (Flory 1947) and causes a remarkable stress upturn after the onset of crystallization. The strain-induced crystallization is reversible, the crystallites disappear at the end of unloading (Murakami, Senoo, Toki, & Kohjiya 2002, Toki, Sics, Ran, Liu, Hsiao, Murakami, Senoo, & Kohjiya 2002). While there are plenty of studies related to quasi-static response of deformation-induced crystallization, there have been relatively few works on SIC under dynamic loading. It has been reported that in moderate
in lled natural rubbers. The orientational dependence of strain-induced crystallization is described by a directional network theory, in which the rubber network is composed of several one-dimensional subnetworks dispersed in different directions d in a micro-sphere. In each subnetwork, there are two types of polymer chains: non-uctuating chains responsible for equilibrium response and uctuating chains in charge of rate-dependent response.
