Strain-induced crystallization, elastomeric

Certain polymers can be crystallized by mechanical stress. Namely, the stress induced elongation decreases the entropy of the chains. For this reason, an additional decrease in entropy, which is required for crystallization is comparatively small. Strain induced crystallization phenomena are of great practical importance since the elastomeric properties can be tailored in some way. In particular, PIB readily undergoes strain induced crystallization already close to room temperature. 

Kloczkowski, A. Mark, J. E. Sharaf, M. A. Erman, B., Theory of Strain-Induced Crystallization in Real Elastomeric Networks. In Synthesis, Characterization, and Theory of Polymer Networks and Gels, Aharoni, S. M., Ed. Plenum Press New York, 1992 pp 227-241. 

Strain-induced crystallization also has a pronounced reinforcing effect within the network, and this increases the ultimate strength and maximum extensibility (26). The capability of crystallizing with extension while remaining elastomeric at low elongations provides an important self-reinforcement mode for natural rubber. The only other elastomer with these properties is cis-polybutadiene. These materials are the elastomers of choice for many heavy-duty applications. 

Mark, J.E. (1979) Effect of strain-induced crystallization on the ultimate properties of an elastomeric polymer network. Polymer Eng. ScL, 19, 6,409. 

Su T-K, Mark JE. The effect of strain-induced crystallization on the elastomeric properties of c 5-1,4-polybutadiene networks. Macromolecules 1977 10 120-5. 

Elastomeric chains of sufficient regularity may crystallize under some conditions. The deformation of a macromolecule is accompanied by a decrease in its conformational entropy. Its tendency to crystallize is therefore increased. The oriented system will crystallize even above the melting point of the isotropic system (and is thus strain-induced crystallization ). The crystallinity reaches a value which depends on the degree of deformation and the crystallization temperature. The melting point = Ai/ /ASm, given by the ratio of the enthalpy of fusion to the entropy of fusion, is thus increased by the network deformation. 

In PTMO based PEBA copolymers where the PE phase dominates the composition and the molecular weight of the PE segments is higher than ca. 2000 g/mole, strain-induced crystallization of the PE phase occurs at ambient temperature when the sample is uniaxially deformed above ca. 500%. Upon strain-induced crystallization, the of the PE phase increases from below room temperature to ca. 45 °C and the initially soft elastomeric sample becomes inelastic until the PE phase undergoes melting. 

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