Polymer crystallization upon strain

Rubber failure upon application of stress is delayed by i) viscoelastic energy dissipation and il) crystallization upon strain of the base polymer. A peculiar rubber failure, namely fatigue failure, is also affected by the same factors. The presence of carbon black or other reinforcing agents does not diminish the contribution of the base polymer on fatigue resistance. 

A characteristic property of amorphous polymers is the ability to sustain large strains. For cross-linked three-dimensional networks the strain is usually recoverable and the deformation process reversible. The tendency toward crystallization is greatly enhanced by deformation since chains between points of cross-linkages are distorted from their most probable conformations. A decrease in conformational entropy consequently ensues. Hence, if the deformation is maintained, less entropy is sacrificed in the transformation to the crystalline state. The decrease in the total entropy of fusion allows crystallization, and melting, to occur at a higher temperature than would normally be observed for the same polymer in the absence of any deformation. This enhanced tendency toward crystallization is exemplified by natural rubber and polyisobutylene. These two polymers crystallize very slowly in the absence of an external stress. However, they crystallize extremely rapidly upon stretching. 

Polymer liquid crystal solutions respond in an interesting way to a transient flow field. For example, a damped oscillatory response was observed upon a flow inception or flow reversal [150,156,157], and the strain was recovered to some extent after a creeping flow [158], These phenomena suggest that liquid crystal solutions have some textured structure, i.e., the spatial variation in the... 

Crystallization of the polymer when the propellant formulation is subjected to low temperatures can be annoying (12). Formation of additional periodic attractions between molecules has the same effect as additional crosslinking. Upon crystallization, the propellant becomes hard and brittle with low strain capability. If the effect is caused by crystallization of the polymer, the original physical properties are obtained when the propellant is heated above the melting point of the polymer. These effects are time-temperature dependent and can have a significant effect on the selection of operating and storage temperatures... 

Wu s explanations for this fundamental observation were couched in field theory, such as overlapping of concentrated stress fields around particles or a transition in local stress state from plane strain to plane stress, and could not furnish a specific material dimension, which, as we present below, depends on the type of the polymer and its crystalline state. The required fully consistent explanation for the discovery was provided by the studies of Muratoglu et al. (1995a), who proposed that the material-specific level of A is a consequence of a preferred form of crystallization of polyamide lamellae near particle interfaces, extending to a certain distance I away from the interface. This results in an anisotropic plastic resistance in this layer, which upon percolation through the matrix and in an... 

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