Strain-induced crystallization modulus properties

The most important parameter controlling the final properties is the draw ratio, X. There is a direct relationship between X and the mechanical properties, in particular the modulus. The maximum draw ratio is generally limited to about 5 for PET and most PA s, about 10 for PP whereas values of 15-20 are common for HDPE. Crystallinity also has a strong effect on final properties. Strain-induced crystallization develops during the orientation process as the molecular order progressively increases upon drawing. 

The orientation in PLLA films depends on the draw rate, temperature, and ratio. High strain rate, low temperature, and high stretch ratio favor strain-induced crystallization during orientation. PLA has excellent optical properties and high modulus. However, it has low elongation, tear, and burst strengths. To overcome these shortcomings, PLA is often coextruded with other polymers to form multilayer structures to enhance its properties [27]. 

Although elastomers are usually amorphous, strain-induced crystallization occurs in rubbers such as cA-l,4-polybutadiene, butyl rubber, and NR. Crystallization under stress, discovered 200 years ago [239], increases the modulus and most failure properties of rubber and is essential to performance in many... 

In some cases, double networks have shown increases in orientability and strain-induced crystallization, as well as improved fatigue resis-tance. ° In fact, some results show that there maybe less of a compromise between failure properties in general and the modulus, which may be due in part to the decreased hysteresis observed for some of these elastomers. There have even been reports of improved thermal stabil-ity, although it is hard to visualize how this would occur. Finally, electrical resistivity is more sensitive to strain in carbon-black reinforced double networks. Better molecular understanding of these observations is being sought with, for example, extensive studies of residual strains and birefringence. ... 

In order to get an insight into the mechanism of strain induced crystallization and its consequences on mechanical properties, a more sensitive method for detecting crystallization has been developed using measurements of the elastic component of the complex modulus of specimens both in undeformed conditipns (isotropic crystallization) and in strained conditions. 

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