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Abstract: The local degree of crystallinity around a crack tip in natural rubber under dynamic load was measured by time-resolved scanning wide-angle X-ray diffraction (WAXD). Using a high-flux synchrotron microbeam, WAXD patterns with less than 20 ms exposure time were acquired while the notched rubber sample was subjected to cyclic dynamic stretching at a frequency of 1 Hz, similar to the loading conditions in tear fatigue experiments. By scanning the continuously cycling sample, a complete crystallinity map at any given strain phase angle was obtained. The crystallinity at the crack tip is considerably reduced compared to static crack tip scans. Further investigations revealed the underlying structural reasons for the well-known relation between R-ratio and crack growth resistance. By performing static crack tip scans on increasingly stretched rubber samples, the mechanisms behind crack deflection and branching were studied. [Copyright &y& Elsevier]

The time-dependence of strain-induced crystallizationin cross-linkednatural rubber was studied using synchrotron wide-angle X-ray diffraction(WAXD) with an unprecedented time resolution in the ms range. In-situdynamic mechanical tests and tensile impact tests were carried out.In tensile impact tests, consisting of a strain step within less than10 ms, it was found that roughly half of the crystallization processwas complete within less than 5 ms, provided the strain is large enough.Afterward, crystallization proceeded on a time scale of a few seconds.This implies that under dynamic loading at frequencies which are typicallyencountered in the application of rubbers, the degree of crystallinityis considerably lower than under equilibrium conditions. This wasdirectly confirmed by in situ dynamic cyclic experiments at a frequencyof approximately 1 Hz. Since crystallization is a major factor contributingto the outstanding mechanical properties of natural rubber, thesefindings can aid in the interpretation and prediction of the frequency-dependenceof mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract: The present work explored the structure-property correlations for the biopolymer poly(l-lactic acid) (PLA) by studying deformation-mediated molecular orientation and crystallization. The structural and morphological variations of amorphous PLA under different strain rates were investigated. The result showed that strain rate significantly influences its strain-hardening behavior. The crystallinity and orientation as well as cavitation of deformed PLA increase with the increase of strain rates. The structure evolution has been divided into three potential stages: (i) at small strains (<100%), the crystallinity of PLA increases by orientation-induced crystallization; (ii) at intermediate strains (100%–160%), the crystallinity of deformed PLA slightly decreases due to the breakage of existing crystals under stress accompanying with newly formed voids and cavities; (iii) at high strains (>160%), the increasing number of oriented chains in the amorphous regions promotes the crystallization of PLA. Our study suggests that strain rate and stretching strain play important roles on modulating the crystallization and orientation of amorphous PLA. [Copyright &y& Elsevier]