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The kinetics of strain-induced crystallization in natural rubber were studied by synchrotron wide-angle X-ray diffraction (WAXD) in a strain jump setup over a broad temperature range. The increase of crystallinity after a strain jump was observed on a millisecond time scale, until a steady state plateau was approached. A novel model, based on chain dynamics scaling laws for diffusion in polymer networks was developed to interpret the time and temperature dependence of the strain-induced crystallization. In contrast to established thermal crystallization models, this model for the strain-induced crystallization proposes that the formation of stretched crystallizable chain segments takes place before the attachment to a growing crystallite. [ABSTRACT FROM AUTHOR]

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 orientation and deformation mechanisms in the filler and in the matrix of filled natural rubber were studied by ultra small-angle X-ray scattering (USAXS) and wide-angle X-ray diffraction (WAXD) performed on rubber samples stretched in situ. Using model fillers that give a signal in the wide and in the small-angle region allowed us to correlate the models developed to interpret the USAXS data with the information obtained from the WAXD data. Due to the simple geometries of aragonite and nano-sized aluminum oxide, which served as model fillers, an analytical description using form factors can well represent the experimental two-dimensional USAXS data of oriented stretched samples. Besides the study of the filler structure, an attempt was made to resolve the local strain in the matrix. We took advantage of the peculiar property of natural rubber, that is, strain-induced crystallization, to unravel the matrix strain by measuring the degree of crystallinity. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 [ABSTRACT FROM AUTHOR]

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]

This paper presents the structure and properties of two multilayered systems where polymers in adjacent layers were either miscible or immiscible. The miscible system consisted of 2, 17, 18, 24 and nominally 288 layers of alternating low-density (LDPE) and low-density/linear-low density (mPE) polyethylene layers with observed thicknesses ranging from 150 nm to 20 μm. The immiscible system consisted of 5 and 19 layer films with a combination of poly(ethylene- co -vinyl alcohol) (EVOH) (thickness: 9 and 1 μm, respectively), LDPE (17 and 7 μm) and a polyethylene adhesive (3 and 1 μm). The purpose of the multi-layering was to increase the crack growth resistance and, in the EVOH-based system, to decrease the oxygen transmission rate. Indeed, the crack growth resistance, as measured on tensile-tested notched films, increased with increasing number of layers. The thinnest polyethylene and polyethylene adhesive layers showed a clear ductile failure when fractured even in liquid nitrogen. Simultaneous synchrotron wide-angle/small-angle X-ray scattering and tensile testing indicated no new deformation features with changes in the layer thickness. The oxygen permeability was the same in the 5- and 19-layer EVOH-based films, but the uptake of n -hexane was strongly reduced in the 19-layer films, demonstrating the effective protective role of the EVOH layers. The n -hexane desorption data of the 2-layer LDPE/mPE film was successfully modeled using the diffusivities and solubilities of the single layers. Crystallization was slower and more confined in the films with thinner layers. The interlayer mixing in the melt (measured by isothermal crystallization from melts of initially layered polyethylene-based systems) was, as expected, significantly faster in the 24- and 288-multilayer films than in the 2-layer film. [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]