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Role of surface phenomena in the strain softening of glassy and crystalline polymers
Published in A. L. Volynskii, N. F. Bakeev, Surface Phenomena in the Structural and Mechanical Behaviour of Solid Polymers, 2018
The phenomenon of strain softening in polymers was detected a relatively long time ago. Figure 5.1 shows the curves of tensile loading and restoration of the dimensions (shrinkage) of the guttapercha [3]. The gutta-percha is a polymer which is a geometrical isomer of natural rubber, but in contrast to the latter it is present at room temperature in the crystalline state [4]. Figure 5.1 shows that this polymer in tensile loading in the first cycle shows the stress– strain curve characteristic of the crystalline polymer (the relatively high modulus, yield stress, the plateau region). This deformation may be carried out to a very high value (to 500 or more percent). However, after interrupting tensile loading and removing the stresses, the tensile loaded polymer restores its dimensions almost completely and spontaneously (Fig. 5.1). In other words, this is accompanied by extensive shrinkage, characteristic of the polymer, which is in the rubbery state. In repeated tensile loading this polymer shows a mechanical behaviour characteristic of the polymer in the rubbery state (low modulus, absence of the yield stress and the plateau region, very low residual strains).
Kinetics of the thermal decomposition of Eucommia ulmoides Oliver leaves and its fermentation products
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2019
Zhihong Wang, Minglong Zhang, Sheng Peng, Qiuling Yang, Mijun Peng
The pyrolysis characteristics of EULU and EULF have been investigated at different heating rates under nitrogen atmosphere. For both samples, the decomposition process could be divided into three stages, and the main weight loss was observed in the second stage. When the heating rate increased from 10, 20, 30, and 40°C, the temperature interval of each pyrolysis zone and peak temperature moved toward high temperature. Meanwhile, the KAS and FWO methods exhibited the similar kinetic characteristics for each sample, respectively. The CR method revealed the second-order model (f(α) = (1-α)2) might be the main thermal decomposition reaction mechanism of the whole pyrolysis process for both samples. However, compared to EULU, the DTG curves of EULF have changed significantly, including major pyrolysis peak, shoulder peaks, tiny peaks, and new peaks produced. At the same time, the average pyrolysis activation energy was basically close. The kinetic characteristics of pyrolysis were significantly changed through fermentation technology, which suggested that biological fermentation technology could cause main changes in chemical composition and lignocellulosic structure. It was worth noting that the pyrolysis activation energy of EUL was higher than that of common biomass plant materials. This might be closely related to the complex composition of EUL and the presence of unique gutta-percha structure (trans-polyisoprene). Therefore, the technology of microbial pretreatment and the pyrolysis model of EUL were worthy of further study.