Degradation Of Molecular Chains Research Articles

Multiscale simulation study on radiation aging of EPDM and preparation of radiation-resistant materials

With the wide application of ethylene propylene diene monomer (EPDM) in the field of nuclear engineering, exploring the radiation aging mechanism of EPDM is of great significance to improve the radiation resistance of EPDM composites. In this paper, the radiation aging mechanism of EPDM was investigated by a combination of experiments and multiscale simulations, and EPDM composites with excellent radiation resistance were prepared. The radiation aging mechanism of EPDM involves the crosslinking and degradation of molecular chains. The aging of EPDM starts from the breakage of α-H in the 5-ethylidene-2-norbornene monomer. In addition, the screening strategy of radiation-resistant additives was proposed. Antioxidants N, N′-diphenyl-p-phenylene-diamine (H), N, N′-ditolyl-p-phenylenediamine (DTPD), and N, N′-di-2-naphthyl-p-phenylenediamine (DNP) were taken as the main research objects, and quantitative analysis was carried out from the aspects of chemical protection and physical protection. The screening results showed that DNP has excellent radiation resistance and migration resistance. The prepared EPDM-DNP composites showed outstanding radiation resistance with 94% tensile strength retention after 300 kGy irradiation. This work contributes to understand the mechanisms of radiation-induced material damage and guides the design of radiation-resistant materials.

New insight into the thermal‐oxidative stability of polyamide 6: A comparison investigation on the effect of hindered amine and CuI/KI

AbstractPolyamide 6 (PA6) and stabilized PA6 with two different thermal stabilizers, (ie, metal salts CuI/KI and hindered amine KYN818 [1,3‐benzendicarboxamide,N,N'‐bis(2,2,6,6‐tetramethyl‐4‐piperidinyl)]) were separately prepared by melt blending and then aged for different time. The effects of aging temperature and aging time on crystallization behaviors of PA6 and stabilized PA6 were systematically investigated by differential scanning calorimetry and X‐ray diffraction. The variations of melting temperature, crystallinity and 2θ values suggest that aging under high temperature will accelerate the formation of crystals initially and then the degradation of molecular chains. The yellowness index and maximum decomposition temperature of stabilized PA6 are higher than that of PA6, which is dominated by different aging mechanisms and properties of stabilizers. In addition, the viscosity of all systems is increased at the preliminary stage, which is caused by the crosslinking network and the post‐polycondensation. The results of dynamic mechanical analysis showed that the glass transition temperature (Tg) decreases because the degradation of molecular chains during the aging process. Moreover, the carbonyl index for stabilized PA6 is lower than PA6, indicating the degradation is hindered by stabilizers. A comparison of the stabilization performance between CuI/KI and KYN818 was investigated to differentiate the long‐term stabilizing efficiency.

High‐performance hybrid polymer based on bis(alkoxy) ortho‐substituted para‐phenylene

AbstractCompared to bis(alkoxy) group para‐substituted para‐phenylene (p‐phenylene) widely used as spacer, the isomer ortho‐substituted p‐phenylene is rarely studied in constructing organic optoelectronic materials. In this manuscript, bis(hexyloxy) group ortho‐substituted p‐phenylene was used to construct hybrid thienylene‐phenylene polymer poly(EDOT‐o‐BE). The density functional theory study indicated the dihedral angles between thienylene and phenylene are 15° for ortho‐substituted EDOT‐o‐BE and 10° for para‐substituted EDOT‐p‐BE. Poly(EDOT‐o‐BE) exhibits the little potential difference between cathodic (0.61 V) and anodic (0.65 V) peaks even at the high potential scan rate, different from bis(alkoxy) para‐substituted control polymer poly(EDOT‐p‐BE). Moreover, the free‐standing poly(EDOT‐o‐BE) film displays better stability than poly(EDOT‐p‐BE) due to the little degradation of molecular chains. Compared to poly(EDOT‐p‐BE), poly(EDOT‐o‐BE) possesses higher ionic transport and different molecular aggregation. These results indicate the positions of side chains have important influence on the properties of conjugated polymer, including molecular configuration, aggregation state, film quality, electrochemical behavior and optical absorption.

Study on Rheological Characterization of Uncured Natural Rubber with Different Initial Moisture Contents Dried by Microwave

In this work, the rheological behavior of uncured natural rubber with different initial moisture contents dried by microwave was studied using RPA under frequency sweep and strain sweep modes. It can be seen from the results that the variation trend of viscous torque S`` versus sweep frequency kept with the trend of elastic torque S` versus sweep frequency. Moreover, the viscous torque S`` values were lower than those for elastic torque S`, a good linear relation curve was obtained by plotting elastic torque S` versus sweep frequency. The curve of storage shear modulus G` versus sweep strain presented a linear relationship, showed a linear viscoelastic behavior of uncured natural rubber with different initial moisture contents. This may be explained by the degradation of molecular chains of natural rubber and the van der waals forces between them.

STUDIES ON DYNAMIC VISCOELASTIC PROPERTIES OF SEVERAL RECENERATED FIBERS

Dynamic viscoelastic properties were measured on several regenerated fibers by means of the vibrating reed method within the frequency range from 50 to 170 cps., in order to make clear the relation between the viscoelastic properties and the internal structure such as crystalinity, size of crystalite, orientation and network structure.The results obtained were as follows:1. The orientation of molecules in the crystal or amorphous region had profound influence on the dynamic viscoelasticity of fibers. Dynamic modulus (E1) and dynamic loss (E2) increased with the orientation, however loss tangent (tanδ) was almost independent of the orientation.2. The growth of crystalines and the increase of networks in amorphous region made increase of E1, which might be due to the increase of intermolecular bonds. E2 and tan δ decreased with those change in fine structure. It could be assumed that the region sensitive to E2 had decreased within the frequency region covered.3. When the degree of polimerization decreased, E1, and E2 also decreased, but tan δ increased perhaps as the result that the degradation of molecular chains was more effective to the decrease of E1 than to the decrease of E2.