A series of random poly(hexamethylene succinate- co -hexamethylene fumarate) (PHSF) samples was synthesized through a two-step reaction process, and their properties were systematically studied using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The crystal modification of polyester changes from monoclinic to orthorhombic after substituting hexamethylene succinate (HS) units by hexamethylene fumarate (HF) units, and the chain conformation changes from all-trans to gauche. The copolyesters were found to be in a particular case as strict isomorphism in a limited composition range, adopting the same crystal modification when HF content ranged from 19 to 100 mol%. The melting point of the copolyester increases almost linearly with increasing molar content of HF, and the melting enthalpy hardly changes. Interestingly, FTIR results suggest that a special hydrogen-bonding interaction, in “C C–H···O C–C C” form, forms in the crystalline phase of PHSF and poly(hexamethylene fumarate) (PHF). The strong hydrogen-bonding interaction might be responsible for the transformation of conformation from all-trans to gauche in hexamethylene segments, and triggers the variation of crystal modification.
Bao Xiao,Qihao Sun,Fangbao Wang,Shiyao Wang,Bin-Bin Zhang,Junjie Wang,Wanqi Jie,Paul Sellin,Yadong Xu
Two-dimensional (2D) organic–inorganic hybrid halide perovskites have recently attracted extensive attention for electronic and optoelectronic applications due to their tunable properties and superior stability compared with their three-dimensional (3D) counterparts. Here, we report two kinds of organic cation (linear butylamine (BA) and branched isobutylamine (i-BA)) tailored CsPbBr3 crystals, namely (BA)2CsPb2Br7 and (i-BA)2CsPb2Br7, grown by the temperature-cooling method. The organic cations' adjustable anisotropic structure, optoelectronic properties and X-ray detection performance have been systematically investigated. By shortening the spacer cation from BA to i-BA, the degree of anisotropy in 2D perovskite crystals is decreased, which may be ascribed to the reduced interlayer distance and barrier height resulting from the enhanced electronic coupling between neighboring organic cations. In particular, the device based on the (BA)2CsPb2Br7 crystal along the ab plane exhibits superior X-ray sensitivity up to 13.26 mC Gy−1 cm−2 at a relatively low electric field of 2.53 V mm−1, owing to the multiple quantum well structure that restricts the charge carrier transport within the ab plane resulting in efficient charge collection. Simultaneously, a superior long-term working stability is obtained under a high X-ray dose rate of 278.4 μGy s−1. We anticipate that these findings will be helpful for the development of Ruddlesden–Popper perovskites for future research and applications.
Jian Hao,Weijie Li,Xidong Suo,Huiqing Wei,Chunxiang Lu,Yaodong Liu
High performance carbon fibers are mostly produced from polyacrylonitrile (PAN). Its chemical structures strongly affect the further steps of carbon fiber manufacturing. In this work, highly isotactic (>60%) PAN was synthesized by template-assisted solid phase polymerization in the presence of magnesium chloride. We systematically studied the effect of isotacticity of PAN on its solution rheology, fiber spinning, thermal stabilization and carbonization . Highly isotactic PAN-based carbon fibers were first produced in this work. The higher isotacticity of PAN was observed to affect the storage stability of PAN solutions, coagulation processes, the structures of PAN fibers , stabilization reactions and carbonization yields. The improvement of PAN isotacticity benefits its thermal stabilization, but also introduces many problems during carbon fiber manufacturing which need to be carefully considered.
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