- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Crystallization, glass transition, and molecular dynamics in PDMS of low molecular weights: A calorimetric and dielectric study
摘要： In this work linear polydimethylsiloxane (PDMS) of five low molecular weights, Wm ～2–8 kg/mol, i.e. mainly below the entanglement threshold, is studied employing differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), a special dielectric technique in the temperature domain, and broadband dielectric spectroscopy (BDS). It is shown mainly via raw data that the crystallization and glass transition for the larger Wms follow expected trends, however, for Wm ～2 k (corresponding to ～22 monomers/chain) crystallization is severely suppressed and slower. Moreover, it is demonstrated that upon isothermal annealing the crystalline fraction (CF) is maximized for all Wms with the exception of the lower one. For the latter, results indicate that during the annealing only the first stages of crystallization (nucleation, ordering of the polymer around the nuclei) take place, while the main crystallization event (formation of crystals) is recorded only upon heating, via the cold crystallization process. The suppressed crystallizability originates from the retarded nucleation. In particular for the lower Wm, the presence of large numbers of chain ends should be at the origins of hindered short range ordering of the polymer chains, as well as in relation to the difficult formation of physical crosslinks. At the same time, in TSDC and BDS uniquely for Wm ～2 k upon annealing and CF～0, next to the relaxation peaks related with the glass transition of the bulk-like polymer (α and αc), an additional quite strong peak (αnew) is recorded at higher temperatures. The overall characteristics of αnew relaxation resemble those of PDMS chains constrained and ordered in the interfacial layer in amorphous silica/PDMS nanocomposites (αint relaxation). Thus, combing the overall results with results from the literature, it is proposed that αnew is a direct recoding of retarded dynamics of the polymer chains being ordered around the crystallization nuclei or around extremely small crystals.
关键词： Polymer chains ordering,Polymer crystallization,Glass transition,Polydimethylsiloxane,Thermally stimulated depolarization currents,Differential scanning calorimetry
更新于2025-09-10 09:29:36
-
O12 scintillator
摘要： The characteristics of a Gd3Al2Ga3O12 crystal scintillator doped with cerium and co-doped with magnesium and titanium have been studied, mainly in view of using it for the Phase II upgrade of the LHCb electromagnetic calorimeter. Samples of the scintillator were irradiated with γ (60Co) to 2 kGy and with 24 GeV protons to 900 kGy. The proton fluence value was ～3·1015 cm?2. It was found that γ-irradiation did not produce any change in the optical transmission of the crystals in the spectral range of the scintillation light, whereas a degradation after the proton irradiation was measurable. For the 1 cm thick sample, a loss of transmission of 3.6% was measured at the wavelength of maximum scintillation (520 nm), and the measured induced absorption coefficient at this wavelength was ～3.6 m?1. The formation of radioisotopes in the crystal at proton irradiation has been analyzed. The formation of isotopes was also simulated with the help of the FLUKA package. The simulation was found to be in a good agreement with experimental results. The results have been used to estimate the expected intensity of parasitic radio-luminescence at high-luminosity operation in a GAGG/W sampling electromagnetic calorimeter.
关键词： Electromagnetic calorimetry,radiation damage,gadolinium-aluminum-gallium garnet,optical transmission,scintillation crystal
更新于2025-09-09 09:28:46
-
Phase Equilibria in Systems DyCuS2–EuS and Cu2S?Dy2S3?EuS
摘要： The phase diagram of system DyCuS2–EuS has been first constructed, and the phase equilibria in the Cu2S–Dy2S3–EuS triangle at 970 K have been studied. Compound EuDyCuS3 (1DyCuS2 : 1EuS), space group Pnma, a = 10.1901(3) ?, b = 3.9270(1) ?, c = 12.8468(3) ?, melts incongruently at 1727 ± 7 K according to the reaction: EuDyCuS3solid ? 0.17 SS EuS (90 mol % EuS, 10 mol % DyCuS2) + 0.83 liq (42 mol % EuS, 58 mol % DyCuS2), ?H = 2.9 ± 0.6 kJ/mol; microhardness of the phase is 3080 ± 35 MPa. Compound EuDyCuS3 is transparent in the range 3000–1800 cm–1. In system DyCuS2–EuS, the solid solution (SS) based on EuS extends from 91 to 100 mol % at 1770 K and from 92 to 100 mol % at 1170 K. In γ-DyCuS2, 2 mol % EuS dissolves at 1487 K. The eutectic is formed between compounds DyCuS2 and EuDyCuS3 at 12 mol % EuS, T = 1487 ± 8 K. In system Cu2S?Dy2S3?EuS, 10 secondary systems have been isolated. At 970 K, tie-lines are located between compound EuDyCuS3 and solid solutions based on compounds β-Cu2S, EuS, DyCuS2, β-(DyCu3S3), and EuDy2S4; between DyCuS2 and the solid solution of α-Dy2S3, DyCuS2, and EuDy2S4.
关键词： complex sulfides,differential scanning calorimetry,phase equilibria,microstructural analysis,incongruent melting
更新于2025-09-09 09:28:46
-
Temperature Dependence of the Luminescence Spectra of a 5CB Liquid Crystal and its Phase Transitions
摘要： The spectroluminescence properties of 4cyano4'pentylbiphenyl CH3(CH2)4(C6H4)2CN (5CB) were studied in the temperature range 4.2–297 K. A red shift of the fluorescence spectrum was noted with increasing temperature. The long-wavelength shifts in these spectra were also analyzed. Comparison of the temperature dependence curves for the emission at the band maxima λmax and their halfwidths Δλ/2 in the fluorescence spectra and the results of a differential scanning calorimetry (DSC) study showed that phase transitions occur in the 5CB liquid crystals at ~230 and ~260 K.
关键词： liquid crystal,phase transition,differential scanning calorimetry,luminescence,low temperature,conformation change
更新于2025-09-09 09:28:46