The melting and crystallization behavior of Vectratm B950, a random liquid crystalline copoly(ester amide), was studied by differential scanning calorimetry (DSC). Nonisothermal experiments showed the existence of some interesting and peculiar transition behavior, which may depend on the crystallization conditions (cooling rate). In isothermal experiments, both the thermal and crystallization behavior have been studied as functions of annealing temperature and annealing time. Different thermal behaviors were observed, indicating the possibility of multiple states of mesophase aggregation In addition, the thermal decomposition of Vectra B950 was also studied by TGA‐FTIR in both nitrogen and air environments. A single‐stage decomposition process was found when heating in nitrogen atmosphere, and the decomposition is due to the easter linkage rupture. A double‐stage decomposition process was found when heating in air, and the decompositions are mainly due to the ester linkage rupture for the first decomposition step and the oxidative reaction for the second decomposition step. Annealing slightly changes the decomposition composition occuring in the early stage of thermal degradation.
Doping provides an
advantage to engineering the optical and electrical
characteristics of transition-metal dichalcogenides (TMDs). Here,
we report the doping of WS2 quantum dots (QDs) with diethylenetriamine
(DETA) using pulsed laser ablation. The synthesized DETA-doped 2H-WS2 QDs with an average size of ∼6 nm have been demonstrated
by transmission electron microscopy. With the introduction of DETA
during pulsed laser ablation, current modulation, carrier concentration,
and field-effect mobility are greatly enhanced, demonstrating a successful
doping in WS2 QDs. The positive shift of the threshold
voltage in gate-dependent conductance measurements reveals p-type
doping for DETA-doped WS2 QDs. A remarkable improvement
in photoluminescence in WS2 QDs by 74-fold has been achieved
after DETA doping. An anomalous dopant-dependent negative photoconductivity
was observed for WS2 QDs, originating from light-induced
desorbing of water (oxygen) molecules on the surface. The proposed
doping approach can provide a vehicle to modulate the optical and
electrical properties in WS2 QDs and could be important
in the performance improvement of WS2-QD-based devices.
This study investigates the synergistic effect of combining multi-walled carbon nanotubes (MWNTs) and Fe2O3nanoparticles on thermal conductivity of nanofluid. Results show that low percentage hybrid fillers loading improve thermal conductivity of water based nanofluid, due to the good dispersion and interfacial adhesion, which is confirmed by scanning electron microscope. Furthermore, the hybrid fillers provide synergistic effect on heat conductive networks. The thermal conductivity enhancement of water based nanofluid containing 0.05 wt % MWNTs and 0.02 wt % Fe2O3nanoparticles is 27.75%, which is higher than that of nanofluid containing 0.2 wt % single MWNTs or Fe2O3nanoparticles.
ABSTRACT:We have studied isothermal transition kinetics and crystallization mechanisms of the Hoechst Celanese liquid crystal polymer Vectra™ A-950 using a differential scanning calorimetry. This material is a random copolymer with a composition made of 73 mol %/27 mol % of HBA (1,4-dihydroxybenzoic acid)/HNA (2,6-dihydroxynaphthoic acid). When comparing our results to previous work on 75/25 HBA/ HNA, we found some similarities as well as dissimilarities. In the case of similarities, both polymers show two types of transition processes in the low-temperature region (below 495 K). One is a fast process, which can be regarded as liquid crystal transition, and is characterized by a heat of fusion that does not vary significantly with annealing time. The other transition is a slow process related to crystal perfection and shows increases in the heat of transition and the transition temperature with increasing annealing time. However, the apparently slight difference in polymer composition also leads to a few surprising results. For example, previous work on 75/25 HBA/HNA two transition peaks (slow and fast processes) were observed after annealing it at 505 K, while in the current work only one transition peak in the case of 73/27 HBA/HNA was observed. In addition, based on the relationship between heat of fusion and annealing time, the LCP made from 73/27 HBA/HNA may perfect faster (in the time scale we study); hence, its posttreatment processes may be accomplished more efficiently than that of 75/25 HBA/HNA. This result also suggests that the former may have a better sequence or a better matching of molecular dimensions for crystallization than that of the latter.
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