The Thermodynamic Behaviour and Miscibility of Discotic Liquid Crystals

This thesis is concerned with the self-organization of molecules that have disc-like shapes. The disc-like molecules may have relatively rigid structures as in polyaromatic systems, or they may have amphiphilic structures with polyaromatics at their cores, and soft outer shells made up of aliphatic chains. This research seeks to explore molecular compatibility and the ensuing structures formed by such disc- like systems through the study of phase diagrams. Thus, the thermodynamic behaviour and miscibilities of discotic liquid crystal materials were investigated by the formation of Gibbs phase diagrams and calculations using the Schröder-van Laar equation for liquid crystals that have structural common features. Polyaromatics were reviewed and investigated as they are hard discs, and are as the central cores of the molecules that form discotic liquid crystals. Existing discotic materials, such as triphenylene hexa-esters, phenyl hexa-esters, phenyl hexa-alkynes, and rod-like compounds such as benzoate esters were studied and analysed using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction where appropriate, for the preparations of mixture studies. Mixture series made by triphenylene derivatives with each other, and with rod-like molecules or star-shape molecules were prepared and examined in order to investigate the potential co-miscibilities for both discotic nematic phase and hexagonal columnar phases. Mixture series made by triphenylene derivatives with polyaromatics were also prepared, and examined by POM and DSC to explore the virtual N-I phase transition temperatures for polyaromatics. These studies appear to show that nematic discotic materials are liquid crystals, whereas columnar materials exhibit 2D crystallinic soft solid phases. In addition, a novel new phase was found to form in the isotropic liquid of mixtures of polyaromatic materials, suggesting the possibility of the cubatic nematic phase being present

Response of seasonal soil freeze depth to climate change across China

Abstract. The response of seasonal soil freeze depth to climate change has repercussions for the surface energy and water balance, ecosystems, the carbon cycle, and soil nutrient exchange. Despite its importance, the response of soil freeze depth to climate change is largely unknown. This study employs the Stefan solution and observations from 845 meteorological stations to investigate the response of variations in soil freeze depth to climate change across China. Observations include daily air temperatures, daily soil temperatures at various depths, mean monthly gridded air temperatures, and the normalized difference vegetation index. Results show that soil freeze depth decreased significantly at a rate of −0.18 ± 0.03 cm yr−1, resulting in a net decrease of 8.05 ± 1.5 cm over 1967–2012 across China. On the regional scale, soil freeze depth decreases varied between 0.0 and 0.4 cm yr−1 in most parts of China during 1950–2009. By investigating potential climatic and environmental driving factors of soil freeze depth variability, we find that mean annual air temperature and ground surface temperature, air thawing index, ground surface thawing index, and vegetation growth are all negatively associated with soil freeze depth. Changes in snow depth are not correlated with soil freeze depth. Air and ground surface freezing indices are positively correlated with soil freeze depth. Comparing these potential driving factors of soil freeze depth, we find that freezing index and vegetation growth are more strongly correlated with soil freeze depth, while snow depth is not significant. We conclude that air temperature increases are responsible for the decrease in seasonal freeze depth. These results are important for understanding the soil freeze–thaw dynamics and the impacts of soil freeze depth on ecosystem and hydrological process. </jats:p

Rods to discs in the study of mesomorphism in discotic liquid crystals

The rational design of calamitic liquid crystals is an area of research that has been intensively explored due to their extensive applications in various devices. The successful methods for design have been, to some extent, mapped on discotic systems such that certain features of the structures of calamitic phases have been superimposed upon those of nematic discotic and columnar phases. In this article, we explore the correlation between nematogenic behaviour of hard rod-like particles and that of hard disc-like systems. We show that for calamitics, nematic behaviour is observed, whereas for discotics this is not the case. Furthermore, we show that nematic discotic materials are miscible, whereas unlike smectics, columnar phases are less likely to be miscible. Indeed, it appears that columnar discotic phases are greater similarities with soft-solids than true liquid crystals

Comparative studies of polymer-dispersed liquid crystal films via a thiol-ene click reaction

In this work, the thiol-ene click reaction is employed to fabricate polymer-dispersed liquid crystal (PDLC) films by photoinitiated polymerization. The PDLC films are prepared by systematic variation of key conditions: variety and content of -ene monomer, liquid crystal (LC) content, curing time, and curing light intensity. We find that both the morphologies and electro-optic properties of these films are adjustable. When increasing the length of alkyl main chain of -ene monomers, the driving voltages reduce, but in turn, the contrast ratio decreases. Increasing -ene monomer content raises the driving voltages as well as the response time, and the increase of LC content lowers the driving voltages but has a negative effect on the contrast ratio. The changes to the curing conditions (both curing time and UV light intensity) can be used to modify the driving voltages, response time, and contrast ratios of PDLC films. These comparative studies will elucidate new insights in commercial applications of intelligent PDLC films

Thiol-ene reaction based polymer dispersed liquid crystal composite films with low driving voltage and high contrast ratio

In this study, polymer dispersed liquid crystal composite films are obtained via a one-step fabrication technique based on photo-initiated polymerization-induced phase separation in a thiol-vinyl ether reaction. The effects of the compositions of two vinyl monomers, curing light intensity, curing light time, liquid crystal content and thiol content on the electro-optic properties of the polymer dispersed liquid crystal films were systematically investigated. It was demonstrated that a composite film with a low driving voltage (33.7 V), high contrast ratio (228.1) and short response time (<5.0 ms) was achieved. This is of great significance for the potential applications of polymer dispersed liquid crystal composite films

Theoretical studies on force titration of amino-group-terminated self-assembled monolayers

The structures of (3-aminopropyl)triethoxysilane (APTES), 4-aminothiophenol (4-ATP) and 4-mercaptopyridine (4-MP) self-assembled monolayers (SAMs) are studied by quantum mechanics in order to explain the force titration curves of these amino-group-terminated SAMs. The surface charges and electrostatic surface potentials derived from the ab initio calculations can give satisfactory explanations for the experimental results. We also propose a simple model to simulate the force titration process. The force between the tip and sample can be estimated according to the slope coef®cient of the curve of energy versus distance. This curve can lead to a better understanding of the force titration curves of amino-group-terminated SAMs.Accepted versio

Integration of Nano-Sized HZSM‑5 with ZnZrO_<i>x</i> as a Bifunctional Catalyst to Boost Benzene Alkylation with Carbon Dioxide and Hydrogen

Benzene alkylation with CO2/H2 is a doubly beneficial option to upgrade benzene toward valuable products, in which benzene can be converted into valuable toluene and xylene, while CO2 can be utilized as an alkylation reagent to mitigate the greenhouse effect. It is quite challenging to ascertain the critical rule of zeolite regulation for the synergistic effect in oxide-zeolite composite (OXZEO) catalysts, a series of bifunctional catalysts were investigated in this work, consisting of ZnZrOx solid solution and nano-sized HZSM-5 with uniform size distribution and distinct acid properties. Key parameters affecting the catalyst performance were analyzed and optimized in this tandem reaction, namely, the mass ratio of two components, the SiO2/Al2O3 ratio of HZSM-5, and the proximity between ZnZrOx and HZSM-5. Due to the increased active site accessibility originating from nano-sized HZSM-5, the combined selectivity of toluene and xylene reaches 94.6% at 31.3% benzene conversion, respectively. Moreover, low ethylbenzene selectivity (<1%) has been achieved. It is worth noting that the long-term stability test indicates a faster deactivation in the nano-sized HZSM-5 system as compared with the micro-sized counterpart. Based on EDS mappings, it is found that nano-sized HZSM-5 experiences a more severe metal migration issue, resulting in lower catalyst stability. The reaction mechanism with formate-methoxy intermediates and the synergistic effect of CO2 hydrogenation-alkylation were further confirmed by in situ diffuse reflectance infrared Fourier transformations spectroscopy and gas chromatography–mass spectrometry. By correlating zeolite size and acidity with catalytic performances, these findings could facilitate the further rational design of OXZEO catalysts toward CO2 catalytic conversions

Spatio-Temporal Characteristics and Differences in Snow Density between the Tibet Plateau and the Arctic

The Tibet Plateau (TP) and the Arctic are typically cold regions with abundant snow cover, which plays a key role in land surface processes. Knowledge of variations in snow density is essential for understanding hydrology, ecology, and snow cover feedback. Here, we utilized extensive measurements recorded by 697 ground-based snow sites during 1950–2019 to identify the spatio-temporal characteristics of snow density in these two regions. We examined the spatial heterogeneity of snow density for different snow classes, which are from a global seasonal snow cover classification system, with each class determined from air temperature, precipitation, and wind speed climatologies. We also investigated possible mechanisms driving observed snow density differences. The long-term mean snow density in the Arctic was 1.6 times that of the TP. Slight differences were noted in the monthly TP snow densities, with values ranging from 122 ± 29 to 158 ± 52 kg/m3. In the Arctic, however, a clear increasing trend was shown from October to June, particularly with a rate of 30.3 kg/m3 per month from March to June. For the same snow class, the average snow density in the Arctic was higher than that in the TP. The Arctic was characterized mainly by a longer snowfall duration and deeper snow cover, with some areas showing perennial snow cover. In contrast, the TP was dominated by seasonal snow cover that was shallower and warmer, with less (more) snowfall in winter (spring). The results will be helpful for future simulations of snow cover changes and land interactions at high latitudes and altitudes