Supramolecular hydrogen-bonded liquid crystals

The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted. Following the first and well-established examples of liquid crystal formation through the dimerization of aromatic carboxylic acids, several classes of compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular systems. In this review the main classes of liquid crystals prepared through hydrogen-bonding interactions are presented, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation. Rigid-rod anisotropic or amphiphilic-type molecules, appropriately functionalized with recognizable moieties, interact in the melt or in solution and lead to the formation of supramolecular complexes that may exhibit thermotropic liquid crystalline character. Depending on the nature, number and position of the groups able to form hydrogen bonds, a diversity of supramolecular structures, both dimeric and polymeric, have been obtained, affording in turn various liquid crystalline phases. The structure and stability of these hydrogen-bonded supramolecular complexes and their relation to the observed liquid crystalline phases are the main topics of this review.     

Thermotropic liquid crystal behaviour of alkaline-earth-metal dihexadecyl phosphate salts

Alkaline-earth-metal dihexadecyl phosphate salts were synthesized, and their thermal stability was evaluated by thermogravimetry. Their thermotropic liquid crystal behaviour was investigated by differential scanning calorimetry, polarizing optical microscopy, dilatometry, and X-ray diffraction. On heating, the calcium, strontium, and barium salts exhibit columnar liquid crystal phases, whereas the less ionic beryllium and magnesium salts melt directly into isotropic liquids.     

Tractable Approximations to Robust Conic Optimization Problems

In earlier proposals, the robust counterpart of conic optimization problems exhibits a lateral increase in complexity, i.e., robust linear programming problems (LPs) become second order cone problems (SOCPs), robust SOCPs become semidefinite programming problems (SDPs), and robust SDPs become NP-hard. We propose a relaxed robust counterpart for general conic optimization problems that (a) preserves the computational tractability of the nominal problem; specifically the robust conic optimization problem retains its original structure, i.e., robust LPs remain LPs, robust SOCPs remain SOCPs and robust SDPs remain SDPs, and (b) allows us to provide a guarantee on the probability that the robust solution is feasible when the uncertain coefficients obey independent and identically distributed normal distributions. The research of the author was partially supported by the Singapore-MIT alliance. The research of the author is supported by NUS academic research grant R-314-000-066-122 and the Singapore-MIT alliance.     

Pyrolytic degradation of common polymers present in packaging materials

Journal of Thermal Analysis and Calorimetry - Pyrolysis of polymers with widespread use, such as PET, HDPE, PVC, LDPE, PP and PS, been utilized extensively as plastic packaging materials, is the...     

Vinylogous Reactivity of Cyclic 2‐Enones: Organocatalysed Asymmetric Addition to 2‐Enals to Synthesize Fused Carbocycles

A method for asymmetric and site selective annulations at the γ and γ′ positions of cyclic 2‐enones with α,β‐unsaturated aldehydes has been developed. The organocatalysed [3+3]‐annulations proceed with high levels of regio‐, diastereo‐, and enantioselectivity, affording a series of high value fused carbocycles. Further elaboration gave key lactones (both bridged and fused).     

N-Acetyl-5-arylidenetetramic acids: synthesis,X-ray structure elucidation and application to the preparation of zinc(II) and copper(II) complexes

A new route to N-acetyl-3-acyl-5-arylidenetetramic acids was developed and applied to the synthesis of a small library of compounds with diversity at carbon 3 of the heterocyclic ring. The 3-acetyl derivative was used as ligand for complexation with zinc(II) and copper(II) acetates. Both the ligand and the zinc complex were studied by X-ray single crystal structure analysis, the zinc complex forms ribbons by H-bonding from the ethanol to a neighbouring molecule.     

On configurational weak phase transitions in graphene

We report a study on configurational weak phase transitions for a freestanding monolayer graphene. Firstly, we characterize weak transformation neighborhoods by suitably bounding the metric components. Then, we distinguish between structural and configurational phase changes and elaborate on the second class of them. We evaluate the irreducible invariant subspaces corresponding to these phase changes and lay down symmetry-breaking as well as symmetry-preserving stretches. In the reduced bifurcation diagram, symmetry-preserving stretches are related to a turning point with a change of stability but not of symmetry. Symmetry-breaking stretches are related to a first-order weak phase transition. We evaluate symmetry-breaking stretches as well as their generating cosets. The reduced bifurcation diagram consists of three transcritical bifurcating curves which are all unstable but can be stabilized producing a subcritical bifurcation. We, also, shortly comment on the hysteretical behavior that might appear in this case.     

Dynamic resource allocation: A flexible and tractable modeling framework

This paper presents a binary optimization framework for modeling dynamic resource allocation problems. The framework (a) allows modeling flexibility by incorporating different objective functions, alternative sets of resources and fairness controls; (b) is widely applicable in a variety of problems in transportation, services and engineering; and (c) is tractable, i.e., provides near optimal solutions fast for large-scale instances. To justify these assertions, we model and report encouraging computational results on three widely studied problems – the Air Traffic Flow Management, the Aircraft Maintenance Problems and Job Shop Scheduling. Finally, we provide several polyhedral results that offer insights on its effectiveness.     

Combined Effect of Temperature and Ternary Mobile Phase Composition on the Retention in Ternary Isocratic and Gradient Elution RP-LC under Isothermal Conditions. Application to the Retention Prediction of Four Macrolide Antibiotics

Retention models considering simultaneously ternary mobile phase organic contents and column temperature (T) were developed by a direct combination of equations expressing separately the dependence of the retention upon each of these factors. Thus, a combination of a linear dependence of the logarithm of the solute retention factor, ln k (T), against 1/T, i.e. ln k (T) = a + b/T, and of either a linear or a quadratic dependence of ln k upon the compositions of a binary mobile phase, gave two different expressions for the logarithm of the solute retention factor in terms of both temperature and organic contents in the ternary mobile phase. The effectiveness of the above models was tested in the prediction of isothermal retention of a mixture of four macrolide antibiotics under ternary isocratic and gradient elution conditions using mobile phases modified by methanol and acetonitrile. The limiting case of using ternary eluents with constant ratio of the concentrations of the two organic modifiers was also tested.