Polymerization of oriented monomers. II. Polymerization of 4-vinylpyridine perchlorate in micellar aggregates

Polymerization of 4-vinylpyridinium perchlorate (I) is studied under isotropic and anisotropic conditions. Anisotropy or orientation of I may be achieved by micelle formation in concentrated aqueous or acid solutions, i.e., above the monomer's critical micelle concentration (CMC). Some properties of the isotropically and anisotropically obtained polymers are comparatively studied with the emphasis placed on the elucidation of their structure by ¹³C-NMR spectroscopy. By the same spectroscopic technique the microstructure of 1,2-polymer is further investigated in conjunction with the proposed polymerization model. It is concluded that polymer structure is affected by the organization of the monomers into micelles whereas their microstructure is not.     

Polymerization of oriented monomers. VIII. Polymerization of allyl and diallyl vesicle-forming quaternary ammonium salts

The synthesis of two novel allyl and diallyl vesicle-forming quaternary ammonium salts is reported. The topochemical polymerization of these monomeric vesicles to their polymerized counterparts was performed by γ-ray irradiation and the observed differentiation in polymerization rates of allyl and diallyl monomers was attributed to different polymerization mechanisms. It was further established that only polymerized vesicles which result from diallyl monomer retain the structure of the monomeric vesicles and exhibit simultaneously higher stability.     

A powerful force-based approach for the limit analysis of three-dimensional frames

For the estimation of the strength of a structure, one could avoid detailed elastoplastic analysis and resort, instead, to direct limit analysis methods that are formulated within linear programming. This work describes the application of the force method to the limit analysis of three-dimensional frames. For the limit analysis of a framed structure, the force method, being an equilibrium-based approach, is better suited than the displacement method and results, generally, to faster solutions. Nevertheless, the latter has been used mostly, since it has a better potential for automation. The difficulty for the direct computerization of the force method is to automatically pick up the structure’s redundant forces. Graph theory concepts may be used to accomplish this task, and a numerical procedure was proposed for the optimal plastic design of plane frames. An analogous approach is developed herein for the limit analysis of space frames which is computationally more cumbersome than the limit analysis of plane frames. The proposed procedure results in hypersparse matrices, and in conjunction with the kinematic upper bound linear program which is solved by a sparse solver, the proposed method appears computationally very efficient. It is also proved that it is much more effective than any displacement-based formulation. The robustness and efficiency of the approach are testified by numerical examples for grillages and multi-storey frames that are included.     

Conformational analysis of ochratoxin a by NMR spectroscopy and computational molecular modeling

Two-dimensional NMR spectroscopy has been used for a complete assignment of the proton and carbon-13 spectra of the metabolite from Aspergillus ochraceus, ochratoxin A. In addition, phase-sensitive nuclear Overhauser effect spectrometry experiments and computational molecular modeling (MM2 and MMFF force field programs) have been employed to examine the conformational properties of ochratoxin A in chloroform solutions. Particular attention has been given to intramolecular hydrogen-bonding formation involving the phenolic group on dihydroisocoumarin, which may be responsible for the toxic mechanism of ochratoxin A.     

Molecular relaxation processes in very dilute systems: A 13C NMR study of alcohols in alkanes

¹³C T₁ relaxation times have been determined for n-butanol in C₆D₁₂ in the concentration range 0.001 ? _x ? 1 (using ¹³C labelled alcohol) and for t-butanol in the range 0.01 ? _x ? 1. In the former case, this has allowed us to probe molecular mobility down to the region of monomeric alcohols. Comparison with previous results from measurements of ¹H chemical shifts, dielectric relaxation, and the picosecond dynamics of electron solvation allows us to build up a detailed picture of molecular clustering and liquid structure in alkane—alcohol mixtures. For n-butanol, the local liquid structure is established by x = 0.2 while t-butanol does not appear to form aggregates larger than trimers until x = 0.5.     

Polymerization of head and tail methacrylate micelle-forming surfactants and thermotropic liquid-crystalline character of these monomers and their polymers

The polymerization in micellar and isotropic media of a head-tail pair of surfactants, i.e. of monomers bearing the methacrylate group either at the polar head or at the end of the lipophilic chain is investigated in this study. Owing to the tailor-made structure of these monomers, any differentiations observed in their polymerization can be primarily attributed to topochemical effects resulting from the different location of the polymerizable group in the two monomers. Both monomers exhibited mesomorphic-like textures, which were examined by optical microscopy and DSC studies. Also, their respective polymers showed mesomorphic behaviour, which was found to be dependent on the position of the backbone relative to the polar head.     

A boundedness result for toric log Del Pezzo surfaces

In this paper we give an upper bound for the Picard number of the rational surfaces which resolve minimally the singularities of toric log Del Pezzo surfaces of given index ℓ. This upper bound turns out to be a quadratic polynomial in the variable ℓ. Received: 18 June 2008