M. Van Beylen

Unreacted Methacrylate Groups on the Surfaces of Composite Resins

The unreacted methacrylate groups on the surface of conventional composites and microfilled composites were studied by means of multiple internal reflection infrared spectroscopy. The influences of polymerization time, temperature, and the effect of polishing were studied. We also attempted to relate the results of some mechanical tests to the degree of chemical unsaturation measured in the different resins. All surface treatments and manipulative variables caused a decrease in double bond content and interfacial bond strength.

Materials Science Characterization of Composite Resins by NMR and TEM

The monomer systems of conventional composites and smooth-surface microfilled composites were studied by means of nuclear magnetic resonance spectroscopy. The monomer composition varied from brand to brand. Obviously, this will result in different physical properties and clinical behavior. Transmission electron microscopy revealed the presence of a free submicron fraction in the monomer system of some restorative resins.

High glass transition chromophore functionalised poly(maleimide-styrene)s for second-order nonlinear optical applications

Nonlinear optical polymers with high glass transition temperatures were prepared by polymer analogous reaction of maleic anhydride copolymers, with aminoalkyl-functionalised azo- and stilbene chromophores. Glass transition temperatures from 178 to 228°C were obtained. Poled films of the polymers were characterised by second-harmonic generation and showed a nonlinear optical response that is stable at elevated temperatures.

The dynamics of electric field effects in ion pairing processes

Abstract The theory of electric field effects in ionic systems is reviewed and their dynamic aspects related to chemical relaxation phenomena in ionic equilibria. The theoretical descriptions are specified for organic, low polar solvents. A critical discussion is given of the field modulation technique, a new relaxation method based on the field dissociation effect and particularly suited for the investigation of ionic equilibria in non aqueous media. The domain of applicability of the method, the inherent limitations and some remaining problems are treated. The value of chemical relaxation measurements for the study of ion pairing phenomena is assessed from experimental investigations. It is shown that the results obtained substantiate the sphere in continuum model as a basic model for ionic interactions; ionic processes in low polar media are well described from a diffusion treatment for hard-sphere ions in a continuous medium. The use of the field modulation technique in carbanion chemistry is described. The way new information is obtained from relaxation measurements on processes such as solvent-separation, ionic dissociation and aggregation is critically discussed.

High glass transition temperature chromophore functionalised poly(phenylquinoxalines) for nonlinear optics

New chromophore functionalised second-order nonlinear optical (NLO) poly(phenylquinoxalines) were prepared by reaction of a bis(1,2-dicarbonyl)chromophore monomer and a tetraamine at room temperature. The heterocyclic polymers have high glass transition temperatures (>195 °C) and can therefore be excellent polymeric materials with high poling stabilities of the NLO effect.

Synthesis of frequency doubling nonlinear optical polymers, functionalized with aminonitro-stilbene dyes. Second harmonic generation in corona poled thin films

Abstract The synthesis and second harmonic coefficients, d 33 , and d 31 as well as the related susceptibilities, χ (2) zzz , of two new series of (NLO-dye methacrylate)-(methyl methacrylate) copolymers were investigated. The NLO-chromophores bound covalently to the polymer chain were 4-nitro-4′-dialkylamino-α-cyano-stilbene, 4-nitro-4′-dialkylamino-stilbene. Thin films were obtained by spin-coating and poled by a positive corona discharge using a needle electrode in order to induce noncentrosymmetry into the systems. Nonlinear susceptibilities, χ (2) zzz , of the films were derived from the analysis of full-angle Maker fringe patterns at 1064 nm. χ (2) zzz values as high as 3.2 · 10 −7 esu could be measured.

Polymers containing nonlinear optical groups in the main chain. Second harmonic generation in corona poled thin films

Abstract Linear polyesters with azobenzene and stilbene chromophores were synthesized and examined for their frequency doubling effect by second harmonic generation measurements. The incorporation of long flexible units results in copolyesters which are soluble in common organic solvents but which show a decrease in Tg; nonlinear optical efficiency χ(2) values as high as 2.6 · 10−9 esu were obtained.

A non-ergodic thermodynamics based not on concentrations but on time fractions. Application to conductance problems of solutions of lithium salts in ethers

ABTRACT Hydrogen bonds or strong dipole-dipole interactions lead to the formation of transient chain-association in solution. The interaction has a given molar energy: AHhnd. In an open chain as for instance in alcohols 0-H---0-H---0-H---0-H---0-H, only the completely inserted molecules are in possession of this energy during the whole time they are inserted. The head and the tail are transient forms during the life-time of which AHbond is transferred from the medium to the molecule or vice-versa. In the ensemble at a given tlme they have no definite energy and their concentrations cannot be calculated by means of a Boltzmann equation. But in a time schedule they can be considered half of the time as possessing AHhnd. and half of the time as free. The classical Guldberg and Waage equilibrium expression using concentrations has to be replaced by another one where time fractions 3 appear. In an alcohol the time fraction 3 during which the molecule escapes from H-bonding and becomes vaporizable is given by the equation (1-3)/~ = KA Cdcohol. The new equations derived from these principles allow to predict correctly the vapor pressures of the alcohols and the solubilities of foreign substances in these solvents and provide a completely new quantitative explanation for the hydrophobicity of alkanes. Non-ergodic association a9qfc:urs between ion pairs as LiCl in erately polar solvents as tetrahydrofuran THF (c 7.53) or tetrahydropyran THP (E ‘ ! d = 5.71), which can form open dimers LiClLiCl or higher associates in these solvents. The formation of these dimers is accompanied by the appearance in the solution of triple anions and cations between which there exists via the neutral dimer a perpetual exchange of a LiCl molecule according to: LiclLi+ + CI* LiclLicl * Li+ + ClLiCl. The triple ions formed in this way cannot be considered as separated thermodynamic entities and do not follow the Guldberg and Waage equilibrium expression. The LiCl group which they have in coininon is sometimes involved in a positive ion and sometimes in a negative one. Only in the neutral dimer it may be considered as in possession of the insertion bond. On the other hand LiCl may also be involved in a dissociation where the Li’ ion becomes specifically solvated by the ether, giving : Licl* LiSn+ + CIHowever this specific solvation is in&mpatibl< with the formation of triple ions because it would prevent the oscillation of LiCl between the triple ions. As a consequence one has to distinguish in the time schedule of a given LiCl a fraction of the time ( O during which it dissociates in an anion and a solvated cation, and the fraction of the time (I-to) during which it is involved in a dimerization process and where triple ions . _ I can be formed. One ks: (1-< “)/E O = p’ (Kd-”)C~i~13’2 where flss is the non-ergodic conshit governing the association of the ion pairs and K d the classical dissociation constant, C ~ I being the concentration of the salt. The quantitative equations based on this t eory allow to explain the peculiarities of the conductance of LiCl solution

Anionic polymerization of methacrylonitrile—I. Polymerization with n-butyllithium as initiator

and the paradoxical effects of the addition of LiCl on the conductivity of Polystyryl-Li solutions and on the kinetics of the anionic polymerization.

Synthesis of frequency doubling nonlinear optical polymers, functionalized with aminocyano- and alkoxycyano-stilbene dyes. Second harmonic generation in corona poled thin films

Abstract The anionic polymerization of methacrylonitrile has been studied at −75° in toluene and with n-butyllithium as initiator. The kinetics of the polymerization were investigated considering the consumption of both monomer and initiator. BuLi disappears relatively slowly and about 50–60% remains unreacted. A simple kinetic scheme cannot therefore be put forward. All possible side reactions have also been examined. The molecular weight study establishes the living character of this system and gives an initiator efficiency of about 0.2. The contribution of low molecular weight products, typical of the polymerization of polar monomers, is also taken into account. In order to obtain a better understanding of the mechanism of this polymerization, in which unreacted initiator is probably engaged in very stable and inactive mixed associated particles, small amounts of THF (known frequently to break down such aggregates) were added to the system. A rather unexpected slow but complete disappearance of the initiator occurs; the conversion at which the rate of monomer consumption levels off depends on the THF concentration.