Viscous Matrices Research Articles

Phosphorescence spectra of bromobenzene in highly viscous glassy matrices

Phosphorescence spectra of bromobenzene have been observed in glycerol, 3-methyloctane and 4-methylheptane glassy matrices. The phosphorescence quantum yield has been determined to be ca. 0.03 in glycerol at 77 K. The phosphorescence is composed of two emissions, from 3 π π ∗ and 3 π σ ∗ to the ground state. The phosphorescence radiative lifetime is 14 times shorter than that of chlorobenzene and this shortening has been explained by the internal heavy atom effect. The non-radiative rate constants were independent of the solvent species but depended only on the solvent viscosity. The non-radiative decay rate constant was 10 2 times larger than that of chlorobenzene.

Time evolution of stress redistribution around multiple fiber breaks in a composite with viscous and viscoelastic matrices

We develop an efficient computational technique, called viscous break interaction (VBI), to determine the time evolution of fiber and matrix stresses around a large, arbitrary array of fiber breaks in a unidirectional composite with a matrix that creeps. The matrix is assumed to be linearly viscoelastic or viscous in shear following a power-law in time (creep exponent 0 ⩽ α ⩽ 1), and interface debonding or slip is not permitted. Such a law is applicable to polymeric matrices over a wide range of temperatures or to a viscous, glassy interphase in a ceramic composite with elongated microstructure. Specifically, we consider an infinitely large, 2-D composite lamina in the shear-lag framework of Hedgepeth, and the multiple break formulation is built on weighted superposition using influence functions based on the response to an isolated break. We apply the method to problems involving large transverse cracks (i.e., aligned, contiguous breaks), fully bridged cracks, and arrays of interacting, longitudinally staggered breaks. In each case we calculate the time evolution of stress concentrations and displacements of individual fibers. In comparing cracks vs spatially staggered breaks, the results reveal interesting contrasts in the time variation of both peak fiber stress concentrations and break opening displacements. In the latter case, we see behavior consistent with the three stages of creep, and show how the local fiber tensile stresses can rise (and subsequently even fall) at rates depending on various microstructural length scales. The motivation for developing VBI is to provide the computational framework for modeling the statistical features of the lifetime of composites in creep-rupture resulting from an accumulation of many fiber breaks and ultimately localization and collapse.

An optical study of silver particles fabricated by ion implantation in a silicon polymer

Polymer layers containing Ag were fabricated by 30 keV Ag-ion implantation into viscous and solid-state silicone organic matrices. The optical properties of the composites are presented by means of transmission spectra in the range from 250 to 870 nm for different ion doses of 0.05- 1.8 1017 ions cm-2. The absorption at the wavelength of the surface plasmon resonance of Ag particles at doses higher than 1016 ionscm-2 was determined. A remarkable red shift in the absorption maximum from 405 to 505 nm was observed with increasing dose and with a change in the viscosity of the irradiated polymer. The optical spectra of the implanted polymer were analysed using the Maxwell-Garnett effective-medium theory. The dose dependences of the calculated value of the metal volume fraction in the implanted polymer suggest the possibility of achieving a higher degree of coagulation of introduced metal ions to metal particles in a viscous polymer than in a solid-state matrix.

Adhesive strength in the interaction of polyamides with aramid fibres

Wetting of organic fibres by polyamide melts was investigated. On the example of Terlon, it was shown that the contact angle of wetting increases in the order PA-610 < PA-6 < < PA-12. The polymers are also in the same order in wetting of a steel wire. Adhesion on the interface was investigated in the same systems. PA-610 in the form of powder exhibited maximum adhesion to steel wire and organic fibres. The necessity of using highly viscous matrices in the form of finely disperse systems for obtaining reliable and reproducible adhesive strength values was demonstrated. The validity of using steel wire as a model substrate was demonstrated and the qualitative characteristics of the change in the adhesive strength under the effect of factors which primarily alter the properties of the polymer matrix were found. The results obtained in studying the shear strength of the composites are correlated with the results of determining the adhesive strength in their unit cell. This allows recommending the shear method for evaluating adhesion in industrial conditions.

Effective properties of porous ideally plastic or viscoplastic materials containing rigid particles

This study deals with the overall yield surface of porous materials containing rigid inclusions and governed by a generalized Gurson model. The dissipation potential of these voided materials can be obtained in a closed form. An estimate for the effective dissipation potential of the composite is derived in terms of the strain energy of a linear comparison solid. The corresponding yield surface is described by a generalized Gurson criterion. These results are extended to voided viscous matrices. The accuracy of the analytical estimates is assessed by comparison with cell calculations performed by the finite element method.

THE FORMATION OF VISCOUS DAMPING MATRICES FOR THE DYNAMIC ANALYSIS OF MDOF SYSTEMS

The paper deals with the representation of dissipative effects by means of equivalent viscous forces. A brief review of the classical analytical treatment of the subject is first presented devoting particular attention to the topics of hysteretic and modal damping. The problem of forming viscous matrices in the case of systems which are non-homogeneous from the point of view of dissipation is then addressed; soil-structure systems are first considered and some accepted techniques for forming the structure contribution to the viscous matrix are reviewed. A different technique is then proposed which avoids some of the drawbacks of the previously quoted methods. In the final section of the paper it is shown how this technique is easily applicable also in the case of systems having internal concentrated dampers of viscous type, this being a situation which is difficult to tackle with usual criteria.

Photochromism of naphthoxazine-spiro-indolines by direct excitation and following sensitisation by triplet-energy donors

Quantum efficiencies of merocyanine formation, (ΦA), have been measured for several naphthoxy-spiro-indolines, (NOSIs) in a variety of solvents as well as in several polyurethane matrices having different glass-transition temperatures. In toluene solution ΦA values range from 0.74 to 0.33 with those compounds substituted with electron-donating substituents in the 6′-position on the oxazine ring giving the highest yields. Quantum yields for merocyanine formation are also sensitive to change of solvent, being the highest in toluene and the lowest in methanol. Interestingly, in highly viscous polyurethane matrices quantum yields are typically in the range 0.1 to 0.2. Surprisingly, replacing the methyl group on the nitrogen in the indoline part of the molecule with an isobutyl group has no effect on the quantum yield in either toluene or in a polyurethane matrix. Triplet-energy donation has been used to establish that a triplet pathway leading to merocyanine formation is available and that the efficiency of merocyanine formation via triplet-state sensitization is close to unity for each of the three derivatives studied in toluene as solvent. By using various triplet-energy donors and measuring the values of the resulting quenching rate constants the lowest triplet energy of 1,3,3-trimethylspiro(indoline-2,3′[3H]naphth[2,1-b]-6′-indolino[1,4]oxazine) has been determined as 200 ± 5 kJ mol–1 which is consistent with sensitization via the triplet state associated with the oxazine part of the molecule. For donors with triplet-state energies > 210 kJ mol–1 quenching constants were shown to reach a plateau value of 4.5 ± 0.3 × 109 dm3 mol–1 s–1.

Kinetic Studies Concerning the Decay of Nitronic Acids in Polymer Matrixes Using a Data Evaluation Method Based on Dispersive Kinetics

Evaluation methods based on dispersive kinetics were applied to derive kinetic parameters for the rearrangement of differently α-substituted nitronic acids generated by flash photolysis (λ inc = 347 nm) of o-nitrobenzyl esters. The rearrangements were performed in polymer matrices of different chemical nature: poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), poly(styrene) (PSt), and poly(dimethylsiloxane) (PDMS). A nonexponential decay behavior was observed in rigid and highly viscous matrices which can be characterized by the average rate constant v and the dispersion factor α

Origin of the decrease in chromatographic resolution induced by the addition of viscous matrices in liquid chromatographic—fast atom bombardment mass spectrometric systems

The monitoring of the chromatographic resolution for three pairs of analytes separated in different chromatographic systems using mobile phases with varying concentrations of viscous fast atom bombardment matrices showed that it exhibits a steady decrease with increasing matrix content in the mobile phase. The decrease in resolution is observed in the partition and ion-pair chromatographic modes at both low and high matrix contents in the eluent for both conventional and capillary chromatographic systems using precolumn addition of glycerol and thioglycerol. Careful examination of the normalized efficiency, capacity factor and selectivity terms contributing to the resolution allowed the identification of the sources of the decrease in resolution in the presence of a matrix in the eluent. Ale efficiency and capacity factor terms show decreases with increasing matrix content in all systems. Subtle variations in the selectivity term observed in the presence of a viscous matrix can increase or limit the decrease in resolution. The variations observed for the efficiency term show similar trends in all systems studied and appear to be independent of the analyte or the chromatographic mode. However, the variations in capacity factor and selectivity induced by the presence of the viscous matrix are dependent on the nature of the analyte, the type of chromatography and the nature of the matrix.

Evaluation of the performance of capillary liquid chromatography—fast atom bombardment mass spectrometry systems with precolumn addition of glycerol as a viscous matrix

A series of experiments were conducted in order to evaluate the boundaries within which precolumn addition of viscous matrices can be systematically used in capillary LC—fast atom bombardment MS systems and to evaluate the broadening caused by components of the systems such as the column, the transfer capillary tube and the probe interface. The effect of the addition of a viscous matrix on the capillary system was studied by monitoring important chromatographic parameters such as capacity factors, selectivity, number of theoretical plates, peak width and resolution. The results obtained indicate that glycerol contents in the mobile phase higher than 5% have a deleterious effect on most chromatographic indicators. The overall effect of the presence of glycerol on the chromatographic system can be rationalized in terms of the modification of the analyte distribution between the mobile and stationary phases and changes in the kinetics of the system created by an increase in the viscosity of the mobile phase. Ale main contribution to band broadening in the system can be attributed to the interface and is related to the formation of a liquid droplet at the tip of the probe. Other contributions such as broadening in the column and in the transfer capillary affect the total variance of the chromatographic system but to a smaller extent.

Limits and effects of precolumn addition of thioglycerol in liquid chromatographic-fast atom bombardment mass spectrometric systems

A series of experiments were conducted in order to determine the effect of the precolumn addition of a viscous matrix such as thioglycerol on the chromatographic performance of liquid chromatographic-fast atom bombardment mass spectrometric (LC-FAB-MS) systems. In those experiments, the concentration of thioglycerol in a mobile phase consisting of acetonitrile-water-trifluoroacetic acid was varied and important chromatographic parameters such as retention times, capacity factors, number of theoretical plates, peak widths, resolution and impedance of separation were monitored for analytes such as met-enkephalin, leu-enkephalin and p-hydroxybenzoic acid. The results obtained indicate that for concentrations of thioglycerol in the mobile phase below 3% most chromatographic indicators are only slightly affected. However, for concentrations of the viscous matrix above that value the capacity factors are significantly decreased, indicating that thioglycerol is behaving as an efficient organic moderator, and peak broadening becomes important, having a detrimental effect on the performance of the system. Van Deemter plots obtained for the analytes at concentrations of thioglycerol in the mobile phase of 0-15% reveal that the major effect of thioglycerol is to reduce the mass transfer efficiency in the chromatographic system at high velocities and concentrations of thioglycerol above 3%. Comparison of the effects of viscous matrices such as thioglycerol and glycerol on the chromatographic performance of LC-FAB-MS systems indicates that the chromatographic efficiency is almost independent of the matrix concentration when the systems are operated near their optimum linear velocities and that high matrix contents and high linear velocities can be used with little decrease in efficiency if the systems are operated at higher temperatures.

Effect of the addition of viscous matrices to the mobile phase on chromatographic performance in liquid chromatography-fast atom bombardment mass spectrometry

The effect of the pre-column addition of a viscous matrix to the mobile phase in liquid chromatographic-fast atom bombardment mass spectrometric experiments was studied with respect to the chromatographic process. A series of experiments, designed to discriminate against the mass spectral components, were conducted with six compounds, ranging in mass from 100 to 1100 daltons and distributed into three chemical classes. Several chromatographic indicators such as retention times, capacity ratios, number of theoretical plates, peak widths, resolution and separation impedance were monitored as a function of the glycerol content of the mobile phase. The results obtained indicate that the retention times and the capacity ratios decrease with increasing glycerol content of the mobile phase. Increasing concentrations of glycerol also reduce the number of theoretical plates in the chromatographic system and generally have a detrimental effect on peak widths for glycerol contents above 5%. However, lower glycerol contents produce negative effects on compounds with smaller capacity ratios but not on compounds with higher capacity ratios such as peptides. Furthermore, the increase in glycerol concentrations reduces the chromatographic resolution for all classes of compounds studied and creates a significant increase in the separation impedance of the system, resulting in higher operation pressures. The overall effect of a viscous matrix in the chromatographic system can be rationalized in terms of the modification of the analyte distribution between phases and changes in the kinetics of the system created by an increase in mobile phase viscosity.

Discrimination between polarity and mobility effects on the charge‐transfer fluorescence of Fluoroprobe in organic matrices

AbstractThe fluorescence of Fluoroprobe, a molecule with an extremely polar excited state, is studied in matrices consisting of (macro)molecules which contain repeating units of different polarity. In matrices of low molecular weight Fluoroprobe displays its typical bathochromic shift with increasing contribution of polar groups, but an hypsochromic shift is observed in viscous or solid matrices of high molecular weight. This remarkable phenomenon is attributed to loss of mobility, which allows fluorescence of the probe to occur prior to full relaxation of the surrounding matrix sites.

Translational diffusion of small molecules in polymers. Laser flash experiments with polydimethylsiloxanes

The quenching of triplet-excited acetophenone by naphthalene served as a probe reaction for studying the mobility of small molecules in polymeric matrices. The rate constants of the quenching process in polydimethylsiloxanes having weight-average molecular weights between 3600 and 86000 were found to depend only slightly on the macroscopic viscosity η at constant temperature: kq ∝η −0,1. In the temperature range from 190 to 298 K (Tg = 152 K), the activation energy of the quenching reaction was determined as Eq = 15,1 kJ/mol, i.e. it is practically equal to the value of the activation energy of viscous flow. The high translational mobility of small molecules in very viscous polymeric matrices is explained on the basis of the concept of microviscosity according to which the trnaslational motion of small molecules in polydimethylsiloxanes at T > Tg is determined by the mobility of free volume elements due to segmental motions rather than by motions of the centers of mass of the macromolecules.

Finite Element Model with Nonviscous Damping

A constitutive law by which structural damping is modeled as a relationship between stress, strain, and strain rate in a material is used in conjunction with the finite element method to develop general integral expressions for viscous and nonviscous damping matrices. To solve the set of nonlinear equations resulting from the presence of nonviscous damping, a solution technique is developed by modifying the Newmark method to accommodate an iterative solution and treat the nonviscous damping as a pseudo-force. The technique is then checked for accuracy and convergence in single- and multi-degree-of-freedom problems, and is found to be accurate and efficient for initial-condition problems with small nonviscous damping.

Selected aspects of photochemistry in polymer media

Abstract A number of selected aspects of the reciprocal interactions of polymers with excited solutes or polymer-bound chromophores are given. The role of free volume, glass-transition temperature (Tg), microscopic viscosity, and polarity or environments to which excited molecules are exposed is emphasized in order to illustrate possible manners in which the photophysical, photochemical, and subsequent chemistries can be influenced. Many of these factors can affect several monomolecular and bimolecular processes encountered in the sensitization of photocrosslinkable polymers: the photophysics of the sensitizer, energy transfer to the reactive sites on the polymer, and the formation of crosslinks. In fact, the triplet yields of aroylnaphthothiazole derivatives, a widely used class of sensitizers, are considerably higher in the more viscous polymeric matrices. These 0. in the polymer approach, however, a value of only ca. 0.7. As a result a search for more efficient triplet sensitizers led to a new class of compounds: 5- and 7-substituted 3-ketocoumarins. As a model for study of the bimolecular processes in polymers, we chose exciplex and excimer probes. The following conclusions were drawn: 1. Several exciplex and excimer emissions in polymeric media are considerably shifted to shorter wavelengths as compared with the maxima measured in fluid media, indicating that interactions are impaired in polymeric matrices. 2. Emissions from the polymer matrix above the glass-transition temperature are similar in wavelength and temperature dependence thereof to those observed in fluid solutions. 3. Improper orientation of the reactants in polymers is responsible for the shift observed in excimer emission and for a part or all of the shift in exciplex emissions. 4. Little if any difference is observed between exciplex emissions in polymers of low and moderate macroscopic polarities. This may be attributed to two causes: a) Due to improper orientation the dipole moment of the exciplex in the polymer is expected to be smaller and, therefore, less solvation energy can be gained. b) The segmental motion of the polymer required to properly solvate the complex is probably too slow at room temperature compared with the lifetime of the exciplex. 5. Bichromophoric molecules, which form exciplexes in fluid media, fail to reach an exciplex configuration when dissolved in polymers.

Generation and properties of banded viscous damping matrices

Numerical analysis of discrete vibratory systems subjected to broad band excitation usually requires specification of a viscous damping matrix. A method of determining the entries in such matrices directly is presented in this paper. The modal damping ratios, which can normally be estimated, are used to produce a fully populated damping matrix. To facilitate numerical integration of the equations of motion, a procedure termed stripping has been established for reducing the matrix to banded form. Additionally, using a procedure termed replacement, small element matrices were identified, extracted from the fully populated damping matrix and utilized (much as element mass and stiffness matrices are) to construct the global damping matrix. The effects of stripping or replacement were evaluated for a number of boundary conditions on plates, beams, and shells. Errors caused by variation of the number of system degrees of freedom, the number of diagonal rows remaining in the stripped matrix, and the magnitude and mo dal distribution of damping ratios were studied. Results were, in general, good. Damping ratios, damped natural frequencies, and eigenvectors were closely approximated by both type of systems.

Electrons in Condensed Media

AbstractThe effects of changes of temperature and pressure and of illumination on the shape and position of the absorption spectrum and ESR saturation curve of electrons trapped in rigid or viscous liquid matrices are described. Data are presented to show that the Arrhenius equation does not adequately describe the influence of temperature on the rates of diffusion‐controlled reactions of the solvated electrons in glassforming liquids, and that pulse‐radiolysis studies of these systems allow a rigorous test of theories of these reactions and in particular demonstrate the reality of the predicted phenomenon of time dependant rate constants.The conclusions drawn from this information are: 1) the wide structureless optical spectrum of es− and et− is partly due to a distribution of electrons in holes of different configurations; 2) that the trapping of electrons is facilitated by the pre‐existence in the medium of “voids” but that reorganisation of some cavities occurs after electron capture; 3) an increase of temperature by increasing the possibility of rotational motion of molecules in the walls of the cavity facilitates both the relaxation of traps and the release of electrons from them and also increases the probability of retrapping a mobile electron; 4) over a large temperature range log k = A ‐ B/(T ‐ T0) where T0 is the temperature below which rotation sufficient to permit translation of molecules is impossible and B is inversely proportional to the thermal expansion coefficient of the liquid; 5) that quantum mechanical tunnelling of solvated electrons occurs and is the major factor influencing the rate of reaction of er− at temperatures in the vicinity of T0 and 6) that absorption of light causes mobilisation of et− and hence photo conductivity and use of light of different wavelengths enables perturbation of the trap distribution.