Dielectric relaxation in polymer solids Part 2: Application of the new model to polyurethane systems

Measurements of the complex permittivity of crosslinked polyurethanes at different temperatures in the frequency range 1–10⁵ Hz are discussed with respect to shape of relaxation curves. Using a new model (published in preceding paper) the shape parameters are related to small and large scale interaction.     

Time domain dielectric spectroscopy. A new effective tool for physical chemistry investigation

The general principles of time domain dielectric spectroscopy (TDS) are summarized. The methods of data treatment and presentation, and different TDS methods which enable one to obtain the permanent spectrum of ^* () in the frequency range of 10⁵–10¹⁰ are given. The examples of TDS application for the investigation of dielectric properties in samples of different nature and structure are considered in this review.     

Studies of polymer solvation by dielectric relaxation spectroscopy I: polyvinyl pyrrolidone in propylene carbonate,dimethyl sulfoxide and tetramethyl urea

The dielectric relaxation behavior of the title solutions of PVP 40000 is measured in the frequency domain (50 MHz to 36 GHz) at 20 °C. The polymer content of the solutions (up to 0.25 mole fraction of monomer units) is such that it does not yet contribute significantly to dielectric loss. The solvent relaxation shows in all cases a bulk and a slowed down contribution, both characterized by concentration-independent relaxation times. The slow contribution is ascribed to the solvate. Solvation numbers for dilute solutions roughly range between 2 and 4 per PVP repeat unit.     

Dilatometric study of glass transition kinetics in polyisoprene

From dielectric measurements the thermal dependence and the distribution function of the relaxation times can be determined. In the present contribution we are testing the correspondence between the relaxation times obtained from the dielectric measurement and the dilatometric behavior of the material in the vicinity of the glass transition temperature. The experimental data used for the comparison were obtained for the sample of polyisoprene.     

Dielectric relaxation of specially prepared polystyrene terminated with rigid polar groups

Two polystyrenes terminated withp-cyanobenzyl andp-cyanobiphenyl groups (=labels) were prepared and their complex dielectric constants were measured in the glass transition region in the frequency range 10^–2–10⁶ Hz. The glass temperaturesT _g (DSC) were considerably different, 92 and 97.5°C, resp., although their molecular weights were very similar (11 000 and 10 000 g/mol, resp.). Their relaxation behavior showed that the cyano groups relax cooperatively with the polymer segments. The cyanophenyl groups were found to relax with shorter relaxation times than the cyanobiphenyl groups. The measured relaxation strengths showed that there was no association between the dipoles. The relaxation mechanisms of the cyano groups in both labels seemed to be different although the only difference between them was an additional phenyl group in the case of the second label.     

Studies of polymer solvation by dielectric relaxation spectroscopy II: Polyethylene glycol in methyl pyrrolidone

Frequency domain dielectric spectroscopy (30 MHz to 72 GHz) is utilized to study solvation in polyethylene glycol (PEG) — methyl pyrrolidone (MPy) mixtures over the whole mixture range at 20 °C, in particular with PEG 200. Further pure PEGs (200 to 400) and butyl glycol (in the pure state and in benzene and n-hexane solution) are considered for comparison to facilitate the assessment of PEG relaxational behavior. It turns out that the PEG-MPy mixtures can be described in terms of a simple solvation model which takes into account only the bulk and the solvating solvent state. The solvation number per repeat unit is about 0.85 in the dilute solution limit. It decreases gradually with increasing PEG concentration in accordance with a solvation equilibrium model.     

Dielectric relaxation of rod-like labels in polyisoprene

Rod-like molecules with a strong dipole in the long axis were mixed with synthetic polyisoprene (IR305) in low concentrations (0.1–5 wt%) and measured dielectrically in the frequency range 10^–2–10⁶ Hz and the temperature range –60°–0°C (glass relaxation range). The measurements showed an additional symmetrical relaxation process slower than the glass process by a factor ranging from 10 to 100 times, depending on the length and structure of the label. The label relaxation process was characterized by a narrow distribution of relaxation times and its strong coupling with the segmental motion. For the labels with only a longitudinal dipole moment, no relevant part of the label relaxation was observed in the relaxation frequencies of the glass process; this may be attributed to the parallelism and compactness of the polyisoprene chains. In contrast, the labels with an additional vertical dipole component showed an additional relaxation process superimposed on the glass relaxation with also almost the same distribution of relaxation times. The slow label relaxation could be attributed to a diffusion or rotational diffusion relaxation process through a multistep relaxation mechanism determined by the segmental motion of the matrix.     

Recent development in dielectric relaxation spectroscopy of polymers

An evaluation strategy for dielectric measurements in the frequency and/or time domain is presented which provides complete information about a relaxation mechanism (intensity, position, and shape of the relaxation function) independent of overlapping with neighboring mechanisms. As an example results on poly-(ethyleneterephthalate) are given.     

Another approach to the interpretation of various macrophenomena simulated in terms of the properties of the model bodies involved in them and their elementary interactions. I. Gaseous media

In this study attention has been focused on the physical nature of molecular interactions regarded as the function of various properties of the involved molecules and giving rise both to individual pressure impulses () and to variations of their frequency at a constant gas concentration and temperature. The corresponding elementary thermal interactions () have also been taken into account. The quantities and are considered to be of the same physical nature but of different magnitudes. The empirical Van der Waals equation of state for the real gases has been reinterpreted in terms of the elementary interactions. The theoretical predictions following from the developed approach have been compared with and satisfactorily confirmed by the corresponding experimental data available in the relevant literature.     

An interlayer model for the complex dielectric constant of composites

The complex dielectric constant of a composite with an interlayer was studied as a function of the volume fractions and the properties of the filler, the interlayer, and the matrix. The theoretical approach is analogous to the calculation of the shear modulus, the bulk modulus, and the termal expansivity of particulate filled polymers using the interlayer model (IM).An analytical expression describing the influence of an interlayer on the generalized dielectric constant of the composite as a function of the volume fraction and interlayer properties is derived.In the case of a composite with non-conductive constituents, the equations for static and oscillatory electric fields are similar. When conductive constituents are present, the complex dielectric constants have to be replaced by the generalized complex dielectric constants.For a composite of non-conductive materials, without interlayer, the obtained relation reduces to the classical Rayleigh equation. In the case of a composite with conductive constituents, also without interlayer, the complete solution of Wagner's theory is found. Special attention has been paid to the case of a water interlayer in a glass-bead filled non-conductive matrix material.     

Thermotropic gelation of ovalbumin 1. Viscoelastic properties of gels as a function of heating conditions and protein concentration at various pH values

A study has been undertaken of stress relaxation in ovalbumin thermotropic gels with a concentration of 8–20%, depending on time and temperature of heating (respectively, 20–60 min, 70°–110°C), at pH 2.5–10.0. In all instances, the dependence of the initial gel elasticity modulus on heating has a single maximum. Gelation conditions corresponding to this maximum are considered optimal. Optimal gelation time is 30 min, regardless of pH. On the other hand, the optimal heating temperature depends on pH. To the right and left of the isoelectric point of protein (2.5pH<4.0 and 5.5G) of gels on heating conditions, pH and protein concentration (X ₁,X ₂,X ₃,X ₄), as well as on time of relaxation (t) may be generally described asG(X ₁,X ₁,X ₁,X ₁,t)=G _e(X ₁,X ₂,X ₃,X ₄)f(t), whereG _e is the equilibrium value of the elasticity modulus, and f(t) the relaxation function. Thus, a change in the parameters only affects the value of the equilibrium elasticity modulus, and exerts no effect on the relaxation time spectrum. For this reason, all the relaxation curves obtained may be transformed into two normalized relaxation functions:f(t)=f(t)/f(1)=G(X ₁,X ₂,X ₃,X ₄,t)/G(X ₁,X ₂,X ₃,X ₄, 1)Each of these normalized functions corresponds to one of the homologous groups. Rheological similarity of gels in each homologous group evidently points to their structural similarity. Invariance of the gel relaxationproperties with regard to protein concentration, leads to a concentration dependence of the equilibrium modulus at various pH values. These dependences are curvilinear on a double logarithmic scale. The slope of the curve exceeds 2 in the entire concentration interval studied. In other words, the dependences obtained cannot be described by the usual law of squares. On the other hand, they adequately match Hermans theoretical relation for a network formed by random association of identical polyfunctional particles without cyclization. This simple model evidently gives a true picture of the major regularities of thermotropic gelation for ovalbumin. An agreement between this theory and experiment was achieved for a protein concentration ofC ^*=6.0±1.0% at the gel point regardless of pH. Invariance of gelpoint position with regards to pH demands further confirmation.List of symbols T _h,t _h heating temperature and time - T _h ^* ,t _h ^* optimal heating temperature and time - C protein concentration - C ^* protein concentration in gel-point - G relaxation modulus - G _e equilibrium modulus - f(t) relaxation function - t time of relaxation - f(t) normalized relaxation function - fT _A (t), f _B (t) normalized relaxation functions of groups A and B - G ₁ T _h,t _h-reduced modulus - G ₂ T _h,t _h, pH-reduced modulus - G ₃ C-reduced modulus - b ₁ T _h, t_h reduction parameter of modulus - b ₂ pH reduction parameter of modulus - b ₃ C reduction parameter of modulus - W_g gel-fraction     

Dielectric analysis of PMMA microcapsules containing an aqueous KCl solution to derive a distribution function of the release rate of KCl

Release of KCl from a batch of PMMA microcapsules containing an aqueous 3 mM KCl solution was studied with the following two methods: 1) dielectric measurements of suspensions of the microcapsules in distilled water. The electrical conductivity of capsule interior was estimated with this method. 2) Measurements of the conductivity of suspending medium of the microcapsule suspensions. These two kinds of experiments provided us with information about the amount of KCl remaining in the capsules and that diffused outside. Results of these experiments showed that the microcapsules varied in their release rate of KCl. A distribution function of the release rates was derived from the results of the dielectric measurements. That distribution function was successful in explaining the results of the measurements of the conductivity of suspending medium. Furrther examination revealed that the release rate increased with the increase in the thickness of capsule wall.     

Selected topics in biomedical polyurethanes. A review

Published literature on biomedical polyurethanes is reviewed. Selected topics concerning chemistry, structure-property relations, tissue-material interactions, surface properties, applications, processing, and sterilization, etc., are discussed.Presented at the SPE Scandinavia Seminar on Medical Polyurethanes, Oxford, UK, September, 1988.     

The salt-induced sphere-rod transition of micelles of dodecylpyridinium iodide in aqueous NaI solutions

Static light scattering has been measured on aqueous NaI solutions of dodecylpyridinium iodide (DPI) over NaI concentrations from 0 to 0.05 M. Reduced intensity of scattered light increases with increasing DPI concentration above the critical micelle concentration. The Debye plot is generally a curve with an initially positive slope and with a weakly convex, upward curvature, when the NaI concentration ranges from 0 to 0.005 M. The molecular weight of the spherical micelle of DPI is 28400 in water, and it increases slightly with increasing NaI concentration. The initial slope decreases with increasing NaI concentration and changes from positive to negative across 0.007 M NaI, which is the threshold for the sphere-rod transition and where the micelle has a molecular weight of 34400. At NaI concentrations from 0.01 to 0.05 M, the Debye plot is a curve with an initially negative slope and with a convex, downward curvature. The magnitudes of slope and curvature are larger, and the rodlike micelles of DPI have larger molecular weight and stronger mutual interaction, as the NaI concentration increases. The linear double logarithmic relationship between molecular weight and ionic strength holds for spherical and rodlike micelles, respectively.     

Spherical shape instability induced changes in the structure of microemulsions

The dispersed phase of a microemulsion in equilibrium with the excess dispersed phase is in the form of spherical globules. A change in the structure of the microemulsion near the transition from two to three phases is predicted on the basis of thermodynamic considerations. This change occurs because the spherical interface of the globules becomes unstable to perturbations. Because this instability is a result of the condition of equilibrium itself, there is a similarity (but not identity) to the behavior near a critical point. The spherical interface can also become unstable to perturbations in some single-phase microemulsions, although it is stable in microemulsions that coexist with the excess dispersed phase. Thermodynamic considerations also indicate that at the transition from two to three phases, the curvature of the spherical globules must be zero. This result is extended to the entire middle-phase microemulsion as the condition of zero mean-time or space-average curvature. Such a condition can be satisfied in the neighborhood of the transition points from two to three phases and again from three to two phases via the oscillations of the globular interface; in the central region of the middle phase, where the volume fraction of oil0.5, this condition is likely to be satisfied via the oscillations of the interface of a bicontinuous sponge.     

Orientation,recognition, and photoreaction of nucleolipids in model membranes

Amphiphiles with nucleobases and nucleosides as headgroups have been synthesized. Their surface behavior was investigated in monolayers at the air/water interface. The double chain nucleolipids form stable monolayers with nearly identical surface pressure-area diagrams, whereas the spreading behavior of the mono chain amphiphiles is dominated by the various nucleobase-headgroups. When measuring the interactions between nucleolipid monolayers and nucleobases (monomeric and polymeric ones), specific base-base effects could be observed: the complementary nucleobases solubilized in the subphase expand the monolayer more than the non-complementary ones. Photodimerization reactions of thymine-amphiphiles were investigated in mono- and multilayers as well as in spin-coated films.     

Chain dynamic parameters for some acrylic polymers

The dielectric relaxation data of Ishida et al. on a number of acrylic polymers are represented in terms of the relaxation function proposed by Havriliak and Negami using the multi-response techniques developed by Havriliak and Watts. Two of the parameters of this function are interpreted in terms of a temperature dependent distribution of relaxation times. In this method of interpretation the breadth of the distribution function is temperature-dependent while the skewness is not. The temperature dependence of the breadth of the distribution function is similar for most of these acrylic polymers.The parameters of the relaxation function are also interpreted in terms of Mansfield's model which represents intra- and inter-molecular interactions in terms of springs and dash pots. Briefly, increasing the side chain length for the methacrylate series increases the inter-molecular relaxation time which may be due to an increase in the entropy of activation for the orientation process. The difference between the one acrylate in this study and the four methacrylates of the series is a reduction in the intra-molecular relaxation time, apparently due to the lack of the alpha methyl group.     

Correlation between transport and equilibrium properties through the ternary interaction parameter for cosolvent and cononsolvent polymeric systems

A study of the ternary polymer systems dimethyl formamide-ethyl acetate-polystyrene, chloroform-1,4 dioxane-polystyrene and tetrahydrofuran-chloroform-polystyrene was carried out by viscosity and light scattering at 298 K. A good correlation has been found between the excess intrinsic viscosity, unperturbed polymer dimensions, second virial coefficient and the excess Gibbs free energy by using a ternary interaction parameter, dependent on molecular weight. This modification enables the conversion between transport and equilibrium properties.