Experimental confirmation of oscillating properties of the complex conductivity: Dielectric study of polymerization/vitrification reaction (original) (raw)
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Physica B: Condensed Matter, 2007
The theoretical generalization of the Jonscher's relationship for the complex conductivity of carriers moving in self-similar medium is derived. It is shown that the correction derived enters to more general expression, which, in turn, we define as the generalized Jonscher's relationship. The basic idea which was used for the derivation of the relationship is based on the supposition that disordered medium has self-similar property. The derived relationship is confirmed on dielectric spectroscopy data related to sodium nitrite embedded to porous glasses. Based on new relationship there is a possibility to extract additional information about relaxation processes of a system of dipoles from the processes related to conductivity. It is important in the cases when the contribution to relaxation peaks is small and unnoticeable on the background of essential domination of processes related to conductivity.
Polymer, 2011
Polymerization kinetics of tri-ethylene glycol dimethacrylate (TrEGDMA)/2,2-azobis-isobutyronitrile (AIBN) mixtures (0.1% w.t.) at different temperatures was investigated by using dielectric relaxation spectroscopy. The dielectric spectra at the polymerization temperatures studied are dominated by high conductivity leading us to employ the electric modulus representation in order to extract information about the evolution of the system during isothermal reaction. An intense peak appears in the imaginary component of the complex dielectric modulus which is related to conductivity. The variation of the strength of this peak and of its relaxation time with the polymerization time allows us to determine the polymerization degree evolution and the moment in which vitrification is attained, which can be compared with results obtained by temperature modulated DSC in a previous work (Viciosa MT, Hoyo JQ, Dionísio M, Gómez Ribelles JL. Journal of Thermal Analysis and Calorimetry 2007; 90:407e414).
Influence of structural chemical characteristics on polymer chain dynamics
The Journal of chemical …, 2008
A comparative study of the dielectric relaxation behavior of two structurally close polymers containing aliphatic-aromatic side groups was carried out in order to get a better understanding on how slight differences in chemical structure affect the molecular responses to perturbation fields. Specifically, chain dynamics of poly͑2-acryloyloxyethyl-2-naphthalene-2-ylacetate͒ and poly͑2-methacryloyloxyethyl-2-naphthalene-2-ylacetate͒ were studied by broadband dielectric spectroscopy in the frequency range of 10 −2-10 8 Hz and temperature window of 298-403 K. Also, the relaxation behavior of ͑2-acetyloxyethyl-2-naphthalene-2-ylacetate͒, model compound of the polymer side groups, was analyzed. The isotherms representing the dielectric loss in the frequency domain show important conductive contributions, especially at high temperature, which hide the low frequency side of the ␣ relaxation. Conductivity also increases the real component of the complex permittivity in the low frequencies region. Retardation spectra were obtained by minimizing the sum of the squares of the difference between the experimental values of the complex permittivity for each frequency and the analytical ones, predicted by the linear phenomenological theory, using a Tikhonov regularization technique. The spectra present an apparent ␣ peak with an excess wing at short time side resulting from the overlapping of the true ␣ relaxation and a  process. Three absorptions, named in increasing order of time ␥, , and ␣ relaxations, are separated by deconvolution methods. The activation energies associated with the ␥ process are 70.0Ϯ 1.8, 68.0Ϯ 1.4, and 74.8Ϯ 0.8 kJ mol −1 for ͑2-acetyloxyethyl-2-naphthalene-2-yl acetate͒, poly͑2-acryloyloxyethyl-2-naphthalene-2-yl acetate͒ and poly͑2-methacryloyloxyethyl-2-naphthalene-2-yl acetate͒, respectively. The respective activation energies associated with the  relaxation are 121.7Ϯ 2.4, 135.3Ϯ 1.4, and 141.6Ϯ 1.3 kJ mol −1. Values of the shape parameters and the strengths of the relaxation processes were obtained as a function of temperature. The dynamic fragility of the polymers and the model compound was studied and compared with that reported for macromolecular and monomeric systems. Also, the evolution of the size of the correlated domains associated with the ␣ relaxation was estimated. Finally, the ␥ relaxation rather than the  absorption obeys the criteria apparently held by the Johari-Goldstein  processes.
Conformational movements explain logarithmic relaxation in conducting polymers
Electrochimica Acta, 1999
Logarithmic relaxation associated with the electrochemical oxidation of conducting polymer ®lms after polarization to high cathodic potentials for long periods of time can be explained by means of the electrochemically stimulated conformational relaxation (ESCR) model. The key idea in the ESCR treatment is that conducting ®lms attain a compacted and closed structure as they are submitted to cathodic polarization. Moreover, longer reduction times promote a higher compactness of the polymeric structure, making dicult the exchange of counterions between the polymer and the solution during further oxidation, which therefore must be preceded by the relaxation of the structure. A logarithmic dependence between the coecient of cathodic polarization (z c) and the wait time was obtained from chronoamperometric analysis. The ESCR model makes use of this result to explain the retard observed in both anodic chronoamperograms and voltammograms as the cathodic polarization time is shifted to higher values.
Investigation of Local Motions in Polymers by the Method of Molecular Dynamics
Macromolecules, 1980
The main features of local motions in linear polymer chains and in cross-linked macromolecules were studied by the method of molecular dynamics. The chain model consists of particles connected by rigid bonds and interacting with each other and with solvent particles with Lennard-Jones potential forces. Chains with various numbers of units were considered at various concentrations and temperatures. The characteristics of local motions (translational and rotational mobility), normal modes, and cooperative motions and the effect of cross-linking on local chain mobility were investigated. The results of numerical experiments (NE) are compared with analytical results for a viscoelastic Hearst-Harris (H-H) model. It is shown that the characteristics of both local motions and normal modes are close to those of viscoelastic models. The dependence of the characteristic relaxation times of normal modes on the wavenumber virtually coincides with that for the H-H model. The present study suggests that in cross-linked systems the translational and rotational mobility of the cross-link and the adjoining chain elements are greatly hindered. The relationship between the characteristic times obtained in NE and experimental results on dielectric relaxation and polarized luminescence is discussed.
The Journal of Chemical Physics, 2005
The relaxation behavior of poly͑2,3-dichlorobenzyl methacrylate͒ is studied by broadband dielectric spectroscopy in the frequency range of 10 −1 -10 9 Hz and temperature interval of 303-423 K. The isotherms representing the dielectric loss of the glassy polymer in the frequency domain present a single absorption, called  process. At temperatures close to T g , the dynamical ␣ relaxation already overlaps with the  process, the degree of overlapping increasing with temperature. The deconvolution of the ␣ and  relaxations is facilitated using the retardation spectra calculated from the isotherms utilizing linear programming regularization parameter techniques. The temperature dependence of the  relaxation presents a crossover associated with a change in activation energy of the local processes. The distance between the ␣ and  peaks, expressed as log͑f max; / f max;␣ ͒ where f max is the frequency at the peak maximum, follows Arrhenius behavior in the temperature range of 310-384 K. Above 384 K, the distance between the peaks remains nearly constant and, as a result, the ␣ onset temperature exhibited for many polymers is not reached in this system. The fraction of relaxation carried out through the ␣ process, without  assistance, is larger than 60% in the temperature range of 310-384 K where the so-called Williams ansatz holds.
Influence of the Mixing Ratio on the Dynamics of Polymer Segments in Polyelectrolyte Complexes
Macromolecular Chemistry and Physics, 2021
as well as the dynamics of the ion pairs formed between the oppositely charged polyelectrolytes. [4] The ion pairs act as temporary cross-links, and their dynamic formation and breaking is influenced by, e.g., ionic strength and pH. [4,5] Dynamics in polymer systems can be studied by electron paramagnetic resonance (EPR) spectroscopy. [6,7] The absence of EPR signals from the most polymeric material provides the opportunity to apply EPR as a selective probe technique. Various studies dealing with the research of dynamics in polymers by EPR spin-label (SL) and spin-probe techniques have been recently reviewed. [8,9] The techniques use nitroxides which are covalently attached to macromolecules or added to the polymer system. We have demonstrated that rotational dynamics of polymer segments in PECs and polyelectrolyte multilayers (PEMs) can be investigated by SL EPR spectroscopy. [10-16] Recently, we have found that the rotational dynamics of the polyanion backbone in PEMs is influenced by the polyelectrolyte in the terminating layer. [16] We have supposed that poly(allylamine hydrochloride) (PAH) molecules are able to diffuse into the PEM during the adsorption process, which is associated with an excess of PAH and a large number of extrinsic PAH sites inside the PEM. The large number of extrinsic sites reduces the density of the intrinsic binding sites, i.e., the density of the temporary crosslinks between the oppositely charged polyelectrolytes decreases, which results in an enhanced rotational mobility of the polyacid chain segments for PAH-terminated PEMs. [16] We suppose that a similar effect also occurs with dispersions of PECs, i.e., an excess of the polycation should result in enhanced rotational mobility of the polyacid in the core of the PEC. The work presented here has been performed to validate this hypothesis by the study of rotational mobility of poly(ethylene-alt-maleic acid) (P(E-alt-MA)) in PECs with PAH and poly(diallyldimethylammonium chloride) (PDADMAC), respectively, as a function of the mixing ratio of polyanion and polycation. 2. Results and Discussion The spectra of the PEC dispersions of PAH and SL-P(E-alt-MA) prepared according to procedure A in buffer solution of pH 4 The rotational dynamics of the spin-labeled polyacid poly(ethylenealt-maleic acid) (P(E-alt-MA)) in polyelectrolyte complexes (PECs) formed with the polycations poly(allylamine hydrochloride) (PAH) and poly(diallyldimethylammonium chloride) (PDADMAC) are studied by electron paramagnetic resonance (EPR) spectroscopy as a function of the mixing ratio of polycation and polyanion. At low mixing ratios a superposition of two spectral components is found. The fast-motion component observed arises from chain segments of the polyacid in excess, which form a stabilizing shell around the PEC particles. The other one is a slow-motion component, which is caused by the interaction of charged chain segments of the polyacid with the oppositely charged segments of the polycation in the core. At high mixing ratios, the spectra are dominated by the slow-motion component, because the polyacid is exclusively located in the core. The study has shown that the outer extrema separation of the slow-motion component decreases with increasing mixing ratio, i.e., the rotational mobility of the polyacid segments in the core increases when the excess of the polycation becomes larger. An increasing number of extrinsic polycation sites within the core of the PECs which reduces the density of intrinsic binding sites are assumed to be the reason.