Dielectric Hole Burning Research Articles

Mechanical spectral hole burning in glassy polymers-Investigation of polycarbonate, a material with weak β-relaxation.

Mechanical spectral hole burning (MSHB) has been used to investigate the nonlinear dynamics in polymers, ranging from melts, solutions, block co-polymers, and glasses. MSHB was developed as an analog to the dielectric spectral hole burning method, which is not readily applicable in polymers due to weak dielectric response. While similar holes were observed in both mechanical and dielectric hole burning, the interpretations were different. In the latter case, it has been argued that the holes are related to dynamic heterogeneity as related to an increase in the local temperature of molecular sub-ensembles (spatial heterogeneity), while in the former case, the holes have been related to the type of dynamics (rubbery, Rouse, etc.). Recent work from our laboratories used MSHB to investigate glassy poly(methyl methacrylate) and showed evidence of hole burning and supported the hypothesis that the origin of holes was related to dynamic heterogeneity as evidenced by the holes being developed near the strong β-relaxation in PMMA. In this work, MSHB is used to study polycarbonate, which has a weak β-relaxation, and the results are compared with those observed in PMMA. We observe that the polycarbonate exhibits weak holes and the nature of the holes with a change in pump amplitude and frequency is different than observed in PMMA. These results support the hypothesis that the hole burning observed in amorphous polymers below the glass transition temperature is related to the strength of the β-transition, which, in turn, is related to molecular level heterogeneity in the material dynamics.

Third order susceptibilities in supercooled liquids and the “box model” theory versus experiments

We give here the exact expression of the time dependent third order polarization predicted by the "box model" when a supercooled liquid is permanently subjected since time t = 0 to a strong electric field E(t) = E(0) sin(ωt), Dielectric Hole Burning experiment. We derive different forms of the time dependent susceptibilities, both at frequencies ω and 3ω, and examine particularly one of them. We also show that its susceptibility at ω should exhibit, at short times, presently undetected oscillations at frequency 2ω that are a signature of the "box model." We finally compare, for a large frequency range, the ω and 3ω time independent susceptibilities with the corresponding measurements on glycerol at 204.7 K. The agreement is good at frequency ω but, as already shown in Ladieu et al. [J. Chem. Phys. 134, 194507 (2011)], it is not the case at 3ω.

Heating liquid dielectrics by time dependent fields

Steady state and time-resolved dielectric relaxation experiments are performed at high fields on viscous glycerol and the effects of energy absorption from the electric field are studied. Time resolution is obtained by a sinusoidal field whose amplitude is switched from a low to a high level and by recording voltage and current traces with an oscilloscope during this transition. Based on their distinct time and frequency dependences, three sources of modifying the dynamics and dielectric loss via an increase in the effective temperature can be distinguished: electrode temperature, real sample temperature, and configurational temperatures of the modes that absorbed the energy. Isothermal conditions that are desired for focusing on the configurational temperature changes (as in dielectric hole burning and related techniques) are maintained only for very thin samples and for moderate power levels. For high frequencies, say ν > 1 MHz, changes of the real temperature will exceed the effects of configurational temperatures in the case of macroscopic samples. Regarding microwave chemistry, heating via cell phone use, and related situations in which materials are subject to fields involving frequencies beyond the MHz regime, we conclude that changes in the configurational (or fictive) temperatures remain negligible compared with the increase of the real temperature. This simplifies the assessment of how time dependent electric fields modify the properties of materials.

Is dielectric hole burning a quantitative method for the study of supercooled liquids?

A brief review is presented of the situation concerning the dielectric hole burning technique applied to supercooled (molecular) liquids. Some of the problems related to recently obtained results are discussed.

Comment on “Measuring the Configurational Heat Capacity of Liquids”

Recently, L. M. Wang and R. Richert [1] proposed a dielectric hole burning technique to measure the configura- tional specific heat,

Mechanical hole burning spectroscopy in an SIS tri‐block copolymer

AbstractWe have previously developed a mechanical hole burning spectroscopy (MSHB) technique that promises to be a powerful tool for the investigation of dynamic heterogeneity of polymeric materials. This is because, unlike its dielectric analogue, MSHB can be used to characterize materials having a weak dielectric response, a particular feature of polymeric materials. However, while both mechanical and dielectric hole burning show behaviors that are consistent with dynamic heterogeneity in the materials, it is still unclear what the relationship between the hole properties, for example, frequency and amplitude, and the actual nature of the heterogeneity and particularly the length scale being probed. Here, we provide first evidence that a known length scale can be probed by the MSHB method by using a tri‐block copolymer as a “calibration” sample. The heterogeneity of a styrene‐isoprene‐styrene copolymer was investigated in the vicinity of the order‐disorder transition temperature (ODT). It was found that the amplitudes of the mechanical holes gradually decrease as the order‐disorder transition is traversed from the region with ordered structures. In the disordered state of the block copolymer, no apparent mechanical holes were detected. In contrast, mechanical holes were burned in the heterogeneous region and the hole amplitude increased as the depth into the ordered structure increased. Hence, the MSHB technique provides a qualitative correlation between the amplitude of the burned holes and the corresponding heterogeneity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3277–3284, 2007

Heterogeneous thermal excitation and relaxation in supercooled liquids

We investigate a phenomenological model which rationalizes the effects of dielectric hole burning on the basis of heterogeneous dielectric and specific heat relaxation in supercooled liquids. The quantitative agreement between model predictions and dielectric hole-burning observations is lost if the assumption of correlated dielectric and thermal relaxation times is removed from the model. This suggests that dynamically distinct domains in real liquids are associated with a time constant which characterizes both the structural and thermal relaxation behaviors. The calculations demonstrate that the observed burn-induced modifications reflect the spectral selectivity and persistence time of the fictive temperatures within these domains, and that 100 or more cycles of the sinusoidal burn field can be required to saturate the heat accumulated in the slow degrees of freedom. It is also shown that the recovery of dielectric holes is entirely accounted for by the model, and that the persistence times do not provide direct insight into rate exchange processes. Additionally, the model predicts that the heating effects considered here are a significant source of nonlinear dielectric behavior, even in the absence of deliberate frequency selective hole burning.

Nonresonant dielectric hole burning in neat and binary organic glass formers

Binary mixtures of the molecular glass former 2-picoline in oligostyrene, in which the dielectric response of 2-picoline exhibits a particularly broad distribution of correlation times, are investigated by nonresonant dielectric hole-burning (NDHB) spectroscopy and the results are compared with NDHB in neat systems, in particular, glycerol. It turns out that in both substance classes spectral selectivity is achieved, which indicates that dynamics is heterogeneous, i.e., slow and fast responses coexist in the material. However, in binary systems the position of the spectral modifications is completely determined by the spectral density of the pump field, and thus shifts linearly with burn frequency as expected, also at pump frequencies around the alpha-relaxation maximum. It is shown that in binary systems the lifetime tau(rec) of the spectral modifications is determined by the burn frequency omega(p) and exceeds its inverse by about one order of magnitude, indicating long-lived dynamic heterogeneity. The data are described in terms of a previously suggested model of dynamically selective heating, which was extended to include intrinsic nonexponential relaxation. It turns out that the spectral broadening in binary mixtures is not only due to pronounced dynamic heterogeneity, but partially also due to intrinsic broadening of the relaxation function.

Mechanical Hole Burning Spectroscopy: Evidence for Heterogeneous Dynamics in Polymer Systems

We have developed a mechanical spectral hole burning (MSHB) scheme that is analogous to nonresonant dielectric spectral hole burning (DSHB). DSHB experiments have been performed close to the glass temperature and interpreted in terms of dynamic heterogeneity. Here we find that holes are burned far above the glass temperature and in the terminal regimes for a branched polymer melt and a polymer solution. The results suggest that MSHB is a potentially powerful tool with which to examine dynamics of complex fluids.

Dielectric hole burning: Signature of dielectric and thermal relaxation time heterogeneity

We have calculated the effects of dielectric hole burning on the basis of the frequency dependent dielectric relaxation ε̂(ω) and specific heat ĉp(ω) of glycerol and assuming heterogeneous dynamics where the dielectric and thermal relaxation times are correlated quantities. The heat generated in the sample is determined from the time dependent electric field and polarization, i.e., without involving steady-state approximations. The calculations mimic the protocol of recent hole-burning experiments on glycerol [K. Duvvuri and R. Richert, J. Chem. Phys. 118, 1356 (2003)], including the high-field burn-process, the phase-cycle employed to eliminate the linear response to the high field, the detection in terms of the electric modulus M(t), and the signal normalization. Without using any fit parameters, this model is capable of quantitatively reproducing the amplitudes, shapes, and peak positions of observed dielectric holes, together with their dependence on the burn frequency and the waiting time between the pump and probe processes. Therefore, frequency selective local heating of the modes of motion associated with the structural relaxation resulting from the dielectric loss fully accounts for the dielectric hole burning effects.

Dielectric hole burning in an electrical circuit analog of a dynamically heterogeneous system

The dielectric properties of a dynamically heterogeneous material are represented by an R– C network in order to study the effects of spectrally selective Joule heating for the situation encountered in (non-resonant) dielectric hole burning (DHB). The energy irreversibly transferred to the system is calculated for a finite number of high electric field sine waves, corresponding to the ‘burn’ cycle preceeding the polarization measurement in a DHB experiment. As a result of the residual polarization, heating continues after the externally applied voltage is set to zero. Modelling temperature-dependent relaxation times by a common negative temperature coefficient for the resistive elements is the only non-linearity of the electrical circuit analog required to reproduce most of the major features observed in DHB experiments.

Dielectric hole burning in the high frequency wing of supercooled glycerol

We have performed dielectric hole burning experiments on supercooled glycerol at frequencies up to five decades above the peak frequency of the dielectric loss. While most observations of heterogeneous dynamics refer to the slower than average relaxation times, this study confirms the existence of independent relaxation time scales across the high frequency wing of the relaxation time distribution. In this short time regime, the persistence time of the spectrally selective modifications is solely a matter of the burn frequency and no longer determined by the structural relaxation time. For a fixed value of the burn field, the amplitudes of the relaxation pattern changes measured along the log t scale (horizontal holes) are independent of the burn frequency ωb and dielectric loss ε″(ωb). This latter feature is observed in a range of burn frequencies which exceed the loss peak frequency by a factor of 10 to 3×105, where ε″(ωb) varies between 16 and 0.17.

The modulus of dielectric and conductive materials and its modification by high electric fields

An alternative to measuring dielectric polarization or conductivity in the time domain under the usual constant field conditions is to record the voltage at constant charge. In this case the voltage is proportional to the dielectric modulus M( t), defined in the frequency domain via M *( ω)=1/ ε *( ω). The method allows the measurement of dynamics at extremely long times, while electrode polarization (EP) effects remain small in M( t) signals. Dielectric hole-burning on vitreous ionic conductors and the elimination of EP can be achieved by combining the modulus technique with a preceeding high sinusoidal electric field.

Heterogeneous and Homogeneous Diffusivity in an Ion-Conducting Glass

The nature of ion diffusivity in the vitreous conductor $2\mathrm{Ca}({\mathrm{NO}}_{3}{)}_{2}\ifmmode \dot{}\else \.{}\fi{}3{\mathrm{KNO}}_{3}$ (CKN) was studied by nonresonant dielectric hole burning. Spectral holes in the electric field relaxation are probed subsequent to a high electric sinusoidal burn field. For sufficiently high pump frequencies we are able to induce spectrally selective modifications in the relaxation of the electric modulus, indicating that ionic diffusivity is a spatially varying quantity in glassy CKN. Homogeneous behavior occurs in the regime of low pump frequencies, in which the resistivity approaches its steady state value. Thus, longer-ranged ionic motions lead to a spatial averaging over the heterogeneity of local ion diffusivities.

A comment on dielectric hole burning

The method of dielectric hole burning is used to distinguish a homogeneous broadened spectrum from a distribution of relaxation times in a dielectric material. After stimulation with a sinusoidal high electric field a time-dependent shift of the response function Φ(t) is observed. This shift is considered to be induced by a frequency selective absorption of energy and an increase of a local fictive temperature. A computer simulation, which is based upon the model of asymmetric double-well potentials for dipole orientations, also shows a time-dependent shift of the response function after applying and removing a sinusoidal high electric field. This behavior is explained considering the time-dependent polarization only. It is shown in this article that it is not necessary to assume an increase of a local fictive temperature caused by a frequency selective absorption of energy.