Technique: Dielectric Modulus M(t) |
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As an alternative to frequency domain impedance methods, we record the time-resolved dielectric modulus by a technique that has been developed in our lab. This is done by measuring the voltage for times 10-3 s to 10+6 s across the sample after applying a charge step, such that the voltage is directly related to the polarization in the material under study [60]. The thermally stimulated version of this method has also been used [63]. The modulus results tell us how molecules or ions respond to electric fields and how their dynamics depend on temperature. |
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Schematic circuit for measuring the electric modulus M(t) directly by recording
E(t) at a time invariant displacement D = D0. Voltage
source, voltmeter, and guarding amplifier are components of the Keithley Model 6517 electrometer.
At time zero, the switch is closed for 10 μs and remains open for
the up to 2 weeks duration of the subsequent voltage recording. [103] |
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The modulus technique is particularly useful for accessing long relaxation times [97], for observing ionic conductivity without the problems associated with electrode polarization [84], and for dielectric hole-burning experiments [91, 98, 112, 115, 135]. For polyvinylacetate, segmental relaxation times of over a year have been made visible, because the modulus signal M(t) decays faster than the equivalent dielectric ε(t) curve. |
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Experimental decay results of the dielectric modulus M(t) normalized to
Ms and M∞ for poly-vinylacetate for temperatures
ranging from 291 K to 323 K in steps of 2 K and in the order from slowest to fastest decay.
Only the contribution associated with segmental motion (α-process) is shown.
The slowest curve originates from dipole reorientation with an average time scale of over 1 year. [60,
97]
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The connection between relaxation, M, and retardation, ε, is simple in frequency domain: M = 1/ε. The average time constants follow the relation τε / τM = εs / ε∞, valid also for dispersive dynamics [160]. |
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Reference numbers refer to the list of publications |
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Experimental techniques:
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Selected projects:
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