Non-exponential NMR relaxation in heterogeneously doped conducting polymers

Mabboux, Pierre-Yves; Beau, B.; Travers, J.P.; Nicolau, Y.F.

doi:10.1016/s0379-6779(96)04243-9

Cited by 5 publications

(2 citation statements)

References 7 publications

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“…The mesoscopic inhomogeneity of conducting polymers and related materials was first addressed in 1987 by Zuo, Angelopoulos, MacDiarmid, and Epstein, who proposed the model of a granular polymer metal on the basis of their dc-conductivity measurements. Later, this model was corroborated by the results of X-ray, microwave frequency-dependent conductivity , and NMR relaxation measurements, and was recently summarized in the paper by Prigodin and Epstein . In terms of this model, such materials are considered as consisting of a network of small (tens of nanometers) conducting/crystalline domains embedded into an insulating/disordered polymer matrix.…”

Section: Introductionmentioning

confidence: 76%

On the Origin of Mesoscopic Inhomogeneity of Conducting Polymers

2007

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The mesoscopic inhomogeneity of conducting polymer films obtained by electropolymerization and spin-coating was studied using Kelvin probe force microscopy (KFM) and current-sensing atomic-force microscopy (CS-AFM). A well-pronounced correlation was established between the polymer morphology, on the one hand, and its local work function (which is related to the polymer oxidation degree) as well as polymer conductivity, on the other. The most conducting regions were associated with the tops of the polymer grains and showed Ohmic behavior. They were surrounded first by semiconducting and then by insulating polymer. The conductivity of the grain periphery could be lower by as much as 2 orders of magnitude. The grain cores also showed consistently higher values of the local work function as compared to the grain periphery. This fact suggested that the grain cores were more oxidized and/or more ordered as compared to the grain periphery, which is in good agreement with the local conductivity data. More uniform morphology corresponded to less variability in the other properties of the polymer. A model is proposed that relates the observed inhomogeneity to preferential deposition of polymer molecules with higher molecular weight at the early stages of the polymer phase formation. The polymer deposition in either electropolymerization or various solution-casting techniques involves the nucleation of a new phase from a solution containing polymer fractions of different molecular weights. The driving force of the nucleation process depends on the solubility of the polymer fractions, which decreases with an increase in the molecular weight. This gives rise to preferential deposition of more crystalline, higher molecular weight polymer at the early stages of the polymer deposition to form the cores of the polymer grains. The fractions with lower molecular weights are deposited later and form less ordered/less conducting grain periphery. On the basis of this model, we conclude that, to ensure the formation of materials with low inhomogeneity and high quality, one should use the starting polymer with as narrow molecular weight distribution as possible. Yet another possibility is to use solvents which would reduce the differences in the solubilities of polymer fractions with different molecular weight.

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Section: Introductionmentioning

confidence: 76%

On the Origin of Mesoscopic Inhomogeneity of Conducting Polymers

2007

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“…The existence of ordered metallic islands in doped conducting polymers was corroborated by experimental measurements using various macroscopic techniques, such as X-ray diffraction [18][19][20][21] (both the X-ray percent crystallinity and coherence length/domain size of crystalline regions were determined), electron paramagnetic resonance (EPR), 17,21,22 electron spin resonance (ESR), 13,19 microwave measurements of the dielectric function, 17,20,21 measurements of thermoelectric power, 10,17,21 infrared reflectance, 13 and nuclear magnetic resonance (NMR). 23 However, despite these observations that supported the existence of conducting nanometre-size domains in conducting polymers, the model of inhomogeneous disorder and especially the accuracy of the experimental observations of anomalous metallic state with very small plasma frequency were a subject of considerable criticism. 7 A further problem was that at that time scanning probe techniques were just making their debut and no direct evidence of structural inhomogeneity and mesoscopic domain structure of conducting and semiconducting polymers was available.…”

Section: Introductionmentioning

confidence: 99%