Mechanical and dielectric relaxation spectra in seven highly viscous glass formers

Buchenau, U.

doi:10.1016/j.jnoncrysol.2007.04.034

Cited by 9 publications

(8 citation statements)

References 33 publications

(68 reference statements)

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“…12,17−20 The origin of this difference is a current topic of investigation. 21,22 The faster mechanical relaxation peak appears to correspond to the dielectric α-relaxation, the mechanical τ M fast superpose on the dielectric τ α , by a 10.5 degrees shift. On the other hand, the τ M slow are longer than the relaxation times of any of the dielectric processes; the corresponding dielectric peak would fall at frequencies for which they would be hidden by the large ionic conductivity and consequent EP.…”

Section: ■ Experimental Sectionmentioning

confidence: 94%

Ion and Chain Mobility in a Tetrazole Proton-Conducting Polymer

et al. 2014

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The morphology, relaxation properties, and conductivity of the statistical copolymer polystyrenic (alkoxy 1H-tetrazole-co-alkoxy nitrile), an anhydrous proton conductor, were measured. The material phase-separates into hard and soft domains, the latter corresponding to a phase richer in the pendant tetrazole groups. Using dielectric and mechanical spectroscopies, two relaxation processes were observed, the slower associated with local segmental dynamics of the backbone and the higher-frequency process involving motion of the tetrazole moieties. The latter is coupled to the ionic conductivity, which means that below the principal glass transition of the material (∼313 K) the conductive mechanism remains active. Thus, the usual compromise in proton exchange membranes between mechanical stability and ion conductivity can be avoided. ■ INTRODUCTIONThe attraction of new energy sources has stimulated development of both new and existing technologies. An obvious enabling factor for devices such as photovoltaic and fuel cells is the combination of high performance and low cost. Polymeric materials are especially promising given, along with their inherent processability, the multitude of chemical structures available to tailor properties. For fuel cells, proton exchange membranes (PEMs) with high proton conductivity but very small electron conductivity are required. The current state-ofthe-art PEMs are made with Nafion, a perfluorinated polymer. However, the proton conductivity of Nafion is strongly dependent on its water content. This limits the operating temperature to below 373 K and requires precise control of hydration levels to obtain optimal cell performance. A better PEM would have the proton conductivity of hydrated Nafion without requiring water molecules for the conduction mechanism.To produce materials possessing the desired conducting properties, the synthetic process must be guided by an understanding of the physical properties. In this work we describe dielectric and mechanical measurements on a protonconducting polymer, the statistical copolymer polystyrenic (alkoxy 1H-tetrazole-co-alkoxy nitrile) (PS-(Tz-co-CN)), the synthesis of which is described elsewhere.1 In anhydrous conditions PS-(Tz-co-CN) has a DC conductivity σ DC = 4 × 10 −12 S/cm at ambient temperature and 2 × 10 −6 S/cm at 388 K, which is comparable to undoped imidazole and triazolecontaining polymers 2,3 and higher than that of anhydrous Nafion (a sulfonic acid polymer). 4 However, this conductivity is well below that of hydrated Nafion (10 −2 S/cm at 353 K), small-molecule-doped materials including organic acid-and base-impregnated polybenzimidazole membranes, 5 strong aciddoped aromatic or heterocyclic polymers, 6 doped polymers structurally similar to PS-(Tz-co-CN) (e.g., imidazoles, triazoles, and tetrazoles), 2 and doped tetrazole-containing polymers, with σ DC as high as 10 −3 S/cm at high T. 7,8 As σ DC of PS-(Tz-co-CN) is comparable to that of other undoped azole-based materials, 2,3 we expect that its conductivity can be ...

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Section: ■ Experimental Sectionmentioning

confidence: 94%

Ion and Chain Mobility in a Tetrazole Proton-Conducting Polymer

et al. 2014

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“…Similar to the issue of different temperature dependences between the viscosity and segmental relaxations (though not for the same reasons), the dielectric and viscoelastic temperature dependences are not identical although they both show super-Arrhenius behavior. Such differences between the dielectric and viscoelastic responses in both the temperature dependences of the relaxation times and the strength of the various relaxations are not unusual for either polymers or small molecular glass-formers − and may arise (but not always) in part from the coupling of the dipole motions to the viscoelastic medium. , …”

Section: Temperature and Pressure Dependence Of The Dynamics In Glass...mentioning

confidence: 99%

50th Anniversary Perspective: Challenges in the Dynamics and Kinetics of Glass-Forming Polymers

2017

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The phenomenology of the glass transition and the associated behavior in the near liquid and glassy states are detailed, including the cooling rate dependence of the glass transition, Kovacs’ three signatures of structural recovery, and enthalpy overshoots. Dynamics in the liquid regime just above T g and the associated temperature dependences are also covered since this behavior is important to understanding the glassy dynamics. The current models of structural recovery and their shortcomings are presented. A number of important unanswered questions are discussed, including how the relaxation time in the glassy state depends on structure, the relationship between the evolution of different properties, the resolution of the Kauzmann paradox, and the behavior of the equilibrium relaxation time below T g. New experimental approaches are needed to make breakthroughs, such as two that are described: one involving 20 Ma amber to test whether the Vogel temperature dependence continues for the equilibrium state below T g and another involving an ideal polymer/pentamer mixture to obtain the entropy of the liquid far below T K in a test of the Kauzmann paradox. An unexplored regime of glassy behavior, characterized by ultrastability, high density, and low fictive temperature, is identified, and experiments to understand the material behavior in this region are motivated.

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“…Parallel studies of the modulus and compliance (dielectric, in particular) relaxations have been done in molecule liquids and polymers [49,[52][53][54], and most of the studies emphasized the correlation of relaxation time. The kinetic decupling of various relaxations has been reviewed [55], while the detailed comparison of the relaxation dispersion profiles is less available [52,56,57].…”

Section: Figmentioning

confidence: 99%