Viscoelastic changes

Swelling phenomena due to solvent uptake by the films may lead to volume and viscoelastic changes, which limits the information that can be obtained on rigid mass as will be discussed below. 

More recently the treatment was extended to piezoelectric devices in contact with viscoelastic media (i.e., liquids and polymers). It was then realised that if the deposited mass was not rigidly coupled to the oscillating quartz crystal, separation of inertial mass and energy losses was not possible with the measurement of the resonant frequency alone. Quartz crystal impedance in the acoustic frequencies was introduced in order to study mass and viscoelastic changes and a full electrical characterization of the crystal behaviour near resonance was employed. 

Fig. 13.1. Schematic crystal impedance responses, plotted as admittance vs. frequency, for (a) purely gravimetric, (b) purely viscoelastic and (c) simultaneous gravimetric and viscoelastic changes in resonator loading. Curves 1 represent the bare crystal and curves 2 the coated...

Quartz crystal microbalance — The quartz crystal microbalance (QCM) or nanobalance (QCN) is a thickness-shear-mode acoustic wave mass-sensitive detector based on the effect of an attached foreign mass on the resonant frequency of an oscillating quartz crystal. The QCM responds to any interfacial mass change. The response of a QCM is also extremely sensitive to the mass (density) and viscoelastic changes at the solid-solution interface [i-vi]. 

As the re on of maximum tack is reached, it has been demonstrated by a variety of measurements that two phases actually exist on the surface. Such a remarkable increase in strength, however, must be the result of a combination of substrate bonding and viscoelastic changes in the bulk polymer. 

An additional driver of viscoelasticity changes is temperature, either for reasons based on free volume at constant film composition (see the brief discussion... 

Viscoelastic effects alter the steady drop shape from being ellipsoidal to drop shapes with more blunt ends. The results reflect a balance between the direct tensile stress contribution of the viscoelastic fluid to the normal stress balance and modifications of the viscous (i.e., Newtonian) stress and pressure due to viscoelastic changes in the flow Hsu and Leal 2009... 

These can then be mounted into an electrochemical quartz crystal microbalance (EQCM) so that mass and viscoelastic changes can be monitored as a function of the applied potential or current the sensitivity is of the order of 4 ng cm . Besides AFM, EIS was also employed for characterisation, as illustrated in Fig. 6.6. which shows the response at the carbon film coated quartz crystal at various potentials and is similar to that at other forms of carbon. The films are robust and are promising for use to study a variety of adsorption and deposition processes associated with the EQCM, in particular for biosensor applications. 

Chlorinated polyethylene-natural rubber blends Viscoelastic changes and thermal degradation — 134... 

QCM of dry films provides estimates of the weight of each layer and monitors the repeatability of the multiple adsorption steps. Drying the films minimizes bias from interfacial viscoelasticity changes. Film mass per unit area MIA (g cm 2) is related to QCM frequency shift AF (Hz) for 9 MHz quartz resonators by [77] ... 

The most commonly measured viscoelastic material function is G (a), T). It is so popular because sinusoidal oscillations can be used to follow viscoelastic changes with time, such as during curing and ciystallization. As discussed below, cross-correlation analysis of the signal can provide accurate G and G" values over a wide range of frequency and signal levels. 

Calvo EJ, Danilowicz C, Etchenique R. Measurement of viscoelastic changes at electrodes modified with redox hydrogels with a quartz crystal device. J Chem Soc. Faraday Trans 1995 91 4083 091. 

Calvo EJ, Forzani E, Otero M (2002) Gravimetric and viscoelastic changes dining the oxidation-reduction of layer-by-layer self assembled enzyme multilayers wired by an Os-containing poly(allylamine) polymer. J Electroanal Chem 538-539 231-241... 

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