Temperature-controlled Quartz Crystal Microbalance Research Articles

Structure Optimization and Performance Evaluation of Dew Point Sensors Based on Quartz Crystal Microbalance

In order to address the instability of the fixation in the temperature controlled quartz crystal microbalance (QCM), a new sensor structure was proposed and evaluated under different dew point conditions. The QCM was fixed by two O-ring thermal gaskets placed on both sides symmetrically with the constraining forces. Simulations by finite-element modeling were performed to determine the O-ring thermal gasket dimensions and the constraining forces. Results showed that the QCM structure had steady vibration properties with high heat transfer rates, when the constraining force value was less than 0.06 N, and the O-ring thermal gasket width ranged from 0.6 to 1.0 mm with a thickness of 0.5 mm. Experiments were carried out to evaluate the accuracy, stability, and repeatability of the sensor and results showed that the designed sensor had good performances compared with the standard instrument.

The effect of nanoparticle location and shape on thermal transitions observed in hydrated layer-by-layer assemblies.

Nanoparticles can have a profound effect on thermal transitions observed in polymer nanocomposites. Many layer-by-layer (LbL) assemblies contain nanoparticles for added functionality, but the resulting effects of nanoparticles on an LbL film's thermal properties are not known. Previously, we have shown that a nanoparticle-free LbL film containing strong polyelectrolytes, poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDAC/PSS), exhibited a single reversible thermal transition much like a glass-melt transition. In the work presented here, nanoparticles of either spherical (SiO2) or platelet (Laponite clay) shape are inserted at varying vertical locations throughout PDAC/PSS LbL films. Temperature-controlled quartz crystal microbalance (QCM-D) and modulated differential scanning calorimetry (MDSC) are applied, for which QCM-D proved to be more sensitive to the transition. All Laponite-containing films possess two thermal transitions. During growth, Laponite-containing films exhibit steady increases in dissipation, which is proposed to arise from mechanically decoupled regions separated by the Laponite nanoparticles. For SiO2-containing films, three transitions are detectable only when the SiO2 nanoparticles are placed in the middle of the film; no transitions are observed for SiO2 placed at the bottom or top, perhaps because of a weakening of the transition. The lowest transition is close in value to that of neat PDAC/PSS LbL films, and was assigned to a "bulk" response. The higher transition(s) is attributed to polymer chains in an interfacial region near the nanoparticle. We propose that nanoparticles restrict segmental mobility, thus elevating the transition temperature in the interfacial region.

Extension of the Enthalpy of Sublimation for Adipic Acid to Temperatures below 80 °C

The enthalpy of sublimation for adipic acid is available from several literature sources (Chemical Rubber Co. Handbook of Chemistry and Physics, 66th ed.; Chemical Rubber Publishing: Boca Raton, FL, 1985-1986. Configurational Thermal Vacuum Stability Test Method Development, TR-427-001; Lyndon B. Johnson Space Center, White Sands Test Facility, January 3, 1989. Afeedy, H. Y.; Liebman, J. F.; Stein, S. E. NIST Chemistry WebBook, NIST Standard Reference Database No. 69; NIST: Gaithersburg, MD, November 1998) for temperatures of ≥80 °C. Those references generally cite the work of Davies and Thomas (Davies, M.; Thomas, G. Trans. Faraday Soc. 1960, 56, 185-192) on the lattice energy of dicarboxylic acids. Measurements made with a 15 MHz temperature-controlled quartz crystal microbalance demonstrate that the enthalpy of sublimation, (129 ± 1.0) kJ.mol -1 , for adipic acid is constant in the range of 25 to 80 °C.