THE MASSCAL G1 NANOBALANCE/MICROCALORIMETER

For over a century scientists and engineers have made thin films, coatings, and membranes with useful properties such as protecting the underlying substrate or allowing certain molecules to pass through. Many important chemical and biological processes occur in these films either in their production or use. When a gas interacts with a solid film it may adsorb or dissolve in the film, or react catalytically at the surface. In a coating containing volatile components, the components evaporate by absorbing heat, the film mass decreases, and the viscous film becomes a glassy solid. In all these processes, heat is generated or absorbed, the thin film gains or loses mass, the viscoelastic properties of the solid film change, and the desired properties may be enhanced or destroyed.

The quartz crystal microbalance/heat conduction calorimeter (QCM/HCC), or nanobalance-calorimeter, was developed to study chemical and biological processes in thin films^1,2,3. Heat conduction calorimetry has been previously used to measure adsorption energetics in solids, and gas sorption instruments or thermal gravimetric analysis can measure mass release in solids. Rheological measurements of shear and loss modulus are usually done at low frequency on large samples with dynamic mechanical analysis instrumentation. Until the nanobalance-calorimeter, however, there has been no instrument that simultaneously measures heat generation, mass uptake or release, and viscoelastic property changes in the same, sub-milligram solid film sample.

The first commercially available nanobalance-calorimeter, the Masscal™G1, is now available from Masscal Corporation. It employs a patented mass/heat flow sensor⁴ with a sensitivity sufficient to detect molecular monolayer formation in all signal channels. The mass sensor used in the G1 is a piezoelectric shear mode resonator made of quartz, termed a quartz crystal microbalance (QCM). When the resonator is electrically driven at its natural acoustical frequency, the decrease in resonant frequency is proportional to the increase in mass per unit area of a thin film deposited at its surface. The thin film may be a polymer, protein, paint or coating, chemical sample, catalyst, or metal, but it must adhere to the QCM surface. Subsequent mass changes are followed as the film is exposed to atmospheric pressure gas mixtures with varying partial pressures of adsorbing or reacting gases⁵.

An uncoated QCM has a sharply defined resonance frequency v₀. The mechanical damping of the quartz that gives rise to this broadening can be determined by measuring an electrical quantity called the "motional resistance" R of the QCM (for typical bare QCM's, R ~ 10 ohms). When thin, stiff films are deposited on the QCM surface the increase in R is small, but softer, thicker films (i.e., rubbery polymers 5-10 microns thick) can increase R by hundreds of ohms. We have shown that the difference in motional resistance of the coated and uncoated QCM is proportional to the shear loss compliance J" of the film3. For organic films gaining or losing volatile components, the changes in J" indicate the extent to which the film is being plasticized.

Figure 1. QCM

In Masscal's mass/heat flow sensor, the QCM is thermally coupled to a heat sink through a Peltier thermocouple plate. Any heat flow generated by processes in the thin film on the QCM's upper surface is detected as a voltage change by the thermocouple plate - the heat conduction calorimetry (HCC) principle. In the Masscal G1 (Figure 2), the mass/heat flow sensor is placed in a quasi-adiabatic thermal environment and is exposed to a slow flow of gas mixture at ambient pressure. Three quantities are measured simultaneously: the thermal power P(t), the mass change m(t), and the change in motional resistance R(t) of the damped oscillator when the sample film takes up, releases, or reacts with the probe gas. Any solid which can be prepared as a thin film on a gold-coated quartz substrate is amenable to study. Films of 1- 2 cm² area and 0.1-10 µm thickness are optimal. Methods of film preparation so far have included spin-coating, spray-coating, and electrochemical deposition. Solids studied have included many polymers, Pd and Pt metal films, the proteins lysozyme and myoglobin, the molecular solids C60 and C60/piperazine, pharmaceutical film-coat materials, and nutrient-containing agar films.

Figure 2. The Masscal G1 Nanobalance-Calorimeter

APPLICATIONS

Here are some key specifications of the Masscal G1™ Nanobalance-Calorimeter:

• Operating temperature from ambient to 100ºC; temperature stability of ±.001 ºC without the use of water baths. Ambient operating pressure.
• Sensitivity of 2 ng in mass measurement and 500 nW sensitivity in heat flow measurements with a time constant of 12 seconds.
• Provisions for the software control of external mass-flow controllers to provide a versatile program of gas composition vs time in the sample chamber.
• Provision for collection of an external analogue signal from a downstream detector such as a relative-humidity meter.

The following applications of the QCM/HCC technique have been performed with the Masscal G1:

• Moisture sorption, transport, and hydrolytic degradation in polylactide films⁶.
• Monitoring the drying and curing of an alkyd spray enamel⁷.
• Gravimetric analysis of the non-volatile residue from an evaporated droplet⁸.
• Sorption isotherms, sorption enthalpies, diffusion coefficients and permeabilities of water in a multilayer PEO/PAA polymer film⁹.
• Energetics of a self-assembled monolayer (SAM) of butylthiol on gold (in preparation).
• Sorption isotherms, sorption enthalpies, and viscoelastic damping produced by water absorption in pharmaceutical film coat materials (in preparation).
• EDC-catalyzed amide bond formation on a carboxylic acid-terminated SAM (in preparation).
• Growth kinetics of alkyl and carboxylic acid SAMs (in preparation).

Because many of the materials now being made and characterized in nanotechnology are ultra-thin films of thickness < 1µm, their thermodynamic and kinetic properties must be measured with methods more sensitive than the normally employed calorimetric or gravimetric techniques. The QCM/HCC technology is ideally suited for such studies. One question of central importance to nanotechnology is the long-term stability of materials with nanostructures in challenging environments, such as high temperature and high humidity, or in the presence of oxidizing agents or solvent vapors. With atomic compositions varying systematically at the nanometer scale, these nanomaterials contain many more contacts between different functional groups and molecular subunits than do typical materials. How stable are these nanomaterials to moisture, to oxidative degradation? Answers to these questions will determine the ultimate usefulness of the many extraordinary new nanomaterials being synthesized today. Knowledge of the thermodynamics and kinetics of these thin-film materials is thus essential in assessing their performance.

References

1. AL Smith, H Shirazi, I Wadso, The OCM/HCC: simultaneous, isothermal, high sensitivity measurements of mass change and heat flow in polymer and fullerene films, Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials. Electrochem. Soc., San Diego, CA, 1998, p. 576-85.
2. AL Smith, HM Shirazi: Quartz microbalance microcalorimetry: a new method for studying polymer-solvent thermodynamics. J. of Thermal Analysis and Calorimetry 59 (2000) 171-86.
3. AL Smith, HM Shirazi: Principles of Quartz Crystal Microbalance/Heat Conduction Calorimetry: Measurement of the Sorption Enthalpy of Hydrogen in Palladium. Thermochim. Acta. 432 (2005) 202-11.
4. AL Smith, Mass and Heat Flow Measurement Sensor, U. S. Patent Office 6,106,149. Allan L. Smith, U. S. A., 2000.
5. AL Smith, SR Mulligan, HM Shirazi: Determining the Effects of Vapor Sorption in Polymers Using the Quartz Crystal Microbalance/Heat Conduction Calorimeter. J. Polymer Sci. Part B Polymer Physics 42 (2004) 3893-906.
6. RA Cairncross, JG Becker, S Ramaswamy, R O'Connor: Moisture Sorption, Transport, and Hydrolytic Degradation in Polylactide. Appl. Biochem and Biotechnology 131 (2006) 774-85.
7. AL Smith, in P. Zarras, B. Richey, T. Wood, B. Benicewicz (Eds.), New Developments in Coating Technology. ACS Symposium Series, Washington DC, 2006.
8. AL Smith: Gravimetric Analysis of the Non-volatile Residue from an Evaporated Droplet, Using the Quartz Crystal Microbalance/Heat Conduction Calorimeter. J. ASTM. Intl. 3, issue 6 (May 2006).
9. AL Smith, JN Ashcraft, PT Hammond: Water Sorption Isotherms, Water Sorption Enthalpies, and Water Diffusion Coefficients In a Multilayer PEO/PAA Polymer Film using the Quartz Crystal Microbalance/Heat Conduction Calorimeter. Thermochim. Acta. 450 (2006) 118-25.