Apparent Mass Uptake Measurements in Thin Polymer Films Using a Quartz Crystal Microbalance: Errors Induced by Film Expa

1. Chemical Engineering Dept. mf tq tf Dm/m Dm 230 nm 0.53 mg 7.57 % 333 mm 7 mg 333 mm 1148 nm 35 mg 2.09 mg 5.79 % Apparent Mass Uptake Measurements in Thin Polymer Films Using a Quartz Crystal Microbalance: Errors Induced by Film Expansion Stresses Lameck Banda, Mataz Alcoutlabi, and Gregory McKenna Department of Chemical Engineering Texas Tech University Lubbock, Texas Results Apparent Mass Uptake Near Tg • Introduction • In our laboratory we have been studying the structural recovery and physical aging responses of polymer glasses subjected to CO2 jumps • The mass uptake measurements during the structural recovery experiments were performed using a quartz crystal microbalance (QCM) fitted with the commonly used AT-cut quartz crystal • The experiments showed that the AT-cut quartz crystal response was affected by a mechanism other than mass change QCM Response: (1) Uncoated Crystal PCO2 and Temperature Calibration Dm corresponds to approximately 2 – 3% of “absolute” mass of polymer coating. As Tg approached/traversed, stresses relieved. (This magnitude of stress relief is similar to the mass uptake of CO2 by PMMA) QCM Response: (2) Coated Crystal PCO2 and Temperature ramps Apparent artifacts • Objectives • Demonstrate that the AT-cut quartz crystal response is significantly impacted by thermal and swelling stresses in the polymer coatings • Show that the AT-cut crystal should not be used for mass uptake measurements for glassy polymers (and other stiff materials) • Confirm the validity of EerNisse’s Caution* on the impact of stress effects for mass uptake measurements using a QCM fitted with an AT-cut quartz crystal unexplained results variability Errors scale with film thickness! • Experiment • Poly(methyl methacrylate) (PMMA), Polystyrene (PS) • Experimental Conditions & Apparatus • Custom-built Environmental Chamber • Quartz Crystal Microbalance (Maxtek) – PC controlled. • Specialty Products Spin Coater • Pressure and Temperature are controlled by using DAQ (NI) Apparent mass change due to a temperature ramp for PMMA 1/3 of signal is hysteresis, is this residual stress in polymer film (relief vs. swelling)? Factors Affecting the Df Response of the QCM • Summary and Conclusions • EerNisse’s Caution is valid • The QCM is clearly sensitive (subnanogram), but may provide inaccurate measurements of mass or mass evolution when the coating changes dimensions and causes stress development in the quartz crystal • Stress induced errors are approximately 2-8% of total film mass • Mass uptake is about 10% of total mass therefore, errors can be • 20-80% or more • Forces in the quartz crystal scale linearly with tf and the mass uptake scales linearly with tf. This implies that relative errors in Dm are independent of tf • Clearly, mass uptake measurements in glassy polymers (and other stiff materials) should not be measured using AT cut quartz crystals: using very thin films does not resolve the problem (stress compensated (SC cut) crystals should be used – expensive and complicated) m = mass, p = pressure, T = temperature, h = viscosity, r = crystal surface roughness In the literature cited above, stress effects are ignored! EerNisse’s Caution*: “certain uses of resonators to measure thin films were plagued with large errors from radial stress in the quartz caused by stresses in the thin film”. *E.P. EerNisse, “Stress effects in quartz crystal microbalances”, Methods and Phenomena, 7, Applications of Piezoelectric Quartz Crystal Microbalances, New York: Elsevier, 1984, pp 125-149. 1Miura et al., Fluid Phase Equilibria 144, 1998, 181, 2Park et al., J.Supercritical Fluids 29, 2004, 203, 3Grant et al., Langmuir, 2004, 20, 3665, 4Weinkauf et al., J. Polym. Sci., Part B: Polym. Phys.Vol. 41, 2003, 2109 G= all other effects, s = stress effect • Regulator B1) Inlet automatic valve B2) Outlet automatic valve C) High pressure pump • D) Filter E) Safety valve F1) Inlet needle valve F2) Outlet needle valve G) Pressure sensor • H) One way valve I) Three- way valve K) Cold trap L) Vacuum pump • We now examine EerNisse’s Caution for mass uptake measurements in polymer glasses Acknowledgements: The authors would like to thank the National Science Foundation for supporting this work under grant numbers DMR-0070052 and DMR-0307084.