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 Chemical Engineering Dept. Objectives 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 Introduction  In our laboratory we have been studying the structural recovery and physical aging responses of polymer glasses subjected to CO 2 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 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 t f and the mass uptake scales linearly with t f. This implies that relative errors in  m are independent of t f  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) Results 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) Factors Affecting the  f Response of the QCM 1 Miura et al., Fluid Phase Equilibria 144, 1998, 181, 2 Park et al., J. Supercritical Fluids 29, 2004, 203, 3 Grant et al., Langmuir, 2004, 20, 3665, 4 Weinkauf et al., J. Polym. Sci., Part B: Polym. Phys.Vol. 41, 2003, 2109 m = mass, p = pressure, T = temperature,  = viscosity, r = crystal surface roughness 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  = all other effects,  = stress effect  We now examine EerNisse’s Caution for mass uptake measurements in polymer glasses QCM Response: (1) Uncoated Crystal PCO 2 and Temperature Calibration 1/3 of signal is hysteresis, is this residual stress in polymer film (relief vs. swelling)? Apparent mass change due to a temperature ramp for PMMA Apparent artifacts A)Regulator B 1 ) Inlet automatic valve B 2 ) Outlet automatic valve C) High pressure pump B)D) Filter E) Safety valve F 1 ) Inlet needle valve F 2 ) Outlet needle valve G) Pressure sensor C)H) One way valve I) Three- way valve K) Cold trap L) Vacuum pump QCM Response: (2) Coated Crystal PCO 2 and Temperature ramps  m 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 CO 2 by PMMA) Apparent Mass Uptake Near Tg In the literature cited above, stress effects are ignored! Errors scale with film thickness! tqtq tftf mfmf mm  m/m 333  m 230 nm 1148 nm 7  g 35  g 0.53  g 2.09  g 7.57 % 5.79 % Acknowledgements: The authors would like to thank the National Science Foundation for supporting this work under grant numbers DMR and DMR variability unexplained results