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

1
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 EerNisses 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)

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
Factors Affecting the Df Response of the QCM

• Summary and Conclusions
• EerNisses 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!
EerNisses 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

1. Regulator B1) Inlet automatic valve B2) Outlet
   automatic valve C) High pressure pump
2. D) Filter E) Safety valve F1) Inlet needle valve
   F2) Outlet needle valve G) Pressure sensor
3. H) One way valve I) Three- way valve K) Cold
   trap L) Vacuum pump

• We now examine EerNisses 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.