The QCM: Mass uptake measurement issues G' B' McKenna, Texas Tech University

1
The QCM Mass uptake measurement issuesG. B.
McKenna, Texas Tech University

• Mass uptake in glassy films subjected to vapor
  (H2O or CO2) can be measured to nominal nanogram
  resolution using the quartz crystal microbalance
  (QCM) because frequency is a sensitive function
  of the mass on the crystal. However, we find that
  mechanical stresses induced in the QCM by the
  swelling of the glassy polymer or by thermal
  expansion can lead to relatively large errors in
  the measurements.
• Figure 1 shows the change in frequency of a QCM
  as a function of temperature for PMMA in air. As
  the glass transition is traversed, the frequency
  changes dramatically due to relaxation of the
  stresses in the film. We see that the frequency
  change corresponds to approximately 2-3 mass
  uptake, which is similar to the amount of mass
  uptake expected. This could result in substantial
  errors in estimated mass changes.

Fig. 1. Change in frequency with temperature of a
QCM coated on one or both sides by a film of PMMA
approximately 1.1 mm in thickness. Upon
traversing the glass transition the film stresses
relax giving an apparent mass change of 2-3.

• Relative mass uptake errors are
• independent of the film thickness
• because the stresses in the film and the
• mass both scale linearly with the film
• thickness. Therefore, the errors of mass
• uptake relative to total film mass of the
  sort
• shown in Figure 1 are independent of film
• thickness
• - Table 1 shows that the relative errors
  are
• similar for two different film
  thicknesses.

Table 1. Mass uptake errors for PMMA in CO2
showing that relative errors are nearly
independent of film thickness
QCM mass measurements for glassy polymer sorption
must be performed with great caution (the
Eernisse Caution)
2
The QCM Mass uptake measurement issuesG. B.
McKenna, Texas Tech University
The work in FY 2004 further examined the
differences between temperature and concentration
glasses. Figure 1 here shows the differences in
creep responses as the retardation time for two
glasses. The blue data points are for a glass
formed from a relative humidity (RH)-jump through
the glass transition and the red points show the
normal path or temperature-jump through the glass
transition. Clearly, for a given volume departure
from equilibrium, the concentration glass is
more stable or has a longer retardation time
than does the temperature-glass. In Figure 2 we
show an interpretation of our results of volume
measurements on a carbon dioxide-created glass
when compared with a temperature hyper quenched
glass. The volume (or enthalpy) of the PCO2
glass does not recover towards equilibrium until
above the nominal Tg while the hyper-quenched
glass begins changing near to or below the
nominal glass transition. The hyper-quench
schematic comes from Berens and Hodge,
Macromolecules, 15, 756 (1982) and we have
modified it to make our point about the PCO2
created glass being different. This figure is in
press at Polymer.
3
Broad Accomplishments-FY2005NSF Grant
DMR-0307084

• Education and Outreach
• Project results presented at national and
  international meetings
• North American Thermal Analysis Society, October
  2004 (L. Banda, grad. Student gave oral
  presentation.His submitted manuscript was awarded
  Best Student Paper Award).
• Society of Rheology in Feb. 2005.(poster
  presentation by L. Banda grad. student oral
  presentation byX. Shi, Ph.D. partially supported
  by this project.)
• American Physical Society, March Meeting, 2005
  (oral presentation by G.B. McKenna, PI)
• Society of Plastics Engineers ANTEC in May 2005
  (one invited and one contributed oral
  presentations by G.B. McKenna, PI)
• Project results presented at one regional meeting
• Society of Plastics Engineers Polyolefins
  Conference (poster L. Banda, graduate student)
• PhD students supported or partially supported.
• Yong Zheng, Received Ph.D. in December 2003.
• Xiangfu Shi, received Ph.D. in December 2004.
• Anny Flory, received Ph.D. in December 2004.
• Shankar Kollengodu-Subramanian, current student.
  Passed qualifying exams, July, 2005.
• Publications
• 4 manuscripts published (Polymer, 45, 5629-5634
  (2004) J. Phys. Condensed Matter, 17, R461-R524
  (2005) Phys. Rev. Lett., 94, 157801-1-157801-4
  (2005) J. Chem. Phys. 122, 114501-1-114501-6
  (2005)).
• 3 proceedings publications
• 1 manuscript in review (J. Polym. Sci. Part B.
  Polymer Physics Ed..)