The QCM: Mass uptake measurement issues G. B. McKenna, Texas Tech University Mass uptake in glassy films subjected to vapor (H 2 O or CO 2 ) 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  m 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 CO 2 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)

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

Broad Accomplishments-FY2005 NSF Grant DMR 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 (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 Xiangfu Shi, received Ph.D. in December Anny Flory, received Ph.D. in December Shankar Kollengodu-Subramanian, current student. Passed qualifying exams, July, Publications –4 manuscripts published (Polymer, 45, (2004); J. Phys.: Condensed Matter, 17, R461-R524 (2005); Phys. Rev. Lett., 94, (2005); J. Chem. Phys. 122, (2005)). –3 proceedings publications –1 manuscript in review (J. Polym. Sci. Part B. Polymer Physics Ed..)