Nanoscale Measurements of Glass Transition Temperature and Temperature-Dependent Mechanical Properties in Polymers

M.P. Nikiforov, S. Jesse, L.T. Germinario (CNMS user, Eastman Chemical Co.),
and S.V. Kalinin

Achievement
We report a novel method for local measurements of glass transition temperatures and the temperature dependence of elastic and loss moduli of polymeric materials. The combination of Anasys Instruments' heated tip technology, ORNL-developed band excitation scanning probe microscopy, and a “freeze-in” thermal profile technique allows quantitative thermomechanical measurements at high spatial resolution on the order of ~100 nm.

Here, we developed an experimental approach for local thermomechanical probing that reproducibly tracks changes in the mechanical properties of polymeric materials based on a combination of band excitation and thermal analysis (BE-TA). Band excitation allows unambiguous determination of the cantilever response amplitude, resonance frequency, and Q-factor, from which mechanical properties and dissipation at the tip-surface junction can be extracted.

In the first variant of the BE-TA method, we developed a procedure that combines atomic force acoustic microscopy and band excitation detection. As a result, this methodology can detect changes in resonance frequency and tip-surface dissipation of a mechanically vibrated sample using a heated tip probe (Fig. 1A). The second approach is based on sending the excitation signal to the heated tip itself while simultaneously measuring the frequency and amplitude response of the oscillations induced by thermal expansion of the material beneath the tip (Fig. 1B). We further developed an experimental protocol that maintains a constant tip/surface pressure and reproducible contact area during a temperature sweep. Control of these parameters is a necessary precondition for quantitative data analysis. Both of these techniques were used to measure the local glass transition and mechanical properties of PET at the nanoscale. This method is now being further developed to allow spatially resolved nanoscale mapping of the thermomechanical properties.

Significance
Polymer thin films and ordered copolymers with thickness in the range of a hundred nanometers are extensively used in many applications such as protective and optical coatings, barrier layers, drug release control, solar cells, and OLEDs. At these dimensions, spatial confinement effects may result in significant deviations in mechanical and thermal properties from those of the bulk. Specifically, Young's modulus as well as the softening temperature of thin polymer layers is of paramount importance for understanding the mechanical stability and functionality of these layers.

The BE-TA method differs in several important ways from previous methods and overcomes many of the shortcomings that have plagued earlier techniques. The most important difference derives from the fact that BE measures changes in the dynamic response of the system, not the slow, melt-induced, plastic deformation at the basis of conventional LTA. Therefore, the onset of phase change can be detected prior to significant damage (i.e. a ~100nm indentation cavity) to the surface permitting high-resolution, non-destructive mapping of thermo-mechanical properties. In addition, because we have shifted our data collection to high-frequency channels, we have made available the low-frequency regime to control force applied by the tip to the surface and thus maintain a constant tip-surface contact area enabling far more quantitative measurements.

Research at the Center for Nanophase Materials Sciences supported by the Scientific User Facilities Division, BES, U.S. Department of Energy.

Publication
S. Jesse, M. P. Nikiforov, L. T. Germinario, and S. V. Kalinin, “Local Thermomechanical Characterization of Phase Transitions using Band Excitation Atomic Force Acoustic Microscopy with Heated Probe,” Appl. Phys. Lett. 93, 073104, (2008).

Fig. 1. Schematics of the atomic force acoustic microscopy (AFAM) (A) and tip heat wave (B) experimental techniques. Amplitude and quality factors of the tip oscillations are presented as a function of temperature for AFAM (C) and tip heat wave (D). The glass transition of polyethylene terephthalate (PET) polymer is clearly seen with both techniques despite the difference in the contrast formation mechanism. The hysteresis curve in the data (C) appears due to re-crystallization of PET, thus, AFAM allows studying the re-crystallization mechanism in polymers.