A New Polymer Testing Surface Requires a New Measurement Method
Researchers at the University of Massachusetts wanted to tackle a question that has bedeviled the polymer field for decades: What is the glass transition temperature of very thin polymer films and is it significantly different from the traditional bulk value? This question became a study conducted by Professor Tom Russell, Post Doctoral Fellow Dr. Haiyun Lu, and graduate student Wei Chen. Their novel approach was to make thermal observations (heating and cooling) of these films floating on a fluid, in particular an ionic liquid, no longer constrained by a substrate such as a silicon wafer.
But they faced the question: How to measure the thickness of such challenging films? They considered using the university's X-ray reflectometer, as well as spectroscopic ellipsometry. However, these methods require samples of several millimeters (mm) and even centimeters in width and the acquisition times (particularly for X-ray) were too slow for the dynamics of the group's polymer system.
Spectral Reflectance Provides Fast and Economical Measurement
Surprisingly, the answer for this testing challenge turned out to be the least expensive and the best solution for measurement: spectral reflectance. With spectral reflectance, only a 1.5mm spot size is needed and the data acquisition takes far less than a second.
To obtain the spectral reflectance measurements, the UMass group had a choice of testing technologies, but each had limitations, according to Dr. Lu. "X-ray reflectivity provided valid results at room temperature, but as the temperature increased, the fluid levels and film thicknesses changed, making the measurements very time consuming and less precise," she explained. Instead, the UMass group used a Filmetrics® F20-UV instrument, coupled to a thermal stage designed by the group to heat the ionic liquid and polymer film uniformly. The Filmetrics F20-UV solution was an ideal choice for this study because it offers capabilities to measure very thin films (to 1 nanometer) in diverse testing media and with very fast testing times (typically measured in milliseconds). "The Filmetrics thin film measurement system is very easy and fast," said Dr. Lu.
Filmetrics also provided consulting support to the UMass group on setting up the testing environment to create the right experiment conditions. Dr. Lu noted, "We got a lot of helpful technical support from Filmetrics for adapting the system to our unique testing needs."
Significant, Publishable Research Results
The polymer thickness measurements conducted by the Filmetrics F20-UV instrument gave the UMass group the data they sought: How thickness changes in polymer films as the films are heated above their bulk-glass transition temperature. (Figure 1) The project results were published as an article in the journal Macromolecules.
The Filmetrics instrument also allowed the group to complete the tests with much less time and effort compared to other testing methods. "What we just did in one half-hour would have taken a full day in an x-ray lab," said Dr. Russell.
An Economical Solution for Polymer Film Research
The department now owns three Filmetrics F20 units, which are used by professors, graduate students, and postdoctoral fellows for projects such as:
- Quick, “walk-up” measurements for members of the Polymer Science and Engineering Department
- Simultaneous GISAXS/SR studies of solvent-swollen polymer films at various national facilities (X-ray beam lines)
- Measuring the parameters of wrinkle relaxation in polymer films
"When I talk to students and other fellows about testing with the Filmetrics F20-UV, I ask them about the thickness range of their films, whether the film is transparent or opaque, and what is the refraction index," says Dr. Lu. "These factors help me determine whether the F20-UV is the right instrument for their measurements."
For researchers at the University of Massachusetts, the Filmetrics F20-UV instrument has proven to be a fast, easy-to-use, and low-cost solution for measuring films in their lab experiments. Moreover, the expert technical support provided by the Filmetrics application engineers helps researchers identify the right testing setup to produce cutting-edge results.
Key Research Issues:
- Identify the best way to measure polymer film thickness on an ionic liquid surface; more challenging than using a solid substrate
- Avoid the lengthy time and testing limitations involved with ellipsometry and X-ray reflectivity measurements
- Filmetrics F20-UV film thickness measurement instruments
- Consulting support from Filmetrics for test environment setup
- Test measurements completed much faster than using alternative methods
- Research study results published in the journal Macromolecules
- Easy-to-use testing instrument for future projects
University of Massachusetts, Department of Polymer Science and Engineering