Transition Temperature Microscopy - Probing Nanoscale Thermal Properties of Polymeric Materials by Anasys Instruments

Topics Covered

Introduction
High Spatial Resolution Thermal Property Mapping via TTM
     Flat Panel Displays
     Characterization of BOPP Films for Packaging
     Analysis of Processing Defects in Fiber-Reinforced Composites
In-Situ Time-Resolved Surface Measurements
     Cure-Rate Measurements in Coating Formulations
     Surface Property Measurements of Weathering Effects on Coatings
Conclusion
Acknowledgements

Introduction

A serious limitation of conventional bulk thermal methods like DSC, TMA and DMA is that they can only measure a sample-averaged response and cannot offer specific information on localized defects, structural non-uniformities or chemical heterogeneities, nor can they give thermal property data of coatings or film surfaces or interfaces that are less than a few microns thick.

TTM extends into an imaging or microscopy mode, the current point measurement technique of nanoscale thermal analysis (nano-TA) which makes use of a thermal probe to locally heat the surface of a sample while simultaneously monitoring the softening of the sample surface under the heated probe. The nano-TA technique is similar to Thermomechanical Analysis (TMA) with the important difference that instead of heating the entire sample, as is done in a TMA experiment, nano-TA probes the thermal response of the material in contact with the probe and therefore can locally determine the transition temperature of the sample on the micro or nanoscale. TTM plots an array of nano-TA measurements to obtain an image or map of the transition temperatures across the region of interest.

Figure 1. An SEM image of the microfabricated nanoscale thermal probe (with tip radius <30nm) used for nano-TA and TTM measurements. The inset is a zoom of the tip which makes contact with the sample surface. Because of its small size, the temperature of the probe can be changed quickly to allow fast throughput as well as provide a large range for variable heating rate experiments. Heating rates can range from 5°C/min to 10,000°C/s.

The basic principle of Transition Temperature Microscopy is outlined in figure 2. At each point of interest on the sample, the probe is brought into contact with the sample surface and heated, while simultaneously monitoring the thermal expansion of the sample under the probe. At a transition temperature, the surface softens allowing the probe to penetrate slightly into the sample. The array of nano-TA measurements is automatically analyzed to determine the transition temperature at each point or pixel within the scanned region. Then a false color map is created where the pixels are shaded according to the measured transition temperatures. The resulting spatial map allows visualization of thermal gradients and can detect the presence of inhomogeneities in a wide range of samples.

Figure 2. Transition Temperature Microscopy (TTM) maps local variations in melting temperatures and glass transition temperatures. A heated probe locally measures the temperature at which softening of the material occurs. Arrays of measurements can be made to assembly a spatially resolved image of the sample.

High Spatial Resolution Thermal Property Mapping via TTM

Flat Panel Displays

A polymer multilayer from a liquid crystalline display (LCD) was microtomed in order to gain access to the different layers within the LCD stack and the cut surface was then imaged using TTM. Figure 3A shows an optical image that reveals the multiple layer construction of the LCD stack which range in size from a few microns to many tens of microns. Below this image is figure 3B, which is the corresponding color-coded TTM image that is scaled to track with the optical image. This figure clearly illustrates the variation in thermal properties across the different layers which are not evident in optical micrographs.

Figure 3. TTM measurements of a microtomed LCD film clearly resolves the different film layers (B) and allow comparison with optical images (A) and visualization of the composite structure in the LCD stack that is not evident in optical micrographs. (C) Data analysis in the form of a histogram generates a plot of the measured distribution of thermal transitions (B) which reveal the degree of uniformity as well as detecting the presence of heterogeneities within the material layers

Characterization of BOPP Films for Packaging

Biaxially Oriented polypropylene (BOPP) is extensively used in the packaging industry with constructions that can be either heat sealable or non-heat sealable. These films can be composed of either uni or multi-layered structures and have typical total thickness of only 15-25 µm. The most common multilayer films consist of a three-layer structure: one thick core layer that is composed of a polypropylene homopolymer, sandwiched between two thin (usually ~1µm thick) skin layers. In the standard three-layer structures, the core layer mainly provides the film's rigidity, whereas the skin provides sealing and/or surface properties. Figure 4 is an example of a cross section of an epoxy embedded film and demonstrates an in-situ localizednano-TA measurement of the transition temperature of the skin layer, core layer and embedding epoxy used to support the multilayer BOPP film.

Figure 4. An AFM image (left) and nano-TA measurements (right) of a microtomed multilayer BOPP film

Analysis of Processing Defects in Fiber-Reinforced Composites

TTM provides a new analytical window for testing heterogeneous, fiber reinforced structures, since interfacial bonding is critical to performance. For example, a polyester fiber reinforced composite was prepared for cross sectional analysis in order to measure the fiber's internal morphology and diameters. Inspection of cross sections by optical microscopy, shown in figure 5, revealed the presence of a skin layer around the microfibers. The TTM map identified the skin layer as having a significantly higher transition temperature than either matrix or fiber. This skin layer was later identified as an unintended artifact that was formed during the microfiber embedding process and was the result of the use of "aged" epoxy resin and catalyst that had hydrolyzed during storage.

Figure 5. An optical microscope image (left), TTM image (center) and histogram (right) of a fiber reinforced composite sample.

In-Situ Time-Resolved Surface Measurements

Cure-Rate Measurements in Coating Formulations

Automotive Refinish clearcoats are crosslinked coatings that are usually cured through a reaction between two or more components. Figure 6 demonstrates how nano-TA can be used to follow the cure kinetics taking place at the coating surface. The softening temperature can be easily measured from these curves and if plotted versus cure times (Figure 6B) provides critical information on crosslinking rates and reaction kinetics . The ability to measure chemical kinetics opens new opportunities to explore the effects of composition, additives and processing conditions on the speed of film drying and mechanical property development at surfaces and interfaces. The ability to measure rates of chemical processes can also yield information about the reaction mechanism, transition states, as well as provide mathematical models that can be used to quantify and describe the time scales of the chemical reactions.

Figure 6. nano-TA measurements of a clear coat measured at three different times after deposition (left) and the plot of softening temperature versus cure time (right).

Surface Property Measurements of Weathering Effects on Coatings

Photodegradation and weathering eects on coatings is another area of potential application of the nano-TA method. Acrylic urethane coatings were exposed 20 and 41 weeks to UV-A and UV-B. Samples were scraped from surfaces and analyzed by modulated DSC (MDSC), while nano-TA analysis was performed, in situ, on the weathered surfaces. Fig 7 shows that nano-TA, due to its surface sensitivity was able to provide a measure of the weathering phenomenon, while MDSC could not differentiate the surface from the matrix.

Figure 7. Comparison of softening temperatures measured for outdoor exposed (0, 20 and 41 week) clear and TiO₂ filled (P25 and R9) acrylic urethane coatings using nano-TA and MDSC. Surface morphology was also analyzed by scanning electron microscopy.

Conclusion

TTM is a technique that combines the benefits and advantages of microscopy with nanoscale thermal probe technology. This combination facilitates the characterization of complex, heterogeneous and multilayered structures by providing high resolution thermal property mapping. The ability to heat and test very small regions of a sample surface enables the TTM technique to be uniquely valuable in applications ranging from coating defect analysis to in-situ characterization of reinforced composites and time-resolved dynamic measurements for coating design.

Acknowledgements

The authors express their gratitude to Drs. Aaron Forster and Stephanie Watson (NIST) for supply of weathered, acrylic-polyurethane coatings. We also thank Dr. Deepanjan Bhattacharya and Mr. Chip Williams of Eastman Chemical for supply of acrylic clearcoats.

Source: Transition Temperature Microscopy: A New Technique for Probing nanoscale Thermal Properties of Polymeric Materials
Author:Kevin Kjoller, David Grandy, William King, Louis T. Germinario, Wolfgang Stein
For more information on this source please visit Anasys Instruments