Thermal techniques such as differential scanning calorimetry (DSC) and modulated temperature DSC (MDSC), thermomechanical analysis (TMA), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) are proven methods for characterizing the composition and morphology of polymers.
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It is often possible to identify and quantify materials by reference to their characteristic transition temperatures and thermal stability. However, a serious restriction of conventional thermal techniques is that they give only a sample-averaged response and cannot provide information on specific features on or within the sample.
A DSC measurement, for instance, may indicate the presence of more than one phase, but the method cannot generally provide any information about the size or distribution of phases. This particularly influences scientists in the field of polymer blends (where the blend morphologies are vital to establishing their material properties), coatings (where imperfections such as gel formations can critically impact performance) and composites.
To address this issue, Reading et al. developed the Localized Thermal Analysis (LTA) technique [1] using a thermal probe to perform the heating very locally on the sample to attain the local thermal response rather than heating the whole sample to acquire a sample-averaged response. This method was limited to micron scale resolution until a breakthrough in the fabrication of nanoscale thermal probes enhanced for atomic force microscopy (AFM) by Prof. William King of the University of Illinois at UrbanaChampaign [2]. These probes have facilitated local thermal analysis to attain sub-100 nm resolution together with an AFM and led to the nano-TA technique from Anasys Instruments.
Nano-TA is an LTA method which integrates the high spatial resolution imaging capabilities of AFM with the ability to attain understanding of the thermal behavior of materials with a spatial resolution of sub-100 nm. To accomplish this, a conventional AFM tip is substituted by a special nano-TA probe that has an embedded miniature heater and is controlled by the specially designed nano-TA software and hardware.
The AFM enables a surface to be visualized at nanoscale resolution with its standard imaging modes, which allows the user to choose the spatial locations at which to examine the thermal properties of the surface. The user then acquires this information by applying heat locally via the probe tip and measuring the thermomechanical response. There have been a number of examples in the literature of the application of sub-100 nm LTA in the field of Polymers and Pharmaceuticals. [3-6].
A recurring consideration for the nano-TA technique is its correlation to bulk thermal analysis. One probable concern is the AFM probe size creates a contact pressure about 10 MPa when in nano-TA mode, two orders of magnitude higher than the contact pressure of bulk themomechanical analysis. TMA. In addition, the larger contact radius from the probe raises questions related to the concept of traceability to the bulk measurements.
This does not essentially mean that bulk and local measurements will or should agree, since the thermal effects at the nanoscale could possess their own dynamics. So as to understand these aspects, recent work has been done to understand the correlation between bulk techniques and the nano-TA measurements and this article reviews the results. (This paper was first published as a featured article in the November 2007 issue of American Laboratory and this article has been excerpted from that paper with the kind permission of the Publishers.)
Experimental Setup
Experiments were done using a Veeco Multimode AFM fitted with an Anasys Instruments nano-TA module and Anasys nanoscale thermal probes.
The images were recorded using tapping mode AFM. The nano-TA data presented is of the probe cantilever deflection (while in contact with the sample surface) plotted against probe tip temperature. Events such as glass transitions or melting that occur during the softening of the material beneath the tip create a downward deflection of the cantilever. So as to verify the tested points of interest, images are regularly recorded after performing the temperature ramp.
Results and Discussion
Figure 1 shows heating rate-dependent deflection curves for these semicrystalline materials (left) and extra amorphous or thermoset systems (right). The curves in Figure 1 reveal deflection of the cantilever because of expansion of the surface underneath the probe until the material yields under the contact pressure through the transition.
All curves in the plot are an average of 3–5 measurements. Heating rates span two orders of magnitude, from 0.1 °C/second (bridging typical TMA and DSC rates) to 10 °C/second. The small thermal volume of the probe makes extremely high heating rates (of up to 10,000 °C/second) accessible. On the whole, the crystalline materials have onsets that are comparatively invariant to heating rate, while the amorphous materials display greater rate dependence as the onsets shift to higher temperatures at higher rates. This is as expected for glass transition or softening of amorphous material.
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Figure 1. Deflection curves for semi-crystalline materials (left) and amorphous systems (on right).
In Figure 2a, the plots offer a least-squares fit of the LTA onset measurements acquired at the three heating rates to the DSC onset values acquired at 10 °C/minute. All the fits are good, with correlation coefficients greater than 0.99.
The LTA measurements are inclined to have positive offsets at all rates compared with the DSC onset measurement. Using a slope and minimum offset criteria, the ideal correlation of LTA to DSC is for the onset acquired at the lowest heating rate, 0.1 °C/ second. In a similar way, in Figure 2b, the LTA results are compared with the TMA onset measurements.
This plot offers a least-squares fit of the LTA onset measurements attained at the three rates to the TMA onset values acquired at 5 °C /minute. Again, the fits are excellent, with correlation coefficients surpassing 0.96. The LTA measurements tend to have somewhat negative offsets at all rates relative to the TMA onset measurement attained at 5 °C/minute. Using slope and minimum offset principles, the ideal correlation of LTA to TMA is for the onset attained at a heating rate of 1 °C/ second. As can be seen from the data, the correlation is good between bulk methods and the LTA method. The offsets of the LTA data compared with the bulk techniques indicate that there is something possibly more subtle about the LTA reaction.
In the case of the DSC, LTA seems to react at a higher temperature than DSC. This could be because of the nature of heat flow in the material as being sourced from the tip versus the ambient. Lower LTA heating rates are clearly closer to the bulk DSC onset temperatures. For TMA, LTA looks to respond at a temperature slightly lower than TMA.
This may be sourced in variations in contact pressure and/or improved deflection sensitivity. Further investigation is required for both of these offsets. The absolute values are clearly different, but not out of the range of the variation between two standard bulk techniques TMA and DSC.
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Figure 2. a) Comparison to DSC b) Comparison to TMA.
Conclusions
The Researchers compared data collected on various homogenous polymeric samples using different experimental conditions with both traditional thermal analysis methods and localized nano thermal analysis. This research has showed an extremely high degree of correlation between nanoscale and bulk thermal analysis.
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Acknowledgments
The Authors express their gratitude to The Dow Chemical Company and to American Laboratory magazine for permission to excerpt portions of their article published in the Nov 2007 issue.
References
1. Hammiche, A.; Reading, M.; Pollock, H.M.; Song, M.; Hourston, D.J. Localised thermal analysis using a miniaturised resistive probe. Rev. Sci. Instr. 1996, 67, 4268–74.
2. King, W.P. ; Kenny, T; Goodson, K; Cross, G; Despont, M; Durig, U; Rothuizen, H; Binning, G; Vettiger, P. Atomic force microscope cantilevers for combined thermomechanical data writing and reading. Appl. Phys. Lett. 2001, 78, 1300–2.
3. Harding, L.; King, W.P.; Craig, D.Q.M.; Reading. M. Nanoscale imaging of partially amorphous materials using local thermomechanical analysis and heated tip pulsed force mode AFM. Pharm. Res. (in press), 2007.
4. Nelson, B.A.; King, W.P. Thermal analysis with nanoscale spatial resolution using heated probe tips. Review of Scientific Instruments (republished on-line in Virtual Journal of Nanoscience & Nanotechnology 15, 2007), 78, 23,702.
5. Nelson, B.A.; King, W.P. Temperature calibration of heated silicon atomic force microscope cantilevers. Sensors and Actuators A, 140, 51-59, 2007.
6. Germinario, L. Nano thermal analysis of polymers, thin films and coatings. Invited paper, presented at Microscopy and Microanalysis conference, Fort Lauderdale, FL, Aug 5, 2007.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces, who has acquired Anasys Instruments. For more information on this source, please visit Bruker Nano Surfaces.