using micro thermal analysis to characterise the nano world

8/14/2019 Using Micro Thermal Analysis to Characterise the Nano World

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THERMAL METHODS such as differential scanningcalorimetry (DSC), thermogravimetric analysis(TGA), thermomechanical analysis (TMA), and

dynamic mechanical analysis (DMA) are well-established techniques for characterizing the morphol-ogy and composition of polymers. It is often possible toidentify and quantify materials by reference to theircharacteristic transition temperatures and thermal sta-bility. By investigating the changes in the measuredproperty (e.g., enthalpy, weight, length, stiffness, etc.)

with temperature, it is also possible to study the effectsof processing on, for example, crystallinity or cross-link density. The introduction of Modulated DSC

(MDSC, TA Instruments, Inc., New Castle, DE),whereby a temperature modulation is superimposed onthe conventional linear heating or cooling program, hasfurther increased the resolution and sensitivity of DSCfor monitoring, in particular, glass-rubber transitions inpolymer blends.14 The recent application of modulatedtemperature programming to TGA has also enhancedthis technique by providing continuous kinetic informa-tion about decomposition processes.5

Despite these advances, conventional thermal meth-ods give only a sample-averaged response. A DSC

measurement, for example, may indicate the presenceof more than one phase (and, with calibration, quantifythe amount of each component), but the technique can-not give any information regarding the size or distribu-tion of phases. In order to obtain spatially resolved in-formation about a sample, the investigator must resortto microscopy. Without employing staining or etchingtechniques, it may be difficult to determine differencesin composition across a specimen. Infrared and Ramanmicrospectrometry may be used to investigate chemicalcomposition on a local scale, but often the resolution(spatially and structurally) is poor. Imaging secondaryion mass spectrometry (SIMS) or X-ray photoelectronspectrometry (XPS) can provide similar information

but also suffer the same drawbacks in addition to re-quiring the sample to be in a high vacuum.

An earlier article6 described the initial results of aca-demic research in bringing together the capabilities ofthermal methods with microscopy. Part of this work hasbeen commercialized in the TA 2990 Micro-ThermalAnalyzer (TA Instruments, Inc.). The TA 2990 (Figure1) is the result of continuing cooperative technical devel-opments with Dr. Mike Reading of Loughborough Uni-

versity (Loughborough, U.K.); Dr. Hubert Pollock andDr. Azzedine Hammiche of Lancaster University (Lan-caster, U.K.); TopoMetrix Corp. (Santa Clara, CA); andLinkam Scientific Instruments, Ltd. (Tadworth, Surrey,U.K.). The instrument combines the visualization powerof atomic force microscopy (AFM) with the characteriza-tion ability of thermal methods by replacing the standardprobe in an AFM with a thermal probe (Figure 2). Thesystem is capable of providing four images or views ofthe surface of a sample: topography, thermal conductivity,modulated temperature (amplitude), and modulated tem-

Reprinted fromAmerican Laboratory August 1998

Using microthermal analysis to

characterize the nanoworld

Dr. Lever is Executive Manager, Europe, TA Instruments Ltd.,Eu-

rope House, Bilton Center, Cleeve Rd., Leatherhead, KT 227UQ, Sur-rey, U.K.; tel.: 44-1-372-360363; fax: 44-1-372-360135; e-mail:[email protected]. Mr. Price is Research Associate, Institute of

Polymer Technology and Material Engineering, Loughborough Uni-versity, Loughborough, U.K.

By Trevor J. Lever and Duncan M. Price

Figure 1 The TA 2990 Micro-Thermal Analyzer, with sampleready for loading.

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perature (phase). After these images have been acquired,any specific location on the sample can be analyzed by

Local Thermal Analysis (LTA) using two new thermaltechniques: Micro-Thermomechanical Analysis(TMA) and Micro-Modulated Differential ThermalAnalysis (MDTA) (both by TA Instruments). The re-sults from these micro techniques are comparable withtheir macro counterparts.

Presented here are several new applications of Micro-Thermal Analysis relating to the visualization and char-acterization of polymeric systems.

Phase miscibility

The characterization of the morphology of polymerblends is of particular interest to the materials scientist.

When two polymers are blended, they can exist eitheras discrete phases, or mix to form a single phase withproperties that are a function of the individual phasesand composition of the blend.

Figure 3 shows the thermal conductivity images oftwo blends of polyethylmethacrylate (PMMA) andpolycarbonate (PC). One sample shows a two-phase do-main structure, while the other clearly shows phasemiscibility. Subsequent -TMA experiments (Figure 4)give the softening points for the two-phase material andthe single-phase material. As one would expect, thesoftening point for the phase miscible material lies be-tween the softening points of the individual phases inthe phase immiscible sample.

Multilayer films

Figure 5 shows the topography and thermal conduc-tivity images for a multilayer packaging film typicallyused in the packaging of convenience foods. The con-struction of the film comprises a central gas and flavorbarrier layer surrounded by HDPE (high densitypolyethylene) and LDPE (low density polyethylene)outer surfaces designed to accept printing ink. The ther-

mal conductivity image shows that there is a centrallayer of polymer flanked by two outer layers, with a pos-sible intermediate tie layer. Figure 6shows the softeningtemperatures obtained from each layer at a heating rateof 500 C/min. Such fast heating rates are possible be-cause of the small sample size and low thermal mass ofthe thermal probe. The softening temperatures are in-dicative of each polymer present in the film. Curves 1and 5 are typical of HDPE; curves 3 and 6 correspond tothe EVOH (ethylene vinyl alcohol) layer; curve 2 (thetie layer) has a lower softening point than HDPE and isprobably MDPE (medium density polyethylene); curve4 is obtained from the EVOH/tie layer interface; and themeasured response is a combination of each materials

individual response.

Gel formation

Figure 7shows four TMA and MDTA scans takenfrom across a section of a polyethylene film containingan imperfection. The middle two runs show the re-sponse of the imperfection and the outer two runs showthe data for the bulk material. The imperfection itself isonly 300 m across. It can be seen that the imperfection

Figure 2 Thermal probe schematic.

Figure 3 Thermal conductivity images of two samples ofpolyethylmethacrylate/polycarbonate blends.

a b

Figure 4 TMA data for two polymethylmethacrylate/polycar-bonate blends.

Reprinted from American Laboratory August 1998 TA-249


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has a lower crystallinity and higher melting point. Asthe indentation of the probe is lower (above the melt-ing point), it can be seen that the imperfection has ahigher molecular weight. All of these results can beinterpreted to indicate that the imperfection is due tounwanted cross-linking of the polyethylene film dur-ing production.

Printing and coatings

Figure 8 shows the topography image of an over-head transparency made by photocopying a page from abook. The image area is 100 100 m and is from thedot of the letter i that appears on the page. The topright-hand side of the image is that of the plastic film,whereas the remainder of the image is largely tonerfrom the copying process. Figures 9a and b show the

TMA and MDTA data from the overhead trans-parency. Three local thermal analysis experiments weremade, two of the plastic film, and a third on an area oftoner. The positions of the scans are labeled in Figure 8.Locations 1 and 3 for the substrate show an expansion

in the TMA signal to around 225 C, followed by soft-ening and melting. This is confirmed by the peak in theMDTA signal with an onset of around 240 C. For lo-cation 2, the toner, there is a softening of the toner com-mencing at 80 C, followed at higher temperatures bythe softening of the polymer substrate. It is interestingto observe a feature in the MDTA trace at around150 C, which corresponds to the fuser temperature ofthe photocopier. From these data, it can be concluded

Figure 5 Thermal conductivity image of a multilayer packagingmaterial.

Figure 6 TMA results from each layer of a multilayer packag-ing material.

Figure 7 TMA and MDTA results obtained at several loca-tions, across the surface of an imperfection within a polyethy-lene film.

Figure 8 Topography image of an overhead transparency,showing toner particles on a polymer substrate.



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that the base film is PET (rather than the cellulose ac-

etate alternative) and the toner is likely to be apoly(styrene-co-butyl methacrylate) base.

Summary

Microthermal analysis provides the polymer scientistwith a unique blend of thermal analysis and microscopyin a single instrument. New applications are already be-ing developed to solve old problems with this technique.

References

1. Reading M. Trends Poly Sci 1993; 1:24853.2. Song M, Hammiche A, Pollock HM, Houston DJ, Reading M.

Polymer 1996; 37:56615.

3. Hourston DJ, Song M, Hammiche A, Pollock HM, Reading M.

Polymer 1997; 38:17.

4. Song M, Pollock HM, Hammiche A, Hourston DJ, Reading M.

Polymer 1997; 38:5037.

5. Blaine R. Am Lab 1998; 30(1):213.

6. Reading M, Hourston D, Song M, Pollock M, Hammiche A. Am

Lab 1998; 30(1):137.

Figure 9 TMA and MDTA results, taken at three locations upon an overhead transparency.

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using micro thermal analysis to characterise the nano world

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