Topics Covered
Introduction
Nanoscale Thermal Analysis
Atomic Force Microscopy -
AFM
Advantages of Nanoscale
Thermal Analysis
AFM
Cantilevers Designed for Nanoscale Thermal Analysis
Polymer
Blends
Multilayer
Films
Coatings
Conclusion
About Bruker Nano Surfaces
Introduction
The Bruker's
Thermal Analysis (VITA) module enables nanoscale thermal analysis (nTA), a
novel technique that allows the determination of the local transition
temperature on the surface of a material with nanoscale spatial resolution. By
providing quantitative characterization, nTA can help identify materials and
their phase separation and component distribution (or aggregation) at the
nanoscale. The technique utilizes a specialized thermal probe to heat a very
small region on the sample surface and locally measure its thermal properties,
including such thermal transitions as melting points and glass transitions. The
thermal probe is similar in geometry and physical characteristics to standard
silicon atomic
force microscopy (AFM) probes, and thus enables the generation of
highresolution sample topography maps using contact mode and TappingMode™
techniques. The AFM image can be used to target locations of interest for thermal
analyses, which can then be executed in a matter of seconds. In this way, nTA
marries the resolution of AFM to the unambiguous and quantitative data of thermal analysis.
This application note describes the technique and demonstrates its benefits in a
number of applications.
Nanoscale Thermal Analysis
Thermal methods, such as differential scanning calorimetry (DSC),
thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA), are
well-established techniques for characterizing the transition temperature of
materials. However, a serious limitation of conventional thermal methods is that
they give only a sample-averaged response and cannot provide information on
localized defects, nor can they give the thermal properties of coatings/films
less than a few microns in thickness. A DSC measurement, for example, may
indicate the presence of more than one phase, but the technique cannot generally
give any information regarding the size or distribution of phases. This
particularly impacts scientists working with polymer blends (where the blend
morphologies are crucial to determining their material properties), coatings
(where imperfections such as gel formations can seriously impact performance),
multilayer films, and composites.
Atomic Force Microscopy - AFM
AFM has been
routinely used to characterize the topography of such materials, as well as the
distribution of their components. In some types of samples, the material, if
known, and its form can be determined from the topography or the mechanical
property variation revealed in AFM images.
Traditionally, this has been accomplished through a number of scanning probe
microscope (SPM) modes, such as lateral force microscopy (LFM), force
modulation, and TappingMode phase imaging. More recently, the introduction of
HarmoniX™ has provided a unique combination of highest resolution, fast,
nondestructive imaging with quantitative mechanical property imaging. HarmoniX
is ideal for mapping nanoscale variations in mechanical properties. Whenever
components or microphases exhibit significant differences in mechanical
properties, these techniques can also provide an unambiguous component and phase
distribution.
Advantages of Nanoscale Thermal Analysis
The advantage of nanoscale thermal analysis (nTA) is that it can provide an
unambiguous nanoscale material identification even in the absence of mechanical
property variations. It allows the determination of local transition
temperatures on the surface of a sample. This is accomplished by bringing a
specialized probe into contact with the sample surface, heating the end of the
cantilever, and measuring its deflection using the standard beam deflection
detection of the AFM. During the measurement, the probe is held at a fixed
location on the surface of the sample. As the cantilever and, in turn, the
sample heat up, the sample will expand, pushing the probe up and causing an
increase in the vertical deflection signal. At a transition temperature, the
material typically will soften such that the force applied by the cantilever can
deform the surface of the sample, allowing the probe to penetrate the sample and
decreasing the deflection of the cantilever. The change in slope of the
deflection signal is an indication of a thermal transition. This technique is
similar to the bulk thermal analysis technique, Thermomechanical Analysis (TMA),
but it can determine the transition temperature of a sample locally on the
micro- and even nanoscale. The transition temperature as measured by nTA
typically correlates well with the transition temperature measured by bulk
techniques, and can be therefore used to identify a material and to determine
whether it is in a crystalline or amorphous form.
AFM Cantilevers Designed for Nanoscale Thermal
Analysis
The specially designed AFM cantilevers used for nTA incorporate MEMS technology to
create a conductive path through the legs of the cantilever and a
high-resistance portion near its end. This causes the end of the cantilever to
heat up when current flows through the conductive path. The cantilever itself is
made of silicon and the path is created by implanting the silicon with different
dopant concentrations. Figure 1 shows an SEM image of the thermal probe used in
nTA. The probe has a similar aspect ratio and end radius to standard etched
silicon probes, allowing high-resolution imaging in either contact mode or
TappingMode. Because the material is doped silicon, the cantilever can withstand
much higher currents than metal film cantilevers, and therefore achieves much
higher temperatures. Controllable heating can be performed up to temperatures as
high as 350-400°C. The high thermal conductivity of silicon enables very high
temperature ramp rates, reaching maximum temperature in less than 100
microseconds, thus allowing for rapid (high throughput and localized) sample
heating. An additional benefit of the cantilevers is their ability to withstand
pulse heating to around 1000°C, which can be used to clean off any contamination
that adheres to the apex of the probe.
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Figure 1. An SEM image of the microfabricated thermal
probe used for nTA measurements. The inset is a zoom of the tip, which makes
contact with the sample surface.
The temperature range accessible with the nTA probes, and the need for
localized sample heating (thus limiting thermal conductivity of the sample)
makes the nTA technique an ideal match for polymers. Thus, nTA applications have
been focused on polymeric and pharmaceutical materials. Following are a number
of applications in these areas showing the utility of nTA to more fully
characterize a material at the nano- or microscale. In addition, the usage of
the heated probes in scanning probe microscopy is continuing to develop new and
interesting techniques and applications, from nanoscale lithography to
temperature-dependent electrical characterization of samples.
Polymer Blends
Polymer blends are used in a wide range of industries due to the fine tuning
of material properties possible through proper component choice. AFM has been
used to help characterize the domain size and distribution in a wide range of
polymer blend samples. As shown in figures 2 and 3, the domains can be
visualized using both topography data and phase imaging. This constitutes an
ideal starting point for nTA, which can then be used to help identify which
domain is which, as well as if the domains are fully phase segregated or are
intermixed. Since the samples in these figures are immiscible blends, the
primary question is which material is which.
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Figure 2. (a) 4µm x 4µm TappingMode AFM image of a
polystyrene - low-densitypolyethylene (PS-LDPE) blend. The red and blue circles
highlight the location utilized for VITA measurements in the PS domains and LDPE
matrix, respectively. (b) VITA nTA measurements showing reproducibly the PS
glass transition temperature inside the domains and the LDPE melting transition
in the matrix, thus identifying the component distribution unambiguously.
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Figure 3. (a) 4µm x 2µm TappingMode AFM image of a
polyethylene oxide - syndiotactic polypropylene (PEO-sPP) blend showing both
topography (left) and phase (right). The red circle highlights a small domain
and the blue circle highlights a similar domain after the nano thermal analysis
was performed. (b) VITA nTA measurement performed at the location of the blue
circle. The curve shows a transition temperature characteristic of PEO, followed
by a sPP melt transition. Apparently, the small features visible in the AFM
images represent shallow PEO domains that are readily traversed, allowing the
probe to sense both the small PEO domain and underlying sPP matrix.
All of the materials in these blends (polystyrene, low-density-polyethylene,
polyethylene oxide, syndiotactic polypropylene) are relatively stiff in
comparison with the cantilever at room temperature, so material identification
based on mechanical property variation can prove unreliable. Transition
temperatures, on the other hand, differ greatly between the components, allowing
for straightforward component identification using nTA. Further information
about domain thickness can be gleaned in the case of the polyethylene oxide -
syndiotactic polypropylene (PEO-sPP) blend, where probe penetration into small
PEO domains is seen to be quickly followed by penetration into an underlying sPP
matrix.
The nTA data presented here (figures 2 and 3) were generated using heating
rates of 5°C per second. While significantly faster than heating rates typically
employed for bulk thermal analysis, this high rate is typical for nTA and it
enables localized heating and high throughput. The unambiguous determination of
blend distribution shown in figure 2 was accomplished within just a few minutes.
The instrumentation allows adjustment of the heating rate over a wide range to
both slower and significantly faster heating rates, as required for the
experiment.
Multilayer Films
Multilayer films represent a standard choice of material for most packaging
applications. The different layers in a multilayer film contribute different
attributes to the final film, including physical rigidity and barrier
properties. While bulk thermal analysis can be used to measure the complete
composite stack, nTA allows individual, in-situ thermal property measurements
within individual layers. This enables the identification of each layer, as well
as the identification of individual defects within any layer. Additionally, the
transition temperature of the individual films can be mapped to detect the
possible presence of transition temperature gradients or inhomogeneities.
Thermal gradients through the thickness of the film can occur during processing
of the film due to differences in thermal history between the two sides. Figure
4 shows an example of a simple multilayer film used for food packaging. The
center ethylene vinyl alcohol (EVOH) film is used as a barrier film and has a
lower transition temperature than the adjacent "tie" layer or the outside
high-density polyethylene layers.
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Figure 4. (a) 25µm x 12µm TappingMode topography image of
a cross-sectioned multilayer film used for food packaging. (b) VITA nTA data
showing distinct thermal transitions in each layer. The blue curves were
obtained in the outer packaging layers (at the left and right sides of the AFM
image) and exhibit the high transition temperatures indicative of high-density
polyethylene. The green curve was obtained in the center layer (center of the
AFM image) and exhibits the much lower transition temperature characteristic of
ethylene vinyl alcohol (EVOH), a typical choice for a barrier layer. The red
curve with its intermediate transition temperature was obtained in the thin
layer surrounding the center layer.
Coatings
Organic polymeric materials are widely used as coatings in a growing number
of applications due to the opportunities they provide for fine tuning
performance, in particular appearance and such surface properties as corrosion
resistance. As the range of applications grows and the requirements become more
demanding, coating complexity is increasing and thickness is decreasing. This
trend towards thin, complex coatings hampers investigation with traditional
thermal analysis equipment. An additional challenge arises from the recent focus
on the curing rate, where environmental regulations and manufacturing cost
considerations are driving a minimization of drying time. Thus, the analysis of
coatings increasingly demands spatial and temporal resolution.
The nTA technique meets all the requirements imposed by modern coating
applications. An individual measurement is performed within seconds, allowing
for the quantification of curing times that are minutes in duration. The
nanoscale spatial resolution afforded by nTA extends thermal analysis to thinner
coatings, and by offsetting the probe a small distance laterally after each
measurement to an undisturbed location, either spatial inhomogeneities or time
dependences can be determined.
Figure 5 shows an example application using VITA nTA to
identify the distribution of materials in a two-component solid lubricant
coating. The two materials were spray deposited together on an aluminum
substrate. Optically, it appeared that the coating was not continuous. However,
neither optical nor AFM
data could differentiate the two materials. By using nTA, the uncoated surface
could clearly be identified because of the lack of probe penetration into the
surface throughout the temperature range, as demonstrated with the green curve
in the VITA data. The two other
components could be identified by their easily distinguished transition
temperatures of ~85°C versus ~125°C. By mapping a number of the islands, it was
also shown that the two components formed separate islands and did not
intermix.
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Figure 5. An optical image (right) of a two-component
solid lubricant coating. The circles indicate locations where nTA data was
taken, and the colors correlate with the curves in the graph (left). The nTA
data in the graph clearly identifies the two different coatings by their
distinct transition temperatures. The complete absence of transition
temperatures in the green curve shows that neither component is present at the
location of the green circle.
Conclusion
By enabling nanoscale thermal analysis, the VITA module
combines the worlds of microscopy and thermal analysis, thus revealing the
spatial distribution of thermal properties and inhomogeneities. This ability
makes the VITA accessory
uniquely valuable in applications ranging from analysis of polymer blends or
composites to in-situ measurements of thin coatings. The technique is made
possible by a microfabricated thermal probe that allows scientists to heat
samples locally and measure thermal properties of regions on the nanoand
microscale.
About Bruker Nano Surfaces
Bruker
Nano provides Atomic Force Microscope/Scanning Probe Microscope (AFM/SPM)
products that stand out from other commercially available systems for their
robust design and ease-of-use, whilst maintaining the highest resolution. The
NANOS measuring head, which is part of all our instruments, employs a unique
fiber-optic interferometer for measuring the cantilever deflection, which makes
the setup so compact that it is no larger than a standard research microscope
objective.
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For more information on this source please visit Bruker
Nano Surfaces.