Nanoscale Thermal Analysis and Characterization of Polymers Using VITA nTA Technique

A number of thermal analysis methods such as dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and differential scanning calorimetry (DSC) are used to determine the transition temperature of materials. However, these methods provide only a sample-averaged result and do not give information on thermal characteristics of coatings and films. Another thermal technique, atomic force microscopy (AFM) has also been utilized to determine the topography and component distribution of materials. Recently, a new technique, PeakForce QNM has offered a nondestructive solution for measuring minute alterations in mechanical properties. All the thermal analysis methodologies discussed above can provide a distinct component and phase distribution whenever the components show considerable change in mechanical properties.

Nanoscale Thermal Analysis (nTA)

The Bruker Thermal Analysis (VITA) enables nanoscale thermal analysis (nTA), which is a revolutionary technique that permits the estimation of local transition temperature at the material surface with a nanoscale spatial resolution. It measures transition temperatures of a sample by using a specialized probe to contact the sample surface.

How Nanoscale Thermal Analysis Works

In this technique, the probe, which is fixed at a certain point on the sample, heats the end of the cantilever and measures the deflection by utilizing AFM’s standard beam deflection detection. When the sample heats up, it expands and pushes the probe in an upward manner, thereby increasing the vertical deflection signal. The material gets softened at the transition temperature and the cantilever’s force deforms the sample surface. This allows the probe to pierce through the sample and reduce the cantilever’s deflection.

The slope change of deflection signal indicates that a thermal transition has taken place. The AFM cantilevers used in nTA feature MEMS technology to generate a conductive path between the cantilever’s legs. The cantilever is manufactured using silicon and the path is generated by implanting the silicon with various concentrations of dopant.

An SEM image of the probe used in this method has been depicted in Figure 1. Silicon features high thermal conductivity, which enables high temperature ramp rates and permits rapid and localized sample heating. The accessible temperature range and the requirement for localized heating make the nTA technique the best method for analysis of polymers.

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.

Applications of Nanoscale Thermal Analysis

The major applications of nTA in the polymer field for full characterization of materials at the nanoscale are detailed below.

Polymer Blends

AFM has been widely used to characterize the distribution and sample size in various polymer blend samples. The domains of the samples can be visualized using phase imaging and topography data techniques, as shown in Figures 2 and 3. nTA is used for identifying the different materials and also determining whether the domains are intermixed or fully phase segregated. The samples used in the figures are immiscible blends, which are stiffer than the cantilever at room temperature. Therefore, material identification based on variations in mechanical property can become unreliable. However, transition temperatures vary substantially between the components and allow direct component identification using nTA.

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Figure 2. (a) 4µm x 4µm TappingMode AFM image of a polystyrene – low-density-polyethylene (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.

Multilayer Films

Multilayer films are widely used for various packaging applications. Individual layers of a multilayer film provide various attributes to the final film. Figure 4 shows a multilayer film that has been used in food packaging. While thermal analysis is used for characterizing the composite stack, nTA enables in-situ measurements of thermal property in individual layers. This allows the identification of every layer, in addition to identifying various defects in any layer. The transition temperature of any single layer can also be mapped to identify any transition temperature gradients.

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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 extensively used as coatings in several applications due to their appearance and corrosion resistance. The increasing trend to use thinner coatings has made it difficult to analyze the coatings with conventional thermal analysis instruments. The nTA technique has been highly successful in thermal analysis of thinner coatings by virtue of its ability to provide nanoscale spatial resolution. Figure 5 shows an application that uses VITA nTA to charecterize material distribution in a two-component solid lubricant coating.

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Figure 5. An optical image (a) 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 (b). 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

The VITA nTA technique combines microscopy and thermal analysis to reveal the spatial distribution of inhomogeneities and thermal properties. This technique determines the transition temperature on the micro- and nanoscale. The main advantage of this technique is unambiguous characterization of materials at the micro- and nanoscale even without significant mechanical property variations. The knowledge of transition temperature can help in identifying materials and determining whether they are in amorphous or crystalline form.The module utilizes a microfabricated thermal probe that allows scientists to heat samples locally and measure the thermal properties of regions on the micro- and nanoscale. This makes the VITA accessory suitable for analyzing polymer blends or composites.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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