Using AFM to Characterize Polymer Thin Films

Atomic force microscopy (AFM) is a powerful characterization tool for polymeric materials. This technique provides imaging morphology as well as nanoscale information about a wide range of physical properties. In order to better understand the properties of polymers, this article describes the various benefits and capabilities of Asylum Research’s Cypher™ and MFP-3D™ AFM for polymer research.

Polymers are the material of choice in many applications. When compared to other materials, polymers are more sustainable, more durable and often less expensive. They can even be designed to have special properties. While developing new polymers, a complete understanding regarding the structure, properties, processing, and performance is very important.

Whether studying basic principles of polymer science (Figure 1) or designing a particular polymer solution (Figure 2), the AFM tool is often used for assessing polymers at small length scales, as its spatial resolution helps in visualizing polymer morphology at sub-micrometer and sub-nanometer levels.

Figure 1. Morphology of PS-PEP diblock copolymer film

Figure 2. Distribution of components in EVA-EPP-carbon black blends

Exploring Morphology and Structure

In polymeric materials, a rich hierarchy of structure is found on micro- and nanometer scales. Larger features include interfacial phases in polymer blends, pores or fillers, and effects of processing on surface finish or roughness.

Smaller features comprise lamellae of crystalline polymers, brush configuration and chain packing of single molecules, and microphase separation in graft or block copolymers. Moreover, polymeric nanodevices, nanofibers and nanoparticles need statistics on individual as well as ensemble shape and size.

The AFM tool delivers data related to structure through nanoscale imaging of topography, which is can be obtained in tapping mode. Images obtained through tapping mode render high spatial resolution and also resolve molecular and atomic structure, as shown in Figure 3.

Figure 3. Molecular and crystalline structure of rubrene

Over the last few years, the speed of AFM imaging has been increased significantly (Figure 4), with the advent of advanced instruments such as Cypher AFMs from Asylum Research. Such systems support the application of smaller cantilevers having higher resonant frequencies. Small-sized cantilevers are also capable of resolving and controlling extremely low forces, a key aspect for polymers that are fine and easily deformed.

Figure 4. PHB/V spherulite crystallization

Measuring Forces and Deformation

The AFM cantilever has high force sensitivity that makes it suitable for determining forces from piconewtons to micronewtons. In single-molecule force spectroscopy, a molecule is stretched between the substrate and AFM tip and the cantilever deflection is then determined.

Based on the understanding of cantilever’s deflection sensitivity and spring constant, a plot of force against distance is acquired. Such force curves offer data regarding intermolecular and intramolecular forces between or within single polymer molecules. Figure 5 illustrates an example of force spectroscopy measurements performed on poly-L-lysine molecules as they desorb from a hydrophobic surface.

Figure 5. Single molecule forces for poly-L-lysine homopolymer

Mapping Nanomechanical Properties

The mechanical properties of polymers play an important role in a wide range of applications such as flexible electronics, food packaging, and so on. In order to improve the mechanical performance, one or more phase-separated fillers or components can be incorporated. The degree of such inclusions requires measurement of mechanical properties with nanoscale spatial resolution.

A number of techniques are offered by Asylum Research for studying nanomechanical properties. These methods range from easy qualitative techniques to more advanced quantitative methods. In many practical applications, these techniques are complementary and hence can be utilized together to gain a better understanding about polymer samples.

Phase imaging is used for characterizing polymer materials as it resolves delicate structural details and differentiates a variety of material components from each other. Biomodal imaging (Dual AC™) is yet another option used for mapping variations of material properties. Among the techniques in Asylum's NanomechPro™ Toolkit, the AM-FM Viscoelastic Mapping Mode is suitable for polymer characterization.

As with bimodal imaging, it utilizes tapping mode and works concurrently at two different cantilever mode frequencies. Figure 6 shows an example of AM-FM mapping on a multicomponent polymer assembly.

Figure 6. Mechanical mapping of polymer sandwich

Force curves are often used for determining elastic modulus; however, this method is more suitable for point measurements and rather slow for mapping. Figure 7 illustrates an example of fast force mapping on a phase-separated polymer blend.

Figure 7. Modulus mapping of PS-PCL blend

Asylum Research's technique, Contact Resonance Viscoelastic Mapping Mode, is used for measuring viscoelastic loss modulus and elastic storage modulus on considerably stiff polymers. This technique can be applied with minimal calibration for quick and qualitative mapping, or can even be calibrated with a known material to obtain better quantitative results.

Measuring Thermal Properties

There are a number of techniques that can be used to study thermal properties at the nanoscale, in addition to monitoring material properties and morphology as a function of temperature. Scanning thermal microscopy (SThM) is one such method that works in contact mode with a unique cantilever. This cantilever monitors the changes in temperature from a local heat source. When the samples’ local surface temperature is measured, produces image contrast that relies on the local thermal conductivity. Figure 8 displays a polymer blend imaged using the SThM technique.

Figure 8. Scanning thermal microscopy on PP-PS-PE ternary blend

Another thermal technique is local thermal analysis (LTA). Instead of detecting the sample temperature like the SThM technique, it uses a heated probe to heat a small amount of the sample. The sample expands or contracts when it heats or cools, respectively in proportion to the local coefficient of thermal expansion. This effect is determined by deflection of the cantilever and results in deflection against temperature curves. Figure 9 shows a ternary polymer blend imaged using LTA technique.

Figure 9. LTA of PP-PE-PS ternary blend

Monitoring Dynamic Processes: Solvent and Thermal Effects

With monitoring of thermal gradient and solvent concentration, latest generation of AFMs such as the Cypher AFM scan rapidly to observe nanoscale dynamic behavior including lamellae formation, chain and brush ordering, decomposition, degradation, melting, and crystallization.

Probing Electrical and Functional Behavior

Unique AFM modes are available to differentiate the nanoscale electromechanical and electrical properties of dielectric, conducting, and semiconducting polymers. Modes such as Kelvin probe force microscopy (KPFM), electric force microscopy (EFM), and conductive AFM (cAFM) offer electrical data such as surface potential, photocurrents, conductivity, and work function.

Asylum AFMs for Topographic Imaging

All AFMs from Asylum Research include closed-loop scanners, which employ position sensors in order to ensure repeatable and precise scanning motion. This not only helps in preventing image distortions, but also allows highly accurate zooms and offsets on particular scan areas. The Cypher ES, Cypher S, and MFP-3D Infinity™ include advanced position sensors with very low noise down to 60pm in X and Y (lateral) and 35pm in Z (vertical).

The Cypher AFM series supports small cantilevers for rapid imaging and normally operates at line scan rates of 10-40Hz. High resolution and low noise levels in small cantilevers ensure that imaging of DNA double helix and single point atomic defects is made much easier.

The GetStarted™ feature automatically optimizes imaging parameters and makes tapping mode simple on the Cypher and MFP-3D Infinity AFM series. Here, a predictive algorithm is utilized to set the parameters even before the tip contacts the sample. This set up ensures that both the sample and tip are unaffected by non-optimal settings.

Conclusion

The range of techniques available from the NanomechPro Toolkit ensures that the most suitable technique can be selected, or the results of two or more techniques can be compared easily. Contact Resonance Mode and AM-FM Mode can be used to determine the viscoelastic response of the sample, besides its elastic response.

In the Cypher AFM, AM-FM Viscoelastic Mapping Mode equipped with small cantilevers facilitates quantitative nanomechanical property mapping at much higher speeds when compared to other techniques, while the NanoRack™ Stretch Stage can be utilized to assess materials under tensile strain using the MFP-3D AFMs.

About Asylum Research

Asylum Research is the technology leader in atomic force probe microscopy (AFM) for both materials and bioscience applications. Founded in 1999,we are dedicated to innovative instrumentation for nanoscience and nanotechnology, with over 300 years combined AFM/SPM experience among our staff. Our instruments are used for a variety of nanoscience applications in material science, physics, data storage and semiconductors, polymers, chemistry, biomaterials, and bioscience, including single molecule mechanical experiments on DNA, protein unfolding and polymer elasticity, as well as force measurements for biomaterials, chemical sensing, polymers, colloidal forces, adhesion, and more.

Technological Leadership and Leading Innovation

Precision and accuracy has been the driving force behind the pioneering AFM instrument innovations in flexure design, lowest-noise closed loop sensors, and full digital control. These are now requirements in any research grade AFM. Our open software, based in IGOR Pro, allows researchers the ease-of-use, and power and flexibility to take their experiments to the next level. These innovations, just to name a few, are offered in our two main product lines-the Cypher™ and MFP-3D™ AFM Families.

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

For more information on this source, please visit Asylum Research - An Oxford Instruments Company.

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