Atomic force microscopy (AFM) has become an essential characterization tool in polymer research due to its unique capacity for multifunctional, nanoscale imaging and measurement. Recent advances have enabled AFM to probe complex polymer structures, functions, and behavior under real-world conditions across various scales. With ongoing advances in polymer complexity, AFM's multifunctional nanoscale imaging is poised to meet emerging challenges and provide unprecedented insights into polymer science.
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Advancing Understanding of Polymer Morphologies and Functional Properties through AFM
Polymers are versatile materials with diverse applications, and their structure and function depend on their chemistry, organization, and chain architecture. However, they exhibit complex multiscale heterogeneity in structure and dynamics, ranging from individual chain conformations to mesoscale self-assembly.
This complexity arises from their chemistry, architecture, organization, and aggregation, leading to emergent functionality in diverse applications. With ongoing advances in polymerization, morphologies are becoming increasingly complex and nanoscale characterization is thus imperative to unlock the full potential of polymers through knowledge-driven design.
Various microscopy techniques, including electronic microscopy, AFM, and X-ray scattering, are commonly used to study the morphologies of polymeric materials. However, AFM stands out because it uniquely provides multifunctional characterization and high-resolution imaging.
Recent AFM advances enable the mapping of topography, mechanics, and electrical properties, with nanometer-scale resolution, even capturing dynamics. This overcomes the limitations of other techniques like sample damage (electron microscopy) or average/non-functional measurements (X-ray scattering).
AFM's nondestructive and real-time imaging abilities and its versatility in operating conditions position it as a valuable and indispensable tool for advancing our understanding of polymer morphologies and their functional properties.
How is AFM Used in the Research and Development of Polymers and Biopolymers?
Exploring Structure and Morphology
Polymers exhibit a complex hierarchical structure at the micro- and nanoscale, with features such as chain packing, crystalline regions, and copolymer phase separation. The AFM, particularly in tapping mode, enables nanoscale imaging of polymer topography with high spatial resolution, including atomic and molecular details.
Recent advancements in AFM technology have increased imaging speed and improved sensitivity to delicate polymers by using smaller cantilevers with higher resonant frequencies, allowing for precise control of low forces.
This capability is crucial for studying the size, shape, and structure of polymeric nanoparticles, nanodevices, and nanofibers at individual and ensemble levels.
Monitoring Dynamic Processes
Observing the evolution of polymer morphology and properties in non-ambient environments, such as solvents or elevated temperatures, provides valuable insights. Advanced AFMs offer fast scanning capabilities that enable the real-time observation of nanoscale dynamic processes, including degradation, ordering, recrystallization, and melting.
By controlling solvent concentration or applying thermal gradients, researchers can study the effects of chemical interactions or elevated temperatures on polymer behavior.
Advancements in AFM imaging methodology and theory have enabled subsurface imaging of polymer systems, revealing the nanostructural organization within their three-dimensional volume.
Subsurface imaging modes, such as nanomechanical, acoustic, and optical methods, have been used to map buried polymer crystallites and nanoparticles and measure the depth of multilayer films. These techniques provide valuable insights into the internal structure of polymers beyond surface characterization.
Nanoscale Characterization of Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE), commonly known as Teflon, is a hydrophobic and non-reactive fluoropolymer.
AFM studies have shown that its hydrophobicity can be reduced by surface modification, offering benefits such as improved wetting, enhanced adhesion, increased heat transfer efficiency, better biocompatibility in biomedical applications, and prevention of fouling
Quantitative AFM imaging reveals the structure of PTFE's pore size, stiffness, fiber density, and membranes, but free-hanging fibers can pose challenges in imaging stability, requiring precise force control for stable imaging conditions.
High-Resolution Compositional Mapping
AFM's TappingMode phase imaging provides a qualitative compositional mapping of polymers by detecting differences in material properties. This enables detailed characterization of features like amorphous versus crystalline regions, nanodomains in blends, phase separation in copolymers, and nanofillers in composites, providing critical insights into structure-function relationships unobtainable by other methods.
Investigating Dynamic Behavior and Processes of Polymers in Real-Time
The dynamics of polymers encompass various processes, from their inherent thermal behavior to interactions with the environment.
Direct real-time observation of structural changes by high-speed AFM can provide insights into underlying mechanisms and kinetics governing crystallization, melting, degradation, chain ordering, and lamellae formation. In addition, fast scanning AFM enables high-speed TappingMode imaging to capture dynamic processes at the nanoscale, regardless of sample size or environment.
This real-space, real-time visualization of morphology evolution sheds light on polymer dynamics unobtainable through static or ensemble-average techniques.
Advancing Biopolymer Development and Optimization
AFM is a versatile tool for studying the development of biopolymers. It enables high-resolution imaging of the topography and morphology of biopolymers such as carrageenan, chitosan, and alginate.
AFM can reveal structural features, such as single chains, fibrous networks, and honeycomb geometries, based on the type and concentration of the biopolymer. It has been used to investigate the effects of different factors, such as temperature and additives, on the biopolymer's nanostructure and surface roughness.
Additionally, AFM techniques like phase imaging and Kelvin probe force microscopy (KPFM) allow for the characterization of interactions, stiffness variations, and surface potential of biopolymers.
These techniques contribute to a deeper understanding of biopolymer properties and their applications in various industries, including medicine, textiles, cosmetics, and catalysis.
Recent Research and Development
AFM Reveals Path to Improved Polymer Solar Cells for Clean Energy
A study published in ACS Applied Polymer Materials investigated the performance enhancement mechanism of all-polymer blend solar cells using photoconductive atomic force microscopy (PC-AFM). The PC-AFM helped image the nanoarchitecture and photocurrents in all-polymer blend solar cells with nanometer resolution.
The researchers found that adding small amounts of solvent additives increases power conversion efficiency by enhancing the ordering and crystallization of the polymers without damaging the phase-separated morphology. The improved polymer microstructure acts like a "highway" to efficiently transport photogenerated charges and increase photocurrents.
This can help optimize the performance of all-polymer-based solar cells, accelerating their widespread application in clean energy technologies.
Conclusion and Outlooks
AFM has revolutionized polymer research and development, providing insights into nanoscale complexity, ordering, and functionality. Ongoing advancements in AFM technology, including high-speed imaging and improved environmental controls, offer new opportunities to investigate polymer dynamics and interactions under real-world conditions.
The multifunctional capabilities of AFM, coupled with its high resolution, make it well-suited for studying increasingly complex polymer materials and their composites. In addition, integrating AFM with other techniques, such as AFM-IR and AFM-mass spectrometry, has enabled nanoscale chemical characterization and spatially correlated imaging.
With continued developments, AFM is poised to advance our understanding of the structure, properties, and heterogeneity of polymer systems in the future.
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References and Further Reading
Bruker. (2023). Quantitative Imaging from Single Molecules to (Bio)Polymers. [Online]. Available from: https://www.bruker.com/en/products-and-solutions/microscopes/bioafm/resource-library/quantitative-imaging-from-single-molecules-to-biopolymers.html
Joshi, J., Homburg, S. V., & Ehrmann, A. (2022). Atomic force microscopy (AFM) on biopolymers and hydrogels for biotechnological applications—Possibilities and limits. Polymers, 14(6), 1267. https://doi.org/10.3390/polym14061267
Murphy, J. G., Raybin, J. G., & Sibener, S. J. (2022). Correlating polymer structure, dynamics, and function with atomic force microscopy. Journal of Polymer Science, 60(7), 1042-1058. https://doi.org/10.1002/pol.20210321
Nguyen-Tri, P., Ghassemi, P., Carriere, P., Nanda, S., Assadi, A. A., & Nguyen, D. D. (2020). Recent applications of advanced atomic force microscopy in polymer science: A review. Polymers, 12(5), 1142. https://doi.org/10.3390/polym12051142
Oxford Instruments Asylum Research. (2015). AFM Applications in Polymer Science and Engineering. [Online]. https://www.alvtechnologies.com.ph/wp-content/uploads/2021/09/AFM-Applications-in-Polymer-Science-and-Engineering.pdf
Wang, D., & Russell, T. P. (2018). Advances in atomic force microscopy for probing polymer structure and properties. Macromolecules, 51(1), 3-24. https://doi.org/10.1021/acs.macromol.7b01459
Yamagata, Y., Benten, H., Kawanishi, T., & Nakamura, M. (2021). Nanoscale observation of the influence of solvent additives on all-polymer blend solar cells by photoconductive atomic force microscopy. ACS Applied Polymer Materials, 4(1), 169-178. https://doi.org/10.1021/acsapm.1c01173