Quantitative Mapping of Viscoelastic Properties Using AFM-nDMA

The search for a technique that can measure local viscoelastic properties with nanoscale or molecular resolution on soft heterogeneous materials such as polymers has been challenging. Now, scientists from Bruker have developed a new method called Atomic Force Microscopy – nano Dynamic Mechanical Analysis (AFM-nDMA) that overcomes the limitations of traditional nanomechanical AFM modes to provide quantitative viscoelastic data that equate directly to bulk DMA measurements.

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Polymers are widely used in materials ranging from basic substances like nylon and PVC to new, advanced materials with exotic properties like self-healing and regulating their own temperature.1,2

Polymers are composed of long molecular chains, combining the features of solids and fluids. As a result, they often exhibit both viscous and elastic characteristics when deformed under stress. Understanding these viscoelastic properties is a crucial aspect of polymer science, optimizing formulations and blends, and developing new materials.3

Dynamic Mechanical Analysis (DMA) Provides Information about Bulk Viscoelastic Properties

The viscoelastic behavior of polymers is typically studied using a technique called dynamic mechanical analysis (DMA). DMA applies small stresses or deformations to a sample in a cyclic manner and monitors the material’s strain as a function of time, temperature, stress, and frequency.4

DMA can be used to determine properties, including storage modulus, loss modulus, damping coefficient, and complex modulus.  It can also be used to locate material transitions like the glass transition temperature.4

DMA is frequently used to understand structure-property relationships in the development of new materials. It is also used as a necessary part of quality assurance in the production of plastics.4

Why We Need to Study Local Viscoelastic Properties at Nanoscale

DMA measures the bulk properties of materials, which is excellent for homogenous materials without any nanostructures. But new materials increasingly rely on composites, blends, multilayers, and nanostructures to create desirable properties.

The bulk characteristics of these heterogeneous materials are often determined by microphases and interphases within the material, where confinement effects and intermolecular interactions alter the structure and viscoelastic properties.5

Understanding the features of microphases, interphases, and how they interact to determine bulk properties is key to designing exciting new materials. But to unlock this new dimension of materials research, we need a technique that can measure local viscoelastic characteristics at the sub-100 nm scale.5

Traditional AFM Modes Can’t Provide Quantitative Measurements of Local Viscoelastic Properties

Nanomechanics is frequently measured using atomic force microscopy (AFM), which provides the high sensitivity and resolution needed to map nanoscale features.  For example, PeakForce QNM provides elastic modulus mapping with high resolution.6,7

But imaging-focused AFM modes are not well suited for quantifying viscoelasticity. Rapid changes in contact area, as well as making and breaking the tip-sample contact, make these measurements intrinsically nonlinear. What’s more, tip-sample adhesion forces are significant, yet they are usually neglected during quantitative calculations. Finally, the frequencies used in AFM imaging techniques are often fixed and far too high for meaningful rheological measurements that can be compared with bulk DMA results. As a result, there is no realistic model for quantifying fundamental viscoelastic properties from imaging-focused AFM modes.5,7

As a result of these limitations, for years, researchers have been striving to develop a quantitative technique for measuring viscoelastic properties at the nanoscale that relate directly to bulk viscoelastic measurements.

AFM-nDMA Can Measure Viscoelastic Properties at the Nanoscale

Scientists at Bruker have now developed a new technique that overcomes the pitfalls of traditional AFM and can measure the viscoelastic properties of materials with 10nm spatial resolution. They demonstrated and published their latest technology, called AFM-nDMA, in The Journal of The Minerals, Metals & Materials Society in July 2019.5

AFM-nDMA uses a new approach to measure viscoelastic properties. First, a non-resonant tip-approach brings the tip into contact without lateral forces and with a known preload.

Subsequently, a series of hold segments are executed wherein the tip-sample force is modulated at well-defined rheological frequencies (0.1-300 Hz, extendible to 20kHz) with low amplitude. Using patented algorithms such as reference segments adapted from Bruker’s Hysitron indenter technology, AFM-nDMA corrects for the intrinsic problem of sample creep changing the contact radius during the measurement.

The retraction of the tip is then modeled using a contact mechanics model, including adhesion, allowing contact radius, storage, and loss moduli to be determined from the raw data. Using pre-calibrated probes with rounded, well-defined tips allows AFM-nDMA to provide accurate quantitative measurements of viscoelastic properties.5,8,9

Demonstrating the Abilities of AFM-nDMA on Real Samples

In their paper published in JOM, the team from Bruker describe how they used AFM-nDMA to measure the viscoelastic properties of a polydimethylsiloxane (PDMS) two-part elastomer, fluorinated ethylene propylene (FEP), a blend of polypropylene (PP) and cyclic olefin copolymer (COC), and an impact copolymer.5

The scientists measured how the storage and loss modulus of PDMS varied with frequency using AFM-nDMA, then compared their results with data from bulk DMA. In a first for AFM, they found excellent agreement with bulk DMA measurements across the entire bulk DMA frequency range and even good repeatability in different laboratories with various AFM systems and operators. What’s more, the AFM-nDMA method allowed access to higher frequencies than traditional DMA, which is typically limited to frequencies below 200 Hz.5

The team also acquired viscoelastic data for FEP as a function of frequency and temperature, enabling them to locate the material's glass transition temperature at different frequencies.  In another first for AFM, they followed industry-standard operating procedures for time-temperature superposition, and were able to generate master curves using bulk DMA and AFM-nDMA with good agreement.5

They also used AFM-nDMA to map the viscoelastic properties of the PP-COC blend, consisting of a PP matrix with COC domains of approximately 1 µm. They collected and mapped storage modulus and loss tangent over a range of temperatures, pinpointing critical thermal and structural transitions of the material that traditional methods would have missed.5

Finally, they mapped an impact copolymer using AFM-nDMA, demonstrating the ability of the technique to measure the local viscoelastic properties of features smaller than 100nm in diameter.5

A New era of Material Design with AFM-nDMA

Designing new materials with precise properties requires an intricate understanding of structure-property relationships. The study by scientists at Bruker demonstrates how AFM-nDMA provides a new dimension of understanding by enabling the quantitative mapping of viscoelastic properties with the spatial resolution only available with AFM.5

Undoubtedly, the new technique will play a central role in developing and optimizing new polymer materials in the coming years.

References and Further Reading

  1. ‘New cutting-edge polymer in development by The University of Nottingham’ https://www.britishplastics.co.uk/materials/new-cutting-edge-polymer-that-cools-down-itself-under-extrem/
  2. ‘Self-healing polymers and composites’ — Mauldin TC, Kessler MR, International Materials Reviews, 2010.
  3. ‘Viscoelastic Behaviour of Polymers’  In: Physicochemical Behavior and Supramolecular Organization of Polymers — Ligia G, Deodato R, Springer, Dordrecht, 2009.
  4. ‘Dynamic Mechanical Analysis: A Practical Introduction, Second Edition’ — Menard KP, CRC Press, 2008.
  5. ‘Nanoscale DMA with the Atomic Force Microscope: A New Method for Measuring Viscoelastic Properties of Nanostructured Polymer Materials’ — Pittenger B, Osechinskiy S, Yablon D, Mueller T, JOM, 2019.
  6. ‘Measuring Nanoscale Viscoelastic Properties with AFM-Based nano-DMA’ https://www.bruker.com/events/webinars/measuring-nanoscale-viscoelastic-properties-with-afm-based-nano-dma.html
  7. ‘Measuring Nanoscale Viscoelastic Properties with AFM-DMA’ https://www.azom.com/article.aspx?ArticleID=18383
  8. ‘Nanorheological Mapping of Rubbers by Atomic Force Microscopy’ — Igarashi T, Fujinami S, Nishi T, Asao N, Nakajima K, Macromolecules, 2013.
  9. ‘AFM-nDMA’ https://www.bruker.com/products/surface-and-dimensional-analysis/atomic-force-microscopes/modes/modes/nanomechanical-modes/afm-ndma.html

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

For more information on this source, please visit Bruker Nano Surfaces.


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