AFM for Polymer Research

Due to their robust, environmentally friendly and cost-effective nature over other materials, polymers have become ubiquitous in research and manufacturing. Polymeric surfaces present in-homogeneities in microscopic scale, which can affect their properties of adhesion, wettability, abrasion and degradation in the environment, which has allowed them to be customized to suit a very wide range of uses. Atomic force microscopy has become a preferred technique for characterizing polymers. Some advantages are that it is not necessary to use vacuum during analysis or conductive covering on the samples; as well as the capability to direct measure height and roughness with atomic resolution. In addition, the AFM technique does not demand prior complex treatment of the sample for the characterization of the morphology, microstructure and crystallinity of different polymer films.

Bruker AFMs enable nondestructive, in situ imaging of polymers with nanometer resolution with TappingMode (compositional mapping, fast scanning of dynamic behavior) and proprietary PeakForce Tapping® (highest resolution imaging, quantitative property mapping). These and other nanoscale characterization techniques cover the full range of structural length scales:

Molecular arrangement/defects
Fibrils, lamellae, brushes
Interphases, phase-separated domains

Modulus image showing the transition between the ULDPE tie layer to the PS/LDPE sealant layer of a cross-sectioned packaging material. Image size 3μm.

High-Resolution Compositional Mapping

Nanoscale distribution of components or multi-phases is critical to polymer function. Physical AFM tip-sample interaction enables simultaneous detection of multiple sample properties. TappingMode phase imaging provides qualitative compositional mapping based on differences in material properties. Bruker's AFMs enable detailed characterization of:

Amorphous, crystalline, mesomorphic regions
Nanodomains in polymer blends
Phase-separated copolymers
Nanofillers in polymer composites

TappingMode topography (left) and phase (right) images of PS-PMMA block copolymer. Phase shows high-resolution contrast based on material properties of the various components. 1.4µm scan.

In Situ Studies of Dynamic Behavior and Processes

Polymer dynamics range from intrinsic thermal behavior to interactions with the environment. Direct observation of structural changes can provide insights into key mechanisms and kinetics governing:

Crystallization/melting
Degradation/decomposition
Chain and brush ordering
Lamellae formation

The tip-scanning FastScan AFM enables high-speed TappingMode imaging of dynamic behavior in real-time and real-space, independent of sample size and environment.

High-speed imaging of poly(diethylsiloxane) (PDES) transitions through solid, liquid, and liquid crystal states as temperatures change.

Nanoscale Imaging of Challenging Polymer Structures

Certain polymer features, such as brush structure, chain-packing, and molecular/point defects, are difficult to image. Direct control of imaging force is critical to resolving these molecular and mesoscale structures. PeakForce Tapping enables <100 pN force control for highest resolution imaging, even on the smallest, most challenging samples. Since this technology maps mechanical properties simultaneously with topography, a comprehensive understanding is much easier to achieve with this proprietary technology.

PeakForce Tapping submolecular-resolution adhesion on iPMMA. 100 nm image. (Sample courtesy of T. Thurn-Albrecht, Martin-Luther-Universitaet Halle-Wittenberg.)

Quantitative Elastic Modulus Mapping

Mechanical performance of polymer materials critical to:

Optimizing formulation/blends
Managing processing conditions
Evaluating product lifetime

Microscopic organization requires characterization of Young’s modulus and stiffness at interphases/interfaces. PeakForce QNM® provides out-of-the-box quantitative elastic modulus mapping with nanometer resolution, enabling researchers to more easily and efficiently characterize polymers.

PeakForce QNM images of layered packaging showing nanoscale structure and modulus values near phase boundaries. These high-resolution maps reveal increased ordering of lamellae near layer interface. 4µm scans.

Quantitative Viscoelasticity Measurements

Viscoelastic properties of polymers are central to many commercial applications, ranging from vibration isolation and noise damping to shock absorption, dirt-resistant coatings, and temperature-dependent requirements. Macroscale properties are governed by nanoscale structure, for example, interphases, interfaces, and domains. Time-temperature superposition is needed to fully understand viscoelastic behavior.

Bruker's AFM-nDMA mode provides fully quantitative viscoelastic property mapping, including storage and loss modulus, as well as loss tangent (separate from adhesion). Rheological temperature and frequency ranges enable mastercurve construction at nanoscales.

High-resolution storage modulus map on four-component (COC, PE, LLDPE, elastomer) polymer (left). Storage Modulus spectra collected at individual points (right).

Molecular Conductivity Mapping of Organic Semiconductors

Conductive polymers are used in many electronic applications (e.g., organic photovoltaics). Charge transport properties and carrier mobilities of organic semiconductors are directly related to level of structural order. The ability to map conductive pathways over multiple length scales is key to improving efficiency. PeakForce-TUNA enables molecular-resolution, quantitative conductivity maps without damaging delicate samples. With this technology, researchers can now directly and reliably correlate topography and nanomechanical properties.