A commercial impact copolymer (ICP), amulticomponent material typically used inautomotive and appliance applications where a balance of stiffness and toughness is needed, was studied with the NanoRack™ Sample Stretching Stage accessory on the MFP-3D™Atomic Force Microscope to investigate material deformation and interface adhesion as a function of tensile stress. Effects of deformation were observed within both the polypropylene  and ethylene-propylene components, as well as at the interface between the two materials. There are no other direct measurement methods available to determine interfacial adhesive strength of polymer blends, and so AFM investigations of micro-domain deformation such as the one described here could be used ultimately to provide a direct determination of interfacial adhesion in complex polymer containing materials such as ICP. Studies of this kind improve our understanding of material structure-propertyrelationships, ultimately enabling manufacture of better quality products.

Application to Impact Copolymer (ICP)

The commercial impact copolymer used for this study is composed of a polypropylene (PP) matrix with micron-sized domains of ethylene-propylene (EP) rubber domains produced in a serial polymerization reactor. Dogbone-shaped samples were molded of the impact copolymer measuring at ~20mm (middle straight part of dogbone) by ~4mm in width by 0.2mm thickness. A portion of the straight part of the dogbone was cryo-faced at -120°C with a cryomicrotome to ensure a smooth sample and to remove the thin polymer

Figure 1: Stress (Newtons) vs. time (seconds) curve of ICP as it is being stretched on the NanoRack.

surface layer that forms during the compression molding process (also referred to as a ‘polymer skin’), leaving a small and smooth surface area in the middle of the dogbone that was suitable for imaging. The sample was mounted into a NanoRack Sample Stretching Stage with smooth grips. The NanoRack is a high-strain, high-travel manual stretching stage that provides two-axis stress control of tensile loaded samples and also allows control of the sample image region under different loads. Automatic load cell calibration provides integrated force measurements with MFP-3D images or other measurements and returns both stress and strain data.

Figure 1 shows real-time stress vs. time curves of the ICP as the sample is being pulled in the NanoRack. The baseline force is

Asylum Research Cypher™ and MFP-3D™ atomic force microscopes (AFMs) provide valuable information for characterizing thin films and coatings. They quantify 3D roughness and texture with unmatched spatial resolution and measure nanoscale functionality including electrical, magnetic, and mechanical behavior.

Thin films and coatings play a critical role in everything from food containers to photovoltaics. To meet such varied needs, they are made from every class of material and by numerous processes including physical and chemical vapor deposition techniques, atomic layer deposition, and sol gel processing.¹ A key step in developing any new film is characterizing its surface structure and physical properties, whether in engineering commercial products (Figure 1) or pursuing fundamental materials science (Figure 2).

The intrinsic dimensions of films (thickness, grain and domain sizes, etc.) make it important to characterize them on sub-nanometer to micrometer length scales. The AFM is a powerful tool for this purpose for many reasons. For instance, it possesses much higher spatial resolution than other stylus or optical-based methods.4 Samples need not be optically reflective or electrically conducting, allowing access to virtually any film. AFMs also provide complementary information to electron microscopes, such as accurate 3D surface profiles, and offer a more flexible operating environment for work at both ambient and non-ambient atmospheres and temperatures.

Here we describe the extensive features of Asylum Research AFMs for thin film characterization and show examples over a range of applications. With today’s AFMs, surface roughness can be measured more accurately, quickly, and easily than ever before. A wider array of built-in analysis tools and automated routines mean higher productivity and greater ease of use. Also, research and instrumentation advances have created a variety of AFM modes for measuring nanoscale film functionality including electrical, magnetic, and mechanical response.

Figure 2: Strain effects in ferroelectric NaNbO3 (NNO) films grownon TbScO3 (TSO) substrates with metal organic chemical vapordeposition (MOCVD). Growth of epitaxial NNO on TSO results in significant anisotropic misfit strain. Understanding relations between strain, crystal structure, and ferroelectric response will enable finetuning
of film properties. The lateral piezoresponse force microscopy (PFM) image on a film with thickness d=11 nm reveals a strong in-plane piezoresponse with highly ordered domains (vertical stripes).
For a thicker film (d=21 nm), distortions in the alignment appear. For an even thicker film (d=66 nm), 90º domains (horizontally striped regions) are observed, indicating a 1D to 2D domain pattern transformation. The graph shows values for the lateral piezoelectric domain width D obtained by PFM and x-ray diffraction (XRD). The dependence of D on d changes from approximately constant to the predicted D∝d 0.5 (dotted line) at d≈20 nm, where the 1D
to 2D transformation occurs. Acquired on the MFP-3D AFM. Adapted from Ref. 3.

Figure 1: Oxygen plasma treatment of polyethylene terephthalate(PET) films. PET fibers coated with a conducting polymer such as polypyrrole could be used in “smart” electronic textiles. However, achieving good coating-to-fiber adhesion remains a key challenge.Images of PET films exposed to oxygen plasma show that RMS surface roughness increased with exposure time. Films processed longer than 60 s displayed surface etching and uniform nanoscale features. The graph reveals a linear dependence of roughness on treatment time after ~30 s. Combined with data on surface chemistry, the results can be used to optimize treatment parameters for improved coating adhesion and conductivity. Scan size 1 μm; height scale 35 nm. Imaged with the Cypher S AFM. Adapted from Ref. 2. AFM Characterization-Thin Films