Mapping of Nanoscale Mechanical Properties of Polymers in Quasi-static and Oscillatory Atomic Force Microscopy Modes
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Mapping of Nanoscale Mechanical Properties of Polymers in Quasi-static and Oscillatory Atomic Force Microscopy Modes Sergei Magonov, Marko Surtchev, John Alexander, Ivan Malovichko and Sergey Belikov NT-MDT Development Inc., 7910 S. Kyrene Rd, Tempe, AZ 85284, U.S.A.
ABSTRACT Recent advances in studies of local mechanical properties of polymers with different atomic force microscopy techniques (contact, Hybrid and amplitude modulation modes) are described in interplay between experiment and theory. Analysis of force curves and time dependencies of probe response to sample compliance, which were recorded on a number of polymer materials at various temperatures, leads to quantitative mapping of specific mechanical properties (elastic modulus, work of adhesion, etc). High spatial resolution of elastic modulus mapping (10-20 nm) is illustrated in measurements of lamellar structures of several polymers. Challenges of examination of viscoelastic properties are pointed out and a possible solution is presented. INTRODUCTION Atomic force microscopy (AFM) provides various means for examination of elastic and viscoelastic properties of polymers. Initially, the deflection-versus-distance curves (DvZ) [1], which are recorded at single sample locations, were used for this purpose. After transformation of DvZ curves to force-versus-deformation (Fvh) dependencies, the latter were used for extraction of elastic modulus (E) and work of adhesion with solid-state deformation models such as Hertz, DMT and JKR [2]. In further step, the arrays of DvZ curves were collected at different sample locations and these data were employed for a construction of spatial maps of these properties. An ability to discern sample locations and structures with different mechanical properties is used for compositional studies of heterogeneous samples. As force curves were usually collected with the ~ 1 Hz rate, the mapping proceeds rather slow.
Figure 1a-b. Height and amplitude images of sPS/PVDF blend obtained in force modulation mode. The contrast in (a) is in the 0-30 nm range; the contrast in (b) – in the relative units.
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In another development - force modulation mode [3] the AFM probe, which scans a sample in the contact mode, was additionally brought into a vertical oscillation at frequency below the probe resonance. In this qualitative mode the probe amplitude, which is measured at the excitation frequency, is larger when the probe hits a stiffer location. The relevant example is given in figure 1a-b, where the images of immiscible polymer blend of syndiotactic polystyrene (sPS) and polyvinylidene fluoride (PVDF) are presented. A darker amplitude contrast of the matrix (figure 1b) allows its assignment to stiffer sPS (E = 5 GPa) and the softer circular domains to PVDF (E = 2 GPa). The limitations of above modes
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