Measuring the Elastic Modulus of Polymers by Nanoindentation with an Atomic Force Microscope (original) (raw)
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Polymer Testing, 2013
Force-deformation curves have been acquired using nanoindentation and atomic force microscopy on two amorphous polymer samples. The shape and size of the indenter tip was characterized using a white light interferometer and AFM. The measured nanoindentation curves were fitted with the Hertz equation to calculate the Young's modulus of the polymers. Once the Young's moduli of the polymers were known, AFM was used to acquire force-distance-curves on the same samples. We also used the Hertz theory for the analysis in this case. As a result, the tip radius of the AFM cantilever tip could be measured. This procedure is proposed as a method to determine the shape and size of AFM tips for the quantitative characterization of surface forces through force-distance curves.
Macromolecules, 2006
The atomic force microscope (AFM), apart form its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well-assessed technique while the characterization of the viscoelastic mechanical properties remains the challenge. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e. on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelastic features which cannot be identified by the force curve alone. For this reason, a viscoelastic characterization, and thus the study of the effects of indentation rate and temperature, was done on model materials where such additional phenomena are not observed. Another time dependence originating from the instrument itself has also been identified and decoupled. In fact, the viscoelastic behavior has been found to be reproducible even if one changes the experimental set-up as far as the preliminary determinations concerning AFM nanoindentations are well performed. The effects of temperature and time scales on the mechanical behavior have also been undertaken. A check on time-temperature superposition is also attempted through the WLF equation and the apparent activation energies for the elementary motions in the rubbery and in the glass transition regions are in good agreement with the expected values.
Measurement Science and Technology, 2009
The atomic force microscope (AFM), apart form its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well-assessed technique while the characterization of the viscoelastic mechanical properties remains the challenge. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e. on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelastic features which cannot be identified by the force curve alone. For this reason, a viscoelastic characterization, and thus the study of the effects of indentation rate and temperature, was done on model materials where such additional phenomena are not observed. Another time dependence originating from the instrument itself has also been identified and decoupled. In fact, the viscoelastic behavior has been found to be reproducible even if one changes the experimental set-up as far as the preliminary determinations concerning AFM nanoindentations are well performed. The effects of temperature and time scales on the mechanical behavior have also been undertaken. A check on time-temperature superposition is also attempted through the WLF equation and the apparent activation energies for the elementary motions in the rubbery and in the glass transition regions are in good agreement with the expected values.
Local Mechanical Properties by Atomic Force Microscopy Nanoindentations
NanoScience and Technology, 2009
The analysis of mechanical properties on a nanometer scale is a useful tool for combining information concerning texture organization obtained by microscopy with the properties of individual components. Moreover, this technique promotes the understanding of the hierarchical arrangement in complex natural materials as well in the case of simpler morphologies arising from industrial processes. Atomic Force Microscopy (AFM) can bridge morphological information, obtained with outstanding resolution, to local mechanical properties. When performing an AFM nanoindentation, the rough force curve, i.e., the plot of the voltage output from the photodiode vs. the voltage applied to the piezo-scanner, can be translated into a curve of the applied load vs. the penetration depth after a series of preliminary determinations and calibrations. However, the analysis of the unloading portion of the force curves collected for polymers does not lead to a correct evaluation of Young's modulus. The high slope of the unloading curves is not linked to an elastic behavior, as would be expected, but rather to a viscoelastic effect. This can be argued on the basis that the unloading curves are superimposed on the loading curves in the case of an ideal elastic behavior, as for rubbers, or generally in the case of materials with very short relaxation times. In contrast, when the relaxation time of the sample is close to or even much larger than the indentation time scale, very high slopes are recorded.
Measuring the Surface and Bulk Modulus of Polished Polymers with AFM and Nanoindentation
2012
A new method to determine the elastic modulus of a material using the atomic force microscope (AFM) was proposed by Tang et al. [Nanotechnology 19, 495713 (2008)] and is used in this study. This method models the cantilever and the sample as two springs in a series. The properties of both the spring and cantilever are determined on two reference samples with known mechanical properties and these properties are then used to find the elastic modulus of an unknown sample. The indentation depth achieved with AFM is in the nanometer range (30-130 nm in this study); and hence when this technique is performed on polymers, whose surface structure is different from their bulk structure, AFM gives a measure of the surface elastic modulus. In the present study, after employing AFM to measure the surface modulus of five polymers, traditional depth-sensing nanoindentation, with penetration depths of about 1 µm, was used to determine the elastic modulus in the bulk. The mean values for elastic modulus from the AFM were within 5-50% of the nanoindentation results, suggesting the existence of a surface modulus for polished polymers.
Elasticity of rubber-like materials measured by AFM nanoindentation
Express Polymer Letters, 2007
We exploit the force spectroscopy capabilities of the atomic force microscope in characterizing the local elasticity of rubber-like materials. Extraction of elastic properties from force curves usually relies on the linear theory pioneered by Hertz. While the Hertzian force-indentation relationships have been shown to be accurate in modeling the contact mechanics at sufficiently shallow indentation depths, the linear deformation regime of the probed material is exceeded in many practical applications of nanoindentation. In this article, a simple, nonlinear force-indentation equation based on the Mooney-Rivlin model is derived and used to fit data from the indentation of lightly crosslinked poly(vinyl alcohol) gels in equilibrium with water. The extracted values of Young's modulus show good agreement with those obtained by both macroscopic compression testing and by fitting truncated portions of the force curves with the Hertz equation.
Indentation modulus and hardness of viscoelastic thin films by atomic force microscopy: A case study
Ultramicroscopy, 2009
We propose a nanoindentation technique based on atomic force microscopy (AFM) that allows one to deduce both indentation modulus and hardness of viscoelastic materials from the force versus penetration depth dependence, obtained by recording the AFM cantilever deflection as a function of the sample vertical displacement when the tip is pressed against (loading phase) and then removed from (unloading phase) the surface of the sample. Reliable quantitative measurements of both indentation modulus and hardness of the investigated sample are obtained by calibrating the technique through a set of different polymeric samples, used as reference materials, whose mechanical properties have been previously determined by standard indentation tests. By analyzing the dependence of the cantilever deflection versus time, the proposed technique allows one to evaluate and correct the effect of viscoelastic properties of the investigated materials, by adapting a post-experiment data processing procedure well-established for standard depth sensing indentation tests. The technique is described in the case of the measurement of indentation modulus and hardness of a thin film of poly(3,4ethylenedioxythiophene) doped with poly(4-styrenesulfonate), deposited by chronoamperometry on an indium tin oxide (ITO) substrate.
Atomic Force Microscope Nanoindentations to Reliably Measure the Young's Modulus of Soft Matter
2000
The analysis of nanomechanical properties is becoming an increasingly useful tool in a large variety of fields, ranging from biology to polymer science. The Atomic Force Microscope, AFM, can bridge the information about morphology, obtained with outstanding resolution, to local mechanical properties. When performing an AFM nanoindentation, the rough force curve, i.e. the plot of voltage output from the photodiode
Journal of Polymer Science Part B, 2017
Nanoindentation is a widely used technique to characterize the mechanical properties of polymeric materials at the nanoscale. Extreme surface stiffening has been reported for soft polymers such as poly(dimethylsiloxane) (PDMS) rubber. Our recent work [J. Polym. Sci. B Polym. Phys. 2017, 55, 30-38] provided a quantitative model which demonstrates such extreme stiffening can be associated with experimental artifacts, for example, error in surface detection. In this work, we have further investigated the effect of surface detection error on the determination of mechanical properties by varying the sample modulus, instrument surface detection criterion, and probe geometry. We have examined materials having Young's moduli from 2 MPa (PDMS) to 3 GPa (polystyrene) using two different nanoindentation instruments (G200 and TI 950) which implement different surface detection methods. The results show that surface detection error can lead to apparent large stiffening. The errors are lower for the stiffer materials, but can still be significant if care is not taken to establish the range of the surface detection error in a particular experimental situation. We have also examined the effect of pressure beneath the probe on the nanoindentation-determined modulus of polystyrene with different probe geometries. V
Atomic force microscopy tip torsion contribution to the measurement of nanomechanical properties
Journal of Materials Science, 2008
The nanomechanical properties of polymethyl methacrylate and indium phosphide were measured with an atomic force microscope and a nanoindentation system. The elastic moduli measured with the atomic force microscope are in good agreement with the values obtained with the nanoindentation system. The hardness is shown to be affected by the tip radius used in our experiments. The cantilever vertical and lateral movements were independently analyzed during nanoindentation, and the tip torsion can be attributed to a change from elastic to plastic deformation regimes of materials during force microscopy nanoindentation. An analysis of the lateral movement of the laser beam associated with the cantilever torsion was used to determine the material yield stress.