0000000000309188
AUTHOR
Davide Tranchida
Combining Atomic Force Microscopy and Depth-Sensing Instruments for the Nanometer-Scale Mechanical Characterization of Soft Matter
Complex materials exhibit a hierarchical structure where a gradient of features on nanometer scale is induced by the synthetic route eventually enhanced by the loading condition. The nanometer scale at which individual components arrange, determining their properties, is a current challenge of mechanical testing. In this work, a survey on nanoindentation is outlined based on the comparison of results obtained by Atomic Force Microscopy and Depth-Sensing Instruments and their combination. An Atomic Force Microscope equipped with a Force Transducer gives indeed the possibility to scan the sample surface in contact mode, thereby allowing one to choose a suitable position for the nanoindentatio…
Linking structure and nanomechanical properties via instrumented nanoindentations on well-defined and fine-tuned morphology poly(ethylene)
Several poly(ethylene) samples with a broad range of morphologies were studied in this work using nanoindentations. The samples had degrees of crystallinity ranging from 30 to 100% while their Young's modulus ranged from few tens of MPa up to several GPa. Experimental conditions for the correct evaluation of Young's modulus were at first identified, choosing a suitable loading rate in order to minimize viscoelastic effects on the unloading. The force curves, i.e., plots of applied load vs. penetration depth, were then analyzed following two common procedures available in the literature. None of these procedures leads to satisfying results when compared to other experimental techniques. Howe…
Relating morphology to nanoscale mechanical properties: from crystalline to mesomorphic iPP
Abstract A nanoindentation technique using an atomic force microscope (AFM) was applied to characterize the mechanical behaviour of several isotactic polypropylene (iPP) samples. The samples were solidified from the melt with a CCT (continuous cooling transformation) procedure spanning a wide range of cooling rates thanks to a fast quenching apparatus developed by the authors. The influence of instrumental parameters on the nanoscale mechanical properties (indentation depth, Young's modulus) shows that for modulus determination one has to rely on simpler methods of force curve analysis based on trace curve alone. Structure homogeneity up to the scale of macroscopic samples used to evaluate …
ORMOSIL thin films: Tuning mechanical properties via a nanochemistry approach
The mechanical properties (hardness and elastic modulus) of organically modified silicate thin films can be finely tuned by varying the degree of alkylation and thus the fraction of six- and four-membered siloxane rings in the organosilica matrix. This opens the way to large tunability of parameters that are of crucial practical importance for films that are finding increasing application in numerous fields ranging from microelectronics to chemical sensing.
On the Use of the Nanoindentation Unloading Curve to Measure the Young's Modulus of Polymers on a Nanometer Scale
Summary: The nanoindentation test is a fundamental tool to assess the link between morphology and mechanical properties. The preliminary results of a more exhaustive study about the applicability to polymers of the most used procedure to determine elastic modulus by indentation are reported in this short communication. A departure of the experimental conditions from the theoretical assumptions and results that give rise to the Oliver and Pharr analysis is shown to occur under a wide range of experimental conditions, with applied loads and penetration depths covering several orders of magnitude and using different indenter geometries. Unloading curves with exponents significantly larger than…
Mechanical Characterization of Polymers on a Nanometer Scale through Nanoindentation. A Study on Pile-up and Viscoelasticity
The analysis of nanoindentation force curves collected on polymers through the common Oliver and Pharr procedure does not lead to a correct evaluation of Young’s modulus. In particular, the estimated elastic modulus is several times larger than the correct one, thus compromising the possibility of a nanomechanical characterization of polymers. Pile-up or viscoelasticity is usually blamed for this failure, and a deep analysis of their influences is attempted in this work. Piling-up can be minimized by indenting on a true nanometer scale, i.e., at penetration depth smaller than 200 nm. On the other side, it is common knowledge that fast indentations minimize the effect of viscoelasticity. How…
Viscoelastic recovery behavior following Atomic Force Microscope nanoindentation of semi-crystalline poly(ethylene)
The residual imprint left behind by the AFM nanoindentation of polymers has been seldom studied in the past. In this work, the evolution of indentations at room temperature performed on a semicrystalline poly- (ethylene) in a broad range of experimental conditions is presented. The study shows that the recovery after 24 h is substantial, although not complete. Moreover, the dynamics of the recovery process is not seen to depend on the magnitude of the applied load for the nanoindentation, but instead on the rate of the indentation used. This points out that viscoelastic processes are minimized when performing fast nanoindentations, while at low loading rates there seems to be a residual vis…
Local mechanical properties by Atomic Force Microscopy nanoindentations
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, …
Some experimental issues of AFM tip blind estimation. The effect of noise and resolution
The convolution of tip shape on sample topography can introduce significant inaccuracy in an AFM image, when the tip radius is comparable to the typical dimension of the sample features to be observed. The blind estimation method allows one to obtain information on the AFM tip through an unknown characterizer sample and thus to perform the deconvolution of the tip shape from an image. When applying the blind estimation method to determine the AFM tip shape, some apparently trivial issues relating to the experimental operating parameters must be taken into account. In this paper, the effects of the operating parameters, e.g., sampling intervals (resolution) and instrumental noise, have been …
Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: viscoelastic characterization of a model material
The atomic force microscope (AFM), apart from 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 viscoelasti…
Nanoscale mechanical characterization of polymers by AFM nanoindentations: Critical approach to the elastic characterization
AFM nanoindentations show a dependence of penetration, i.e., the relative motion between the sample and the tip (indenter), on material elastic properties when using the same load. This elationship becomes visible by using of samples being homogeneous down to the scale of nanoindentation. They were prepared from materials covering a broad range of mechanical behavior: from rubbery networks to glassy and semicrystalline polymers. The elastic modulus can be obtained applying Sneddon’s elastic contact mechanics approach. To do this, some calibrations and instrumental features have to be measured accurately. All the polymers tested show that the contact between the tip and the sample is dominat…
Ion bombardment of polyethylene—influence of polymer structure
Abstract Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (Mw), molecular weight distribution (Mw/Mn) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate p…
Accurately evaluating Young’s modulus of polymers through nanoindentations: a phenomenological correction factor to the Oliver and Pharr procedure
The Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure is a widely used tool to analyze nanoindentation force curves obtained on metals or ceramics. Its application to polymers is, however, difficult, as Young’s moduli are commonly overestimated mainly because of viscoelastic effects and pileup. However, polymers spanning a large range of morphologies have been used in this work to introduce a phenomenological correction factor. It depends on indenter geometry: sets of calibration indentations have to be performed on some polymers with known elastic moduli to characterize each indenter.
Mechanical properties of the nanometer scale pre-crystalline order of a poly (ethylene terepthalate) / poly (ethylene naphthalene) blend
A previous study carried out on PET has shown that this polymer undergoes a continuous structural modification over a wide cooling rate interval when solidified from the melt[1] assuming a semi-crystalline structure below 2 K s 1 and a completely amorphous one above 100 K s 1. Most important was the existence of a state of intermediate order between the above cooling rates which was evidenced by the absence of crystalline reflections in the WAXS patterns and the occurrence of SAXS maxima[2] and exothermic peak areas (DSC) in the cooling rate range above 2 K s 1. Microhardness (MH) measurements revealed that this phase affects the mechanical properties[3] plausible if one thinks of crystalli…