Mechanical properties soft modes

Normal mode analysis of the mechanical properties of a triosephosphate isomerase-barrel protein suggests that the region between the secondary structures plays an important role in the dynamics of the protein. The beta-barrel region at the core of the protein is found to be soft in contrast to the helical, strand and loop regions [62]. A detailed discussion of other properties of proteins is mechanically highly non-linear systems is given by Kharakoz [63]. 

IntGrmittGnt Contact ModG SFM. Intermittent contact mode SFM, also called tapping mode SFM, has been developed to reduce shear forces (36), which can cause sample deformation or even destruction of soft samples in contact mode SFM (for a recent review see Ref 19). In addition, this mode can be useful to probe adhesive or surface mechanical properties, and thus achieve contrast between different materials. Intermittent contact mode SFM uses an oscillating cantilever/tip assembly to minimize contact time and contact forces, and to especially eliminate shear forces between tip and sample surface. This mode relies on a similar setup as contact mode SFM. However, in intermittent contact mode (in air) a stiffer cantilever is used ik = 20-80 N/m). This cantilever is excited to vibrate near its resonance frequency by an excitation piezo. Close to a sample surface, the vibration is damped as a result of tip sample interactions. The original amplitude Aq is hence reduced to A, depending sharply on the distance to the surface (Fig. 2c). The... 

In the local microscopy s field, a large effort is dedicated to study the mechanical properties which can be accessed with a nanotip. Within this context, soft materials are well adapted to probe mechanical response at the nanometer scale. After a discussion of some experimental and technical key points, we present three different types of experiments done on one model polymer polystyrene films with different molecular weights. In the experiments, the tip may scan the sample surface (friction loops), or move upward and downward in the vicinity of the sample -in contact mode (force curve) or in an oscillating mode (approach-retract curves)-. The comparison of the results shows the sensitiveness of the tip to local mechanical properties. New routes to explore mechanical properties without touching the sample are proposed. 

In this paper we will compare different experiments in contact and with an oscillating tip to show their contribution for the study of soft material. In static contact mode, force curves and friction loops are recorded while in tapping a systematic investigation of approach-retract curves is presented. A model sample is used monodisperse polystyrene films of different molecular weights (MJ bulk mechanical properties and molecular weight dependence of the glass transition temperature. In order to emphasize the inherent difficulties encountered with an AFM, we begin with a detailed discussion of the technical conditions. 

Only for the PDMS elastomer is the stiffness of the surface on the order of the cantilever stiffness used to probe the mechanical properties. All other surfaces show negligible penetration. The sensitivity to measure mechanical properties of polymer surfaces using AFM under static deflection depends on the relative stiffness of the cantilever and surface (14). The soft (k contact mode cantilevers were not able to distinguish between materials with bulk modulus in excess of IGPa (2). 

Static deflection AFM can be used to measure local mechanical properties of polymer surfaces, but only after consideration of the relative stiffness of the cantilever and the surface under study. Cantilevers with stiffness in excess of 50 N/m are necessary to indent materials with a bulk modulus in excess of 1 GPa (10 N/m ). Soft levers with a spring constant less than 1 N/m are sufficient to indent elastomers. Conventional staining techniques used in electron microscopy provide a viable way to harden unsaturated, hydrocarbon elastomers for imaging with soft cantilevers. Alternatively, low bulk modulus polymers (E < 1 MPa or 10 N/m ) require resonant imaging techniques such as Tapping Mode for direct imaging. 

Helen, W., de Leonardis, P., Uhjn, R.V., et al. Mechanosensitive peptide gelation mode of agitation controls mechanical properties and nano-scale morphology. Soft Matter 7, 1732 (2011). doi 10.1039/c0sm00649a... 

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