Mechanical property measurement theory

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

For the materials scientist the measurement of mechanical properties provides information about internal structure and such measurements afford an opportunity to test theories of structure. In consequence a range of tests has been developed by materials scientists which supplement the, usually simpler, engineering tests and which in many cases provide information of no obvious value to the engineer. There are, however, many ad hoc tests used by the practical man which provide information, applicable by the materials scientist, but often in a confusing or complicated way. 

The mechanical properties of single hydrated dextran microcapsules (< 10 pm in diameter) with an embedded model protein drug have also been measured by the micromanipulation technique, and the information obtained (such as the Young s modulus) was used to derive their average pore size based on a statistical rubber elasticity theory (Ward and Hadley, 1993) and furthermore to predict the protein release rate (Stenekes et al., 2000). 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

Density functional theory is used for band structure calculations of hydrogen storage materials. This method has been applied to a variety of hydrides such as ABs [77-79], AB [77], transition metals [53, 80], Laves phases [81], and complex hydrides [82[. Theoretical investigation is not only useful for the prediction of the heat of formation but it could also assess the elastic and mechanical properties of these materials, properties which are usually difficult to measure in the case of hydrides [78[. 

Professor Wakeham is interested in the relationship between the bulk thermophysical properties of fluids and the intermolecular forces between the molecules that comprise them. Thus, at one extreme, he is involved in the determination of intermolecular forces from measurements of macroscopic properties and the development and application of the statistical mechanics and kinetic theory that interrelate them. He is also actively involved in the measurement of the thermophysical properties of fluids under a very wide variety of thermodynamic states. The same thermophysical properties find application in the process industries within the design of a plant. A part of Professor Wakeham s activities are therefore concerned with the representation and extension of a body of accurate information on thermophysical properties in a fashion that allows their use with software packages for process simulation. 

The mechanical behavior of the hydrogels can be described by the theories of rubber elasticity and viscoelasticity, which are based on time-independent and time-dependent recovery of the chain orientation and structure, respectively. Mechanical properties due to rubber elastic behavior of hydrogels can be determined by tensile measurements, while the viscoelastic behavior can be determined through dynamic mechanical analysis. 

To measure the hardness and elastic modulus of thin films while avoiding the influence of the substrate, peak indentation depth cannot exceed about 30% of the film thickness.Because commercial nanoindenters can make a minimum penetration depth of 10-15 nm, hardness and elastic modulus of films thinner than 30 nm cannot be measured. Clearly, new techniques for fabricating sharper indenters and new nanoindentation theories are needed to extend this technique. For film thicknesses less than 30 nm, nanoscratch tests are widely accepted to evaluate the mechanical properties (discussed later). Alternatively, assuming the hardness and elastic modulus of a film do not change with thickness, thicker films can be used. 

In crystalline semiconductors, the investigation of electronic transport properties (measured as a function of temperature or electric or magnetic field strength) can provide information on the scattering mechanism, carrier concentration and mobility, Fermi-level position, etc. Yet none of these latter quantities is obtained directly from the experiment Theoretical models for the transport are fitted to the experiment data to obtain transport parameters. The validity of both transport theories and transport parameters is checked by the quality of the fit and by a comparison of parameters obtained from different transport experiments. 

The adhesion properties of all types of polyolefins are not easy to explain because these properties are affected by different phenomena. Using of a single theory or mechanisms based on the physical and chemical adhesion manifestations is difiicult for the description of interdisciplinary nature and diversity. There is considerable information to discuss each of the adhesion mechanisms. Therefore, it is not possible to select only the thermodynamic theory of adhesion that is the best to describe the surface free energy of the polyolefin. All mechanisms and adhesion theories are implied by the diversity of polymer systems, which are embraced in combination with research for the analyses of adhesion properties. The physical and chemical composition in the first atomic layers dictates the adhesion and some other properties of the polymer materials. This layer represents underneath layer and this subsurface partially controls the outer layers. The double bonds and cross-linked stmctures limit the mobility macromolecules of polyolefins in the subsurface layers, which results in the functional group stabilization on the surface. Other basic research is necessary for an examination of the polymer subsurface layer and explanation of its effect changes of the surface properties. Moreover, for the improvement of quantitative measurements of adhesion, additional investigation is required. 

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