Film/coating properties modulus

In this section we will describe how a proper accounting for film dynamics, based on a model of the thin-film/acoustic-wave interactions, can be used to quantitatively evaluate the shear modulus values as a function of temperature. As described in Section 3.1, an equivalent-circuit model can be used to relate the measured TSM electrical characteristics to the elastic properties, density, and thickness of a polymer film coating the device. Consequently, measurements made with polymer-coated TSM devices can be used to extract the shear elastic properties of the film. 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

Mechanical Tensile Properties, measured according to ASTM D638 on 3-mil thick cured films of the conformal coating, are modulus of 1750 psi, tensile of 2000 psi, and elongation-at-failure of 285%. 

In sensor applications, greater care should be taken in the selection and application of polymer films to minimize modulus effects. The effect of elastic properties of the coating materials must be taken Into consideration in the interpretation of SAW sensor data. Alternatively, the sensor community ml t take advantage of the SAW sensitivity to elastic properties to devise more sensitive sensors, and to expand the applications of these sensors. 

The mechanical properties of biomaterial thin films depend strongly on factors such as the deposition process and the microstmcture of the deposited film. Accurate techniques are needed to measure the properties of these films because their properties such as hardness and Young s modulus can be different from the equivalent bulk material and the underlying substrate on which they are deposited. It is essential to have tools such as scratching and nanoindentation available for the characterization of sub micron coating properties. 

Comparing an uncrosslinked emulsion polymer film and a highly cross-linked automotive top coat at temperatures well below their respective glass transition temperatures as an example, both polymeric films have tensile modulus values of approximately 2 x 10 dyncm [8]. Nevertheless, these two polymer systems show very different mechanical properties in the rubbery plateau region. The tensile modulus values are approximately (5-10) x 10 and (2-8) X 10 dyncm for the emulsion polymer and automotive top coat, respectively. Thus, one can certainly improve the physical properties of this latex... 

Tensile properties of importance include the modulus, yields, (strength at 5% elongation), and ultimate break strength. Since in many uses the essential function of the film may be destroyed if it stretches under use, the yield and values are more critical than the ultimate strength. This is tme, for example, where film is used as the base for magnetic tape or microfilm information storage. In some cases, the tensile properties at temperatures other than standard are critical. Thus if films are to be coated and dried in hot air ovens, the yield at 150°C or higher may be critical. 

It has been also shown that when a thin polymer film is directly coated onto a substrate with a low modulus ( < 10 MPa), if the contact radius to layer thickness ratio is large (afh> 20), the surface layer will make a negligible contribution to the stiffness of the system and the layered solid system acts as a homogeneous half-space of substrate material while the surface and interfacial properties are governed by those of the layer [32,33]. The extension of the JKR theory to such layered bodies has two important implications. Firstly, hard and opaque materials can be coated on soft and clear substrates which deform more readily by small surface forces. Secondly, viscoelastic materials can be coated on soft elastic substrates, thereby reducing their time-dependent effects. 

This section examines the dynamic behavior and the electrical response of a TSM resonator coated with a viscoelastic film. The elastic properties of viscoelastic materials must be described by a complex modulus. For example, the shear modulus is represented by G = G + yG", where G is the storage modulus and G" the loss modulus. Polymers are viscoelastic materials that are important for sensor applications. As described in Chapter S, polymer films are commmily aj lied as sorbent layers in gas- and liquid-sensing applications. Thus, it is important to understand how polymer-coated TSM resonators respond. 

Admittance-vs-frequency measurements made at several temperatures on a polyisobutylene-coated TSM resonator were fit to the equivalent-circuit model of Sections 3.1.3 and 3.1.9 to determine values of G and G for the film [66]. These extracted values are shown in Figure 4.4, along with 5-MHz values obtained from the literature for polyisobutylene having an average molecular weight of 1.56 X 10 [44]. We note excellent agreement between the extracted and literature values of G from —20°C to 60°C, and in G" from —20°C to 10°C. Above 10°C, the extracted G" values are approximately 30% higher than the literature values. These results illustrate how AW devices can be used to quantitatively evaluate the viscoelastic properties of polymer films. Similar models for other AW devices, such as the model for SAW devices coated with viscoelastic layers (Section 3.2.7 and [61]), can enable these other devices also to be used to determine modulus values. However, the pure shear motion of the TSM does simplify the model, and the evaluation of the modulus values as compared with the more complex displacements of other AW devices such as the SAW device (a comparison of the models of Section 3.1.9 for the TSM and Section 3.2.7 for the SAW demonstrates this point). 

The expansive internal stress in a plasma polymer is a characteristic property that should be considered in general plasma polymers and is not found in most conventional polymers. It is important to recognize that the internal stress in a plasma polymer layer exists in as-deposited plasma polymer layer, i.e., the internal stress does not develop when the coated film is exposed to ambient conditions. Because of the vast differences in many characteristics (e.g., modulus and thermal expansion coefficient of two layers of materials), the coated composite materials behave like a bimetal. Of course, the extent of this behavior is largely dependent on the nature of the substrate, particularly its thickness and shape, and also on the thickness of the plasma polymer layer. This aspect may be a crucial factor in some applications of plasma polymers. It is anticipated that the same plasma coating applied on the concave surface has the lower threshold thickness than that applied on a convex surface, and its extent depends on the radius of curvature. 

China clays and calcium carbonates can be used to impart anti-blocking properties to films produced from polyester, and cellulose acetate. Figure 19.19 shows the effect of coated ground calcium carbonate. Even such a small addition as 10 wt% has a substantial effect on blocking. Many other properties such as impact strength, modulus of elasticity, and opacity are improved. ... 

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