Micromechanical adhesion

Properties of this material have been reported. It has been found to bond to enamel and dentine, and to do so reliably and with good durability [107]. Results from X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) show that this bonding is the same as for conventional glass-ionomers, and involves the formation of chemical bonds to calcium in the mineral phase of the tooth. There is also evidence of micromechanical adhesion in this material [107]. 

Capability of dental adhesives is dependent on two conditions. First, the adhesive must bond to enamel and dentin, and second, the adhesive must adhere to the fining composite. The second condition has been shown to derive from a process of copolymerization of residual double bonds (-C—C-) in the oxygen inhibition layer. Bonding to enamel and dentin is believed to be by micromechanical adhesion as the main adhesive mechanism. This happens by an exchange process during which inorganic tooth... 

Packaging (paper and plastic) packaging adhesives release coatings barrier coatings Photochemical machining (89) micromechanical parts optical waveguides... 

Barquins, M. and Maugis, D., Tackiness of elastomers. J. Adhes., 13, 53-65 (1981). Creton, C. and Lakrout, H., Micromechanics of flat-probe adhesion tests of soft vi.scoelas-tic polymer films. J. Polym. Sci. B Polym. Phys., 38(7), 965-979 (2000). 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

Step 3. The set of fracture properties G(t) are related to the interfaee structure H(t) through suitable deformation mechanisms deduced from the micromechanics of fracture. This is the most difficult part of the problem but the analysis of the fracture process in situ can lead to valuable information on the microscopic deformation mechanisms. SEM, optical and XPS analysis of the fractured interface usually determine the mode of fracture (cohesive, adhesive or mixed) and details of the fracture micromechanics. However, considerable modeling may be required with entanglement and chain fracture mechanisms to realize useful solutions since most of the important events occur within the deformation zone before new fracture surfaces are created. We then obtain a solution to the problem. 

Other aspects of interfacial science and chemistry are examined by Owen and Wool. The former chapter deals with a widely used chemistry to join disparate surfaces, that of silane coupling agents. The latter chapter describes the phenomenon of diffusion at interfaces, which, when it occurs, can yield strong and durable adhesive bonds. Brown s chapter describes the micromechanics at the interface when certain types of diffusive adhesive bonds are broken. The section on surfaces ends with Dillingham s discussion of what can be done to prime surfaces for adhesive bonding. 

Although the importance of Buonocore s discovery cannot be overemphasized, micromechanical attachment cannot be regarded as true adhesion. True adhesion must be on the molecular level and must involve chemical or physicochemical bonds. 

Cements based on phytic add set more quickly than their glass polyalkenoate or dental silicate cement cormterparts, but have similar mechanical properties (Table 8.2). They are unique among add-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone... 

Thus the study of surfaces has emerged as an important focus in the chemical sciences, and the relationship between surfaces of small systems and their performance has emerged as a major technological issue. Flow in microfluidic systems—for example, in micromechanical systems with potential problems of stiction (sticking and adhesion) and for chemistry on gene chips—depends on the properties of system surfaces. Complex heterogeneous phases with high surface areas—suspensions of colloids and liquid crystals—have developed substantial... 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

In spite of the imperfections of the approach, the reversible work of adhesion can be used for the characterization of matrix/filler interactions in particulate filled polymers. Debonding is one of the dominating micromechanical processes in these materials. Stress analysis has shown that debonding stress (a ) depends on the reversible work of adhesion [8], i.e. ... 

Micromechanical force measurement apparatus (Taylor, 2006 Taylor et al., 2007) Particle adhesive forces Yes Adhesive forces vs. time (min) 15 psi >5 pm Adhesive forces between hydrate—hydrate particles, hydrate particle-surface... 

A micromechanical force apparatus has been developed at CSM to measure directly the adhesive forces between hydrate particles or between a hydrate particle and a surface (Yang et al., 2004 Taylor et al 2007). Similar micromechanical force apparatus designs have been applied to measure adhesive forces between ice particles (Hosier, 1957 Hosier and Hallgren, 1961 Fan, 2003). This apparatus... 

Figure 6.9 A schematic of the micromechanical force measurement (left) and video images of hydrate particles during each stage of the adhesive force measurement. (From Taylor, C.J., Adhesion Force between Hydrate Particles and Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, MS Thesis, Colorado School of Mines, Golden, CO (2006). With permission.)...

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

Micromechanical experiments made so far can be roughly divided into two parts (i) design of special techniques to measure and evaluate separately different contributions in the net force, such as adhesion, friction, deformation, and (ii) imaging of various heterogeneous surfaces such as blends, composites and microphase separated structures by conventional SFM s to collect statistical information and understand the origin of the mechanical contrast. Many of the micromechanical experiments were discussed elsewhere [58, 67, 68, 381, 412-414]. Here we will focus on recent advances in analytical applications of the active probe SFM. 

Gutowski, W. V. 2003. Interface/interphase engineering of polymers for adhesion enhancement Review of micromechanical aspects of polymer interface reinforcement through surface grafted molecular brushes. Journal of Adhesion 79 445-82. 

Bohse, J., Krietsch, T., Chen, J., Brunner, A.J., (2000) Acoustic Emission Analysis and Micromechanical Interpretation of Mode I Fracture Toughness Tests on Composite Materials , Proceedings ESIS Conference on Fracture of Polymers, Composites and Adhesives, ESIS Publication 27, pp. 15-26, Elsevier, Oxford. 

Mastrangelo, C.H. Adhesion-related failure mechanisms in micromechanical devices. Tribology Lett. 1997, 3 (3), 223-238. 

Keywords QCM Cell-substrate interactions Cell adhesion Cell spreading Extracellular matrix Cellular micromechanics Cytoskeleton Cell elasticity ... 

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