Molecular modelling adhesion

Advanced adhesives are composite liquids that can be used, for example, to join aircraft parts, thus avoiding the use of some 30,000 rivets that are heavy, are labor-intensive to install, and pose quality-control problems. Adhesives research has not involved many chemical engineers, but the generic problems include surface science, polymer rheology and thermodynamics, and molecular modeling of materials... 

Fundamental adhesion is connected with the nature of the bonds producing cohesion between two media. These bonds may be classified into two categories, namely strong bonds (polar, covalent and metallic bonds) and secondary bonds (hydrogenous and Van der Waals bonds). Different atomic or molecular models have been proposed to describe the electronic structure of interfaces. None however, is sufficient for calculating the intensity of adhesion forces for systems of practical interest. 

D. Taylor and J.E. Rutzler, Adhesion Using Molecular Models, Industrial Engineering... 

Kruszynski, M, Nakada, M T, Tam, S H, Taylor, A H, Fieles, W E, Heavner, G A, Determination of the core sequence of an antagonist of selectin-dependent leukocyte adhesion and correlation of its structure with molecular modeling studies. Arch. Biochem. Biophys., 331, 23-30, 1996. 

Jacobson, S.H. Molecular modeling studies of polymeric transdermal adhesives structure and transport mechanisms. Pharmaceut. Technol. 1999, September, 122-130. 

The molecular model is confined to the case of weak adhesion (5 < 0) to ensure homogeneous contact [48, 65]. In this case, two sources contribute to the frictional stress of a gel elastic deformation of an adsorbing polymer chain Cei and the lubrication of the hydrated layer of the polymer network ffvis, which can be represented as follows (Fig. 12) ... 

Therefore, using moisture sorption, microcalorimetric, IGC, molecular modelling and other techniques, the consequences of the particle size reduction process can be assessed. Moreover, surface energetics can be measured directly and predictions made about the nature of the surface, which ultimately could affect properties such as the flow of powders or adhesion of particles (Podczeck et al. 1996b). 

Lignin, adhesive, docking, lignin-ceUulose interaction, molecular modeling... 

Figure 4.2 Self-assembling peptide amphiphiles (PA) used for biomimetic mineralization of HA/PA nanocomposite, (a) Chemical structure of the PA, comprising 5 regions (1) a hydrophobic alkyl tail (2) four cysteine residues that can form disulfide bonds to polymerize the self-assembled structure (3) a flexible linker region of three glycine residues (4) a single phosphorylated serine residue that was able to interact strongly with calcium ions and help direct mineralization of HA (5) the cell adhesion ligand ROD. (b) Molecular model of one single PA molecule, (c) Schematic showing the self-assembly of PA molecules into a cylindrical micelle.

Figure 17.13. Three advances in computer theory of adhesion molecular modeling. Brownian modeling, and continuum mechanics.

White blood oells oontinually patrol the circulatory system and interstitial spaces, ready for mobilization at a site of trauma. The frontline scouts for leukocytes are carbohydrate groups on their surface called sialyl Lewis acids. When injury occurs, cells at the site of trauma display proteins, called selectins, that signal the site of injury and bind sialyl Lewis acids. Binding between selectins and the sialyl Lewis acids on the leukocytes causes adhesion of leukocytes at the affected area. Recruitment of leukocytes in this way is an important step in the inflammatory cascade. It is a necessary part of the healing process as well as part of our natural defense against infection. A molecular model of a sialyl Lewis acid is shown below, and its structural formula is given in Section 22.16. 

Self-assembled monolayers (SAMs) of alkanethiolates on metal surfaces constitute a class of molecular assemblies formed by the spontaneous chemisorption of long-chain functionalized molecules on the surface of solid substrates. Due to their ease of preparation, long-term stability, controllable surface chemical functionality, and high, crystal-like, two-dimensional order, SAMs represent suitable model surfaces to study molecular adsorption, adhesion, wetting, lubrication, and the interaction of proteins and cells with artificial organic surfaces. The latter phenomena are of crucial importance to the fields of biomaterials, biosensors, and medical devices. 

The interesting thing about this molecular model of adhesion is that energy and force of the molecular bonds do not decide the adhesion. Instead it is evident that molecular adhesion depends on the product of molecular adhesion energy and range. 

The strength of an adhesive joint is a system property dependent on the properties of the adhesive, the adherend(s), and the interphase and requires an understanding of mechanics, physics and chemistry. Accordingly, this chapter will provide a brief overview into proposed mechanisms (or molecular models) responsible for adhesion before embarking on a description of a number of test methods and discussing the meaning of some test results. 

Due to the difficulties in devising experiments to study the CNT-polymer interface, molecular modeling may serve to elueidate the importance of various factors constituting the interfacial characteristics of CNT reinforced polymer composites. To extend our understanding on CNT-polymer interactions, the interfacial adhesion characteristics between CNTs and a group of polymers (Table 13.2) are studied through molecular mechanics simulations. In this study, we are only concerned with non-bond interactions. 

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