Chemical bond order,covalency

Most structural adhesives depend upon the formation of chemical bonds (mainly covalent but some ionic and static attractive bonds may also be present) between the adherent surface atoms and the compound constituting the adhesive (Kinloch, 1987). Prior to the rehabilitation or retrofitting of RC and PC structures their surfaces to be bonded must be prepared, and likewise the surface of the FRP composite. The purpose of the surface preparation of concrete is to remove the outer, weak and potentially contaminated skin together with poorly bound material, in order to expose small- to medium-... 

Let us define the covalence of atom A as the sum of the chemical-bond orders (Wiberg indices) between atom A and all other atoms of the crystal. Using the idempotency relation (4.138), we have... 

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

It would be desirable to achieve a quantitative version of the Hammond postulate. For this purpose we need a measure of progress along the reaction coordinate. Several authors have used the bond order for this measure.The chemical significance of bond order is that it is the number of covalent bonds between two atoms thus the bond orders of the C—C, C==C, bonds are 1, 2, and 3,... 

The physical and chemical properties of any material are closely related to the type of its chemical bonds. Oxygen atoms form partially covalent bonds with metals that account for the unique thermal stability of oxide compounds and for typically high temperatures of electric and magnetic structure ordering, high refractive indexes, but also for relatively narrow spectral ranges of transparency. 

Inelastic shearing of atoms relative to one another is the mechanism that determines hardness. The shearing is localized at dislocation lines and at kinks along these lines. The kinks are very sharp in covalent crystals where they encompass only individual chemical bonds. On the other hand, in metal crystals they are often very extended. In metallic glasses they are localized in configurations that have a variety of shapes. In ionic crystals the kinks are localized in order to minimize the electrostatic energy. 

Some molecules exist where the bonding electrons cannot be assigned to atom pairs, but belong to more than two cores, e.g. in the polyboranes. In these cases the model concept of covalently bound atom pairs as a rep-resention basis for chemical constitution using binary relations can be sustained by the assignment of fractional bond orders. 

CNT can markedly reinforce polystyrene rod and epoxy thin film by forming CNT/polystyrene (PS) and CNT/epoxy composites (Wong et al., 2003). Molecular mechanics simulations and elasticity calculations clearly showed that, in the absence of chemical bonding between CNT and the matrix, the non-covalent bond interactions including electrostatic and van der Waals forces result in CNT-polymer interfacial shear stress (at OK) of about 138 and 186MPa, respectively, for CNT/ epoxy and CNT/PS, which are about an order of magnitude higher than microfiber-reinforced composites, the reason should attribute to intimate contact between the two solid phases at the molecular scale. Local non-uniformity of CNTs and mismatch of the coefficients of thermal expansions between CNT and polymer matrix may also promote the stress transfer between CNTs and polymer matrix. 

In order to overcome this drawback, there are two main approaches for the surface modification of carbon nanostructures that reoccur in the literature. The first one is covalent functionalization, mainly by chemical bonding of functional groups and the second one is noncovalent functionalization, mainly by physical interactions with other molecules or particles. Both strategies have been used to provide different physical and chemical properties to the carbon nanostructures. Those that will be presented here are only a few examples of the modifications that can be achieved in carbon nanostructure surfaces and composite fabrication. 

Covalent functionalization of fullerenes has also been used to obtain surface-modified fullerenes that are more compatible to polymer matrices in order to fabricate composites. In this context, four basic strategies were developed. The first one allows the fullerenes to react during the monomer polymerization, so that the fullerene can be attached to the polymer chain [111, 112]. Second, an already synthesized polymer is treated using specific conditions that allow the chemical reaction with fullerenes [113,114]. Third, the fullerenes are chemically bonded to a monomer which is polymerized or co-polymerized to obtain the modified monomer [115,116]. Fourth, a dendrimer can be synthesized around a fullerene which then acts as a nucleus [117,118]. 

The obvious deduction from these observations is that the orbital energy splitting is not primarily of a simple electrostatic nature, but reflects rather the much shorter range effects to be expected of covalence in chemical bonding to the immediate donor atoms. The conclusion is reinforced by the fact that when the known interionic separations are used together with free ion 3d-orbital wave functions to evaluate Dq for first transition series ions in an MF2 lattice, values too small by an order of magnitude are obtained.6 20-22... 

Mesophases of supermolecular structure do not need a rigid mesogen in the constituent molecules. For many of these materials the cause of the liquid crystalline structure is an amphiphilic structure of the molecules. Different parts of the molecules are incompatible relative to each other and are kept in proximity only because of being linked by covalent chemical bonds. Some typical examples are certain block copolymers50 , soap micelles 51 and lipids52. The overall morphology of these substances is distinctly that of a mesophase, the constituent molecules may have, however, only little or no orientational order. The mesophase order is that of a molecular superstructure. 

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