Cohesion mechanism

Due to the closed-shell electronic structure of C60,the cohesive mechanism of pristine solid C60 is considered to be via van-der-Waals-type relatively weak in-... 

C60 is shown in Fig. 6b. It is evident that the fully occupied tlu band, a new valence band of semiconducting K6C60, is considerably wider than that of the empty tlu band of the pristine bcc C60. This band widening upon introduction of K atoms takes place also for the tIg conduction band. Such band widening is considered to be due to the presence of hybridization between the C60 and K electronic states [32]. Therefore, the cohesive mechanism in semiconducting bcc A6C60 is primarily ionic with additional cohesion through the electronic hybridization. 

S. Saito and A. Oshiyama, Cohesive Mechanism and Energy Bands of Solid Ceo, Phys. Rev. Lett. 66, 2637-2640 (1991). 

Lord, C.J., Johlman, C.L. Rhett, D.W. Jul. 1998. Is capillary suction a viable cohesive mechanism in chalk SPE 47310, the SPE/ISRM Eurock 98, Trondheim, Norway. 

Spinline failure appears to occur through a cohesive mechanism, which has received little theoretical attention (49,50). A simple scaling model that predicts a critical value of the recoverable strain as a fimction of stretch rate captures the qualitative features of the limited data on melt failure in extension (50,51). 

Saito S and Oshiyama A, Cohesive mechanism and energy bands of solid C , Phys Rev Lett, 1991 66(20) 2637-2640. 

If the concentration of junction points is high enough, even branches will contain branches. Eventually a point is reached at which the amount of branching is so extensive that the polymer molecule becomes a giant three-dimensional network. When this condition is achieved, the molecule is said to be cross-linked. In this case, an entire macroscopic object may be considered to consist of essentially one molecule. The forces which give cohesiveness to such a body are covalent bonds, not intermolecular forces. Accordingly, the mechanical behavior of cross-linked bodies is much different from those without cross-linking. 

There are two mechanisms by which arching can occur particle interlocking and cohesive strength. The minimum outlet size required to prevent mechanical interlocking of particles is directly related to the size of the particles. The diameter of a circular outlet must be at least six to eight times the particle size, and the width of a slotted outlet must be at least three to four times the particle size. These ratios normally only govern the outlet size of mass flow hoppers if the particles are at least 0.6 cm or larger. 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories (7) quantum mechanical forces, pure electrostatic... 

The transformation of bulk Hquid to sprays can be achieved in many different ways. Basic techniques iaclude applying hydraulic pressure, electrical, acoustic, or mechanical energy to overcome the cohesive forces within the Hquid. 

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

Toothpastes are packaged in flexible tubes, other flexible containers, and mechanically operated pump dispensers. They are usually extmded as cylindrical ribbons of a cohesive, smooth paste, approximately 2.54 cm in length and weighing approximately 1.5 g. New or modified dispensing devices are continually introduced to increase consumer interest. 

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. 

Usually tj/ is very much larger than Fq. This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than works of adhesion or cohesion. However, a modest increase in Fq may result in a large increase in adhesion as and Fo are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

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. 

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