Molecular bundle

Jullien, L. Lehn, J.-M., (1988) The chundle approach to molecular channels. Synthesis of a macrocycle-based molecular bundle Tetrahedron Lett. 3803-3806. 

Recently, Gillespie (2001) introduced an approximate approach, termed the r-leap method, for solving stochastic models. The main idea is the same as in the WP-KMC method. One selects a time increment r that is larger than the microscopic KMC time increment, and multiple molecular bundles of fast events occur. However, one now samples how many times each reaction will be executed from a Poisson rather than a uniform random number distribution. Prototype examples indicate that the r-leap method provides comparable noise with the microscopic KMC when the leap condition is satisfied, i.e., the time increments are such that the populations do not change significantly between time steps. 

On the basis of the foregoing and other recent evidence, it seems reasonable to assume that there is a basic structural unit in the so-called amorphous polymers. This is the molecular bundle, which may or may not include some chain folding. If one postulates the existence of such... 

Crazes occur perpendicular to the Stress direction shortly before a destructive break. They may be up to 100 fitn long and up to 10 /im wide. Crazes are not hairline cracks, that is, they ate not totally void between the break surfaces. The spaces between the break surfaces in crazes mostly contain molecular bundles or lamellar material stretched in the stress direction. Consequently, in contrast to genuine breaks, crazes possess a structural and mechanical continuity. Because certain materials whiten on crazing, crazes arc often called white breaks. 

Figure 4.7 Birth of a vacuole, pull-out of molecular bundle and growth of crack.

According to this model the first step is the generation of a vacuole, which becomes deformed in the direction of stress. The wall between the crack and the deformed vacuole becomes thinner and thinner, eventually breaking into molecular bundles. This is a typical model but the reality may be quite different, depending upon whether the material is in the glassy state, glass-rubber transition, rubbery state or fluid state. 

The pull-out of molecular bundle in fracture of gum rubber is difficult to demonstrate, because the bundle is in a fluid state such that it tends to flatten out. The retraction dne to elasticity may also obscure the evidence of the pull-out. 

The sample was a 50/50 by weight mixture of NBR and polyvinyl chloride (PVC). The NBR contains 33% acrylonitrile and was miscible with PVC. The presence of stiffer PVC chains presumably slowed retraction of the molecular bundle. The fracture was a result of the elongation at room temperature at 0.15 s strain rate. 

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