Factors Controlling Molecular Fracture

The approach taken in this section will be to consider in turn the empirical factors which govern the occurrence of molecular fracture and its consequences. The incidence of main-chain fracture in polymers under tensile stress is dependent on a wide range of variables, including polymer chemical structure, physical structure or morphology, additives, environment, time, temperature, orientation and physical state, as well as the more obvious variables of stress and strain. 

It is helpful to think of main-chain fracture as a competitive response to deformation if the deformation can be accommodated more readily by molecular flow or by crystallographic processes, then main-chain fracture will be less evident. This competitive situation underlies many of the phenomena reported in what follows. 

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The present paper is organized as follows. We start by describing our molecular model for the study of the factors controlling the drawability (Fig. 1) of flexible polymer chains. Effects such as polymer molecular weight, density of entanglements and temperature of drawing are explicitly taken into account. We then present our model for the perfectly oriented fiber (Fig. lb) and its mode of fracture. Our approach allows for both covalent and non-covalent bonds to break during deformation. 

The fracture resistance of a homopolymer is controlled by (i) the number of tie-molecules, which act as stress transducers between the crystallites and (ii) by the disentanglement resistance of the individual chains [112-115]. The higher the molecular weight, the more likely both factors are high fi-nucleated polypropylene does not infringe this rule [33,72,74,116]. 

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