Craze surface tension

This craze widening stress, which can be considered to be the crazing stress of the polymer, is only weakly dependent on the craze interface velocity but proportional to the geometric mean of the flow stress and the craze surface tension per unit width of the 

The surface tension F of the void ceiling that appears in the capOlarity equation (Eq. (12)) is the key quantity to be specified in understanding the effects of network strand density v on the craze widening stress. For the moment suppose this network is comprised entirely of crosslinked drains. Then to create a surface requires the scission of a certain number of strands per unit area, a geometricaUy necessary strand loss, which is given by (1/2) vd. The energy required to create this surface is then 

It is worthwhile to derive the second term a different way using a force argument since F is also a force per unit length. On the advancing interface there will be a certain number of network strands, stretched almost to their breaking force f which is approximately 

A schematic of a single polymer chain stretched along the crown of the void surface, b Imaginary tube surrounding this chain its intersection with the plane of separation at a distance x from one of the chain ends represents a stagnation point, c Plot of the force on the chain as a function of x 

Chains where x x will break rather than disentangle. 

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Fig. 14 a, b. The experimentally determined a and theoretically predicted b temperature dependence of the craze surface tension F(= SDJS) for crazes grown in a 1,800,000 molecular weight PS. (Part a from Ref. courtesy J. Polymer Sci.-Polymer Phys. (Wiley))... 

Length of cavitational process zone Surface free energy, surface tension Aspect ratio of a craze, = a/b... 

The failure of plastic specimens by crazing, both in air [12] and in the presence of any type of environmental agent [13,14], requires the creation of new surface area, and is therefore resisted by the surface tension of the polymer. 

Formations with enhanced resistance to craze formation have been described. Increasing the molecular weight has little influence on suppressing varnish-induced crazing in PAI Aims. However, craze resistance is associated with curing and crosslinking. The ratio of imide to amide and the surface tension of the film coatings influence the tendency to craze formation. 

Recently, Brown and Kramer have reported a study of the rise in stress after changing the environment of crazed polystyrene specimens under load from methanol, water or their mixtures to air [12]. They derived an equation relating the change in the surface component of the stress to the surface tension and the craze fibril geometry. 

The stress sensitivity of hard elastic polymers to changes in environmental surface tension has been well documented [6,10,11]. Because of their exposure to the environment, the surface contribution to the stress is primarily due to the microfibrils. Brown and Kramer [12], using a cylinder as a model for craze fibrils related the change in the surface component of the stress in crazed polystyrene to the change in surface tension, as shown below ... 

Vlymer/air Vlymer/1 iquid craze fibril diameter. The above predicts that the stress depression is simply a function of the surface tension and fibril geometry of any polymer. Thus, Equation 1 should be applicable to all microfibrillated polymers. 

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