Craze fracture

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

From these results and those in Figures 7a and 7b the following can be concluded about the effects of silicone block introduction at intermediate temperatures the resistance to craze initiation is reduced by 60-70%, the resistance to shear deformation by 40-50%, and the resistance to craze fracture by about 40%. Thus both modes of plastic deformation are made easy relative to brittle failure. The origins of... 

Fig. 18a and b. Patchwork pattern on the craze-fracture surface of PP with a ower and b higher MW... 

We expect that when craze sources are plentiful, resulting in a high active craze front length Q, the craze velocity that needs to be maintained to match the imposed strain rate can be proportionally lower resulting in a lower craze flow stress and an increased craze fracture time tf. The contrary will hold when craze sources are few, requiring high velocities and high craze flow stresses to be maintained. There is very little information available currently on the important dependence of the craze fracture time tf on the applied stress implied by Eq. (9). 

In PEI the DCG process, as in any polymer, is active. The epsilon CTPZ, however, was not observed. No plane strain shear bands have yet been observed. Some form of localized crack tip shear process can be activated, however, as evidenced by the inversion transition that occured at higher stresses (at the higher temperatures). The fracture surface did not show fracture to occur on a slanted 45 degree plane. The fracture plane was still normal to the leading direction. The fracture surface, however, was not smooth, as seen with craze fracture, but has a definite roughened texture which is associated with active localized shearing. This texture is often described as honeycomb or tufted. 

Finally, our interest will be limited here exclusively to the phenomenon of crazing in heterogeneous polymers. Thus, apart from the considerations of improving toughness by manipulation of the processes that govern the craze flow stress and, thus, rendering the extrinsic flaws inoperable that result in craze fracture, we will not consider the mechanics of fracture of crazable polymers. A brief survey of this subject related to the crazing process can be found elsewhere... 

In contrast to equilibrium thermodynamics, the thermodynamics of irreversible processes portray the application of thermodynamic methods as dynamic and therefore time-dependent procedures. The name Prigo-gine must be mentioned in relationship to this—he received for his work in this area the Nobel Prize in the year 1977. A new, very complex thermodynamics originated from his examination method for chemical reactions, and was developed by us, to come to a successful description of heterogenous multiphase polymer systems. This theory interprets crazing fracture energy dissipation and fracture mechanism in a totally new way on the basis of dissipative structures in polymer blends and their dynamics, For a list of abbreviations used in this section sec page 610,... 

In this Section we will present the new model for impact modification in polymer blends. This new model is derived from the new non-equilibrium thermodynamical description of heterogeneous polymer systems and interprets crazing fracture energy dissipation and the fracture mechanism in a new way on the basis of dissipative structures in polymer blends and their dynamics. 

Fracture toughness and tensile properties are summarized in Table 1 below. The ductile IPNs showed substantial drawing and yielding in the impacted test>pieces compared with either smooth or sometimes locally crazed fracture surfaces for the more brittle polymers. 

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