Argon theory

The Argon theory has successfully interpreted the yield behavior of a large number of amorphous thermoplastic polymers (3,4). For thermosets,... 

This paper rerports an investigation of the yield behavior of several amine and anhydride cured DGEBA resin systems. The Argon theory is used to assess the controlling molecular parameters from the experimental results. Such parameters are then compared with the known chemical structures of the resins. The mechanisms of plastic flow in thermoset polymers such as epoxies is demonstrated. 

Comparison of Experiments and The Argon Theory. The measured compression modulus E and yield stress O were first converted into JA, and T. by using the... 

One important feature of the Argon theory is to allow the parameters A and B to be determined from the straight lines in Figure 3. Such parameters in turn give the critical activated molecular segment dimension which can be compared with the known molecular structures of the materials. Equation 6 can be immediately rearranged to give... 

The A, B, a and z values obtained from the experimental results based on the Argon theory are given in Table II. The molecular parameters should be related to the natural molecular hinges in the materials. For comparison, the mean spacing 1 of the natural molecular hinges are estimated (Table II) for the systems studied in the manner described in (3). 

All the systems studied appear to have similar A values, which are however lower than A=0.169 ( 1 = 0.35) according to Equation 5. This difference can be attributed to the real molecular cross-sections different from the circular one assumed in the Argon theory (3). In general, all the systems have molecular parameters (a, z ) similar to the DGEBA/TETA systems studied by Yamani and Young (5) as shown in Table II. 

The Argon theory, therefore, consistently interprets the yield behavior of both thermosets and thermoplastics. This indicates that crosslinks in thermosets do not introduce appreciable deviation to the kink formation process described. This point is also supported by Ygmani and Young s finding of the molecular parameters, z and a, being insensitive to crosslinking density for DGEBA cured with different amount of TETA. 

The plastic deformation in several amine and anhydride cured epoxy resins has been studied. The experimental results have been reasonably interpreted by the Argon theory. The molecular parameters determined from the data based on the theory reflect the different molecular structures of the resins studied. However, these parameters are in similar enough range to also show the structural similarity in these DGEBA based systems. In general, the mechanisms of plastic deformation in epoxy resins below T are essentially identical to those in amorphouE thermoplastics. The yield stress level being related to the modulus that controls the intermolecular energy due to molecular deformation will, however, be affected by the crosslinks in the thermosets. 

The existence of submicrocracks in polymers has aheady been mentioned in connection with the argon theory of craze initiation. Zhurkov, Kuksenko and Slutsker [92] have use small-angle X-ray scattering to establish the presence of such submicroscopic cracks. Although it has been proposed by Zakrevskii [93] that the formation of these submicrocracks is associated with a cluster of free... 

Figure 12.29 Ratio of shear yield stress to shear modulus as a function of temperature at different strain rates, for amorphous polyethylene terephthalate. Points from unpublished data of Foot and Ward, curves from Argon theory. Strain rates X, 1.02 x CPs +, 21.4 s - V,...

A final consideration is that the Argon theory essentially regards yield as nucleation controlled, analogous to the stress-activated movement of dislocations in a crystal produced by the applied stress, aided by thermal fluctuations. The application of the Eyring theory, on the other hand, implies that yield is not concerned with the initiation of the deformation process, but only that the application of stress changes the rate of deformation until it equals imposed rate of change of strain. The Eyring approach is consistent with view that the deformation mechanisms are essentially present at zero stress, and are identical to those observed in linear viscoelastic measurements (site model analyses in Section 7.3.1). Here, a very low stress is applied merely to enable detection of the thermally activated process, without modification of the polymer structure. 

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