Epoxy polymers molecular characteristics

Polymer mechanical properties are one from the most important ones, since even for polymers of different special-purpose function a definite level of these properties always requires [20]. Besides, in Ref [48] it has been shown, that in epoxy polymers curing process formation of chemical network with its nodes different density results to final polymer molecular characteristics change, namely, characteristic ratio C, which is a polymer chain statistical flexibility indicator [23]. If such effect actually exists, then it should be reflected in the value of cross-linked epoxy polymers deformation-strength characteristics. Therefore, the authors of Ref [49] offered limiting properties (properties at fracture) prediction techniques, based on a methods of fractal analysis and cluster model of polymers amorphous state structure in reference to series of sulfur-containing epoxy polymers [50]. 

The use of fractal analysis makes it possible to relate molecular parameters to characteristics of supermolecular structure of polymers. Figure 11.12 illustrates the linear correlation between D and df [dj was estimated from Equation (11.27)] for epoxy polymers. When the molecular mobility is suppressed (D = 1), the structure of the polymer has the fractal dimension df = 2.5, which corresponds to p. = 0.25. The given value of the Poisson coefficient corresponds to the boundary of ideally brittle structure at p< 0.25, the polymer is collapsed without viscoelastic or plastic dissipation of energy [3]. This is fnlly consistent with the Kansch conclnsion [117] stating that any increase in the molecular mobility enhances dissipation of the mechanical energy supplied from the outside and, as a conseqnence, increases plasticity of the polymer. When D = 2 the df value is equal to 3, which corresponds to p = 0.5, typical of the rubbery state. 

Hence, formation of the structure and properties of epoxy polymers during curing is determined by fundamental physical principles. This is accompanied by the change in the characteristic ratio C (molecular characteristics), although the structure of the macromolecule remains invariant. The use of the above physical principles even in the simplest version provides a correct description of the structure and properties of network polymers. 

Table 11.6 Calculated molecular characteristics of epoxy polymers [8] ...

The values (p j and can be determined according to the Eqs. (4.66) and (2.16), respectively. The theoretical dependence of (where - 1) on the ratio curing agent - oligomer AT, obtained by the indicated mode, is adduced in Fig. 7.9 (the shaded line). Its comparison with the experimental data shows the Eq. (7.15) inadequacy for epoxy polymers considered series (. estimation. Since the same equation describes well the data for a linear pol5uners number [51], then the comparison of the data of Fig. 7.9 and the results of Ref [51] assumes adequate usage of this method in the case of pol5uner molecular characteristics invariability only. 

In Fig. 7.9, the comparison of values 6p calculated according to the Eq. (7.11) and obtained experimentally, is adduced. Now the good coirformity of theory and experiment is observed, both quantitative and qualitative. This confirms postulate that molecular characteristics change in epoxy polymers curing process actually occurs and, if this factor is not taken into account, the Ejprediction can give incorrect results. 

In turn, the macromolecule effective diameter dj can be determined from Bragg s interval value d according to Equation 4.10. Simulating the crosslinked epoxy polymer macromolecule as a cylinder and using experimental values and d, the important molecular characteristics 5 and can be estimated [23]. 

The Curing Influence on Molecular and Structural Characteristics of Epoxy Polymers... 

Mechanical properties of polymers are among the most important, since a certain level of these properties is always required even for polymers of different special-purpose functions [50]. In papers [38, 51] it has been shown that the curing process of the chemical network of epoxy polymers with the formation of nodes of various density results in a change in the molecular characteristics, particularly the characteristic ratio C. If such an effect actually exists, then it should be reflected in the deformation-strength characteristics of crosslinked epoxy polymers. Therefore the authors [49] offered methods of prediction of the limiting properties (properties at fracture), based on the notions of fractal analysis and the cluster model of the amorphous state structure of polymers, with reference to a series of sulfur-containing epoxy polymers [52, 53] (see also Section 5.4). 

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