Cross-linking quantitative structure-property

Model Networks. Construction of model networks allows development of quantitative structure property relationships and provide the ability to test the accuracy of the theories of mbber elasticity (251—254). By definition, model networks have controlled molecular weight between cross-links, controlled cross-link functionality, and controlled molecular weight distribution of cross-linked chains. Silicones cross-linked by either condensation or addition reactions are ideally suited for these studies because all of the above parameters can be controlled. A typical condensation-cure model network consists of an a, CO-polydimethylsiloxanediol, tetraethoxysilane (or alkyltrimethoxysilane), and a tin-cure catalyst (255). A typical addition-cure model is composed of a, co-vinylpolydimethylsiloxane, tetrakis(dimethylsiloxy)silane, and a platinum-cure catalyst (256—258). 

Epoxy thermosets are typical densely cross-linked polymer materials. They are used in a wide variety of practical applications and thus have been studied extensively. However, the quantitative dependence of physical properties, such as strength, stiffness, and fracture toughness, on network microstructure are largely undetermined. This can be attributed, in part, to the lack of adequate techniques for characterizing densely cross-linked network structure. Several microstructiu e variables that have been studied with some success are (1) cross-link density,... 

Fig. 8. Illustration of simple quantitative structure-property relationship given by equation (4) for the dependence of Tg on cross-linking. Tg(oo) is the Tg of the uncross-linked limit (n->-oo, where n is the number of repeat units between cross-links). Alrot is a number of rotational degrees of freedom per repeat unit parameter. Each curve is labeled by the value of Alrot-...

As it has been noted above, at present it is generally acknowledged [2], that macromolecular formations and polymer systems are always natural nanostructural systems in virtue of their structure features. In this connection the question of using this feature for polymeric materials properties and operating characteristics improvement arises. It is obvious enough that for structure-properties relationships receiving the quantitative nanostructural model of the indicated materials is necessary. It is also obvious that if the dependence of specific property on material structure state is unequivocal, then there will be quite sufficient modes to achieve this state. The cluster model of such state [3-5] is the most suitable for polymers amorphous state structure description. It has been shown, that this model basic structural element (cluster) is nanoparticles (nanocluster) (see Section 15.1). The cluster model was used successfully for cross-linked polymers structure and properties description [61]. Therefore, the authors of Ref [62] fulfilled nanostmetures regulation modes and of the latter influence on rarely cross-linked epoxy polymer properties study within the frameworks of the indicated model. 

The analysis of a large amount of experimental data collected from the literature ((1), for a more detailed discussion see (55)), led to the simple quantitative structure-property relationship given by equation 4 (illustrated in Figure 8), where n (defined by equation 5) is the average number of repeat units between cross-links. is the average molecular weight between cross-links. M is the... 

Because of their known structures, such model elastomers are now the preferred materials for the quantitative characterization of rubberlike elasticity. The properties of PDMS networks have been of interest to a variety of groups. " Such specific cross-linking reactions are also useful in the preparation of some of the liquid-crystalline elastomers,discussed in chapter 3. 

The polymers physical aging represents itself the structure and properties change in time and is the reflection of the indicated materials thermodynamically nonequilibriiun nature [61, 62], As a rule, the physical aging results to polymer materials brittleness enhancement and therefore, the ability of structural characteristics in due course prediction is important for the period of estimation of pol5mier products safe exploitation. For cross-linked polymers the quantitative estimation of structure and properties changes in physical aging process was conducted in Refs. [63, 64] within the frameworks of fracture analysis [65] and cluster model of polymers amorphous state structure [7, 66]. The authors of Ref. [67] use the indicated theoretical models for the description of PC physical aging. Besides, for PC behavior closer definition in the indicated process such theoretical notions were drawn as structure quasiequilibrium state [68] and the thermal cluster model [69], which is one from variants of percolation theory. 

The efficient utilization of any polymeric material requires a detailed molecular understanding of its unique properties. In its most useful form, such information consists of quantitative relationships between the physical properties of interest and the structural characteristics of the material that determines them. In the case of elastomeric materials, the molecular feature of surpassing importance is the interlinking or cross-linking of the polymer chains into a macroscopic, three-dimensional network structure ". Such networks can not be molecularly dispersed in a solvent, and the usual solution characterization techniques can not be applied to obtain the required structural information. For this reason, it has been exceedingly difficult to obtain reliable structure-property relationships for elastomeric materials ... 

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