Mechanical properties, glassy polymers

Thus, although some degree of local organization may indeed occur in amorphous systems, and may even have some effect on the mechanical properties of polymers in the glassy state, the influence on the mechanical properties of melts, concentrated solutions and networks appears to be negligible. 

Molecular modeling techniques have been used to predict and interpret mechanical properties of polymers [88-95]. Theodorou and Suter [88, 89] found that the internal energy contribution to the elastic response is much more important than the entropic contribution for glassy polymers by a thermodynamic... 

While due to their well-known plastic deformation properties glassy polymers provide excellent model systems for fracture studies, most engineering plastics are semicrystalline. Nevertheless, the molecular mechanisms of reinforcement of interfaces between semicrystalline polymers are much less well understood and the first systematic studies on the subject have only appeared recently [16, 30,96-99]. The reasons for this are mainly twofold ... 

Porter [212] applied his group interaction modeling approach to both the nonlinear strain and the long-term mechanical properties of polymers. He showed that both problems can be explained qualitatively by his concept of discrete energy levels in a polymer. His key argument is that a fraction of rubberlike states coexists with the predominantly glassy state of a polymer below Tg is a function of the temperature, time and mechanical energy of deformation and... 

Boyer (85) argued in 1968 that polymer scientists must understand mechanical properties of polymers in terms of the molecular structure and molecular motions in order to be able to design better plastics. The same may be said of imderstanding physical, electrical, and transport properties of glassy polymers, and the chapters contained in this book present current research efforts toward that end. 

There is still a number of topics related to mechanical properties of polymers which we did not cover. One of them is aging in the glassy state tending toward equilibrium, the material increases its density, and thus lowers its free volume. We do not have space for it, but aging is understandable in terms of CRC as explained in Section 24.1, and is discussed in some detail by Robertson and Kim [72]. 

The length of the main chain does not affect the d30iamic mechanical properties of polymers in the glassy state, where molecular motions are of restricted extent, but the glass transition temperature is depressed at very low relative molecular masses... 

Macromolecular modifiers are used to improve some physical properties, mainly mechanical properties of polymers. They have found wide-spread application in the reduction of basic defects of polymers in their glassy state, mainly their resistance to low-energy impact, and they are used also to modify rheological properties of polymer melts [1]. 

An alternative and more radical approach is to use the preparation techniques described in Section 5.3 to prepare polymer samples directly in the glassy state. This method has been used for modeling polypropylene and polyethylene. In the latter case careful control over the preparation procedure was used to construct amorphous samples of the same polymer in different configurational states in order to examine the relation between mechanical properties and polymer structure. Such control is only possible, of course, for nonequilibrium states where the structure ean be immediately frozen in. The transformation to equilibrium liquid behavior from sueh samples can be observed by heating. ... 

Apparently local motions indicating differences in packing are closely related to the mechanical properties of glassy polymers. One of the puzzling features of the P-relaxation in PC as in other glassy polymers 3 6 76 77) is that it often is suppressed if the glass transition temperature is lowered by adding a plasticizer. The material then becomes brittle, which severely limits the applications of such polymers. Such low... 

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