Glassy polymers molecular order

Scientists and engineers pursue development and application of theories for the physics of glassy polymers in order to understand vitrification and the glassy state in molecular solids. This understanding is coupled with the investigation of glassy state phenomena and properties so that new materials and new applications are continuously developed to improve the quality of life. Examples of this approach are foimd in the following chapters and an illustrative example can be found in Chapter 25 by Forsyth... 

Using such techniques as 2H-NMR and 2D-13C-MAS-NMR, Spiess and coworkers (Boeffel and Spiess, 1989) have been able to determine the molecular order of the different parts of the molecule. The decrease of order from the mesogenic group to the polymer chain was found. For example, in the glassy state with a frozen-in molecular order of the liquid crystalline phase, the order parameters for the polymer (3.52) (Table 3.12) were found to be 0.88, 0.52 and 0.25 respectively for the mesogenic unit, the spacer, and... 

This chapter describes the microstructures of the main types of polymer, concentrating on features used later to explain physical properties. The order of magnitude of elastic moduli for rubbers, glassy polymers and polymer crystals will be related to their molecular mobility and inter-molecular forces. These values will be used in Chapter 4 to predict the moduli of semi-crystalline polymers. 

In order to deal with the four non-crystalline forms in a unified way, we define a network chain in a crosslinked system, as the section of network between neighbouring crosslinks (Fig. 3.6). The shape of both a network chain in a rubber, and a molecule in a polymer melt, can be changed dramatically by stress, and both can respond elastically. However, when the polymer is cooled below Tg, the elastic strains are limited to a few per cent (unless a glassy polymer yields), so the molecular shape is effectively fixed. If the melt or rubber was under stress when cooled, the molecular shape in the glass is non-equilibrium. This molecular orientation may be deliberate, as in biaxially stretched polymethylmethacrylate used in aircraft windows, or a by-product of processing, as the oriented skin on a polystyrene injection moulding. Details are discussed in Chapter 5. 

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. 

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