Relaxation polymer cryogenics

Direct experimental verifications of the temperature dependences of the elastic moduli of perfect crystals of polyethylene in the chain-extended form, as represented in Table 4.3, present great difficulties, first because they relate to a perfect crystalline material and second because they are based on the anharmonic atomic interactions in such perfect material. Polymeric solids, even those that are highly crystalline, incorporate a variety of crystal imperfections that permit thermally assisted relaxations under stress. These dramatically attenuate the elastic properties that, at all but the lowest cryogenic temperatures, mask the temperature dependence of elastic interactions of the perfect crystal, particularly the stiffest intra-molecular interactions along the C—C backbone. In the vast majority of cases the elastic moduli of polymers reflect the soft intermolecular interactions, and the temperature dependence of these overwhelmingly dominates the intramolecular variety at all but the lowest temperatures. 

Cryogenic temperature measurements of both longitudinal and shear speeds have been made for many polymers. For polyethylene, polytetrafluoroethylene, polyformaldehyde (acetal resin), and polyamides (114,115) at low temperature, there is a plateau in the temperature dependence where sound speed becomes independent of temperature. All relaxation processes have become frozen out. For poly(methyl methacrylate), however, there is no plateau (116). Even at very low temperature, the methyl group attached to an ether link can rotate. Fluoropoly-mers also exhibit more complicated behavior (117). Some measurements in the range 0.2-2 K on two epoxy polymers indicate a peak rather than a plateau (118). 

Table 20.8 contains a compilation of literature entries on the voltammetry of conducting polymer films. The scope of these studies is similar to that of the transient experiments discussed in Section V.A in terms of the types of electrodes and media employed. Both cyclic and hydrodynamic voltammetry have been used as shown in Table 20.8. Other aspects under discussion include the mathematic modeling of cyclic voltammo-grams [277,278], the occurrence and origin of prewaves in the cyclic voltammograms [319], the use of very fast scan rates [220], structural relaxation effects and their manifestation in voltammetry [304,317,320], the inactivation of polymer electroactivity when driven to extreme potentials, and the so-called polythiophene paradox [225,226,306,321]. Unusual media and cryogenic temperatures have also been employed for the volta-mmetric observation of doping phenomena [322-325]. Dual-electrode voltammetry (Section II.1) has been performed on derivatized polypyrrole [290] in an attempt to deconvolute the electronic and ionic contributions to the overall conductivity of the sample as a function of electrode potential. Finally, voltammetry has been carried out in the solid state , i.e., in the absence of electrolyte solutions [215,323].

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