PIEZOELECTRICITY PYROELECTRICITY AND THERMOELECTRICITY OF POLYMER FILMS

Some electrical phenomena in amorphous and partially crystalline polymers are studied. The phenomena are phenomenologically similar to those in crystalline materials, but their origin is concluded to be space charges embedded in polymers. Voltage is induced across the surfaces of polymer films, i.e., polystyrene, poly(methyl methacrylate), polyethylene, and polypropylene, when they are vibrated in elongation (elongational piezoelectricity) and in bending (bending piezoelectricity). The former is attributed to a space charge distribution which is antisymmetrical about the median plane of the film and the latter to symmetrical distribution. A general description of piezoelectricity of a system in which space charge is embedded in homogeneous continuum is developed. Output current from poly(vinyl chloride) films at high temperatures (above 125°C) was measured under the following conditions: (1) under potential gradient (electrical conduction), (2) under temperature gradient (thermoelectricity), and (3) when the film is uniformly heated (pyroelectricity). Pyroelectricity is attributed to drift of space charge to electrodes on account of electrostatic force by image charge. Depolarization current of a poly(vinyl chloride) film which has been polarized at 125°C prior to measurement (electret) is interpreted in terms of dipole disorientation plus space charge drift. Copyright © 1968 John Wiley & Sons, Inc.