No abstract

It is well known that electrostrictive strains are proportional to the square of the applied electric field. It therefore appeared reasonable to assume that for some polymeric materials, a large acoustic thickness response, dT, \[ \left( {d_T = \frac{{dS}}{{dE}}} \right) \] might be obtained by application of high dc bias fields, ∼ 20 MV/m, to a film while driving the film with an ac signal to access the high slope region of the electrostrictive strain vs. applied field curve. Previous studies of crystallizing poly(vinylidene fluoride) (PVF2) from solution under high electric fields have demonstrated that gel‐like samples of PVF2 with high content of the plasticizer tricresyl phosphate (TCP) could be subjected to electric fields as high as ∼ 100 MV/m. Using this type of heavily plasticized PVF2 dT values ∼ 4 Å/V were obtained. Values of 9 Å/V were obtained for a certain class of thermoplastic elastomer (i.e., a polyurethane). These dT values are considerably greater than those obtained from conventional piezoelectric ceramic materials. In addition, large elastic strains (> 3%) were observed as a function of applied dc field. © 1994 John Wiley & Sons, Inc.

Samples of very low molecular-weight fractions of isotactic polypropylene have been crystallized by means of a solvent-evaporation technique as well as by melt recrystallization. The crystalline entities obtained by these methods have been examined in terms of their internal structure and associated morphology. The particular polymorphic form observed for these fractions is dependent on both the length of the molecule and the particular crystallization conditions employed. The morphological characteristics of the samples have been found to be qualitatively similar to those previously reported for unfractionated polypropylene and high-molecular-weight fractions, in spite of the fact that the molecular lengths of the samples investigated are sufficiently short to prevent chain folding. Selected-area electron-diffraction patterns have been obtained from the γ form and from mixed α and γ form of the isotactic polymer. The diffraction data have resulted in a modification of the previously proposed triclinic unit cell for the γ phase, and a conclusion that the γ→α transformation in isotactic polypropylene is martensitic in nature. The mechanism of this solid-solid transformation involves relative motion between the (040) planes which are found to be invariant in the two crystal phases.

Cold‐drawn Nylon‐11 films, which were prepared by stretching the melt‐quenched films to a draw ratio of 2.8: 1 at room temperature, were found to exhibit an electric displacement versus electric field hysteresis loop. The results confirmed that the Nylon‐11 film exhibited ferroelectric behavior at or below room temperature. The coercive fields were 65, 98, 125, 160, and 215 MV/m at 20, 0, −20, −40, and −60°C and the remanent polarization at −20°C (where there was considerably less dc conduction) was 56 mC/m2. Switching of the polarization was almost completed within 20 ms.

The pressure dependence of the melting and crystallization temperatures of poly(vinylidene fluoride) is determined by high-pressure differential thermal analysis (DTA). These results are used in the investigation of the polymorphic crystal form obtained by pressure quenching molten poly(vinylidene fluoride): The resulting crystal form depends on both the initial and final pressures. The pressure-quenching experiments were performed in a high-pressure piston-cylinder system and in a high-pressure DTA system; a comparison is made of the results obtained by both methods.