T. T. Wang

Sub‐sub Tg structural relaxation in glassy polymers

Structural relaxation in a temperature range from 32 to 62° K glass transition has been investigated, for the first time, for a nearly monodisperse polystyrene using a differential scanning calorimeter and a thermal mechanical analyzer. Low temperature anneals in the vicinity of 320 K stabilize the glassy structure and lead to volume contraction. Upon heating, the annealed samples show an excess endothermic peak and exhibit a gradual expansion associated with structural recovery above the annealing temperature. Significantly, the samples recover the initial heat content and volume without reheating through the glass transition temperature. This sub‐Tg structural relaxation behavior is in many respects distinct from that commonly observed near the glass transition. Kinetics of relaxation processes are also discussed.

Physical properties of poly(vinyl chloride) ‐copolyester thermoplastic elastomer mixtures

A study has been made on the compatibility, thermal behavior, and mechanical properties of the system poly(vinyl chloride) and a copolyester thermoplastic elastomer (under the trade name of Hytrel by duPont). Results from NMR, linear thermal expansion, tensile test, and dynamic mechanical measurements indicate extensive mixing of PVC and the soft segments of the thermoplastic elastomer when the mixture is rapidly cooled from around 150 °C to ambient temperature. The hard segments of the elastomer crystallize under these conditions, as revealed by the calorimetric measurements. Upon isothermal annealing at 130 °C the mixture shows signs of phase separation in dynamic mechanical measurements. These observations suggest the existence of upper critical solution temperatures for the system. The associated miscibility gap encompasses room temperature and reaches a maximum temperature somewhere between 130 and 150 °C. The phase domains in the annealed sample are extremely fine and have been estimated to be no more than 100 A in size from the pulsed NMR data.A study has been made on the compatibility, thermal behavior, and mechanical properties of the system poly(vinyl chloride) and a copolyester thermoplastic elastomer (under the trade name of Hytrel by duPont). Results from NMR, linear thermal expansion, tensile test, and dynamic mechanical measurements indicate extensive mixing of PVC and the soft segments of the thermoplastic elastomer when the mixture is rapidly cooled from around 150 °C to ambient temperature. The hard segments of the elastomer crystallize under these conditions, as revealed by the calorimetric measurements. Upon isothermal annealing at 130 °C the mixture shows signs of phase separation in dynamic mechanical measurements. These observations suggest the existence of upper critical solution temperatures for the system. The associated miscibility gap encompasses room temperature and reaches a maximum temperature somewhere between 130 and 150 °C. The phase domains in the annealed sample are extremely fine and have been estimated to be no mo...

Ferroelectriclike dielectric behavior in the piezoelectric amorphous copolymer of vinylidenecyanide and vinyl acetate

The alternating copolymer of vinylidenecyanide and vinyl acetate is the first amorphous polymer to show a combination of excellent properties among the several known piezoelectric polymers, e.g., high piezoelectric activity (d31=7 pC/N), high electromechanical constant (kt=0.38), and high thermal stability (glass transition temperature, Tg=170 °C). Our dielectric studies have uncovered for the first time several unusual characteristics which strongly suggest the copolymer to be a ferroelectric glass. The ferroelectric nature is manifested in the dielectric anomalies (a fourteenfold increase in the dielectric constant), the Curie–Weiss behavior and, more importantly, the positive and negative divergence of the third‐order dielectric susceptibility around the transition temperature. Significantly, these transitional phenomena occur at temperatures slightly above the Tg of the copolymer. The microscopic origin of the dielectric anomalies has been attributed to the cooperative interactions of C≡N dipoles whic...

Piezoelectricity in β‐phase poly(vinylidene fluoride) having a ’’single‐crystal’’ orientation

β‐phase PVF2 films having a ’’single‐crystal’’ orientation (single‐crystal analog) were prepared by a combination of drawing, rolling, and annealing, applied in sequence. X‐ray diffraction patterns revealed that the polar b axis was preferentially oriented normal to the surface of the film while the a and c axes were in the plane of the film, lying along two mutually orthogonal axes. Piezoelectric outputs (d13) of the films were compared with those of drawn‐only films under the same poling conditions (about 500 kV/cm) and found to be on average about 25% higher than the latter. X‐ray and dynamic mechanical data showed that the improvement depends primarily on the preferred alignment of the a and b axes in the plane normal to the draw and rolling direction.

Polarization behavior during high field poling of poly(vinylidene fluoride)

The polarization process which takes place during electric poling of poly(vinylidene fluoride) is investigated by measuring the time dependence of currents in poling circuits with varying RC time constants. For applied voltages between 3 and 8 kV (average field strengths 1.2–3.2 MV/cm) and RC values between 10 and 10−3 s, the current‐time behavior, normalized by the corresponding RC time constants and initial current values, can be described by a single curve (quasi‐steady state). Such a representation indicates that the switching of polarization is simply a voltage‐ or field‐induced effect. A phenomenological description of the poling process is presented which allows one to determine the dependence of polarization with respect to the voltage across the sample. The corresponding density function shows a steep rise in polarization at about 2400 V (average electric field strength 1 MV/cm) followed by an exponentially decaying tail toward higher voltages. Implication of such a distribution is discussed briefly.

High electric field poling of electroded poly(vinylidene fluoride) at room temperature

Room temperature electrode poling is by far the simplest and the most convenient way of preparing piezoelectric films of poly(vinylidene fluoride) PVF2. A shortcoming of this poling technique is, however, the relatively low electric field (<3.5 MV/cm) that can applied to the film without incurring instantaneous flashover or arcing around the film edge. We demonstrate here that by simply adding a rubber O ring and a plastic plate to the regular electrode poling setup, it is possible to pole biaxially oriented PVF2 films with fields up to about 9 MV/cm for at least 5 s without incidence of breakdowns. The piezoelectric e31 coefficient of the poled film rises sharply with increasing field but, after reaching a value of about 21 mC/m2 (d31=7.5 pC/N) at 2.5 MV/cm, it levels off gradually to a value of about 31 mC/m2 (d31=11 pC/N) at fields above 6 MV/cm.

Properties of piezoelectric poly (vinylidene fluoride) films irradiated by γ-rays

Abstract The influence of γ-ray irradiation on short-term and long-term properties of uniaxially stretched and poled PVF2 films has been investigated. Significant improvements were observed in the high temperature retention characteristics of both piezoelectric property and film dimensions in the irradiated samples. Additionally, stress relaxation tests at elevated temperatures revealed that the inverse relaxation (or stress buildup) behavior, commonly observed in the uniaxially stretched polymers, is suppressed markedly by irradiation. However, the film showed a steady decrease in its tensile strength with increasing dose and became embrittled in the direction perpendicular to the initial draw direction as the γ-doze exceeded 50 Mrad. The structural changes in the irradiated films were probed by means of differential scanning calorimetry, X-ray diffraction, and gel fraction analysis. The results indicate that the improvement of thermal stability is associated with the formation of crosslinks in the polym...

Stress relaxation and its influence on piezoelectric retention characteristics of uniaxially stretched poly(vinylidene fluoride) films

Piezoelectic films of uniaxially stretched poly(vinylidene fluoride) were found to exhibit an inverse stress relaxation behavior along the initial draw direction when exposed to elevated temperatures (≳40 °C) under a fixed strain. Further, films aged under a fixed strain were found to display a slower piezoelectric decay than films aged in a stress‐free state, especially at high temperatures (e.g., 90 °C). The slower decay in stressed films is believed to be induced by dimensional constraints (rather than by the buildup of film tension) which retard the depolarization by inhibiting molecular relaxation in the films.

High field poling of poly(vinylidene fluoride) films using a current limiting circuit

A high field room temperature poling technique has been developed for preparing poly(vinylidene fluoride), PVDF, films with high piezoelectric activity. The technique utilizes a resistor (typically 5 × 107 to 1011 Ω) which is inserted between the power supply and the polymer film to control the voltage buildup across the polymer film and to limit the current flow through the film in case of incipient breakdowns. Application of the technique to 25 μm-thick biaxially oriented films (with provision to prevent edge flashover) showed that the films could be poled without breakdown failure for a period of one to 105 s depending on the external resistance and the applied field. The piezoelectric e31 coefficients of the poled films were comparable to those obtained with the breakdown field poling method recently developed by Wang and West; however, the values obtained with the present method were in general more consistent and reproducible than the values obtained with the latter method. It is shown that physical...

Dielectric hysteresis behavior in form I poly(vinylidene fluoride)

The dielectric hysteresis behavior of polymer ferroelectrics has until now been described by an energy approach in which the redistribution of the crystal dipole orientation under an applied field is determined by the probability for the molecular dipoles to rotate about the long chain axes from one energetically stable position to the other. We propose an alternative approach in which the orientation of the dipole is determined by the equilibrium between the field‐induced torque and the elastic restoring torque generated by the excursion of the dipoles from their initial stable positions. Measurements of hysteresis loops performed on both uniaxially and biaxially oriented films of form I poly(vinylidene fluoride) reveal that all the important features of the loops can be accounted for by our model. An added advantage of our model is that it allows for description of switching experiment where inertial effects become important.