The antipolar crystal form of poly(vinylidene fluoride) can be made piezoelectric and pyroelecric by the temporary application of electric fields in excess of 1 MV/cm at room temperature. Infrared and x-ray diffraction data reveal that the polarization occurs in two stages. At fields near 1 MV/cm, a phase transition to a polar form II occurs with presumably no change in chain conformation. Fields near 5 MV/cm cause a change in conformation to produce form I. Our results indicate that at least a portion of the residual polarization occurs within the crystal phase of the polymer.

The purpose of this work was to determine the influence of chemical food-simulating liquids on the wear of various commercial dental composite restoratives. In many cases, pre-conditioning the restoratives in these liquids for one week produced swelling of the polymer matrix and considerable surface damage. The resulting degradation reduced the hardness and enhanced the wear as measured by a pin-and-disc apparatus. Four kinds of commercial composites were investigated: a conventional quartz-filled, a strontium-glass-filled, a visible-light-activated, and a microfilled composite. The liquids employed were heptane and several aqueous solutions of ethanol with solubility parameters, delta, ranging from delta = 1.5 to 4.8 X 10(4) J1/2m-3/2. With all restoratives, the decline in hardness during pre-conditioning maximized at about delta = 3 X 10(4), which corresponds to a 75% ethanol solution. The wear behavior was considerably more complicated and variable, as discussed in the text. For the most part, the increase in wear rate from pre-conditioning corresponded to the fall in hardness. A notable exception was for the strontium-glass-filled composite pre-conditioned in pure water. Here the wear was enhanced considerably, with no decrease in hardness. In this case, the degradation mode is assumed to be different from the others in that it is attributed to stress corrosion of the glass filler.

A description is given of the molecular and morphological structure of polyvinylidene fluoride and from this description a classical model is proposed for calculating the piezoelectric and pyroelectric properties. The model consists of an array of crystal lamellae with a net moment from aligned dipoles in the crystals and compensating space charge on the crystal surfaces. The results for no compensation and complete compensation essentially bracket experimentally observed results and indicate that the largest contribution to the activity of this polymer arises from bulk dimensional changes rather than from changes in molecular dipole moments.

The wear resistance of a commercial dental restorative composite exposed to different chemicals was measured with a pin-disc apparatus. The initial wear rate increased dramatically once the composite had been immersed in chemicals known to soften the cross-linked matrix material, which is a copolymer derived from BIS-GMA and diluent monomer. The increase of wear rate in a chemically softened composite was found to persist over a depth beyond 160 mu for specimens immersed over two wk prior to wear testing.Excessive wear of dental restorative composites is a major problem encountered in their use in stress-bearing dental applications. Interaction with the many substances of the oral environment may be a factor contributing to the durability of restorations in such applications. Intra-oral conditions can be expected to be more complex than those achieved by employing 37°C distilled water in laboratoryaccelerated wear measurements, as is frequently done. It has been shown that BIS-GMA base polymer is highly susceptible to chemical softening. Its surface layer can be softened by chemicals with a solubility parameter ranging from 1.82x 104 to 2.97 x 104 (J/m3)/2.1 This range of solubility parameters embraces a wide variety of chemicals, e.g., it covers more than half of the 412 chemicals listed in Table 2, chapter IV, of the Polymer Handbook.2The purpose of this work is to demonstrate the effects of some of these chemicals on the wear resistance of BIS-GMA base dental composites.The chemical substances within the oral environment exist jointly with water; therefore, a set of wear tests was also conducted with specimens soaked in ethanol-water mixtures of different concentrations. Materials and methods.The unfilled resin sample consisted of 69.4% BIS-GMA, 29.5% triethylene glycol dimethacrylate, 0.9% benzoyl peroxide, and 0.2% N,N-bis(2-hydroethyl)-P-toluidine. The specimens were cast between paraffin-coated glass slides separated by 1-mm-thick spacers and were stored at 37 + 1°C for 24 h prior to any tests.The composite specimens used for wear measurements were made from a commercial restorative* and were mixed according to the manufacturer's specifications. One day after curing, the wear surfaces were abraded sequentially 1180 with 240, 320, 400, and 600 grit SiO2 paper, with the final polishing being 0.05/ A1203. Great care was taken to avoid embedding abrasive particles in the softer polymeric composites. The shape of the specimen was a disc of 18 mm diameter and 2.5 mm thick. Each of the specimens was immersed for two wk at 37°C in the indicated chemical.Test procedures. -The wear test was made using a classical pin and disc configuration. The rotating discs were comprised of the 18-mm-diameter composite specimens mentioned in the last section. The wear pins were made of AISI type 303 stainless steel. The pin and disc track diameters were 2 and 12 mm, respectively. All the wear tests were carried out with 37°C distilled water flowing over the wear surfaces. The amount of wear was determined by measuring the wear ...

PVT meas ure me nts we re mad e o n liquid and glassy poly(vinyl ace tate) ove r ra nges of -30 to 100°C and 0 to 800 ba r (gage press ure). Th e data we re ob tained by three diffe rent th e rmodynami c hi stories: (a) variable formation press ure. (b) co nsta nt formati on press ure at one a tm os ph e re, a nd (c) co nstant formation press ure at 800 bar. In all of th ese th e glass was form ed by iso ba ri c coo lin g at 5°C/h. Th e salie nt c harac te ri sti cs res ultin g from the diffe re nt his tories a re th e followin g. Hi story (a) produces a glass of stru c ture varying with form ati on pressure and, he nce, does not necessa ri ly , give th e pro pe r th e rm od yna mic prope rti es of a "sin gle ph ys ical sub sta nce." Howeve r, th e liquid·glass inte rsection te mp erature, Tu( P ) , is a n importa nt kinetic, or relaxa tional , prope rt y whi c h a pprox im ates an isovi scous sta te. Accordingly, the values of dTu ldP a re in close agree me nt with th ose obta in ed from d yna mic mec ha nical a nd die lec tric tim e·te mpe ra ture· press ure s upe rpositio n. Co nstant formation hi stori es (b) a nd (c) give proper the rmodynami c properti es of th e gla sses, but very littl e inform a tio n with res pect to kin eti cs. In c reasing the press ure a t whic h th e glass is formed inc re ases th e de nsit y of th e glass (a t the give n cooling ra te) conside rably in co ntrast to the e ntropy (from oth e r work), whi c h a ppears to be essenti ally ind e pe nde nt of formation press ure.A co ns id erable part of th e pa pe r is de finitional. The res ults a re re lated to oth e r PVT, dynamic mecha ni cal , dielec tri c, a nd th e rm od ynamic measure me nts. Interpretations are giv e n in te rm s of both ph e nom e nological and molec ular models.