The pressure and temperature phase diagrams of DPPC bilayers have been obtained by DTA and dilatometry. Under atmospheric pressure DPPC shows three phase transitions at 42°C, 36°C and around 18°C, corresponding to the main-, pre- and sub-transitions respectively. Throughout our experiments we discovered very interesting phenomena in these transitions due to the effect of pressure and are as follows. First, the presence of a tricritical point on the pre-transition line has been found at about 2.5 kbar and 78°C, where the first order transition changes to a second order transition. Second, the sub-transition could be observed under high pressure without annealing procedure which required to stock DPPC at about 0°C for three or four days under atmospheric pressure. Third, a new fourth transition below the sub-transition temperature has been discovered.

High pressure ultrasonic and D-E hystresis measurements were performed on a vinylidene fluoride-trifluoroethylene (VDF/TrFE) copolymer with 54 mol% VDF content to investigate the pressure effect on physical properties accompanying wiht a ferroelectric phase transition. Differing from the results at atmospheric pressure where the ferro-to-paraelectric phase transition proceeded simply in one-step, this copolymer exhibited at 350 MPa two-step temperature variations in ultrasonic velocity and absorption, and remanent polarization upon the ferroelectric phase transition. These results agree well with the previous results of DTA and X-ray diffraction experiments, suggesting that the changes in the nature of the ferroelectric phase transition is attributed to the pressure-induced structural transformation.

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