Susumu Hirakawa

Transitions and Phases of Polytetrafluoroethylene

Behavior of transitions and the nature of high-pressure phase in polytetrafluoro ethylene are studied using methods of ultrasonic waves, thermal expansion, differential thermal analysis and X-ray diffraction. A phase diagram is obtained in pressure range from 1 atm. to 6,500 kg/cm2 and in temperature range from room temperature to 165?C. Three phases are found; phase I above 30?C, phase II below 20?C, both at 1 atm. and phase III above 5,000 kg/cm2. The triple point is located at 75?C and 5,000 kg/cm2. Pressure dependence of transition temperatures is obtained as 15 and 9 degrees per 1,000 kg/cm2 for the 20?C and 30?C transitions at low pressure, 9 degrees per 1,000 kg/cm2 for both transitions at intermediate pressure range from about 3,000 to 5,000 kg/cm2, -13 and 50 degrees per 1,000 kg/cm2 for the II?III and the III?I transitions, respectively. It is suggested that the high-pressure phase (phase III) may be a closely packed but disordered phase.

Pressure Dependence of Transitions and Relaxations in Polytetrafluoroethylene

The ultrasonic absorption and linear thermal expansion behavior of polytetrafluoroethylene in a hydrostatic high pressure vessel (piston and compound cylinder type) capable of achieving 16,000 kg/cm2 are measured in the temperature from -30°C to 200°C and in the pressure from 1 atm. to 5,000 kg/cm2. Ultrasonic (5 Mc/s carrier frequency) attenuation maxima at atmospheric pressure are found at about -12°C, 30°C, 50°C, 130°C and 140°C, and pressure dependence of these maxima is found as about 10, 9, 28, 13, and 25 degrees per 1,000 kg/cm2, respectively. From these facts and the dispersion map, the third, fourth and fifth peaks indicate important informations of existence of segmental motion in amorphous regions (primary dispersion), crystalline dispersion and grain-boundary dispersion, respectively. By a thermodynamical consideration and the thermal expansion behavior, the second peak is defined as the one due to the 30°C transition.

Formation of α Form Nylon 12 under High Pressure

The formation conditions of α form Nylon 12 under high pressure from the melt and from the quenched state were investigated by means of high-pressure differential thermal analysis and wide-angle X-ray diffraction. The α form was formed by crystallization from the melt above 200 MPa, and the γ form disappeared in the sample obtained above 500 MPa. The quenched state was transformed into the α form at lower temperature than the melting point of the α form under high pressure, whereas the γ form was hardly transformed at all. The thermal behavior of the α form at atmospheric pressure was also studied. Two endotherms due to the meltings of the α and γ forms and one exotherm due to the α to γ form transformation were observed. This transformation occurred without melting of the α form at a slow heating rate and was regarded as a monotropic transition.

Mechanical Anisotropy in Oriented Nylon 12

To investigate the mechanical anisotropy in oriented nylon 12, the direction of the hydrogen bond in the a-c plane of the α form was determined by wide-angle X-ray diffraction measurements under various pressures. The hydrogen bond of the α form is nearly parallel to the rolled plane, unlike in the γ form. The mechanical properties of unoriented and doubly-oriented samples were investigated by viscoelastic measurements. In all doubly-oriented samples, the temperature dependence of the storage moduli at 0° and 90° to the chain direction in the rolled plane exhibited a cross-over, which is interpreted in terms of the manner of connection between crystalline and amorphous regions. The storage modulus of the α-form sample in the 90° direction was higher than that of the γ form because of the difference in the direction of the hydrogen bond.