Tetuo Takemura

The II-I crystal transformation of poly(vinylidene fluoride) under tensile and compressional stresses

Abstract The mechanism for a crystal transformation in poly(vinylidene fluoride) by a tensile deformation at atmospheric pressure was investigated in the temperature range 25–150 °C. Simultaneous X-ray and stress-strain relationship measurements during the drawing process revealed that the crystal transformation from Form II to Form I occurred at the temperatures below 130°C where the sample deformed by cold-drawing, and always initiated at the deformation stage where necking was completed at the centre of the tensile sample. Above 140°C, on the other hand, the sample deformed uniformly without necking and did not perform the crystal transformation. Thus, it was suggested that a heterogeneous stress distribution in the sample played a critically important role in the crystal transformation phenomenon. A uniaxial compressional deformation also caused the crystal transformation from Form II to Form I in this sample. The crystal conversion ratio varied with the conditions of compressional pressure and temperature, and the molecular orientation in resultant samples depended on the shapes of the anvils employed in the compression experiment. By applying a high d.c. voltage during the compressional deformation, a highly uniaxially oriented Form I film with prominent piezoelectric properties was obtained.

X-Ray Studies of Polyethylene under High Pressure

The phase diagram, growth processes of extended-chain crystal and high pressure phase, and the temperature dependency of lattice parameter in polyethylene have been studied under high pressure up to 6000 kg/cm2 using a new high pressure X-ray diffraction apparatus. The high pressure phase diagram obtained by micro-DTA measurement agrees with that by X-ray measurement. The crystallization process of the high pressure phase seems to be a homogeneous and diffusion controlled two dimensional growth. The growth process of the extended-chain crystal is very rapid, and the direct formation of the extended-chain crystal without passing through the high pressure phase is clarified. The temperature dependency of the lattice parameters of the extended-chain crystal and the high pressure phase at 5000 kg/cm2 were obtained, and the shrinkage of the c-axis in the high pressure phase which shows only a sharp (100) diffraction pattern is discussed with the aid of dilatometric data.

Direct Observation of Crystal Transformation Process of Poly (vinylidene fluoride) under High Pressure by PSPC X-Ray System

A PSPC (position sensitive proportional counter) X-ray system was employed to directly observe the crystal transformation processes of poly (vinylidene fluoride) at high pressures and high temperatures. X-ray diffraction measurements performed with a heating rate of 5°C/min have revealed that Form I structure is formed after the melting of Form II structure at high pressures above 3,500 kg/cm2. This result provides a solid proof for the origins of multiple endothermic peaks observed in the DTA melting trace at high pressures. Changes in X-ray diffraction patterns during annealing process at a temperature below the melting temperature of Form II structure under 4,000 kg/cm2 were followed and it has been shown that Form II structure performs a crystal transformation into Form I structure with an activation energy of about 30 kcal/mol.

FINE STRUCTURE OF THE III-I TRANSITION AND MOLECULAR MOTION IN POLYTETRAFLUOROETHYLENE.

The fine structure in the III-I transition region and the molecular motion of polytetrafluoroethylene under high pressure have been studied by X-ray diffraction and nuclear magnetic resonance. In the transition region from the high pressure phase III to phase I, two kinds of crystals were observed to coexist over a considerably wide temperature range. This fact suggests that the abnormal attenuation of ultrasonic waves reported in this transition region is due to a density fluctuation of these crystals. Also, a detailed study on the molecular chain conformation and the molecular motion in phase I revealed that there exists in the higher temperature region of phase I a new region, where the molecules have apparently a planar zigzag conformation as a result of right-handed and left-handed helical thermal motions in molecular chains.

Crystal Structure of High Pressure Phase of Polytetrafluoroethylene

The crystal structure of high pressure phase of polytetrafluoroethylene (PTFE) has been determined by X-ray methods. A high pressure X-ray diffraction apparatus which enables to get the fiber pattern of crystalline sample under purely hydrostatic high pressure at high temperature has been constructed. A small specimen of PTFE fiber was held in the bore of a beryllium window for X-ray. The fiber pattern of high pressure phase of this polymer has been recorded on a cylindrical film at 5500km/cm2, 78°C. It is found that in this phase the molecules have a planar zigzag arrangement different from that of a helix at atmospheric pressure. The unit cell is orthorhombic, with a=8.73 A, b=5.69 A, c=2.62 A. The result of the X-ray diffraction studies agrees with that of the other physical properties of PTFE in high pressure phase.

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.

One Dimensional Electric Conduction Mechanism in Mixed-Valence Complexes

Abstract The d.c. conductivity a along the chain of —M(II)—XM(IV)X— was observed with crystals of [M(II)(AA)23]·[M(IV)(AA)2X2]Y4; M=Pt, Pd, Ni, X=Cl, Br, I, (AA)=(en), (tn), Y=ClO4, BF4. Systematic change of [sgrave] from 10−8 to 10−15 (Ω-cm)−1 with M and X was characterized by an inter-ionic distance δ as [sgrave]∞exp [-αδ]. Its temperature change is expressed as [sgrave](T)=[sgrave]0 exp [-ΔE/kT] with ΔE=0.4 to 1.4 eV, which is in hν>2ΔE relation for the specturm absorption. Hydrostatic pressure on single crystal induces lattice contraction and [sgrave] enhancement of 103±1 times only by 7 kbar, suggesting dominant role of the orbital overlapping on [sgrave]. ESR singnal of gav=2.15±0.03 was observed for M=Pt, and its anisotropy with c-axis as gτ>g// indicates dz2 band contribution. The signal intensity change down to 77 K provides the carrier activation energy Δe=12±1 meV (X=I) and scarce activation number 1 per 104 Pt ion. Hence the conduction is considered as dominantly regulated by hopping process a...

Microscopic observations of the crystallization processes of polyethylene under high pressure

Abstract Microscopic observations of the three-stage crystallization processes of polyethylene at 5000 kg/cm 2 reported previously as those of ‘unknown structure’, ‘extended-chain crystal’ and ‘folded-chain crystal’ have been performed precisely using a diamond-anvil cell. At first, a lens-like and structureless crystal grows at the crystallization temperature of unknown structure. This lens-like crystal develops into the form of 10 μm diameter and 3 μm width with time. Secondly, the lens-like and structureless crystal forms a band-like structure (band width 1 μm) rapidly at the crystallization temperature of extended-chain crystal. Thirdly, a slight gap between band structures is filled up rapidly by many fine spherulites at the crystallization temperature of folded-chain crystal.

NMR and Raman Scattering Studies of MBBA and EBBA

Abstract The refined broad line NMR spectra of MBBA and EBBA and the Raman spectra in the low frequency region (100 ∼ 650 cm−1) of EBBA are given. The band associated with the accordion mode vibration of the end butyl chain of these molecules is assigned to 280 cm−1 in Raman spectrum. The analysis of accordion mode vibration indicate that the end butyl chain EBBA molecule tends to take all trans conformation, and that that of MBBA molecule tends to take the conformation which has constraint, such as gauche conformation. Assuming that the whole molecular conformation of each sample in the nematic phase, assignments between protons attached to each molecule and each line in the NMR spectrum are given. Molecular and segmental motions, molecular packing, and orientational order in the nematic phase of these samples are discussed.

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.