Keiji Ohara

Temperature and friction speed dependence of frictional electrification between polymer films. Contribution of molecular motion of polymers to frictional electrification

Abstract The temperature dependence of the density of electrostatic charge, produced by friction between polystyrene and nylon 6, polystyrene and polycarbonate, and polyvinyl chloride and polycarbonate films, has been measured at various friction speeds. The charge density exhibited a maximum at about the glass transition temperature of polymers. The peak temperature shifted to a higher temperature region and the peak value became smaller with increasing friction speed. The apparent activation energy for the process of generation of electrostatic charge was 138, 214 and 226 kJ/mol for respective sample systems. It was concluded that the molecular motion of polymers affects greatly the generation of frictional charge. A series of experimental results indicates the existence of the electrostatic relaxation phenomenon (possibility of application of the time—temperature superposition principle) in the process of frictional electrification.

Frictional electrification between flat surfaces of polymers and of Langmuir–Blodgett layers

Abstract A friction apparatus was constructed for measuring the frictional electrification due to reciprocating friction between flat materials such as polymer films or LB layers deposited on flat substrates. The apparatus is equipped with four electrodes by which it is possible to measure generation and decay processes of charges on both surfaces simultaneously. The frictional electrification of LB layers is useful for constructing a frictional electrification series of atomic groups that elucidates the effect of surface conditions of molecular aggregates like polymers and that is useful for designing a new type of antistatic material.

Relationship between frictional electrification and molecular motion of polymers

Abstract Frictional electrification between polycarbonate films and a metal was investigated in the temperature and friction speed ranges of 10 to 100°C and 0.001 to 40 mm s −1 , respectively, in order to study the effect of molecular motion of polymers on the frictional electrification. The electrification charge showed a maximum at a temperature and a friction speed inherent to the sample. Applying the principle of time—temperature superposition to the results obtained, it was found that the electrification behaviour can be described by a single master curve and is affected by the molecular motion which is related to the relaxation process of the deformed molecular chains.

A method of exponential function analysis of contact and frictional electrification curves

Abstract A method of fitting a double exponential function to experimental data which increase and saturate with time is shown. An expected saturation value, two time constants and two pre-exponential factors could be determined. It is not necessary to expand the function in the present method. The method was applied to contact electrification data obtained in various atmospheric pressures and frictional electrification results.

Stick-slip motion and surface deformation

Abstract The relation between stick-slip motion and surface deformation in the frictional contact of polymer films was studied by multiple-beam interferometry. The stick-slip motion is caused by the combined deformation behavior of semi-macroscopic contact regions within the apparent contact area.

Observation of surface profiles and the nature of the contact between polymer films by multiple beam interferometry

Abstract The nature of the contact between polymer films (polyethylene terephthalate and nylon 6 films against a polystyrene film) was observed by multiple beam interferometry under varying load, tangential force and temperature. Three types of contact, microscopic (apparent), semi-macroscopic and microscopic, were considered. The number of contact points or points liable to contact increased, but not infinitely, with increasing load. When a tangential force was applied, deformation of the contact (hill) was observed in the direction of the tangential force. The hill deformed elastically. The stick-slip motion between polymer films subjected to tangential force was explained by this type of deformation. The nature of the contact changed abruptly at the glass transition temperature of the films.

Displacement currents generated during compression of fatty acid and phospholipid monolayers at the water-air interface

Abstract Displacement currents generated during the compression of monolayers of stearic acid and phospholipids (DSPC and DPPE) on the surface of water were investigated. Current peaks were observed not only in the surface pressure range larger than zero but also in the zero surface pressure range. Changes in the current in the zero pressure range were larger than those in the higher pressure range. The results suggested that the state of existence of molecules on the water surface changes remarkably, even in the zero surface pressure range, and that two processes contributed to the generation of the currents: one involving the movement and aggregation of molecules and the other concerning the movement of the aggregates. The effective dipole moments of the molecules on the water surface were determined to be 172 mD, 626 mD and 247 mD at 20 °C for stearic acid, DSPC and DPPE respectively.

Effective surface charge densities on two surfaces of polystyrene films charged by friction and corona discharge and their polarity

Abstract Effective surface charge densities (σ∗) were measured for both surfaces of polystyrene (PS) films charged by friction with nylon 6 (Ny) films and corona discharge. In the films charged by friction, the polarity of σ∗ on charged (S1) and uncharged (S2) surfaces was negative. The absolute value of σ∗ on S1 was larger than that on S2. After rinsing the charged films with ethyl alcohol, S1 carried negative charge but S2 positive charge. The series of frictional electrification predicts the polarity of charge on PS rubbed by Ny before rinsing. However, it should be noted that the films were polarized distinctly with a small net charge. In temperature dependence of σ∗, a peak was found at about 40°C and an increase of σ∗ was noticed in the higher temperature region up to 100°C. They were explained by thermal motion of PS molecules. In corona electrification, the polarity of σ∗ on both surfaces changed when the distance between the film and an earthed electrode was varied: when negative corona was emitted to the film put directly on the earthed electrode S1 carried negative charge but S2 positive charge. When the film was suspended 3.7 mm above the electrode, S1 and S2 carried negative charges. Charge decay was measured for the films charged by friction and corona discharge and analyzed using a three-exponential-sum equation to obtain relaxation times. Two mechanisms of charge decay were considered: one caused by surface charges and the other by charges injected into surface layer. Thermally stimulated depolarization current was also investigated. All the data showed that the behaviour of charges on S1 was different from that on S2.

Effective surface charge densities on two surfaces and thermally stimulated depolarization currents of polyvinyl chloride films charged by friction and corona discharge

Abstract Effective surface charge density (σ ∗ ) on charged (S1) and back (S2) surfaces of polyvinyl chloride films charged by friction with polystyrene films and negative corona discharge and thermally stimulated depolarization current (TSDC) of these films were investigated. In the films charged by friction, the polarity of σ ∗ on S1 and S2 was negative. The absolute value of σ ∗ on S1 was larger than that on S2. After rinsing the charged films with ethyl alcohol, S1 carried negative charge but S2 positive charge. In temperature dependence of σ ∗ of the charged and alcohol-treated films, a peak was observed at about 63°C. This shows that charges generated by friction entered into the surface layer of the films. In corona electrification, the polarity of σ ∗ on S2 changed depending on the distance, d , between the film and an earthed metal plate. Charge density on S1 and S2 of the film charged at d = 0 mm decayed in negative and positive quadrants, respectively, but unsymmetrically. The polarity of σ ∗ on S2 of the film charged at d = 6 mm was negative but changed to positive after a certain period of decay. The same tendency was observed in the film charged by friction. The decay data were analyzed using a three-exponential sum equation. Three relaxation times and corresponding three charge mobilities were estimated. In TSDC of corona charged films, three peaks were found which decreased with an increase in the period in which the charged films were left in air after the electrification. The polarity of the peak caused by the motion of CCl polar groups changed from negative to positive in the decay process. Poling temperature dependence of TSDC showed that orientation of polar groups occurred in frictional and corona charging process.

Thermally stimulated currents in LB films composed of molecules in which various polar groups are bonded to linear hydrocarbon chains

Thermally stimulated current (TSC) was investigated in as obtained and corona-charged multi-layer LB films, composed of molecules in which various polar groups, such as -COOCH/sub 3/, -CH/sub 2/NH/sub 2/, -CH/sub 2/OH and -CONH/sub 2/, were bonded to linear hydrocarbon chains, -(CH/sub 2/)/sub 16/CH/sub 3/. In the as obtained LB films of these materials, several TSC peaks were observed below and above the melting point of bulk materials. TSC peaks observed below the melting point were attributed to the pre-melting and the melting of two-dimensional LB layers. Peaks of TSC above the melting point of bulk materials may be caused by the aluminum oxide films formed on the electrode and by the reorientation of residual water molecules in the layers. In the corona-charged LB layers, the same types of TSC curves were obtained. However, the current values were larger and TSC were clearer. The activation energy for pre-melting peaks was obtained to be 135 kJ/mol in average and that of water in the one layer stearyl alcohol film was 77 kJ/mol. It is concluded that the head polar groups have an effect on the TSC appearance and peak position.