Masayuki Iijima

Pyroelectricity in poled thin films of aromatic polyurea prepared by vapor deposition polymerization

Aromatic polyurea films with a thickness of less than 1 µm were prepared by vapor deposition polymerization using different kinds of aromatic diamine and diisocyanate monomers. After poling above the glass transition temperature, the films acquired a pyroelectric activity of (1.0–2.5)×10-5 C/m2K, which is comparable to that of polyvinylidene fluoride. The temperature limit for the activity was 150 to 200°C, depending on the kind of monomer. The multilayer films consisting of aromatic polyurea and aluminum electrodes showed a high sensitivity proportional to the number of layers.

Piezoelectric properties of thin films of aromatic polyurea prepared by vapor deposition polymerization

Aromatic polyurea films of a few hundred nanometer thickness were prepared by vapor deposition polymerization of the monomers 4,4’‐diaminodiphenylmethane and 4,4’‐diphenylmethane‐diisocyanate. Piezoelectric activity was observed after poling the films under an electric field of 100 MV/m at 210u2009°C for 10 min. The piezoelectric stress constant (polarization/strain) remains constant at about 15 mC/m2 over the temperature range from −50 to 200u2009°C. The multilayer film produced by the alternate deposition of polyurea layers and aluminum electrodes exhibited a response proportional to the number of layers. The residual polarization caused by alignment of the dipoles of the urea bond is believed to be the origin of the effect.

Dependence of Piezoelectric and Pyroelectric Activities of Aromatic Polyurea Thin Films on Monomer Composition Ratio

Thin films of aromatic polyurea were prepared by simultaneous deposition of 4,4-diamino diphenylmethane (MDA) and 4,4-diphenylmethane diisocyanate (MDI) on a polyimide film substrate. By changing the evaporation temperature of the monomers, the monomer molar ratio of MDA to MDI of the deposited film was varied to yield MDA-rich, balanced, and MDI-rich films. After poling, the balanced films exhibited the maximum values of pyroelectric and piezoelectric constants, which were stable up to 200°C. Elemental analysis confirmed that oligomers consisting of about five monomers were present in as-deposited films. In the balanced films, an exothermic peak appeared at about 110°C in the first run of DSC measurements and the IR absorption peak due to isocyanate groups disappeared after heat treatment. These results suggest that during the poling process polymerization of oligomers, which forms polar urea bonds, takes place at about 110°C under a high electric field. The large residual polarization thus produced leads to high pyroelectric and piezoelectric activities.

Piezoelectric and ferroelectric properties of polyurea‐5 thin films prepared by vapor deposition polymerization

Submicron films of aliphatic polyurea‐5 were synthesized by vapor deposition polymerization. The structure of films depended on the partial pressure ratio of evaporated monomers. Excess of diamine monomer over diisocyanate monomer yielded lower crystalline structure, to be poled easier at low temperatures. The maximum value of the piezoelectric stress constant e31 was 10 mu2009C/m2, twice that of polyurea‐9. The pyroelectric constant was about 15 μC/m2 K. Ferroelectric D‐E hysteresis was observed. Three relaxations were observed for the temperature dependence of the elastic constant, the dielectric constant, and the piezoelectric constant. The poling dynamics involving urea dipoles and ionic species was discussed.

Heat resistant polyimide films with low dielectric constant by vapor deposition polymerization

Abstract Aromatic polyimide was prepared by vapor deposition polymerization(VDP), reacting a diamine monomer with a dianhydride monomer. Evaporation characteristics of monomers were investigated and several polyimide films with low dielectric constant were prepared by the VDP method, focusing on chemical structures as the means to raise the heat resistant temperature. The rod-like (3,3′-4,4′-biphenyl tetracarboxylic dianhydride(BPDA)/o-tolidine(OTD)) polyimide films have dielectric constant (ϵ)=2.9, and thermal stability above 500°C. The semi-rod-like (pyromelliticdianhydride(PMDA)/2,2-bis[4-(4-aminophenoxy)phenyl] propane(BAPP)) polyimide films have ϵ=2.7, and thermal stability above 400°C. The polyimide films have other good electric properties such as low leakage current density, high dielectric breakdown strength and no-ageing dielectric constant.

Piezoelectric and Dielectric Properties of Aromatic Polyureas Synthesized by Vapor Deposition Polymerization.

Four different kinds of aromatic polyurea were synthesized by vapor deposition of monomers. As a diisocyanate, [4,4-diphenyl methane diisocyanate] (MDI) was used. As a diamine, [4,4-diamino diphenyl methane] (MDA), [4,4-diamino-3,3-dimethyl diphenyl methane] (MeMDA), [4,4-diamino diphenyl ether] (ODA), and [4,4-diamino diphenyl sulfide] (SDA) were used. Poled thin films of P(MDA/MDI), P(MeMDA/MDI), P(ODA/MDI), and P(SDA/MDI) exhibited piezoelectric activity which remained up to almost 200° C. The stable value of the piezoelectric constant for P(ODA/MDI) was 26 mC/m2. The piezoelectric constant plotted against the evaporation temperature of MDI showed a maximum, where the stoichiometrically balanced composition of MDI and diamine monomer was obtained. The curve was sharp for P(MDA/MDI), but broad for P(ODA/MDI) and other polyureas. It was suggested that the high probability of production of balanced oligomers might be due to the longer time of residence of ODA molecules on the substrate.

Determination of Evaporation Rate and Vapor Pressure of Organic Monomers Used for Vapor Deposition Polymerization

Thermogravimetry (TG) and the Langmuir equation have been employed for the determination of the temperature dependence of the evaporation rate and saturated pressure of monomers used in vapor deposition polymerization (VDP). The measuring process utilizes the evaporation of monomers in vacuum. The ranges of the evaporation rate and saturated pressure of monomers as determined by the TG technique are from 10-2 to 10-5 mol/m2s and from 1 to 10-3 Pa, respectively. Comparison of the evaporation rate vs temperature relation and the melting point of monomers determines whether the evaporation involves sublimation from a solid surface or evaporation from a liquid surface.

Synthesis of Aliphatic Polyurea Films by Vapor Deposition Polymerization and Their Piezoelectric Properties

Aliphatic polyurea 9 films were prepared by vapor deposition polymerization of the monomers 1,9-diisocyanatononane and 1,9-diaminononane on a glass substrate cooled to various temperatures below 0° C. When the temperature of the substrate was low, the residence time of deposited monomers was sufficiently long to allow addition polymerization. The piezoelectric constant of poled films depended on the poling electric field and the poling temperature. Three relaxation processes were observed at -150° C (γ), -50° C (β) and 100–140° C (α) in the measurement of the temperature dependence of the elastic and dielectric constants. The thermal molecular motins at these three relaxations influenced the crystallization, poling process and thermal stability of piezoelectric activity.

VAPOR PRESSURE AND MEAN ADSORPTION TIME OF PYROMELLITIC DIANHYDRIDE AND 4,4'-OXIDIANILINE

Thermogravimetry (TG) measurements taken at pressures ranging from 10-1 to 10-3 Pa are used to calculate the vapor pressure of pyromellitic dianhydride (PMDA) and 4,4-oxidianiline (ODA). The vapor pressure calculated for PMDA is closely agrees with to those reported by Pethe et al.., Elshazly and Salem. For ODA, the calculated vapor pressure fits exactly on the curve obtained by Salem. The enthalpy is also calculated. The activation energy of desorption is found to be nearly equal to the enthalpy. The adsorption and desorption rates for both PMDA and ODA at various substrate temperatures are also observed. The mean adsorption time, τ calculated for ODA is several orders of magnitude larger than that for PMDA. 1:1 doses of ODA and PMDA therefore produce imine bonds which lead to the formation of thermally unstable films.

Dielectric Relaxation in Polyurea Thin Films Prepared by Vapor Deposition Polymerization

Polyurea thin films with the thickness of 500 nm were prepared by the vapor deposition of 4,4-diamino diphenylmethane (MDA) and 4,4-diphenylmethane diisocyanate (MDI) on a polyimide film substrate. The dielectric constants observed at 10 Hz for poled MDA-rich and MDI-rich films were about 10 and 3.5, respectively, and almost independent of temperature up to 200°C. The dielectric constant at 10 Hz for a poled balanced film was 4 at room temperature and increased with increasing temperature. A large dielectric relaxation was observed for the poled balanced film at the temperature range from 152 to 192°C and the frequency range from 10-2 to 102 Hz. The estimated relaxation strength was about 700. It was presumed that polar domains consisting of about 20 monomers were formed by the poling process, in which urea bonds were aligned in parallel and linked by hydrogen bonds.