Anisotropic Decomposition Research Articles

Anisotropic Decomposition of Polyimide Molecules Induced by Irradiation of Linearly Polarized UV Light

We have investigated the anisotropic decomposition of polyimide (poly[bis(4,4′-oxydiphenylene)-dimetylmetyl-pyromellitimide]) molecules induced by exposure to linearly polarized ultraviolet light (LPUVL) of wavelength ∼ 300 nm. The decomposition of the polyimide molecule was monitored by measuring the polarized infrared (IR) absorption spectra as a function of LPUVL exposure. We propose an empirical equation that describes the relation between the orientational distribution of polyimide chains and LPUVL exposure. We consider two decomposition rates β/ and β⊥ for the polyimide chains oriented parallel and perpendicular to the polarization direction of LPUVL, respectively. By taking account of the increase of the decomposition rates around 30 J·cm−2, the IR absorption data could be reproduced with β/β⊥ = 1.23 ± 0.02. The decomposition rate β/ is (4.5 ± 1.0) × 10−3J−1·cm2 for the LPUVL exposure range up to -30 J·cm−2, and (1.6 ± 0.1) × 10−2 J−1·cm2 beyond -30 J·cm−2.

Dichroism of a Polyimide Chain for Ultraviolet Light

We have determined the absorption coefficient ratio a‖/a⊥ of a poly [4, 4′ -oxydiphe-nylene-pyromellitimide] (PMDA-ODA) chain in the wavelength range from 240 nm to 380 nm, where a‖ and a⊥ are the absorption coefficients of the polyimide chain for ultraviolet (UV) light polarized parallel and perpendicular to the chain direction, respectively. To accomplish that, we have measured the polarized UV absorption spectra of a rubbed polyimide film whose molecular orientation was previously determined by polarized infrared absorption spectroscopy. The a‖/a⊥ ratio becomes maximum in the wavelength region from 290 nm to 330 nm : a‖/a⊥ ≃ 5. The absorption coefficient peaks at ∼290 nm. These results sugest that linearly polarized UV light at 290 nm is most suitable for causing anisotropic decomposition of the polyimide chain. At 266 nm the a‖/a⊥ ratio was compared with the ratio of the photoinduced decomposition rates of the polyimide chain oriented parallel and perpendicular to the polarization direction of UV light. The anisotropy of the photoinduced decomposition rates of the polyimide chain is much smaller than that of the UV absorption coefficients.

Surface anisotropy of polyimide film irradiated with linearly polarized ultraviolet light

Using polarized infrared (IR) absorption, we have investigated the surface anisotropy of a poly [4, 4′-oxydiphenylene-pyromellitimide] (PMDA-ODA) film that arises from anisotropic decomposition of the polyimide chain during irradiation with linearly polarized ultraviolet (LPUV) light. To monitor the surface anisotropy, we designed the sample structure so that the polyimide films decomposed uniformly over the entire film thickness. The surface anisotropy has a maximum at an irradiation energy of 105 J/cm2. For PMDA-ODA, the maximum surface anisotropy is significantly smaller than the surface anisotropy generated by rubbing. By analyzing the irradiation energy dependence of an IR absorption band, we found that the decomposition rate of the polyimide chain oriented parallel to the polarization direction of the LPUV light is greater only by ∼23% than that oriented perpendicular to it. This is the reason for the small surface anisotropy induced by the LPUV light irradiation.

Structural evolution of Au[001]/Ni MBE thin films and Au 1− cNi c solid solutions with temperature: a HREM study

The evolution with temperature of Ni MBE (molecular beam epitaxy) ultra-thin films embedded between Au layers is described. Strong oscillations in the contrast, with a period of 4 atomic planes in the growth direction, appear on high-resolution electron microscopy (HREM) images. These oscillations are related to chemical modulations. For comparison, the thermal stability of solid solutions has been studied, where the same feature is found. The main conclusion is that anisotropic decomposition occurs only when the film or the solid solution is under an in-plane stress.

Spinodal decomposition and pattern formation near a crystalline surface

The kinetics of spinodal decomposition and morphological evolution near a crystalline surface (an edge in two dimensions) were investigated by microscopic master equations in both the point and pair approximations and a second-neighbor interaction model in a two-dimensional model system. It is shown that, in the presence of a surface, spinodal decomposition initially involves surface segregation, followed by anisotropic decomposition in the near-surface region, with subsequent isotropic decomposition in the bulk. It is demonstrated that, due to segregation, a surface spinodal decomposition may take place for alloys whose overall average compositions are outside the bulk spinodal. It is found that the presence of a surface results in a dominant concentration wave, which produces interesting transient morphological patterns such as distorted hexagonal precipitate lattices for relatively low-volume fractions and straight stripes at high-volume fractions in the near-surface region. The effect of pair correlations on the kinetics of spinodal decomposition and morphologies was studied.

High-energy Fe-Cr-Co permanent magnets with (B H)max ≃8–10 MG Oe

A series of new high-energy Fe-Cr-Co magnet alloys with (BH)max≃8–10 MG Oe have been developed. An Fe-22Cr-15Co alloy with equiaxed grain structure achieves (BH)max=8.3 MG Oe utilizing the abnormal shape of the miscibility gap of the system. The addition of Mo to the alloys enhances the anisotropic decomposition and the development of <100≳ columnar grains further improves the magnetic properties up to (BH)max=9.5 MG Oe.

Effects of Scale Porosity, Second‐Phase Oxides, and Doping in the High‐Temperature Oxidation of Cobalt and Dilute Cobalt‐Chromium Alloys

The oxidation mechanisms of high‐purity cobalt and dilute Co‐Cr alloys have been investigated over a 950°–1350°C temperature range in oxygen atmospheres of 10–760 Torr. The 99.999% cobalt exhibited parabolic oxidation behavior for weight gains of up to 30 mg oxygen/cm2 rectangular specimen surface. Parabolic kinetics were interrupted when fissures developed in the oxide scale as a result of mechanical stresses at the metal/scale interface or through anisotropic decomposition along grain boundaries. Molecular oxygen then short‐circuited to the porous inner scaling layer in accelerated attack of fresh Co surfaces. Diffusion of Co2+ through the scale is considered to be the primary rate determining process during periods of steady‐ state oxidation. The effect of porosity at the metal/scale interface prior to perforation of the outer scaling layer is slight, but is believed to result in a small reduction of the oxidation rate due to a reduction in the contact surface through which cations enter the oxide. Co alloys containing 0.5, 3, 7, and 10 w/o (weight per cent) Cr also obey parabolic kinetics during formation of thinner scaling layers. Porosity and localized loss of scale adherence are aggravated by the presence of and particles in the inner scaling layer to the extent that scale rupture mechanisms quickly obscure parabolic kinetics for chromium contents greater than 3 w/o at temperatures of 1150°C and above in 100 Torr oxygen. Under conditions where initial periods of steady‐state parabolic oxidation were discernible on thermobalance curves, maximum reaction kinetics are associated with 1–2 w/o Cr additions (which approach the Cr3+ solubility limit in ) as rationalized by Wagner's semiconductor valence theories.

Heat capacity study of linear polyurethanes

A r~ARG~, group amongst the considerable number of urethane-type polymers is formed by the linear polyurethanes (PI~), which can crystallize. The X-ray studies of the formation and decomposition of the crystal structure of the PU based on hexamethylene diisocyanate and various low molecular weight (m.w.) glycols led to the conclusion that the main feature of the crystal structure is the distinct anisotropy of the crystal lattice, which is due to the considerable number of hydrogen bonds present [1-5]. The crystal structure therefore consists of a number of fiat lattices, in which adjacent macromolecules are regularly linked by hydrogen bridges, while those superimposed on each other form a three-dimensional network linked by Van I)er Waals forces. The different nature of the bonds inside the lattices and between them results in an anisotropic decomposition of the crystal lattice, in which the Van Der Waals forces are gradually disrupted first; this is followed by a relatively quick disruption of the hydrogen bonds. The same investigations also established tha t linear PU easily form pseudo-crystalline structures which can rearrange themselves, under certain conditions, into a three-dimensional network. On this basis it is thought that the main features of the melting and crystallization processes of linear PU are determined by the hydrogen bonds. These should naturally also affect the nature of the changes of thermodynamic properties during the formation and decomposition of crystal lattices, and in particular the temperature dependence of heat capacity, which is the subject of this investigation. Two linear P U of the following structures were used in our work: