Two-phase Domain Structure Research Articles

Structure and bonding in amorphous iron carbide thin films

We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe1−xCx (0.21 ⩽ x ⩽ 0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe1−xCx films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy, and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From x-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15–25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit an increasing number of unoccupied 3d states and a decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. The four-point probe resistivity of the Fe1−xCx films increases exponentially with carbon content from ∼200 μΩ cm (x = 0.21) to ∼1200 μΩ cm (x = 0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through the control of amorphous domain structures.

Small-angle neutron scattering investigation of polyurethane aged in dry and wet air

The microstructures of Estane 5703 aged at 70°C in dry and wet air have been studied by small-angle neutron scattering. The samples were swollen in deuterated toluene for enhancing the contrast. The scattering data show the charac- teristic domain structure of polyurethanes consisting of soft and hard segments. Debye-Anderson-Brumberger function used with hard sphere structure factor, and the Teubner-Strey model are used to analyze the two-phase domain structure of the polymer. The combined effects of temperature and humidity have a strong disruption effect on the microstructures of Estane. For the sample aged at 70°C in wet air for 1 month, the domain size, described by the correlation length, increases from 2.3 to 3.8 nm and their distance, expressed by hard-sphere interaction radius, increases from 8.4 to 10.6 nm. The struc- ture development is attributed to degradation of polymer chains as revealed by gel permeation chromatography. The hydrol- ysis of ester links on polymer backbone at 70°C in the presence of water humidity is the main reason for the changes of the microstructure. These findings can contribute to developing predictive models for the safety, performance, and lifetime of polyurethanes.

Studying silane mobility on hydrated mica using ambient AFM

The mobility of the mono-functional aminosilane, aminopropyldimethylmethoxysilane (APDMMS), on mica has been investigated under ambient relative humidity (RH) using tapping-mode (TM) AFM. Silane layers were formed from vapour phase at various, controlled humidities and then imaged at ambient laboratory conditions (typically 30–40% RH). At low RH of formation (<25%) films without any resolvable sub-structure were formed, and these were stable to the imaging probe. At high RH of formation (>25%), where islands of phase II water are known to exist on mica, a two-phase domain structure was seen as two height levels and the surface area of the higher domains correlated with the RH. Sequential images taken over a 30–60 min time period show that these domains are mobile and at intermediate to high RH the domains coalesce under the influence of the scanning AFM tip. The results suggest the APDMMS resides at the air–water interface (rather than interacting with the mica) and that it preferentially interacts with the mobile phase II water as opposed to the phase I water tightly bound to the mica.

Coupling of Chemical Cross-linking, Swelling, and Phase Separation in Microencapsulation

In the interfacial polymerization of poly(urea−urethane) membranes that result in the formation of microcapsules, the mechanism of microencapsulation can be complex. To clarify this process, a series of organic solvents with different degrees of compatibility between the solvent and the polymer, consisting of diethyl phthalate (DEP), dibutyl phthalate (DBP), and dioctyl phthalate (DOP), was used to study the polymerization of triisocyanate in both film and microcapsule formation. The physical properties of both the films and of the microcapsules were then determined by using a combination of differential scanning calorimetry, optical microscopy, and atomic force microscopy. The glass transition temperatures of films with limiting amounts of DEP, DBP, and DOP and without the organic solvent were, respectively, 98, 115, 123, and 124 °C. The triisocyanate + DOP system had a lower critical solution temperature of about 5 °C, with a two-phase domain structure for the membrane synthesized with DOP above the critical solution temperature. The shape of the microcapsules was found to be spherical, biconcave, and biconcave with a hump for compositions with weight fraction of DOP, ws, being 0, 0.3−0.5, and 0.78, respectively. By taking into account the compatibility between the solvent and the polymer, the combined results suggest that chemical cross-linking, swelling, and phase separation can be coupled to control the mechanism in microencapsulation of poly(urea−urethane) membranes to form microcapsules. A homogeneous elastic membrane is formed with organic solvents that are compatible with the wall-forming monomers, whereas the membrane becomes brittle and heterogeneous with less compatible solvents due to phase separations.

Domain structure of the antiferromagnetic insulating state in Nd0.5Sr0.5MnO3

Optical reflectivity studies of the ferromagnetic metal (FMM) to antiferromagnetic insulator (AFI) phase transition are performed on Nd0.5Sr0.5MnO3 manganite in a wide temperature and magnetic field range. The formation of a domain structure in the AFI state during the FMM−AFI phase transition is observed. It is shown that the two types of domains observed are energetically equivalent states. On the basis of the experimental results and symmetry analysis we conclude that these domains are crystal twins. The twin domain structure of the AFI state in the Nd0.5Sr0.5MnO3 is visible in reflected unpolarized light due to a different tilting of the surface in the domains. The two-phase domain structure FMM+AFI formed in the vicinity of the phase transition is also studied. It is found that a thermodynamically equilibrium two-phase stripe domain structure does not develop. The absence of the magnetic intermediate state is due to the large energy of the interphase wall, which results in the stripe structure period being much larger than the size of the sample.

Effect of magnetostatic pressure on the stabilization of a two-phase domain structure in thin garnet ferrite films

The coexistence of two domain phases, namely, a spiral domain and a bubble-domain lattice, is investigated experimentally in thin garnet ferrite films with uniaxial anisotropy. The condition for this coexistence is shown to be the equality of the magnetostatic pressures of the two phases. It is also shown that the possible formation of a spiral domain structure is determined by the magnetostatic pressure.

A water vapor-permeable membrane from block copolymers of poly(butylene terephthalate) and polyethylene oxide

Water vapor-permeable membranes are finding application in clothing. In this paper membranes prepared from block copolymers of polybutylene terephthalate (PBT) and polyethylene oxide (PEOX) are introduced from a structural perspective. The block copolymers are processed into dense membranes by melt-extrusion. In the solid state the PBT and PEOX blocks are segregated into a two-phase domain structure in which both phases are continuous. The polymer shows a spherulitic-like morphology caused by the crystalline PBT submatrix, which provides strength, stability and processibility. The flexible and polar PEOX submatrix provides for a high permeability of water and an elastomeric character. The polymer composition (block ratio and block length) strongly affects the final membrane properties and can be used for optimizing conflicting properties like permeability and mechanical stability.

Application of defocus transmission electron microscopy to polymers

Abstract Image formation in polymer electron microscopy is discussed with particular reference to the defocus-contrast technique in which interfaces between regions with different electron densities can be imaged. This method is applied to semi-crystalline polymers, where the difference in density between amorphous and crystalline regions gives rise to the defocus contrast. While other techniques of imaging semicrystalline polymers (i.e., staining, ion etching, etc.) suffer from the fact that during the preparation uncontrolled changes in the structure of the polymer can occur, the defocusing technique permits the direct observation of the polymer structure. Furthermore, sequences of micrographs from the very same area can give information about molecular processes although the technique is limited by beam damage. Results are reported concerning lamellar crystallization, lamellar thickening during annealing, and the two-phase domain structure in segmented polyurethane, in addition to several general appli...

Steady Flow and Dynamic Viscosity of Branched Butadiene-Styrene Block Copolymers

Abstract Steady flow and dynamic viscosities were determined for symmetrical linear and star-branched block copolymers of butadiene and styrene above their upper (polystyrene) glass transition. Block structures examined were B-S-B, (B-S-)3, S-B-S, (S-B-)3 and (S-B-)4. At constant molecular weight and total styrene content viscosities were greater for polymers terminating in styrene blocks, irrespective of branching. Branching decreased the viscosity of either polybutadiene-terminated or polystyrene-terminated block polymers, compared at equal Mw. However, comparisons at equal block lengths showed that the length of the terminal blocks, not the total molecular weight, governs the viscoelastic behavior of these polymers to a surprisingly good approximation. This unusual result is rationalized in terms of the two-phase domain structure of these polymers, which persists to a significant degree in the melt. Below the glass transition of the polystyrene blocks the effects of branching were masked by differences in the morphology of the domain structure unrelated to branching.