Effects of Nonrubber Components on the Crystallization Behavior of Natural Rubber

The crystallization behavior of highly purified natural rubber (NR) was investigated by polarized light microscopy. NR was purified by enzymatic deproteinization followed by transesterification to remove nonrubber components present in the rubber. Nucleation rate and growth rate were estimated from a number of spherulites of different sizes observed in the course of crystallization for the purified NR. The slope of a linear line drawn in a plot of the logarithmic growth rate vs. 1/TΔT for the purified rubber was identical to that for the untreated rubber, suggesting that the lateral surface free energy was not a function of the nonrubber components. In contrast, nucleation was found to be promoted in the presence of nonrubber components, such as the fatty acids. This demonstrated that the crystallization of NR was significantly promoted by the nucleating effect of the fatty acids.     

Heterogeneous nucleation in CFD simulation of flashing flows in converging–diverging nozzles

Flashing flow is an important phenomenon in many industrial contexts; however simulation of these flows remains difficult. CFD simulations are able to describe the distribution and evolution of 3D structures in the flow but are dependent on good closure relations for interphase transfer. Nucleation during flashing flow is often neglected in CFD simulation where a minimum starting vapour fraction and a constant bubble number density are given. Models that include nucleation have used wall nucleation terms from 1D system code models, averaged over the domain. In this work, three models for wall nucleation are tested and compared with experimental data from a converging–diverging nozzle. Nucleation is applied at the walls of the domain, and various models are investigated. Good agreement with the critical flow rate and axial profiles are found, but agreement with the radial void fraction data is not satisfactory. Methods of addressing this are explored, and it is found that including a small bulk heterogeneous nucleation term gives the best agreement with the radial profiles, with negligible impact on the axial average properties.     

Formation of carbon composite structures on the Ge(110) surfaces

We present our first-principles calculation of the adsorption and diffusion of a carbon adatom on the H-terminated and clean Ge(110) surfaces, which are essential processes in the nucleation and growth of a monolayer graphene on Ge(110) by chemical vapor deposition. On the H-terminated surface, the C adatom spontaneously substitutes H atom(s) to form a monohydride structure (CH) or a dihydride structure (CH₂) and makes direct bonds with the substrate Ge atoms. The resulting diffusion barriers of the C adatom are 2.67 and 6.45 eV parallel to and perpendicular to the zigzag Ge chains of the surface, respectively. On the clean surface, the C adatom embeds into the zigzag Ge chain with nearly no barrier, kicking out a Ge atom out of the chain at the same time. The kicked-out Ge atom, instead of the C adatom, becomes a diffusion species with the barrier less than 0.63 eV. The formation of the C composite structures makes the C adatom difficult to diffuse both on the H-terminated and clean Ge(110) surfaces, which suggests that the nucleation and growth of the graphene islands from C seeds is much suppressed. We propose a growth mechanism of graphene monolayer going round the diffusion of the C adatoms on the Ge(110) surfaces.     

Accelerated surface flattening by alternating Ga flow in hydride vapor phase epitaxy

GaN films were grown on c$c$-plane sapphire substrates by using hydride vapor phase epitaxy (HVPE) with a pulsed flow of HCl over Ga metal. NH₃${\mathrm{NH}}_3$ gas supply was controlled to flow in a constant rate or in a modulated way. The surface morphology dependence of these films on the various flow modulation schemes was investigated. Depending on the duty cycle of NH₃${\mathrm{NH}}_3$ flow, the surface morphology of GaN films was sensitively modified. This sensitive response of surface morphology of GaN films to the flow modulation was attributed to diffusion efficiency variation of Ga species under different gas environment. Under proper modulation conditions, flattened top-surface morphology of nucleated domains was found to be obtained.     

High-field magnetic phase diagram of CsCoCl3 revised

A fine structure of high-field magnetization curves of CsCoCl₃ and related compounds is discussed. It is shown that a precursor of a high-field phase transition originates from nucleation of an intermediate high-field ferrimagnetic (IFM) phase. Since the IFM state is degenerate its growth starting from few nucleation centers gives rise to domain walls formation. Their creep also makes a contribution to the magnetization dynamics similar to slow relaxation in Ca₃Co₂O₆. An experimental verification of the model is proposed.     

The Kinetics of Emulsion Polymerization: Some Controversial Aspects

Abstract

Advances in the study of emulsion polymerization (EP) have allowed the clarification of some aspects of the mechanism and kinetics involved. However, such advances have provided controversial information in other aspects. In this work, the authors give a personal view of the state‐of‐the‐art of some of the most controversial issues of the EP kinetics: particle formation (nucleation) mechanism, particle size distribution, evolution of the rate of polymerization, and free‐radical capture.     

Structure of a spin-crossover Fe(II)–1,2,4-triazole polymer complex dispersed in an isotactic polystyrene matrix

Isotactic polystyrene (i-PS) was employed as a matrix to disperse a metallo-organic polymer of [Fe(II) (4-octadecyl-1,2,4-triazole)₃(ClO₄)₂] in order to obtain novel functional materials exhibiting thermal spin-crossover transition. A detailed investigation of the structure of the metallo-organic polymer and metallo-organic polymer/iPS blends has been carried out by DSC, WAXD and SAXS techniques as a function of temperature and metallo-organic polymer/iPS proportion.The results obtained confirm on the one hand that a structural transition associated with a change in the magnetic susceptibility of the metallo-organic polymer is preserved in the presence of i-PS. This transition was found to be associated to both, an inter-conversion of lamellar structures into hexagonal structures and to an increase of inter-sheet distances within the lamellar structures in metallo-organic polymer films prepared by casting from toluene solutions. On the other hand, an increase of the degree of crystallinity of the iPS is observed in the presence of the metallo-organic polymer which suggests some nucleating effect of the metallo-organic polymer in the crystallization of isotactic polystyrene.     

Monte Carlo study of the effect of perturbations on critical droplets

We present a Monte Carlo study of the effect of perturbations on critical or nucleation droplets in both classical and spinodal nucleation. Locating the saddle point with an intervention technique, we determine that the effect of perturbations at the saddle point depends on their location in the droplet. We find that the most effective perturbations occur at the location of the maximum growth rate where the droplet is allowed to nucleate and grow unperturbed. Moreover, the decay of sufficiently perturbed droplets follows a path that can be best characterized as a growth mode in reverse, specifically the decay of classical droplets is at the surface and that of spinodal droplets at the center independent of the location of the perturbation.     

Evidence for spinodal singularities in high-dimensional nearest-neighbor ising models

Conventional theories of nucleation predict that the metastable state has an average lifetime which monotonically decreases as the system is quenched further from the condensation point. However, theories based on the coarsegrained Ginzburg-Landau free energy functional seem to indicate that for systems above six dimensions there is a sharp spinodal dividing the metastable and unstable regimes where the lifetime of the metastable state diverges. Monte Carlo simulations are used to investigate this discrepency. Both nucleation rates and bulk susceptibility measurements seem to support the prediction of the Ginzburg-Landau theories.