Sufen Chen

Effects of molecular weight of PVA on formation, stability and deformation of compound droplets for ICF polymer shells

Sphericity and wall thickness uniformity are some of the hardest specifications to fulfill, as required by inertial confined fusion (ICF) research for polymer shells prepared by the microencapsulation technique. Driven by the need to control the deformation of compound droplets, the effects of the molecular weight of poly(vinyl alcohol) (PVA) on the formation and stability of the droplets, as well as the sphericity and wall thickness uniformity of the resulting shells, were investigated. On increasing the molecular weight of the PVA, the densities of the external water phases (W2) are almost the same, but the viscosity of the W2 phase increases more quickly than the interfacial tension. This makes the detaching force increase more quickly than the upward one, causing the formation of compound droplets and detachment from the oil tube. On the other hand, the increase in interfacial tension makes the maximum pressures ( P max) in the O phase (O) of the compound droplets increase, causing them to rupture easily and decreasing their stability. However, for PVA with the same molecular weight, the viscous shear force in the flowing field reduces the role of gravity and makes the inner water droplet move towards the center of the compound droplet, decreasing its P max in the flowing field and improving its stability. Moreover, during the solidifying process, the viscous shear force increases more quickly than the interfacial tension force due to the quicker increase in viscosity with an increase in the molecular weight of the PVA. The increase in the viscous shear force can make the droplets deform, resulting in a decrease in their sphericity. However, the appropriate viscous shear force can also center the compound droplet—although they become decentered when the viscous shear force is too large, leading to the wall thickness uniformity increasing at first before decreasing quickly. The results presented in this work provide a more in-depth understanding of the formation, stability and deformation of compound droplets, to the benefit of preparing polymer shells with the high sphericity and uniform wall thickness needed in ICF experiments.

Preparation of polystyrene-poly (vinyl alcohol) double-shell

This paper presents preparation of polystyrene-poly(vinyl alcohol) (PS-PVA) double-shell with micro-encapsulation method. In order to improve the gas-barrier property of the PVA shell, a mixture of 2wt% PVA solution and borax are used as matrix for PVA coating. The pH value of the matrix are kept less than 8 to prevent the PVA coating from formation of patch which arise from local gelation and ruin the uniformity of the PVA shell. The resultant PVA coating ranges from 2 μm to 15 μm with uniformity better than 95%.

Influence of sucrose on the stability of W1/O/W2 double emulsion droplets

Large sized W1/O/W2 double emulsion droplets with uniform wall thickness and diameters were prepared by adopting the emulsion microencapsulation method. Via visually online counting the number of double emulsion droplets over observing time, the stability of these compound droplets solidified in a constant rotating flow field was comprehensively studied. Combining emulsion kinetics, the adsorption radius of polymer chains, surface tension and rheological properties of the outer water phase (W2), the influence of sucrose concentration on the stability of double emulsion droplets was systematically investigated. Combining the change of surface tension, the size of adsorbed polyvinyl alcohol (PVA) chains and storage modulus of the W2 phase, mechanisms of the interactions between sucrose molecules and PVA chains were fully elucidated in this paper. Moreover, the results showed that the stability of the double emulsion droplets was improved when the contents of sucrose was less than 1.0 wt%. This is probably due to the proper match between shear force and surface tension of the W2 phase, and less inward force caused by osmotic pressure in dilute sucrose concentrations.

Fabrication of Polyacrylonitrile Microcapsules for ICF Targets

Abstract Polyacrylonitrile (PAN) microcapsules for inertial confinement fusion targets were fabricated. First, density-matched double emulsions were generated with a double T-junction microfluidic device. Second, the solvents in the middle phase of the double emulsions were driven off with a rotary evaporator. The radii of the obtained PAN microcapsules with smooth surfaces were in the region of 50 to 500 μm, with shell thickness from 30 to 300 μm. Influences of solvents, flow speeds, and solidification conditions on the microcapsules were studied.

An antireflection method for a fluorinated ethylene propylene (FEP) film as short pulse laser debris shields

Debris mitigation is a major challenge for all high peak power laser system. Thus, fabrication of special polymer films to protect from target debris is significant. Fluoride polymers, representative of fluorinated ethylene propylene (FEP), have excellent ultraviolet-visible transmission, laser-induced damage thresholds and mechanical properties, and stand a good chance to be used as debris shields. However, the transmittance of FEP is still lower than that of fused silica glass and needs to be improved before it is really used in the laser field. The difficulty is that modifying on inactive fluoride polymers is very difficult. In this study, we developed a simple method to create large-scale porous silica antireflection layers on inactive FEP film. The method combines oxygen plasma processing and the sol–gel process, and the maximum transmittance of the coated FEP film reaches 99.28% as compared to 96.83% for the uncoated FEP film. Aiming at the wavelength of 351 nm, the transmittance of the coated FEP film reaches 97.48% compared to 93.42% for the uncoated FEP film. This antireflection FEP film has considerable applications in the high peak power laser field.

Investigation of Craze and Cracks of Polystyrene Shells During Drying Process

Abstract Drying is one of the most important processes to prepare the hollow polystyrene (PS) shells which meet the requirements for the inertial confined fusion experiments. A tracing experiment was taken by white light interferometer to explore the drying process. The results indicate that the inner water drop passed through the PS shells with the state of water stream molecule. During the experiment, three structures were observed by digital microscope: the structure of craze, mixture of craze and cracks, and cracks. With ongoing drying, the decrease in the interfacial energy was regarded as the inducing factor for the formation of craze, while the residual stress inside the PS shells was the primary cause. Once the craze formed, it not only reduced the strength of the PS shells but also served as the stress concentration point. In the function of adequate time and stress, the voids of craze would coalesce resulting in the cracks formation. High-temperature treatment to the PS shells at 75°C for 3 h was taken to eliminate the residual stress so that the integrated PS shells would be produced. In addition, the comparison of surface roughness between all of the drying conditions is discussed.

Fabrication of thick-walled polyacrylonitrile (PAN) with high uniformity by an easily assembled double-T droplet generator

Low density carbon shells are required in inertial confinement fusion (ICF) experiments. Polyacrylonitrile (PAN), as a common raw material of carbon, was studied and adopted to prepare thick-walled microspheres with the outer diameter ranging from 300 μm to 800 μm; wall thickness around 50–120 μm; sphericity (OOR): <2 μm; and wall thickness uniformity (ΔTw): <9 μm. The preparation of PAN microspheres was based on an assembled double T-junction droplet generator. The major challenge in this experiment was to simultaneously meet the requirements and restrictions on both wall thickness and its uniformity. In order to improve the wall uniformity of thick-walled PAN microspheres, two major factors, the viscosity and the temperature which affect the density-matching solutions between the O1/W compound droplet and the O2 phase, were tested. The calculated results of OOR and ΔTw showed that the optimal density gap between the O1/W compound droplet and the O2 phase would be around 0.015 g cm−3 when the temperature is at 10 °C or the viscosity ratio λ is near 3.6 times (the viscosity ratio λ is defined as the ratio of the W phase to the O2 phase). Under this circumstance, the wall thickness uniformity and sphericity were largely improved. Furthermore, the experiment established that wall thickness uniformity was more sensitive to the temperature and the viscosity than the sphericity.