Jonghwi Lee

Inorganic particle toughening I: micro-mechanical deformations in the fracture of glass bead filled epoxies

General characteristics of micro-mechanical deformations found in the fracture of various glass bead filled epoxies are investigated. Among the various types of step features on fracture surfaces, the basic longitudinal texture is not influenced by the existence of glass beads, but most other steps formed are significantly affected. Various microscopic investigations show that microcracking does not extensively occur in the fracture of glass bead filled epoxies. Microcracking other than debonding of glass beads is seldom observed, and furthermore, debonding is found to occur only on and near the fracture surface. Micro-shear bands are clearly identified and distinguished from microcracks found in unnotched tensile specimens. Based on examination of micro-mechanical deformations, three types of fracture processes are proposed for glass bead filled epoxies having different glass bead contents and interfacial strengths.

Fracture of glass bead/epoxy composites: on micro-mechanical deformations

To understand the fracture behavior of inorganic particle filled polymers, glass bead filled epoxies having different glass bead contents and sizes were prepared as model systems. Although their macroscopic fracture behavior was brittle, diffuse matrix shear yielding and micro-shear banding were found to occur around crack paths. Besides these plastic deformations, debonding of glass beads, step formation on fracture surface, and birefringence due to thermal residual misfit between glass beads and matrix were identified and studied. The fracture toughness and modulus of composites generally increased with increase in the volume fraction of glass beads. Micro-shear band zone size, debonding zone size, and the areal density of steps also followed increases in the volume fraction. The effect of glass bead size on fracture toughness and modulus was not significant, but the areal density of steps was found to increase as the size decreased. Differential thermal contraction between glass beads and matrix was found to cause the thermal residual misfit, resulting in birefringence around glass beads. Microscopy studies revealed that this thermal residual misfit might not have an extensive influence on crack propagation.

Inorganic particle toughening. II: toughening mechanisms of glass bead filled epoxies

Abstract Based on the previously established knowledge about the micro-mechanical deformations occurring during the fracture of glass bead filled epoxies, the major energy dissipation mechanisms are investigated. Correlation studies between the fracture toughness of composites and the size of micro-mechanical deformation zones (or areal density of deformation) are used to assess the contributions of the deformations to toughening. Among the deformations found in the fracture of glass bead filled epoxies, i.e. micro-shear banding, debonding of glass beads/diffuse matrix shear yielding, and step formation, micro-shear banding is established as the major and most effective toughening mechanism. In terms of this mechanism, the negligible effect of surface treatments of glass beads on the fracture toughness of glass bead/thermoset composites can be explained successfully. This mechanism is expected to give more detailed and fundamental understanding of inorganic particle toughening than the crack front bowing mechanism.

Role of inherent matrix toughness on fracture of glass bead filled epoxies

Abstract Some polymers can be toughened by rigid inorganic fillers. In this study, the effect of inherent matrix toughness on the fracture of filled thermosets is systematically investigated using glass beads and epoxies. The toughening effect of glass beads, i.e. the relative increase of toughness due to the incorporation of glass beads, follows an increase in the molecular weight of epoxides. Inherent matrix toughness is found to increase with the increase of the epoxide molecular weight. Microscopy studies reveal that the sizes of debonding and micro-shear banding zones are increased as the fracture toughness of composites increases. Matrix shear yielding is identified as one of the major energy absorbing mechanisms for unmodified and modified epoxies.

Phase transition and elasticity of protein-based hydrogels.

Reported are specific materials characterizations of three protein-based polymers comprised of repeating pentapeptide sequences, namely (GVGVP)251, (GVGIP)260 and (GVGVP GVGVP GEGVP GVGVP GVGVP GVGVP)n](GVGVP) where G = glycine, V = valine, P = proline, I = isoleucine, and E = glutamic acid, which had been previously prepared and γ-irradiation cross-linked into elastic matrices. These polymers exhibit a hydrophobic folding and assembling transition on raising the temperature above a critical temperature, designated by Tt. Their equilibrium swelling ratio, uniaxial tensile and dynamic shear behavior were studied. The effect of the transition on swelling and mechanical properties was demonstrated. Equilibrium swelling ratio below the transition temperature decreased with the increase of γ-irradiation dose. Above the transition temperature, hysteresis and frequency dependence were found in tensile loading-unloading tests and dynamic shear measurements, respectively. The rubber elasticity theory of random chain networks was applied to the data only below the transition temperature, where they may properly be considered random network hydrogels, to estimate molecular weight between cross-links and the Flory-Huggins interaction parameter.

L -Ascorbic Acid Microencapsulated with Polyacylglycerol Monostearate for Milk Fortification

Efficiency was examined of microencapsulating L-ascorbic acid by polyglycerol monostearate (PGMS), and changes in the chemical and sensorial aspects of L-ascorbic acid and/or iron-fortified milk during storage were evaluated. The selected core materials were ferric ammonium sulfate and L-ascorbic acid. The highest efficiency (94.2%) of microencapsulation was found with the ratio of 5:1 as the coating to core material. The release of ascorbic acid from the microcapsules increased sharply from 1.6 to 6.7% up to 5 d of storage. The TBA value was the lowest in the milk sample with added encapsulated iron and unencapsulated L-ascorbic acid up to 5 d of storage in comparison with the other treated samples. A sensory analysis showed that most aspects were not significantly different between the control and fortified samples encapsulated with ascorbic acid after 5 d of storage. The results indicate that L-ascorbic acid microencapsulated with PGMS can be applied to fortify milk and acceptable milk products can be prepared with microencapsulated L-ascorbic acid and iron.

Fracture behavior of glass bead filled epoxies: Cleaning process of glass beads

Inorganic particles are commonly cleaned with solvents such as alcohols before being incorporated into thermoset polymers as fillers or tougheners, but the role of the cleaning process has never been examined. In this study, the effect of the cleaning process on the fracture behavior of particulate composites is investigated using glass bead filled epoxies as model systems. The cleaning process is shown to be a simple method to strengthen the interface between the glass beads and the epoxy matrix. Although the chemistry of the glass bead surface is unlikely to be altered by the cleaning process, submicron particles that exist on the glass bead surfaces are removed by cleaning with distilled water or ultrasonic vibration. The removal of submicron particles increases the interfacial strength between the glass beads and the matrix and changes the tensile strength of the composites. However, the modulus and fracture toughness of the composites is not significantly dependent on the cleaning process. Thus, it may be the case that debonding of the glass beads is not one of the major energy dissipating mechanisms in the fracture of glass bead filled thermoset systems.

Effect of rubber interlayers on the fracture of glass bead/epoxy composites

The effectiveness of rubber interlayers between inorganic particles and polymer matrix for toughening has been a controversial subject. In this research, a series of rubber-encapsulated glass beads and its epoxy composites were prepared, and underlying mechanisms which can connect material parameters related with rubber interlayers with energy dissipation mechanisms, were investigated. The critical stress intensity factor (KIC) and critical strain energy release rate (GIC) of rubber-encapsulated glass bead filled epoxies were found to insignificantly depend on the existence and thickness of rubber interlayers. Microscopy studies on fracture process identified four different micro-mechanical deformations which can dissipate fracture energy: step formation, micro-shear banding, debonding of glass beads, and diffuse matrix shear yielding. It was found that the first two became less extensive and the others became more extensive as the thickness of rubber interlayers increases. This offsetting effect of micro-mechanical deformations seems to be the reason for the absence of significant toughening effect of rubber interlayers.

The microencapsulated ascorbic acid releasein vitro and its effect on iron bioavailability

The present study was carried out to examine the stability of microencapsulated ascorbic acid in simulated-gastric and intestinal situationin vitro and the effect of microencapsulated ascorbic acid on iron bioavailability. Coating materials used were polyglycerol monostearate (PGMS) and medium-chain triacylglycerol (MCT), and core materials were L-ascorbic acid and ferric ammonium sulfate. When ascorbic acid was microencapsulated by MCT, the release of ascorbic acid was 6.3% at pH 5 and 1.32% at pH 2 in simulated-gastric fluids during 60 min. When ascorbic acid was microencapsulated by PGMS, the more ascorbic acid was released in the range of 9.5 to 16.0%. Comparatively, ascorbic acid release increased significantly as 94.7% and 83.8% coated by MCT and PGMS, respectively, for 60 min incubation in simulated-intestinal fluid. In the subsequent study, we tested whether ascorbic acid enhanced the iron bioavailability or not. In results, serum iron content and transferring saturation increased dramatically when subjects consumed milks containing both encapsulated iron and encapsulated ascorbic acid, compared with those when consumed uncapsulated iron or encapsulated iron without ascorbic acid. Therefore, the present data indicated that microencapsulated ascorbic acid with both PGMS and MCT were effective means for fortifying ascorbic acid into milk and for enhancing the iron bioavailability