Transitions From Bicontinuous To Discrete Microemulsion Structures

Author : Brian H. Shoop
Publisher :
ISBN 13 :
Total Pages : 121 pages
Book Rating : 4.:/5 (52 download)

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Download or read book Applications of Sugar-based Microemulsion Glasses written by Brian H. Shoop and published by . This book was released on 2009 with total page 121 pages. Available in PDF, EPUB and Kindle. Book excerpt: A microemulsion is an optically clear, thermodynamically stable mixture of a hydrophilic species (typically water) and a hydrophobic species (oil) stabilized by a flexible surfactant film. They exist primarily as dispersions of swollen micelles of oil in water (O/W), swollen reverse micelles of water in oil (W/O), or as bicontinuous structures. While there are many applications for these formulations, one that has received much attention is the use of microemulsions as structure-directing agents for polymerization reactions. Of particular interest is arresting the structure of a bicontinuous microemulsion in polymer form for use in synthesizing ultra filtration membranes. However, previous attempts to do so using aqueous microemulsions have fallen short due to surfactant film rearrangement, non-uniform reaction initiation, and breakthrough of the template. Replacing water in typical microemulsion formulations with sugar has been shown to mimic traditional microstructure of aqueous microemulsions but in a solid, glassy form. These microemulsion glasses have been used as templates for free-radical polymerizations with 1-to-1 copies of the microemulsion structure formed as a polymer membrane. However, in the interest of broadening their potential applications, two different research projects were proposed. First, the glass transition temperature of the initial system of sucrose, trehalose, sucrose ester surfactant, and limonene oil was determined to be 570C. While this is acceptable for low-temperature applications, higher temperature applications run the risk of crystallization and phase separation of the template. In order to combat this, it was proposed that sodium citrate, having been shown to increase the glass transition temperature of sucrose significantly, replace trehalose in the formulation in hopes that the glass transition temperature increases for the system. Phase behavior studies confirmed that bicontinuous microemulsion glasses can be formed with the citrate replacement. However, temperature-modulated differential scanning calorimetry experiments showed that the replacement of trehalose with citrate, and the concentration of citrate within, did not have a significant effect on the glass transition temperature of the sample. Second, a step-growth polymerization scheme was proposed, whereby a diisocyanate reacts with the hydroxyl groups on the sugar and surfactant head groups, cross-linking the surfactant film and arresting the structure. Limonene was replaced by allowing 1,6-diisocyanatohexane to diffuse into the extruded glass film, forming a solid, clear, polymer membrane. Ultra small-angle x-ray scattering experiments confirmed that bicontinuous structure is preserved during polymerization and washing away of the template. However, attempts to characterize the hydraulic permeability and volume porosity of the membrane were fruitless. It is hypothesized that a surface polymerization is taking place, by virtue of the highly hydrophobic surface of the pressing material causing surfactant reorganization as well as a finite amount of sugar solubilizing into the isocyanate at the surface and reacting immediately. This surface reaction effectively destroys all porous structure.