Epoxy-acrylic core-shell particles by seeded emulsion polymerization.

We developed a novel method for synthesizing epoxy-acrylic hybrid latexes. We first prepared an aqueous dispersion of high molecular weight solid epoxy prepolymers using a mechanical dispersion process at elevated temperatures, and we subsequently used the epoxy dispersion as a seed in the emulsion polymerization of acrylic monomers comprising methyl methacrylate (MMA) and methacrylic acid (MAA). Advanced analytical techniques, such as scanning transmission X-ray microscopy (STXM) and peak force tapping atomic force microscopy (PFT-AFM), have elucidated a unique core-shell morphology of the epoxy-acrylic hybrid particles. Moreover, the formation of the core-shell morphology in the seeded emulsion polymerization process is primarily attributed to kinetic trapping of the acrylic phase at the exterior of the epoxy particles. By this new method, we are able to design the epoxy and acrylic polymers in two separate steps, and we can potentially synthesize epoxy-acrylic hybrid latexes with a broad range of compositions.

Extraction of guided modes from organic emission layers by compound binary gratings.

The extraction of guided modes from a 100 nm organic emission layer by compound binary gratings with multiple superimposed periods at different ratios is investigated. We measure angle-dependent photoluminescence from samples with double-period (350 and 450 nm), triple-period (350, 400, and 450 nm), and multiperiod (350, 400, 450, and 500 nm) gratings and show that each period component produces two outcoupling features due to first-order Bragg scattering of the TE(0) guided mode. The averaged angular color change is reduced by up to a factor of 11 compared to a single-period grating structuring.

Interplay between gelation and phase separation in aqueous solutions of methylcellulose and hydroxypropylmethylcellulose.

Thermally induced gelation in aqueous solutions of methylcellulose (MC) and hydroxypropylmethylcellulose (HPMC) has been studied by rheological, optical microscopy, and turbidimetry measurements. The structural and mechanical properties of these hydrogels are dominated by the interplay between phase separation and gelation. In MC solutions, phase separation takes place almost simultaneously with gelation. An increase in the storage modulus is coupled to the appearance of a bicontinuous structure upon heating. However, a thermal gap exists between phase separation and gelation in the case of HPMC solutions. The storage modulus shows a dramatic decrease during phase separation and then rises in the subsequent gelation. A macroporous structure forms in the gels via "viscoelastic phase separation" linked to "double phase separation".

Direct measurement of the time-dependent mechanical response of HPMC and PEO compacts during swelling.

Mechanical indentation is used to measure the time dependent mechanical properties of three model compact formulations during swelling in aqueous media. The formulations are based on polyethylene oxide (PEO), hydroxypropyl methylcellulose (HPMC) and a PEO/HPMC blend. The technique is sensitive to changes in compact thickness and mechanical response and is used to characterize changes in the mechanical properties of the model compacts during the swelling process. The gel thickness and the effective elastic modulus of the gel layer are obtained from the load/displacement relationship during initial indentation. The HPMC and hybrid compacts showed significantly more swelling (110%) than the PEO compact (67%). Viscoelastic properties of the gel layer are determined throughout the swelling process by an oscillatory indentation method. Results show the complex modulus of all three compacts decreasing by approximately an order of magnitude over the course of swelling for 6h. The measurement techniques presented here can easily be extended to more complex systems.