Date of Award

Winter 2002

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

W Rudolf Seitz


Derivatized lightly crosslinked polymer microspheres that swell and shrink as a function of pH have been investigated to develop polymeric transduction elements for fiber optic sensors and other sensors. The microspheres were immobilized in hydrogels forming a membrane. The membranes look turbid because of the difference between the refractive index of hydrogel and the refractive index of microspheres. As the microspheres swell as a function of pH, their refractive index decreases approaching the refractive index of the hydrogel. As a result, the turbidity of the membrane decreases. The change in the turbidity of the membrane was monitored by a UV-Vis-NIR spectrophotometer.

Optical measurements in our research involve turbidity, light reflection and light scattering. Therefore the conditions for turbidity and light scattering have been investigated. The turbidity of monodisperse polystyrene microspheres with diameters from 0.5 to 1.5 microns, suspended in water or embedded in hydrogels, has been studied. The turbidity increased with increasing particle size at longer wavelengths. At shorter wavelengths (i.e. shorter than 600 nm) the turbidity of smaller particle increased sharply while the turbidity of large particles decreased. The turbidity of microparticles increased with increasing particle concentration at all wavelengths. Scattered light intensity increases with decreasing particle size.

Microspheres of 2-(dimethylamine)ethyl methacrylate (DMAEMA) and 4-vinyl pyridine (4VP) were synthesized with various comonomers including 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA) and styrene. Microparticles made of DMAEMA and HEMA did not generate a measurable turbidity with pH changes. When MMA was added as a third monomer in the copolymer, swelling properties improved. Particles of DMAEMA, HEMA and MMA responded pH between 5.5 and 8. The apparent pKa of the membrane was 7.10 for decreasing pH and 7.24 for increasing pH. Membrane response times were less than 20 seconds.

Microspheres of 4VP-HEMA and 4VP-HEMA-Styrene were synthesized with 4VP contents from 10% to 50%. All formulations responded to pH between 3.5 and 5.5. The response time of 4VP-HEMA microparticles increased with increasing 4VP content but was less than 60 seconds for all formulations. Addition of styrene to 4VP-HEMA copolymer improved the performance of the microparticles. An approximately 50% decrease was observed in turbidity for all formulations.