Thermorheologic characterization of a thermal gelling polymer

Christopher William Spancake, Purdue University

Abstract

Poloxamines (Tetronics$\sp{\rm R}$) are a series of block copolymers some of which exhibit the property of reverse thermal gelation, that is, aqueous solutions of the polymer undergo a sol-gel phase transition upon an increase in temperature. The initial objective of this research project was to investigate the rheologic properties of Tetronic 1508 as a function of temperature in order to evaluate the potential use of this polymer as a novel drug delivery system. Rotational viscometry studies indicated the existence of a sol-gel phase transition at low temperatures ($<$25$\sp\circ$C) and a gel-sol phase transition at high temperatures ($>$60$\sp\circ$C). Upon the sol-gel transition, the system changed from a Newtonian liquid with a viscosity of 100 mPa s to a very rigid gel with a viscosity of 10,000,000 mPa s. Between the two transition temperatures the gels exhibited plastic rheologic behavior and maintained a relatively constant viscosity. As the temperature was increased through the gel-sol transition temperature, the system changed from a viscous gel back to a low viscosity Newtonian fluid. The practical and theoretical implications of the thermorheologic profiles are discussed. Based on the macroscopic viscosity, a hypothesis was proposed which predicted that the rate of aspirin hydrolysis in a Tetronic gel should be greatly reduced. To test the hypothesis, the hydrolysis of aspirin was followed as a function of polymer concentration and pH. The data from the aspirin hydrolysis study, however, refuted the original hypothesis and suggested the presence of a low viscosity "micro" environment within the rigid gel structure. The presence of a low viscosity microenvironment was investigated utilizing an Electron Spin Resonance (ESR) spin probe technique. While the Tetronic gels may have a macroscopic viscosity as high as 10,000,000 mPa s, the ESR studies indicate that the viscosity in the microenvironment of the spin probe is approximately 10 mPa s. In addition, it was observed that while the macroscopic viscosity increased with temperature, the viscosity within the microenvironment decreased with temperature. Finally, it is apparent that an understanding of both the macroscopic and microscopic rheological properties is essential in describing the thermorheologic behavior of the Tetronic polymer.

Degree

Ph.D.

Advisors

Mitra, Purdue University.

Subject Area

Pharmacology

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