Rheology and Structures of EO-PO-EO Block Copolymers in Aqueous Solutions

Aqueous solutions of a Pluronic copolymer P104 have been investigated by rheological techniques in order to characterize its gelation and its gel properties. P104 solutions have a low viscosity at low temperatures but form an elastic gel at physiological temperatures. Elastic moduli are of the order 10 kPa with a significant yield stress. Possible use of P104 to prevent flow in pipes or blood veins is discussed. INTRODUCTION Most materials become softer when heated and this is used in many practical applications of polymers, and in the food industry thermo-setting gels such as pectin or gelatin gels are important. Some protein solutions and suspensions of starch granulates undergo a transition from a Newtonian liquid to a solid state when heated. Boiling an egg is a good example. However, these systems are not thermoreversible i.e. they maintain their elastic properties when cooled again. For many technical and medical applications it is desirable to have systems which are low viscosity liquids at low temperatures but form solid elastic states when heated. Such systems may be potentially useful in preventing flow in pipes at selected temperatures and block flow in blood veins during surgery or stop blood circulation to damaged organs. Block copolymers of ethylene oxide (EO) and propylene oxide (PO) in the form of EO-PO-EO copolymers, called Pluronics or Poloxamers have some of the desired properties. They are low toxicity compounds which are approved for medical applications and used for delayed drug release. Copolymers are dissolved as single molecules, unimers, in water at low temperatures but form micelles, with a core of PO and a mantle of EO, at higher temperatures. The hydrophobic core of PO is used to give the delayed release of hydrophobic drugs. At higher concentrations and temperatures some Pluronic solutions form thermoreversible physical gels. These gels consist of liquid crystals of packed either spherical or rodlike micelles. 1,2 In order to block flow in a pipe a heatsetting material should have a low viscosity at low temperatures so the pipe can be filled easily. At the higher temperature of interest, 37 °C for most biomedical applications, it should form a gel with a high elastic modulus as well as a significant yield stress. If a pressure, P, is applied to a pipe of length, L, and radius, R, the stress σ exerted is given by