REVERSIBLE THERMOASSOCIATION OF WATER-SOLUBLE POLYMERS

In various industrial fields, water soluble polymers are commonly used as thickening agents to control the reology of aqueous fluids. Nevert heless, their properties are weakened as the temperature increases. In order to overcome this problem, the concept of ''thermoassociative'' water soluble polymers was developed. Such new amphilic systems can be obtained by grafting on an hydrophilic backbone, side chains which be come non water soluble above a Lower Critical Solution Temperature (LC ST). Semidilute solutions of these copolymers present reversible thick ening properties as the temperature increases and reaches a critical v alue, close to the side chains LCST. This behaviour can be related to the agregation of the grafts above their LCST, into ''hydrophobic micr odomains'', inducing the formation of a three dimensional network. At higher temperature, the viscosity of the solution sheared at a constan t rate reaches a maximum value. This can be interpreted in terms of th e reorganisation of the physical network under shear, from a structure with mainly ''intermolecular associations'' to a system with an incre asing number of ''intramolecular associations''. Owing to the diversit y of water soluble polymers exhibiting a phase separation on heating ( LCST), different ''thermoassociative'' copolymers were realized [1] an d [2], using either polyelectrolyte or neutral hydrophilic backbone. I n the aim of applications of such systems in the oilfield industry, co polymers containing 2-acrylamido 2-methyl propane sulfonic acid (AMPS) were developed, using polyethylene oxide (PEG) as LCST grafts. A gene ral description of their rheological behaviour will be given here. The ir properties can be controlled either by varying the solution charact eristic (polymer concentration, salinity, etc.) or by modifying the ch emical structure of the copolymer (grafting ratio, molecular weight of the backbone, etc.). This rheological study showed the potentiality o f the ''thermoassociative'' system, particularly towards the oilfield industry [3]. It is now possible to have thin cement slurries at ambie nt temperature and still viscous slurries at bottom hole temperature, as desired. The reversibility of the phenomenon joint to the shear thi nning properties of the polymer solution are adding a plus for cement slurries and drilling fluids designs. The thermoassociative phenomenon was also studied by Small Angle Neutrons Scattering (SANS, using copo lymers composed by a deuterated polyacrylate backbone and protonated p olyethylene oxide grafts. The scattering properties of our system were studied by varying the temperature, the salinity and the polymer conc entration of the solution. A good agreement between the microscopic ch aracteristics of the solutions and their rheological properties was fo und.