Water‐soluble polymers with tunable temperature sensitivity: Solution behavior

Controlled polymer solubility in water is of great interest, with promise of use in applications requiring smart/responsive materials such as sensors and actuators, cell patterning, and smart/controlled drug-delivery systems. Temperature-sensitive solubility usually originates from the existence of a lower critical solution temperature (LCST) beyond which the polymer becomes insoluble in water. Such behavior is typical for the polymers that form hydrogen bonds to water. Driven by the high promise for biomedical applications, polymers that exhibit a response in water at about 37 °C are of particular interest. Taylor and Cerankowski predicted that LCST of a water-soluble polymer can be varied by controlling the balance of hydrophilic and hydrophobic segments in the polymer chain. However, most polymers that have been examined are based on a single homopolymer [poly(N-isopropylacrylamide), PNIPAM] that exhibits LCST at 32 °C, and efforts to change its LCST mostly involved modifications through the addition of hydrophobic branches. These branched polymers exhibit cloud points (CPs) that do not correlate with the hydrophobic/hydrophilic balance of the polymer. This behavior originates from the branched molecular architecture of these materials that results in a coil to micelle “phase transition” rather than a polymer solution (LCST) phase transition. Bokias et al. showed that increasing the length of the hydrophobic side chains can shift the LCST of PNIPAM, but now the phase transition broadens and occurs over a wide temperature range. Virtanen et al., who investigated PNIPAM modified with poly(ethylene oxide) (PEO) grafts, also found the same broadening of phase transition that they attributed to the collapsed aggregate formation—a micelle that consists of a PNIPAM/PEO core with a PEO shell. Such coil-to-micelle transitions make it difficult to predict the behavior of branched-modified thermosensitive polymers on the basis of the balance of hydrophilic and hydrophobic molecular segments, thus limiting dramatically the ability to design polymers with tailored temperature response in aqueous solutions. A sharp LCST transition that is determined by the hydrophobic/hydrophilic balance dictates a linear polymer architecture where the hydrophobic and hydrophilic segments are not lumped together in blocks, which could facilitate the formation of collapsed polymers upon precipitation from solution. This is the driving force for this study that aims to design watersoluble polymers with a controlled temperature response in aqueous solutions and tailor their phase separation through the balance of hydrophilic and hydrophobic segments. For this purpose, we synthesized polymers on the basis of monomers with a controlled stoichiometry of ethylene/(ethylene oxide) with the intention to tailor the polymer LCST by controlling the hydrophobic/hydrophilic balance within the monomer. We report the first experimental results on the solubility phase diagram and the LCST dependence of the monomer composition. *Contribution from the March 2002 Meeting of the American Physical Society—Division of Polymer Physics, Indianapolis, Indiana Correspondence to: E. Manias (E-mail: manias@psu.edu)