Adjusting the low critical solution temperature of poly(N-isopropyl acrylamide) solutions by salts, ionic surfactants and solvents: A rheological study

Abstract

The control over the Low Critical Solution Temperature (LCST) and thickening of thermosensitive polymers is of crucial interest for applications such as drug delivery systems and temperature sensors. Commonly the design of a new drug delivery system of a temperature sensor requires the synthesis of new thermosensitive polymers that respond to a desired target temperature. In this study we follow the LCST by rheology and report examples of how the onset of LCST can be adjusted by chemical species that are able to change polymer solubility, such as salts, organic solvents or surfactants.For this purpose we used the thermo-responsive polymer, poly(N-isopropyl acrylamide) or PNIPAM, and surfactants, salts and ethanol as additives. The LCST was investigated by means of rotational rheology, as it has been shown a good correlation between the LCST and the onset of the temperature-induced thickening. From this study we observe that salts can both increase or decrease the temperature at which the LCST occurs. This is attributed to the affinity of the different salts with the polymer. More hydrophobic salts such as KSCN shifted the LCST of PNIPAM to higher temperature values while the opposite scenario is observed in the case of more hydrophilic salts such as KCl. When surfactants are added to the system, it is observed that, in both cases (SDS and CTAB) and at the concentrations employed, the LCST was shifted to higher temperatures. Surfactants interact with the polymeric chains, converting the macromolecule into a polyelectrolyte and retarding the occurrence of clouding or phase separation. Major effects were also observed by changing the polarity of the bulk: the presence of 10% ethanol shifted the LCST of PNIPAM solution to lower temperatures, forming a co-non-solvent pair with water. No phase transition was observed when PNIPAM was dissolved in absolute ethanol. These observations highlight the role of hydrophobic associations and the entropy of counterions.