NEW SWELLING PROPERTIES OF CROSSLINKED POLY(N-ISOPROPYLACRYLAMIDE) GELS IN NONAQUEOUS SOLUTIONS.

M.V. MARTINEZ; MARIA MOLINA; CESAR BARBERO

Congreso; XXVII INTERNATIONAL MATERIALS RESEARCH CONGRESS; 2018

The hydrophilic nature of poly(N-isopropylacrylamide) (PNIPAM) networks is very attractive howeverit is an important disadvantage at the moment of encapsulating hydrophobic drugs. In this context,we have studied the swelling in different solvent mixtures and also in pure nonaqueous solvents.The analysis of the swelling kinetics in pure solvents employed the half life time (t1/2) suggests thatthe relaxation of the polymer chains is directly related to the viscosity of the solvent. The maininteraction between the solvent and the polymer chain seems to involve the hydrogen bonding withthe amide group, according to the calculated Hansen parameters. Accordingly, PNIPAM gels swellstrongly in highly polar solvents (e.g. chloroform) but it does not swell in slightly polar solvents(e.g. toluene). However, it is possible to swell the gel in binary or ternary mixtures containingtoluene. This property was used to load polystyrene inside PNIPAM employing ethanol-water-toluenemixtures, then the solvents were removed and the hydrogel re-swelled in water. As a proof ofconcept, a pH sensor in non-aquous media is developed, using methyl orange a polar water solublemolecule and in the opposite way a Cu+1 sensor by loading the bathocuproine ligand (insoluble inwater). Taking into account that PNIPAM swells strongly in CHCl3 (solvent immiscible with water), twoliquid phases can be set inside a gel piece. A near infrared absorbing dye (IR800) soluble in CHCl3 isloaded in only half of a PNIPAM cylinder. Then, the solvent is removed and the whole gel piece isswelled in water, recovering the thermosensitive properties, since the gel does not suffer phasetransition in CHCl3. Upon NIR light irradiation, only the region where the dye is present heats updriving the phase transition of PNIPAM. The observed properties of PNIPAM gels allow to describethem as solvogels (swellable in strongly different solvents) or amphigels (swellable both in water andnonaqueous solvents).