Aggregation induced emission based fluorescence pH and temperature sensors: probing polymer interactions in poly(N-isopropyl acrylamide-co-tetra(phenyl)ethene acrylate)/poly(methacrylic acid) interpenetrating polymer networks

Aggregation induced emission (AIE) active copolymers P1-P6 with high molecular weights (14 000 - 17 000) and low polydispersity indices (1.3-1.4) were prepared through copolymerization of N-isopropyl acrylamide (NIPAM) and tetra(phenyl) ethene (TPE)-based acrylate monomers. Copolymers P1-P6 show comparable thermal stability to poly(N-isopropylacrylamide) (PNIPAM), while their glass transition temperatures are higher by 7-9 degrees C than those of pristine PNIPAM. Copolymers P1-P6 are soluble in common organic solvents as well as in water. They retain a similar thermal sensitivity to PNIPAM, but their lower critical solution temperatures (LCST) are reduced with increase of TPE content. By changing the molar ratio of P1-P6/poly(methacrylic acid) (PMAA) and pH, complexes P1-P6-PMMA were studied by fluorescence spectroscopy and dynamic light scattering (DLS). The complexes are non-emissive in THF, and their fluorescence can be turned on upon addition of water. Moreover, their fluorescence is enhanced with the decrease in pH values due to the formation of interpenetrating polymer networks (IPNs) through inter-polymer hydrogen bonding. Fluorescence spectroscopy and DLS results also reveal that the phase transition behaviour of IPNs upon heating could be significantly modified by pH change. Reduction in the pH value from 7.0 to 4.0 leads to the decrease in LSCT of IPNs by up to 5 1C with respect to PNIPAM. By tuning the pH value to dissociate the formed inter-polymer hydrogen bonds, the formed IPNs would be able to fold cooperatively to a compact structure without a loss of solubility at temperatures below the LCST. Thus, these novel IPNs with AIE active moieties would be used as drug delivery systems, in which the release process could be readily monitored by fluorescence spectroscopy.