Pyrene-Labeled Graft Copolymers of N-Vinylcaprolactam: Synthesis and Solution Properties in Water

Thermosensitive homopolymers of N -vinylcaprolactam and graft copolymers of N -vinylcaprolactam and poly(ethylene oxide)−alkyl methacrylate macromonomer were synthesized with different amounts of hydrophobic fluorescent labels, pyrene, and naphthalene. Association of the polymers in dilute aqueous solutions was studied below and above the LCST using dynamic light scattering, microcalorimetry, and various fluorescence spectroscopy techniques, including nonradiative energy transfer (NRET) and quenching experiments. At room temperature the labeled PVCL homopolymers associate via hydrophobic interactions between labels of different chains, while the amphiphilically grafted PVCL forms mainly intrapolymeric micelles. Upon heating, the polymers aggregate in water and form stable nanosized particles. Results from quenching experiments indicate that the PEO containing grafts are able to create a hydrophilic environment in the vicinity of pyrene units, even at temperatures higher than the LCST. The absence of energy transfer from naphthalene-labeled polymers to pyrene-labeled polymers confirms that, on the contrary to labeled homopolymers, the grafted copolymers do not mix at room temperature, even after the heating cycle. The consequences of amphiphilic modification are discussed in terms of the mechanism of stabilization and of the possible applications of PVCL samples carrying both hydrophobic groups and poly(ethylene oxide) groups.