Cononsolvency-Induced Micellization of Pyrene End-Labeled Diblock Copolymers of N-Isopropylacrylamide and Oligo(ethylene glycol) Methyl Ether Methacrylate

Abstract

Pyrene end-labeled double hydrophilic diblock copolymers, poly(N-isopropylacrylamide)-b-poly(oligo(ethylene glycol) methyl ether methacrylate) (Py-PNIPAM-b-POEGMA), were synthesized via consecutive reversible addition-fragmentation chain transfer polymerization using a pyrene-containing dithioester as the chain transfer agent. These diblock copolymers molecularly dissolve in pure methanol and water, but form well-defined and nearly monodisperse PNIPAM-core micelles in an appropriate mixture of them due to the cononsolvency of PNIPAM block. 1H NMR, laser light scattering, fluorescence spectroscopy, and transmission electron microscopy were employed to characterize the cononsolvency-induced PNIPAM-core micelles. When the volume fraction of water, phi water, in the methanol/water mixture is in the range of 0.5-0.8, the sizes of micelles are in the range of 20-30 nm in radius for Py-PNIPAM50-b- POEGMA18. At phi water = 0.5, the formed micelles possess the highest overall micelle density and the largest molar mass. The effects of varying the block lengths of Py-PNIPAM-b-POEGMA diblock copolymers on the structural parameters of PNIPAM-core micelles have also been explored. Although we can observe the immediate appearance of bluish tinge upon mixing the diblock copolymer solution in methanol with equal volume of water (phi water = 0.5), which is characteristic of the formation of micellar aggregates, the whole micellization process apparently takes a relatively long time to complete, as revealed by monitoring the time dependence of fluorescence emission spectra. The excimer/monomer fluorescence intensity ratios, IE/IM, continuously decrease with time and then reach a plateau value after approximately 20 min. The decrease of IE/IM after the initial formation of pseudo-equilibrium micelles should be ascribed to the structural rearrangement and further packing of PNIPAM segments within the micelle core, restricting the mobility of pyrene end groups and decreasing the probability of contact between them. Compared to the conventional cosolvent approach employed for the micellization of block copolymers in selective solvents, the reported cononsolvency-induced unimer-micelle-unimer transition of Py-PNIPAM-b-POEGMA in methanol/water mixtures has been unprecedented.