Thermoresponsive Self-Assembly of Twofold Fluorescently Labeled Block Copolymers in Aqueous Solution and Microemulsions

A nonionic double hydrophilic block copolymerwith a long permanently hydrophilic and a small thermoresponsiveblock is synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT). By employing a specificallydesigned chain-transfer agent, the polymer is functionalized withcomplementary end groups which are suited for Fo??rster resonanceenergy transfer (FRET). The end group attached to thepermanently hydrophilic block of poly(N,N-dimethylacrylamide)pDMAm is designed as a permanently hydrophobic segment(sticker) comprising a long alkyl chain and the 4-aminonaphthalimidefluorophore. The other end attached to thethermoresponsive block of poly(N-isopropylacrylamide) pNiPAm incorporates a coumarinfluorophore. The temperature-dependent self-assembly of the twofoldfluorescently labeled copolymer is studied in pure aqueous solution as well as in an o/wmicroemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), andfluorescence spectroscopy. It iscompared to the behaviors of the analogous twofold-labeled pDMAm and pNiPAm homopolymer references. Thefindings indicatethat the block copolymer behaves as a polymeric surfactant at low temperatures, with one relatively small hydrophobic end block andan extended hydrophilic chain forminghairy micelles. At elevated temperatures above the LCST phase transition of the pNiPAmblock, however, the copolymer behaves as an associative telechelic polymer with two nonsymmetrical hydrophobic end blocks, whichdo not mix. Thus, instead of a network of bridgedflower micelles, large dynamic aggregates are formed. These are connectedalternatingly by the original micellar cores as well as by clusters of the collapsed pNiPAm blocks. This type of structure is even morefavored in the o/w microemulsion than in pure aqueous solution, as the microemulsion droplets constitute an attractive anchoringpoint for the hydrophobic dodecyl sticker but not for the collapsed pNiPAm chains

Automated summary

A nonionic double hydrophilic block copolymer is synthesized using RAFT, with one hydrophilic and one thermoresponsive block. The copolymer acts as a polymeric surfactant at low temperatures, forming hairy micelles, and as an associative telechelic polymer at elevated temperatures, forming large dynamic aggregates, especially in a microemulsion.