Association of fluorocarbon and hydrocarbon end-capped poly(ethylene glycol)s: NMR and fluorescence studies

Poly(ethylene glycol)-based associating polymers with fluorocarbon and hydrocarbon hydrophobes (F-PEG & H-PEG) were synthesized. Their association behavior in aqueous solutions was investigated by NMR and a fluorescent probe technique. NMR results demonstrate that besides the hydrophobe, the urethane linkage connecting the hydrophobe and PEG chain is incorporated into the hydrophobic core as well. Thus, the F-PEG micelles consisting of both the fluorocarbon chain and urethane linkage possess a dual WP character, whereas a pure H one exists for H-PEG micelles. Three types of fluorescent probes, that is, pyrene, 9-(anthrylmethyloxymethyl)pyrene (AMOP), and 1-(perfluorooctanoyl)pyrene (PyCORf), have been used to monitor the association, with emphasis on comparing their probing abilities for different microdomains. As for H-PEG, both pyrene and AMOP are effective in monitoring the association and measuring the critical. aggregation concentration (CAC). In addition, AMOP presents information about the microviscosity of the micelles. Nevertheless, pyrene and AMOP cannot serve as effective probes for the association of small-molecular fluorocarbon surfactant FC143 because of their limited solubility in pure fluorocarbon microdomains. Compared with the surfactant case, the probing abilities of both pyrene and AMOP are markedly improved for F-PEG. This is attributed to the dual H/F character of F-PEG micelles, which enhances their affinity to the probes. PyCORf, the probe carrying a fluorocarbon chain, is effective and informative in the studies of F-PEG association as a result of its good affinity to fluorocarbon microdomains. The variation of excimer intensity of PyCORf at 465 nm is suggested as a criterion for association and estimating the CAC of F-PEG.