The structure of poly(methyl methacrylate)-block-poly(n-butyl methacrylate) (PMMA-b-PnBMA) micelles in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), a selective solvent for the PMMA block, has been studied using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). A series of seven PMMA-b-PnBMA diblock copolymers were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization, in which the degree of polymerization of the PMMA block was kept constant while the PnBMA block length was varied. All the polymers formed spherical micelles at ambient temperature in dilute solution; their hydrodynamic radius (Rh) and core radius (Rc) were obtained by DLS and SAXS, respectively. It was found that Rc and the degree of polymerization of the core block, NB, followed a power law relationship in which Rc ∼ NB0.71±0.01. The corona thickness (Lcorona), given by the difference of Rh and Rc, does not show any apparent dependence on NB. These results were compared to scaling theory, and were found to be only in partial agreement with the star model proposed by Halperin et al. However, the mean-field calculations of micellar dimensions by Nagarajan and Ganesh were in excellent agreement with the data. This comprehensive experimental study provides precise quantification of the Rc and Lcorona dependence on core block lengths, due to the use of seven different block copolymers with identical corona block lengths.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (NSF) through Award DMR-1206459. The SAXS experiments were conducted on the DuPont?Northwestern?Dow Collaborative Access Team (DND-CAT) Synchrotron Research Center located at Sector 5 of the Advanced Photon Source. DND-CAT is supported by E. I. DuPont de Nemours & Co., Dow Chemical Company, the U.S. National Science Foundation through Grant DMR-9304725, and the State of Illinois through the Department of Commerce and the Board o Higher Education Grant IBHE HECA NWU 96. Use of the dvanced Photon Source was supported by the U.S. Depart-ment of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38.