3 research outputs found

    Chain Exchange Kinetics in Diblock Copolymer Micelles in Ionic Liquids: The Role of χ

    No full text
    Chain exchange kinetics of diblock copolymer micelles with lower critical micellization temperature (LCMT) phase behavior were investigated using time-resolved small-angle neutron scattering (TR-SANS). Three nearly identical isotopically substituted pairs of poly­(methyl methacrylate)-<i>block</i>-poly­(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) diblocks were used in mixtures of the room temperature ionic liquids 1-ethyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide and 1-butyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide. In this case, the <i>h-</i>PnBMA and <i>d</i><sub>9</sub>-PnBMA blocks form the micellar cores. The results are consistent with previous measurements in other systems, in that the barrier to chain extraction scales linearly with the core block length. By varying the ratio of the two homologous solvents in the mixture, the value of χ between the core block and the solvent is varied systematically. The results show that the solvent selectivity has a remarkable effect on the chain exchange rate, as anticipated by a previous theory. However, in contrast to an assumption in previous studies, we find that the barrier to chain exchange is not simply proportional to χ. Accordingly, we propose a more elaborate function of χ for the energy barrier, which is rationalized by a calculation in the spirit of Flory–Huggins theory. This modification can account for the chain exchange behavior when χ is relatively modest, i.e., in the vicinity of the critical micelle temperature

    Poly(methyl methacrylate)-<i>block</i>-poly(<i>n</i>-butyl methacrylate) Diblock Copolymer Micelles in an Ionic Liquid: Scaling of Core and Corona Size with Core Block Length

    No full text
    The structure of poly­(methyl methacrylate)-<i>block</i>-poly­(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-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-<i>b</i>-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 (<i>R</i><sub>h</sub>) and core radius (<i>R</i><sub>c</sub>) were obtained by DLS and SAXS, respectively. It was found that <i>R</i><sub>c</sub> and the degree of polymerization of the core block, <i>N</i><sub>B</sub>, followed a power law relationship in which <i>R</i><sub>c</sub> ∼ <i>N</i><sub>B</sub><sup>0.71±0.01</sup>. The corona thickness (<i>L</i><sub>corona</sub>), given by the difference of <i>R</i><sub>h</sub> and <i>R</i><sub>c</sub>, does not show any apparent dependence on <i>N</i><sub>B</sub>. 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 <i>R</i><sub>c</sub> and <i>L</i><sub>corona</sub> dependence on core block lengths, due to the use of seven different block copolymers with identical corona block lengths

    Exchange Kinetics for a Single Block Copolymer in Micelles of Two Different Sizes

    No full text
    The effect of micellar size on the chain exchange kinetics in spherical micelles consisting of poly­(methyl methacrylate)-<i>block</i>-poly­(<i>n</i>-butyl methacrylate) (PMMA-<i>b</i>-PnBMA) in a mixture of ionic liquids (1-ethyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide, [EMIM]­[TFSI], and 1-butyl-3-methyl­imidazolium bis­(trifluoro­methyl­sulfonyl)­imide, [BMIM]­[TFSI]) was investigated using time-resolved small-angle neutron scattering (TR-SANS). Two spherical micelles with different core sizes were prepared from a single block copolymer by using different protocols. In one case the micelles were formed in the presence of a cosolvent, while in the other a polymer thin film was directly dissolved in the ionic liquid. Interestingly, the micelle core size prepared from the latter method is ∼30% larger than that obtained in the former case. TR-SANS experiments reveal that the rate of single chain exchange in the micelles with a larger core size is slowed down by ∼3 times compared to the smaller core radius. This can be possibly attributed to the smaller interfacial area per chain, and larger corona density, for micelles with a larger core dimension. These geometrical factors can potentially lead to changes in both the attempt time and activation barrier for chain expulsion during the unimer exchange process. Our results clearly suggest that, in addition to the molecular characteristics of the block copolymer and solvent, the geometrical structure of the micelle plays an important role in the unimer dynamic exchange processes in block copolymer micelles
    corecore