Browsing by Author "Perrier, S"

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    Ordered microphase separation in thin films of PMMA-PBA synthesized by RAFT: effect of block polydispersity
    (American Chemical Society, 2009-04-28) Sriprom, W; James, M; Perrier, S; Neto, C
    The microphase separation of diblock copolymers synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization, containing one monodisperse block (poly(methyl methacrylate), PMMA) and one polydisperse block (poly(butyl acrylate), PBA), was investigated in thin films (<100 nm). The formation of ordered microphase-separated domains was observed by atomic force microscopy (AFM) and resulted in four morphologies, depending on composition and film thickness: parallel lamellae, hexagonally packed perforated lamellae (PL), parallel cylinders (C∥), and hexagonally packed spheres, and in C∥-to-PL-to-C∥ transitions. Polydispersity of the PBA block shifts the phase boundaries toward higher PBA volume fraction values with respect to those expected for monodisperse block copolymers and stabilizes the perforated lamella morphology. Neutron reflectivity data confirmed that lamellae parallel to the substrate form at a very low PBA volume fraction, fPBA = 0.23. Polydispersity of the PBA block also has the effect of stabilizing each microphase domain over a film thickness regime larger than expected for monodisperse blocks. For the first time RAFT-polymerized block copolymers are shown to microphase separate with high reproducibility and with excellent degree of order, hence proving to be ideal systems to test the effect of polydispersity on microphase separation. © 2009, American Chemical Society
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    Temperature- and pH-responsive micelles with collapsible poly(N-isopropylacrylamide) headgroups
    (ACS Publications, 2014-06-18) FitzGerald, PA; Gupta, S; Wood, K; Perrier, S; Warr, GG
    We have studied the micelle formation and phase behavior of a series of temperature- and pH-responsive surfactants prepared by controlled radical (RAFT) polymerization. These C12NIPAMm surfactants consist of a dodecyl tail, a poly(N-isopropylacrylamide) (polyNIPAM) headgroup with average degrees of polymerization of between 7 and 96, and an ionizable carboxylate group. In the un-ionized state, these surfactants phase separate on warming toward a lower critical solution temperature (LCST), which decreases as the length of the NIPAM group is decreased. This is in agreement with the behavior of conventional nonionic poly(ethylene oxide)-based surfactants but is very different from that of polyNIPAM oligomer solutions. Small angle neutron scattering (SANS) shows that these surfactants self-assemble into micelles consisting of a nearly spherical hydrophobic core surrounded by a “hairy” polyNIPAM shell far below their LCST. Upon warming, the micelles undergo a sphere-to-rod transition induced by the collapse of the polyNIPAM shell, causing a reduction in the headgroup area. In the un-ionized state the demixing follows at the LCST, but a single charge on the free polymer end completely suppresses phase separation, allowing micelles to undergo a shape change but remain dissolved. © 2014, American Chemical Society.