title: Effect of side-chain asymmetry on the intermolecular structure and order-disorder transition in alkyl-substituted polyfluorenes.
abstract: We study relations among the side-chain asymmetry, structure, and order-disorder transition (ODT) in hairy-rod-type poly(9,9-dihexylfluorene) (PF6) with two identical side chains and atactic poly(9-octyl-9-methyl-fluorene) (PF1-8) with two different side chains per repeat. PF6 and PF1-8 organize into alternating side-chain and backbone layers that transform into an isotropic phase at T^{ODT}(PF6) and T_{bi}^{ODT}(PF1-8). We interpret polymers in terms of monodisperse and bidisperse brushes and predict scenarios T^{ODT}<T_{bi}^{ODT} and T^{ODT}∼T_{bi}^{ODT} for high and low grafting densities (the side-chain length above or below the average grafting distance). Calorimetry and x-ray scattering indicate the condition T^{ODT}(PF6)∼T_{bi}^{ODT}(PF1-8) following the low grafting prediction. PF6 side chains coming from the alternating backbone layers appear as two separate layers with thickness H(PF6), whereas PF1-8 side chains appear as an indistinguishable bilayer with a half thickness H_{bilayer}(PF1-8)/2≈H(PF6). The low grafting density region is structurally possible but not certain for PF6 and confirmed for PF1-8.
authors.affiliation: Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Knaapila
authors.affiliation: Materials Science Centre, DSM Research, 6160 MD Geleen, The Netherlands.
authors.forename: R
authors.initials: R
authors.lastname: Stepanyan
authors.affiliation: Department of Physics, University of Helsinki, 00014 Helsinki, Finland.
authors.forename: M
authors.initials: M
authors.lastname: Torkkeli
authors.affiliation: MAX IV Laboratory, Lund University, 22100 Lund, Sweden.
authors.forename: D
authors.initials: D
authors.lastname: Haase
authors.affiliation: Macromolecular Chemistry Group (buwmakro), Bergische Universität Wuppertal, 42119 Wuppertal, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Fröhlich
authors.affiliation: Macromolecular Chemistry Group (buwmakro), Bergische Universität Wuppertal, 42119 Wuppertal, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Helfer
authors.affiliation: Macromolecular Chemistry Group (buwmakro), Bergische Universität Wuppertal, 42119 Wuppertal, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Forster
authors.affiliation: Macromolecular Chemistry Group (buwmakro), Bergische Universität Wuppertal, 42119 Wuppertal, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Scherf