title: Structural study of helical polyfluorene under high quasihydrostatic pressure.
abstract: We report on an x-ray diffraction (XRD) study of helical poly[9,9-bis(2-ethylhexyl)fluorene] (PF2/6) under high quasihydrostatic pressure and show an effect of pressure on the torsion angle (dihedral angle) between adjunct repeat units and on the hexagonal unit cell. A model for helical backbone conformation is constructed. The theoretical position for the most prominent 00l x-ray reflection is calculated as a function of torsion angle. The XRD of high molecular weight PF2/6 (M(n)=30 kg/mol) is measured through a diamond anvil cell upon pressure increase from 1 to 10 GPa. The theoretically considered 00l reflection is experimentally identified, and its shift with the increasing pressure is found to be consistent with the decreasing torsion angle between 2 and 6 GPa. This indicates partial backbone planarization towards a more open helical structure. The h00 peak is identified, and its shift together with the broadening of 00l implies impairment of the ambient hexagonal order, which begins at or below 2 GPa. Previously collected high-pressure photoluminescence data are reanalyzed and are found to be qualitatively consistent with the XRD data. This paper provides an example of how the helical π-conjugated backbone structure can be controlled by applying high quasihydrostatic pressure without modifications in its chemical structure. Moreover, it paves the way for wider use of high-pressure x-ray scattering in the research of π-conjugated polymers.
authors.affiliation: Physics Department, Institute for Energy Technology, Kjeller NO-2027, Norway. matti.knaapila@ife.no
authors.forename: M
authors.initials: M
authors.lastname: Knaapila
authors.forename: Z
authors.initials: Z
authors.lastname: Konôpková
authors.forename: M
authors.initials: M
authors.lastname: Torkkeli
authors.forename: D
authors.initials: D
authors.lastname: Haase
authors.forename: H-P
authors.initials: HP
authors.lastname: Liermann
authors.forename: S
authors.initials: S
authors.lastname: Guha
authors.forename: U
authors.initials: U
authors.lastname: Scherf
mesh_terms: D003198:Computer Simulation; D005449:Fluorenes / Q000737:chemistry*; D006874:Hydrostatic Pressure; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation
chemical_list: D005449:Fluorenes