Staff profile
Publication details for Professor Andy Monkman
Knaapila, M, Stepanyan, R, Lyons, BP, Torkkeli, M, Hase, TPA, Serimaa, R, Guntner, R, Seeck, OH, Scherf, U & Monkman, AP (2005). The influence of the molecular weight on the thermotropic alignment and self-organized structure formation of branched side chain hairy-rod polyfluorene in thin films. Macromolecules 38(7): 2744-2753.- Publication type: Journal papers: academic
- ISSN/ISBN: 0024-9297
- Keywords: LIQUID-CRYSTALLINE POLYFLUORENE; BLUE POLARIZED ELECTROLUMINESCENCE; STRUCTURE-PROPERTY RELATIONSHIPS; FIELD-EFFECT TRANSISTORS; X-RAY-DIFFRACTION; CONJUGATED POLYMERS; COPOLYMER FILMS; PHASE-BEHAVIOR; MORPHOLOGY; PHOTOLUMINESCENCE
Author(s) from Durham
Abstract
We report on the influence of the molecular weight (M) on the alignment
and structure of poly(9,9-bis(ethylhexyl)fluorene-2,7-diyl) (PF2/6) in
thin films on rubbed polyimide in the equilibrium. The degree of
alignment has been studied using optical spectroscopy and compared to
theoretical arguments. The structure of PF2/6 has been studied using
grazing-incidence X-ray diffraction. PF2/6 realizes a threshold
molecular weight M-n* = 10(4) g/mol separating a low M-n (LMW, M-n <
M-n*) and a high M-n (HMW, M-n > M-n*) region. LMW materials show only
the nematic (Nem) phase while HMW compounds show hexagonal (Hex) and
Nem phases at low and high temperatures, respectively. LMW samples
align equally well at any temperature above the glass transition, and
the dichroic ratio, R, increases with M-n. HMW material aligns well in
the Nem regime, and R drops exponentially with M-n, which is in
agreement with the theory. The orientational order is maximized near
M-n*, and the orientational. order parameter is > 0.9. HMW samples,
whether aligned or not, reveal two kinds of coexistent Hex crystallites
distributed in the sample plane having one crystal axis a perpendicular
or parallel to the surface. Local order within the triaxially aligned
Hex phase surpasses that of the in-plane aligned Hex phase.