Formation of Single-Crystal-like Poly(9,9-dioctylfluorene) Thin Film by the Friction-Transfer Technique with Subsequent Thermal Treatments

Abstract

A liquid-crystalline polymer, poly(9,9-dioctylfluorene) (PFO), was found to form highly oriented films by a friction-transfer technique. The polarized UV−vis absorption and photoluminescence spectra of the films formed showed strong dichroism with a dichroic ratio of approximately 10 in the drawing direction of friction transfer. Subsequent thermal treatments of the friction-transferred PFO films were specifically effective for the improvement of the physical properties, structure, and morphology of the films. By rapid and slow cooling from the liquid-crystalline melted states of the friction-transferred films, liquid-crystalline and crystalline films were prepared, respectively. Both thermally treated films showed enhanced optical anisotropy with a dichroic ratio of approximately 25 in photoluminescence spectra. The electron diffraction pattern of the crystalline film showed a large number of sharp diffraction spots as seen in a single crystal. The liquid-crystalline polymer was found to form a single-crystal-like thin film by the friction-transfer technique with subsequent thermal treatments.

Single-Crystal-like Structure of Poly(9,9-dioctylfluorene) Thin Films Evaluated by Synchrotron-Sourced Grazing-Incidence X-ray Diffraction

“…This indicates that the alkyl side chains (b-axis) are oriented orthogonal to the friction direction. All of the results of in-plane GIXD are in full agreement with those of electron diffraction; 25 the q x and q y scans in GIXD correspond to the meridian and equator in electron diffraction, respectively.…”

“…We assume the mechanism of above-mentioned perfect orientation as follows. 25 First, polymer chains close to the substrate are probably highly oriented and firmly fixed to the substrate by the friction-transfer technique. Second, the orientation of the immobilized polymer chains at the interface induces the homogeneous bulk alignment above the LC transition temperature.…”

“…[19][20][21][22][23][24] Recently, highly oriented thin films of poly(9,9-dioctylfluorene) (PFO) was found to be formed by this technique. 25 Furthermore, we have succeeded in fabrication of polarized polymer LEDs (polarized PLEDs) utilizing the oriented PFO films. 26 Evaluation of film structure is important for the application of oriented films to optoelectronic devices.…”

“…26 Evaluation of film structure is important for the application of oriented films to optoelectronic devices. In our previous study, 25 we investigated the structure of friction-transferred PFO films by using transmission electron microscope (TEM) and found that subsequent thermal treatments were specifically effective for improvement of molecular orientation. In this study, we have quantitatively investigated the distribution of molecular orientation by using synchroy To whom correspondence should be addressed (Tel: +81-29-861-6252, Fax: +81-29-861-6303, E-mail: masahiro.misaki@aist.go.jp, yuji.yoshida@aist.go.jp).…”

Enhanced charge-carrier mobility inβ-phase polyfluorene

“…5,6 Furthermore, crystalline or ␣-phase regions can form in the glassy PFO matrix upon specific thermal treatment. 7,8 With this morphological diversity, PFO offers a unique possibility to study the optoelectronic properties for different supramolecular organizations, without the need to chemically modify the polymer. The extent to which the conductive properties of conjugated polymers are determined by the order on a supramolecular scale is nicely illustrated by the hole mobility in glassy films of PFO: The ͑al-ready relatively high͒ time of flight mobility of 3 ϫ 10 −4 cm 2 / V s for isotropic glassy films 3 can be increased by an order of magnitude to 8.5ϫ 10 −3 cm 2 / V s by homogeneous alignment.…”