Polymer Research Center
Arts & Sciences
• Gordon Research Conference
PINO2009 submission deadline July 9,
meeting in Spring 2009
ACS meeting in Autumn 2008,
Symposium on Polymer Chemistry 2008, Hefei China
National Meeting, Aug.2007 Boston
Conference on Frontier of Polymer and Advanced Materials.
ACS Mar.2007 Chicago
Research Society 3:45 PM C5.6
Hydrophobic Dielectrics of Fluoropolymer / BaTiO3
Nanocomposites for Low-Voltage and Charge-Storing Electrowetting Devices.
Murali K. Kilaru, Gui Lin, James E. Mark and Jason Heikenfeld; Univ.
of Cincinnati, Cincinnati,
The field of electrowetting is experiencing an explosion of interest in
applications for optics, electronic-paper, and lab-on-chip devices.
Electrowetting of an electrically-conductive liquid occurs when a bias is
applied between the liquid and an underlying dielectric-coated electrode. If
electrowetting is to achieve its fullest potential, new materials should be
developed. Critical needs include dielectrics with low-voltage operation
(higher capacitance) and reduced-contact angle saturation. Other
applications, such as electronic paper would benefit from development of new
bi-stable electrowetting approaches. Our working group is developing new
composite hydrophobic dielectrics consisting of high-k BaTiO3
nanopowder dispersed in fluoropolymer. The BaTiO3 nanopowder has a
mean particle diameter of 56 nm and is stabilized in fluoropolymer using a
fluorosilane. Thin ~1 µm films are prepared by spin coating of the composite
systems dissolved in fluorosolvent. Various compositions ranging from 1:0 to
1:37 vol. % fluoropolymer:BaTiO3 were evaluated. In order to
decrease the voltage required for electrowetting, films with increased BaTiO3
content exhibit a dielectric constant increase from ~2 (fluoropolymer) to ~20
(fluoropolymer: BaTiO3). The films with higher loading of BaTiO3
also show a desired increase in initial liquid contact angle from
θc~104° to 121°. According to the Cassie-Baxter relationship, this
corresponds to a surface fill factor of 0.38 for the 1:37 composition. Under
a DC bias of ~100 V, the film of 1:1 composition exhibits an improved minimum
wetting angle of θc~54° before electrowetting saturation occurs. This is
a 20° improvement over the θc~70° saturation observed for 1:0
composition of the same film thickness. Operating voltage for electrowetting
decreases by ~30 V for films with higher BaTiO3 content, but this
is less than that expected given the large increase in film capacitance.
Charge injection and storage in the composite has been observed as the
droplet remains wetted even after removing the applied voltage. Charge storage
time approaches ~120s before de-wetting transitions through θc~90°. This
is partly due to the 50X increase in surface area between the fluorpolymer
and BaTiO3 in the nanocomposite as compared to a film of
fluoropolymer laminated on continuous film of BaTiO3. Applying a
smaller and reverse-polarity voltage results in instantaneous de-wetting.
This is a major step forward for creating electrowetting pixels in displays
and e-paper which can hold their optical state even after the voltage is
removed. Also attractive for reflective e-paper displays, a diffuse
reflectance of ~50% is achieved for films based on a 1:4 nanocomposition. The
authors gratefully acknowledge support provided by the Univ.
of Cincinnati Institute for Nanoscale Science and Technology.