2 research outputs found
Resistance of POSS Polyimide Blends to Hyperthermal Atomic Oxygen Attack
Copolymers
of polyhedral oligomeric silsesquioxane (POSS) and polyimide (PI)
have shown remarkable resistance to atomic oxygen (AO) attack and
have been proposed as replacements for Kapton on the external surfaces
of spacecraft in the harsh oxidizing environment of low Earth orbit
(LEO). POSS PI blends would be an economical alternative to the copolymers
if they also resisted AO attack. Thus, blends of trisilanolphenyl
(TSP) POSS and PI with different weight percentages of the Si<sub>7</sub>O<sub>9</sub> POSS cage were cast into films and exposed to
a hyperthermal AO beam, and they were characterized in terms of their
recession, mass loss, surface morphology, and surface chemistry. In
order to compare the AO resistance of the blends with POSS PI copolymers,
samples of previously studied copolymers were also investigated in
parallel with the blends. For all POSS PI materials, the AO resistance
increased with increasing AO fluence and with increasing POSS cage
loading. At similar POSS cage loadings and exposure conditions, the
TSP POSS PI blends showed comparable erosion yields to the POSS PI
copolymers, with specific samples of blends and copolymers achieving
erosion yields as low as 0.066 Ă 10<sup>â24</sup> cm<sup>3</sup> atom<sup>â1</sup> with an AO fluence of 5.93 Ă
10<sup>20</sup> O atoms cm<sup>â2</sup>. SEM and XPS analyses
indicated that passivating SiO<sub><i>x</i></sub> layers
were formed on the surfaces of all POSS-containing polymers during
AO exposure. Thus, a TSP POSS PI blend is proposed as a low-cost variant
of a POSS polyimide for use in extreme oxidizing environments, such
as LEO
Reactive-Atom Scattering from Liquid Crystals at the LiquidâVacuum Interface: [C<sub>12</sub>mim][BF<sub>4</sub>] and 4âCyano-4â˛-Octylbiphenyl (8CB)
Two complementary
approaches were used to study the liquidâvacuum
interface of the liquid-crystalline ionic liquid 1-dodecyl-3-methylÂimidazolium
tetrafluoroborate ([C<sub>12</sub>mim]Â[BF<sub>4</sub>]) in the smectic
A (SmA) and isotropic phases. O atoms with two distinct incident translational
energies were scattered from the surface of [C<sub>12</sub>mim]Â[BF<sub>4</sub>]. Angle-dependent time-of-flight distributions and OH yields,
respectively, were recorded from high- and low-energy O atoms. There
were no significant changes in the measurements using either approach,
nor the properties derived from them, accompanying the transition
from the SmA to the isotropic phase. This indicates that the surface
structure of [C<sub>12</sub>mim]Â[BF<sub>4</sub>] remains essentially
unchanged across the phase boundary, implying that the bulk order
and surface structure are not strongly correlated for this material.
This effect is ascribed to the strong propensity for the outer surfaces
of ionic liquids to be dominated by alkyl chains, over an underlying
layer rich in anions and cation head groups, whether or not the bulk
material is a liquid crystal. In a comparative study, the OH yield
from the surface of the liquid crystal, 8CB, was found to be affected
by the bulk order, showing a surprising step increase at the SmAânematic
transition temperature, whose origin is the subject of speculation