4 research outputs found
Cross-Linked Block Copolymer/Ionic Liquid Self-Assembled Blends for Polymer Gel Electrolytes with High Ionic Conductivity and Mechanical Strength
PolyÂ(propylene
oxide)–polyÂ(ethylene oxide)–polyÂ(propylene oxide) (PPO–PEO–PPO)
block copolymers (BCPs) with cross-linkable end groups were synthesized,
blended with an ionic liquid (IL) diluent, and cross-linked to form
polymer gel electrolytes. The IL prevented crystallization of PEO
at high concentrations, enabling fast ion transport. In addition,
the IL was selective for the PEO block, inducing strong microphase
separation in what are otherwise disordered or weakly ordered BCP
melts. Cross-linking the BCPs in the presence of the IL resulted in
the formation of solid, elastic gels with high ionic conductivitiesî—¸greater
than 1.0 mS/cm at 25 °C for some compositions. However, it was
found that neither the presence or absence of microphase separation
nor the BCP composition of the microphase separated gels substantially
influenced ionic conductivity. Increasing the cross-link density through
the use of phase-selective PEO- and PPO-based cross-linking reagents
was also evaluated. It was revealed that confinement of cross-links
to the PPO rich domains through the use of PPO-based diacrylates enhanced
the mechanical strength of the gels without detriment to the ionic
conductivity. Conversely, cross-linking in the PEO-rich domains through
the use of PEO-based acrylates significantly reduced conductivity.
Isolation of cross-links within a minor nonconducting domain in a
microphase separated gel is a viable strategy for mechanical property
enhancement without a large sacrifice in conductivity, effectively
decoupling ionic conductivity and mechanical strength. This approach
yielded solid-like gel electrolytes fabricated from BCPs that can
be produced inexpensively, with ionic conductivities of 0.64 mS/cm
at 25 °C and a frequency independent storage modulus of approximately
400 kPa
Nanostructured Block-Random Copolymers with Tunable Magnetic Properties
It was recently shown that block copolymers (BCPs) produced
room-temperature
ferromagnetic materials (RTFMs) due to their nanoscopic ordering and
the cylindrical phase yielded the highest coercivity. Here, a series
of metal-containing block-random copolymers composed of an alkyl-functionalized
homo block (C<sub>16</sub>) and a random block of cobalt complex-
(Co) and ferrocene-functionalized (Fe) units was synthesized via ring-opening
metathesis polymerization. Taking advantage of the block-random architecture,
the influence of dipolar interactions on the magnetic properties of
these nanostructured BCP materials was studied by varying the molar
ratio of the Co units to the Fe units, while maintaining the cylindrical
phase-separated morphology. DC magnetic measurements, including magnetization
versus field, zero-field-cooled, and field-cooled, as well as AC susceptibility
measurements showed that the magnetic properties of the nanostructured
BCP materials could be easily tuned by diluting the cobalt density
with Fe units in the cylindrical domains. Decreasing the cobalt density
weakened the dipolar interactions of the cobalt nanoparticles, leading
to the transition from a room temperature ferromagnetic (RTF) to a
superparamagnetic material. These results confirmed that dipolar interactions
of the cobalt nanoparticles within the phase-separated domains were
responsible for the RTF properties of the nanostructured BCP materials
Binary Short-Range Colloidal Assembly of Magnetic Iron Oxides Nanoparticles and Fullerene (nC<sub>60</sub>) in Environmental Media
Colloidal
assembly of nC<sub>60</sub> fullerene with naturally
abundant magnetic iron oxide NPs will affect their fate and transformation
in environmental media. In solution, fullerene association to aggregating
iron oxide NPs/clusters greatly enhanced the overall colloidal stability.
Development of depletion-mediated structured fullerene layers between
pure and surface modified ÎłFe<sub>2</sub>O<sub>3</sub> NPs possibly
resulted in such stabilization. Here, we also report that on air–water
interface, association of fullerene to pure and humic acid (HA7) coated
ÎłFe<sub>2</sub>O<sub>3</sub> NPs led to the formation of ordered
assemblies, e.g., binary wires and closed-packed “crystalline”
and “glassy” structures in the presence and absence
of electrolytes suggesting immobilization of the former. The interaction
of fullerene to Fe<sub>3</sub>O<sub>4</sub> NPs and clusters also
produced ordered assemblies along with amorphous aggregates. Fullerene
interaction with Fe<sub>3</sub>O<sub>4</sub> NPs in low concentration
of HA1 and Na<sup>+</sup> at pH 6 formed dendritic growth and polycrystalline
circular assemblies on air–water interface. HRTEM study further
revealed that the monolayer circular assemblies were highly ordered
but structural degeneracy was evident in multilayers. Therefore, interfacial
assemblies of fullerene with iron oxide NPs resulted in short-range
periodic structures with concomitant immobilization and reduction
in availability of the former, especially in soils or sediments rich
in the latter
Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability
Solar
cells fabricated using alkyl ammonium metal halides as light
absorbers have the right combination of high power conversion efficiency
and ease of fabrication to realize inexpensive but efficient thin
film solar cells. However, they degrade under prolonged exposure to
sunlight. Herein, we show that this degradation is quasi-reversible,
and that it can be greatly lessened by simple modifications of the
solar cell operating conditions. We studied perovskite devices using
electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-,
formamidinium (FA)-, and MA<sub><i>x</i></sub>FA<sub>1–<i>x</i></sub> lead triiodide as active layers. From variable temperature
EIS studies, we found that the diffusion coefficient using MA ions
was greater than when using FA ions. Structural studies using powder
X-ray diffraction (PXRD) show that for MAPbI<sub>3</sub> a structural
change and lattice expansion occurs at device operating temperatures.
On the basis of EIS and PXRD studies, we postulate that in MAPbI<sub>3</sub> the predominant mechanism of accelerated device degradation
under sunlight involves thermally activated fast ion transport coupled
with a lattice-expanding phase transition, both of which are facilitated
by absorption of the infrared component of the solar spectrum. Using
these findings, we show that the devices show greatly improved operation
lifetimes and stability under white-light emitting diodes, or under
a solar simulator with an infrared cutoff filter or with cooling