11 research outputs found
Decoupling Mechanical and Conductive Dynamics of Polymeric Ionic Liquids via a Trivalent Anion Additive
The mechanical and
conductive properties of a polymeric ionic liquid
(PIL) are decoupled through the addition of a fraction of trivalent
anions to a chloride single-ion conductor. Trivalent phosphate ions
strongly coordinate with polymer-bound imidazoliums, producing an
increase in both the ionic conductivity and the polymer viscosity.
Both the viscosity and the ionic conductivity increase with phosphate
content, and the conductivity is superior to that of the neat PIL
at larger trivalent anion concentrations. The interaggregate spacing
(determined by X-ray scattering), glass transition temperature (measured
by calorimetry), and free volume (estimated by rheology) are each
sensitive to the presence of trivalent ions but not to changes in
the phosphate concentration. Thus, the presence of a fraction of trivalent
anions qualitatively changes the structure and interaction of ions,
resulting in modified macroscopic properties of the PIL. We hypothesize
that this step change in properties upon introducing phosphate ions
is due to a densification of ion aggregates by the trivalent ion,
which strongly binds to imidazolium ions. This provides a new mechanism
for creating PILs with tailored conductive and rheological behavior
Visualizing ion transport in polymers via ion-chromic indicators
There is growing interest in polymers with high ionic
conductivity
for applications including batteries, fuel cells, and separation membranes.
However, measuring ion diffusion in polymers can be challenging, requiring
complex procedures and instrumentation. Here, a simple strategy to
study ion diffusion in polymers is presented that utilizes ion-chromic
spiropyan as an indicator to measure the diffusion of LiTFSI, KTFSI,
and NaTFSI within poly(ethylene oxide)-based polymer networks. These
systems are selected, as these are common ions and polymers used in
energy storage applications, however, the approach described is not
specific to materials for energy storage. Specifically, to enabling
the study of ion diffusion, these salts cause the spiropyran to undergo
an isomerization reaction, which results in a significant color change.
This colorimetric response enables the determination of the diffusion
coefficients of these ions within films of these polymers simply by
optically tracking the spatial-temporal evolution of the isomerization
product within the film and fitting the data to the relevant diffusion
equations. The simplicity of the method makes it amenable to the study
of ion diffusion in polymers under a range of conditions, including
various temperatures and under macroscopic deformation
Molecular-Weight Dependence of Center-of-Mass Chain Diffusion in Polymerized Ionic Liquid Melts
Polymerized ionic liquids (PILs) with flexible polymer
chains and
weakly interacting ionic liquid (IL) groups have received great attention
for their desirable properties in electrochemical applications such
as ionic conductivity. Less is known about their dynamic properties
such as center-of-mass chain diffusion and how it depends on molecular
weight in the presence of IL groups. In this work, a series of acrylic
PILs with imidazolium cations and bis(trifluoromethanesulfonyl)imide
(TFSI) anions (TFSI-f-PILN) were synthesized
via reversible additionâfragmentation chain-transfer polymerization
with degrees of polymerization N ranging from 40
to 236. A fluorescent acrylic monomer with the 7-nitrobenzofurazan
group was copolymerized at trace levels as a probe of chain motion,
and the diffusion coefficient (D) of TFSI-f-PILN was determined by fluorescence recovery
after photo bleaching at Tg + 45 K. Within
the uncertainty of 3â20%, a scaling relationship of D ⌠Nâ2 was observed
which is the same as the scaling of linear neutral polymers. Wide-angle
X-ray scattering exhibited no peak at âŒ5 nmâ1, indicating no long-range imidazolium-TFSI ionic correlations. Our
results indicate that the molecular weight dependence of center-of-mass
diffusion is not affected by electrostatic interactions of IL groups.
No transition from a Rouse regime (D ⌠Nâ1) to reptation regime (D ⌠Nâ2) was observed within
the studied N range
Ion Transport in Dynamic Polymer Networks Based on MetalâLigand Coordination: Effect of Cross-Linker Concentration
The
development of high-performance ion conducting polymers requires
a comprehensive multiscale understanding of the connection between
ionâpolymer associations, ionic conductivity, and polymer mechanics.
We present polymer networks based on dynamic metalâligand coordination
as model systems to illustrate this relationship. The molecular design
of these materials allows for precise and independent control over
the nature and concentration of ligand and metal, which are molecular
properties critical for bulk ion conduction and polymer mechanics.
The model system investigated, inspired by polymerized ionic liquids,
is composed of polyÂ(ethylene oxide) with tethered imidazole moieties
that facilitate dissociation upon incorporation of nickelÂ(II) bisÂ(trifluoroÂmethylsulfonyl)Âimide.
Nickelâimidazole interactions physically cross-link the polymer,
increase the number of elastically active strands, and dramatically
enhance the modulus. In addition, a maximum in ionic conductivity
is observed due to the competing effects of increasing ion concentration
and decreasing ion mobility upon network formation. The simultaneous
enhancement of conducting and mechanical properties within a specific
concentration regime demonstrates a promising pathway for the development
of mechanically robust ion conducting polymers
Harvesting Waste Heat in Unipolar Ion Conducting Polymers
The
Seebeck effect in unipolar ion-conducting, solid-state polymers
is characterized. The high Seebeck coefficient and sign in polymer
ion conductors is explained via analysis of thermogalvanic multicomponent
transport. A solid-state, water-processeable, flexible device based
on these materials is demonstrated, showcasing the promise of polymers
as thermogalvanic materials. Thermogalvanic materials based on ion-conducting
polymer membranes show great promise in the harvesting of waste heat
Molecular Design of Multimodal Viscoelastic Spectra Using Vitrimers
Imparting
multiple, distinct dynamic processes at precise time
scales in polymers is a grand challenge in soft materials design with
implications for applications including electrolytes, adhesives, tissue
engineering, and additive manufacturing. Many competing factors, including
the polymer architecture, molecular weight, backbone chemistry, and
presence of a solvent, affect the local and global dynamics and in
many cases are interrelated. One approach to imparting distinct dynamic
processes is through the incorporation of dynamic bonds with widely
varying kinetics of bond exchange. Here, statistically cross-linked
polymer networks are synthesized with mixed fast and slow dynamic
bonds with 3 orders of magnitude different exchange kinetics. Oscillatory
shear rheology shows that the single component networks (either fast
or slow) exhibit a single relaxation peak while mixing fast and slow
cross-linkers in one network produces two peaks in the relaxation
spectrum. This is in stark contrast to telechelic networks with the
same mixture of dynamic bonds, where only one mixed mode is observed,
and here we provide molecular design guidelines for having each dynamic
bond contribute a distinct relaxation mode. By comparing the polymer
architecture and the difference in the number of dynamic bonds per
chain, we have elucidated the role of network architecture in imparting
multimodal behavior in dynamic networks. A highly tunable and recyclable
material has been developed with control of rubbery plateau modulus
(through cross-link density), relaxation peak locations and ratio
(through cross-linker selection and molar fractions), and tan ÎŽ
(through the relationships of the rubbery plateau and relaxation peak
locations)
StructureâConductivity Relationships of Block Copolymer Membranes Based on Hydrated Protic Polymerized Ionic Liquids: Effect of Domain Spacing
Elucidating the relationship between
chemical structure, morphology,
and ionic conductivity is essential for designing novel high-performance
materials for electrochemical applications. In this work, the effect
of lamellar domain spacing (<i>d</i>) on ionic conductivity
(Ï) is investigated for a model system of hydrated diblock copolymer
based on a protic polymerized ionic liquid. We present a strategy
that allows for the synthesis of a well-defined series of narrowly
dispersed PS-<i>b</i>-PIL with constant volume fraction
of ionic liquid moieties (<i>f</i><sub>IL</sub> â
0.39) and with two types of mobile charge carriers: trifluoroacetate
anions and protons. These materials self-assemble into ordered lamellar
morphologies with variable domain spacing (ca. 20â70 nm) as
demonstrated by small-angle X-ray scattering. PS-<i>b-</i>PIL membranes exhibit ionic conductivities above 10<sup>â4</sup> S/cm at room temperature, which are independent of domain spacing
consistent with their nearly identical water content. The conductivity
scaling relationship demonstrated in this paper suggests that a mechanically
robust membrane can be designed without compromising its ability to
transport ions. In addition, PIL-based membranes exhibit low water
uptake (λ â 10) in comparison with many proton-conducting
systems reported elsewhere. The low water content of the materials
described herein makes them promising candidates for electrochemical
devices operating in aqueous electrolytes at low current densities
where moderate ion conduction and low product crossover are required
Anisotropic Thermal Transport in Thermoelectric Composites of Conjugated Polyelectrolytes/Single-Walled Carbon Nanotubes
We
report a method to determine the thermal conductivities of polymer
composites with single-walled carbon nanotubes (SWNTs) using time-domain
thermoreflectance. Both through-plane and in-plane thermal conductivities
were determined. Two types of CPEs used in these studies are of the
same conjugated backbone but with either cationic (CPE-PyrBIm<sub>4</sub>) or anionic (CPE-Na) pendant functionalities. The CPE-Na/SWNT
composites are p-type conductors, whereas the CPE-PyrBIm<sub>4</sub>/SWNT counterparts exhibit n-type charge transport. The CPE/SWNT
films were prepared through a filtration method that preferentially
aligns the SWNTs in the in-plane direction. Attaching the composites
onto glass substrates with a precoated heat transducer allows one
to measure the through-plane thermal conductivity of materials with
rough surfaces. The in-plane thermal conductivity can be measured
by embedding thick samples into epoxy followed by microtoming to expose
the relatively smooth cross sections. The thermal conductivity along
the in-plane direction is found to be higher than that along the through-plane
direction. Indeed, the anisotropy factor of thermal conductivity in
these composites is approximately an order of magnitude, favoring
in-plane direction
Colloidal Semiconductor Quantum Dots with Tunable Surface Composition
Colloidal CdS quantum dots (QDs) were synthesized with
tunable
surface composition. Surface stoichiometry was controlled by applying
reactive secondary phosphine sulfide precursors in a layer-by-layer
approach. The surface composition was observed to greatly affect photoluminescence
properties. Band edge emission was quenched in sulfur terminated CdS
QDs and fully recovered when QDs were cadmium terminated. Calculations
suggest that electronic states inside the band gap arising from surface
sulfur atoms could trap charges, thus inhibiting radiative recombination
and facilitating nonradiative relaxation
Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes
Vitrimers are an important class of materials offering
advantages
over conventional thermosets due to their self-healing properties
and reprocessability. Vitrimers are ideal candidate materials for
solid polymer electrolytes because their viscoelasticity and conductivity
can be independently tuned by salt addition in distinct ways from
linear polymer electrolytes while further providing resistance to
lithium dendrite propagation. In this work, the chemical and physical
properties of vinylogous urethane (VU) vitrimers were characterized
by using a combination of experiments and simulations to develop molecular
design rules for controlling material properties. A series of VU vitrimers
containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt
were synthesized by precisely controlling the VU cross-linking density
using defined linker lengths of ethylene glycol (xEG, x = 2, 3, 4, 6, or 12), thereby enabling control
over the dynamic bond-to-EG ratio. Viscoelastic measurements show
that the characteristic relaxation time Ï* of VU vitrimers containing
salt decreased by a factor of âŒ70 relative to neutral vitrimers
due to Li-ion coordination and catalysis of VU bond exchange. Stress
relaxation times and shear moduli decrease with lower cross-linking
densities in VU vitrimers. Solid-state 7Li NMR further
reveals that VU vitrimers with longer linker lengths prefer lithium-ethylene
oxide (Li-EO) solvation, whereas shorter linkers cannot sufficiently
solvate the cation, and Li-VU coordination is preferred. Density functional
theory (DFT) simulations were used to elucidate the dominant binding
mode of Li-ion interaction as a function of linker length. The preferential
partitioning of Li at the VU site leads to an order of magnitude decrease
in stress relaxation times with a negligible impact on the conductivity
after normalizing to the glass transition temperature Tg. Interestingly, our results show universal behavior
for Tg-normalized ionic conductivity data
regardless of linker length. Overall, this work provides new avenues
for orthogonal tuning of bulk dynamics, recyclability, and conductivity
in vitrimer electrolytes