6 research outputs found
Interfacial Polymerization on Dynamic Complex Colloids: Creating Stabilized Janus Droplets
Complex
emulsions, including Janus droplets, are becoming increasingly important
in pharmaceuticals and medical diagnostics, the fabrication of microcapsules
for drug delivery, chemical sensing, E-paper display technologies,
and optics. Because fluid Janus droplets are often sensitive to external
perturbation, such as unexpected changes in the concentration of the
surfactants or surface-active biomolecules in the environment, stabilizing
their morphology is critical for many real-world applications. To
endow Janus droplets with resistance to external chemical perturbations,
we demonstrate a general and robust method of creating polymeric hemispherical
shells via interfacial free-radical polymerization on the Janus droplets.
The polymeric hemispherical shells were characterized by optical and
fluorescence microscopy, scanning electron microscopy, and confocal
laser scanning microscopy. By comparing phase diagrams of a regular
Janus droplet and a Janus droplet with the hemispherical shell, we
show that the formation of the hemispherical shell nearly doubles
the range of the Janus morphology and maintains the Janus morphology
upon a certain degree of external perturbation (e.g., adding hydrocarbon–water
or fluorocarbon–water surfactants). We attribute the increased
stability of the Janus droplets to (1) the surfactant nature of polymeric
shell formed and (2) increase in interfacial tension between hydrocarbon
and fluorocarbon due to polymer shell formation. This finding opens
the door of utilizing these stabilized Janus droplets in a demanding
environment
Role of Mechanical Factors in Controlling the Structure–Function Relationship of PFSA Ionomers
Ion-conducting polymers are ideal solid electrolytes
for most energy
storage and conversion devices where ion transport is a critical functionality.
The system performance and stability are related to the transport
and mechanical properties of the ionomers, which are correlated through
physiochemical interactions and morphology. Thus, there exists a balance
between the chemical and mechanical energies which controls the structure–function
relationship of the ionomer. In this paper, it is reported how and
why thermal treatments result in different water uptakes and nanostructures
for a perfluorinated sulfonic acid (PFSA) membrane. The nanostructure
of the PFSA membrane is characterized using small- and wide-angle
X-ray scattering experiments. These changes are correlated with water
content and mechanical properties and result in fundamental relationships
to characterize the membrane with different thermal histories. Moreover,
quasi-equilibrium water uptake and domain spacing both decrease with
predrying or preconstraining the membrane, thereby suggesting that
similar mechanical energies govern the structural changes via internal
and external constraints, respectively. The findings suggest that
heat treatments alter the balance between the chemical–mechanical
energies where the interplay of the morphology and mechanical properties
controls the structure–function relationship of the membrane.
Finally, a model is developed using an energy-balance approach with
inputs of the mechanical and structural properties; the dependence
of water uptake on pretreatment is successfully predicted
Predicting the Mechanical Properties of Organic Semiconductors Using Coarse-Grained Molecular Dynamics Simulations
The
ability to predict the mechanical properties of organic semiconductors
is of critical importance for roll-to-roll production and thermomechanical
reliability of organic electronic devices. Here, we describe the use
of coarse-grained molecular dynamics simulations to predict the density,
tensile modulus, Poisson ratio, and glass transition temperature for
polyÂ(3-hexylÂthiophene) (P3HT) and its blend with C<sub>60</sub>. In particular, we show that the resolution of the coarse-grained
model has a strong effect on the predicted properties. We find that
a one-site model, in which each 3-hexylÂthiophene unit is represented
by one coarse-grained bead, predicts significantly inaccurate values
of density and tensile modulus. In contrast, a three-site model, with
one coarse-grained bead for the thiophene ring and two for the hexyl
chain, predicts values that are very close to experimental measurements
(density = 0.955 g cm<sup>–3</sup>, tensile modulus = 1.23
GPa, Poisson ratio = 0.35, and glass transition temperature = 290
K). The model also correctly predicts the strain-induced alignment
of chains as well as the vitrification of P3HT by C<sub>60</sub> and
the corresponding increase in the tensile modulus (tensile modulus
= 1.92 GPa, glass transition temperature = 310 K). We also observe
a decrease in the radius of gyration and the density of entanglements
of the P3HT chains with the addition C<sub>60</sub> which may contribute
to the experimentally noted brittleness of the composite material.
Although extension of the model to polyÂ(3-alkylÂthiophenes) (P3ATs)
containing side chains longer than hexyl groupsî—¸nonyl (N) and
dodecyl (DD) groupsî—¸correctly predicts the trend of decreasing
modulus with increasing length of the side chain measured experimentally,
obtaining absolute agreement for P3NT and P3DDT could not be accomplished
by a straightforward extension of the three-site coarse-grained model,
indicating limited transferability of such models. Nevertheless, the
accurate values obtained for P3HT and P3HT:C<sub>60</sub> blends suggest
that coarse graining is a valuable approach for predicting the thermomechanical
properties of organic semiconductors of similar or more complex architectures
Efficient Characterization of Bulk Heterojunction Films by Mapping Gradients by Reversible Contact with Liquid Metal Top Electrodes
The ways in which organic solar cells
(OSCs) are measured and characterized
are inefficient: many substrates must be coated with expensive or
otherwise precious materials to test the effects of a single variable
in processing. This serial, sample-by-sample approach also takes significant
amounts of time on the part of the researcher. Combinatorial approaches
to research OSCs generally do not permit microstructural characterization
on the actual films from which photovoltaic measurements were made,
or they require specialized equipment that is not widely available.
This paper describes the formation of one- and two-dimensional gradients
in morphology and thickness. Gradients in morphology are formed using
gradient annealing, and gradients in thickness are formed using asymmetric
spin coating. Use of a liquid metal top electrode, eutectic gallium–indium
(EGaIn), allows reversible contact with the organic semiconductor
film. Reversibility of contact permits subsequent characterization
of the specific areas of the semiconductor film from which the photovoltaic
parameters are obtained. Microstructural data from UV–vis experiments
extracted using the weakly interacting H-aggregate model, along with
atomic force microscopy, are correlated to the photovoltaic performance.
The technique is used first on the model bulk heterojunction system
comprising regioregular polyÂ(3-hexylthiophene) (P3HT) and the soluble
fullerene derivative [6,6]-phenyl C<sub>61</sub> butyric acid methyl
ester (PCBM). To demonstrate that the process can be used to optimize
the thickness and annealing temperature using only small (≤10
mg) amounts of polymer, the technique was then applied to a bulk heterojunction
blend comprising a difficult-to-obtain low-bandgap polymer. The combination
of the use of gradients and a nondamaging top electrode allows for
significant reduction in the amount of materials and time required
to understand the effects of processing parameters and morphology
on the performance of OSCs
Insights into Magneto-Optics of Helical Conjugated Polymers
Materials with magneto-optic (MO)
properties have enabled critical
fiber-optic applications and highly sensitive magnetic field sensors.
While traditional MO materials are inorganic in nature, new generations
of MO materials based on organic semiconducting polymers could allow
increased versatility for device architectures, manufacturing options,
and flexible mechanics. However, the origin of MO activity in semiconducting
polymers is far from understood. In this paper, we report high MO
activity observed in a chiral helical poly-3-(alkylsulfone)Âthiophene
(<b>P3AST</b>), which confirms a new design for the creation
of a giant Faraday effect with Verdet constants up to (7.63 ±
0.78) × 10<sup>4</sup> deg T<sup>–1</sup> m<sup>–1</sup> at 532 nm. We have determined that the sign of the Verdet constant
and its magnitude are related to the helicity of the polymer at the
measured wavelength. The Faraday rotation and the helical conformation
of <b>P3AST</b> are modulated by thermal annealing, which is
further supported by DFT calculations and MD simulations. Our results
demonstrate that helical polymers exhibit enhanced Verdet constants
and expand the previous design space for polythiophene MO materials
that was thought to be limited to highly regular lamellar structures.
The structure–property studies herein provide insights for
the design of next-generation MO materials based upon semiconducting
organic polymers
Stretchable and Degradable Semiconducting Block Copolymers
This
paper describes the synthesis and characterization of a class of highly
stretchable and degradable semiconducting polymers. These materials
are block copolymers (BCPs) in which the semiconducting blocks
are based on the diketoÂpyrrolopyrrole (DPP) unit flanked by
furan rings and the insulating blocks are polyÂ(ε-caprolactone)
(PCL). The combination of stiff conjugated segments with flexible
aliphatic polyesters produces materials that can be stretched >100%.
Remarkably, BCPs containing up to 90 wt % of insulating PCL have the
same field-effect mobility as the pure semiconductor. Spectroscopic
(ultraviolet–visible absorption) and morphological (atomic
force microscopic) evidence suggests that the semiconducting blocks
form aggregated and percolated structures with increasing content
of the insulating PCL. Both PDPP and PCL segments in the BCPs degrade
under simulated physiological conditions. Such materials could find
use in wearable, implantable, and disposable electronic devices