accurate-determination-equilibrium-surface-tension-values-with-area

Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests

We demonstrate two robust protocols for determining the equilibrium surface tension (EST) values with area perturbation tests. The EST values should be ...

Purdue University

Macromolecules with aliphatic backbones that bear pendant stable radical groups (i.e., radical polymers) have attracted much attention in applications where a supporting electrolyte is capable of aiding charge transport in solution; however, the utilization of these materials in solid state applications has been limited. Here, we synthesize a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), through a controlled reversible addition-fragmentation chain transfer (RAFT) mediated polymerization mechanism to generate well-defined and easily-tunable functional polymers. These completely amorphous, electronically-active polymers demonstrate relatively high glass transition temperatures (Tg ∼170 °C) and, because of the aliphatic nature of the backbone of the radical polymers, are almost completely transparent in the visible region of the electromagnetic spectrum. Additionally, we quantify the conductivity of PTMA (∼1×10(-6) S cm(-1)) and find it to be on par with pristine π-conjugated polymers such as poly(phenylene vinylenes) (PPVs) and poly(3-alkylthiophenes) (P3ATs). Furthermore, we demonstrate that the addition of small molecules bearing stable radical groups provides for more solid state charge hopping sites without altering the chemical nature of radical polymers; this, in turn, allows for an increase in the conductivity of PTMA relative to neat PTMA thin films while still retaining the same high degree of optical transparency and device stability. Because of the synthetic flexibility and easily-controlled doping mechanisms (that do not alter the PTMA chemistry), radical polymers present themselves as promising and tunable materials for transparent solid-state plastic electronic applications.

Controlled radical polymerization and quantification of solid state electrical conductivities of macromolecules bearing pendant stable radical groups.

Polymer thermoelectric devices are emerging as promising platforms by which to convert thermal gradients into electricity directly, and poly(3,4-ethylene dioxythiophene) doped with poly(styrenesulfonate) (

Tuning the Thermoelectric Properties of a Conducting Polymer through Blending with Open-Shell Molecular Dopants.

The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials. 

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells.

Membrane adsorbers are a proposed alternative to packed beds for chromatographic separations. To date, membrane adsorbers have suffered from low binding capacities and/or complex processing methodologies. In this work, a polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) triblock polymer is cast into an asymmetric membrane that possesses a high density of nanopores (d ∼ 38 nm) at the upper surface of the membrane. Exposing the membrane to a 6 M aqueous hydrochloric acid solution converts the PDMA brushes that line the pore walls to poly(acrylic acid) (PAA) brushes, which are capable of binding metal ions (e.g., copper ions). Using mass transport tests and static binding experiments, the saturation capacity of the PI-PS-PAA membrane was determined to be 4.1 ± 0.3 mmol Cu(2+) g(-1). This experimental value is consistent with the theoretical binding capacity of the membranes, which is based on the initial PDMA content of the triblock polymer precursor and assumes a 1:1 stoichiometry for the binding interaction. The uniformly sized nanoscale pores provide a short diffusion length to the binding sites, resulting in a sharp breakthrough curve. Furthermore, the membrane is selective for copper ions over nickel ions, which permeate through the membrane over 10 times more rapidly than copper during the loading stage. This selectivity is present despite the fact that the sizes of these two ions are nearly identical and speaks to the chemical selectivity of the triblock polymer-based membrane. Furthermore, addition of a pH 1 solution releases the bound copper rapidly, allowing the membrane to be regenerated and reused with a negligible loss in binding capacity. Because of the high binding capacities, facile processing method implemented, and ability to tailor further the polymer brushes lining the pore walls using straightforward coupling reactions, these membrane adsorbers based on block polymer precursors have potential as a separation media that can be designed to a variety of specific applications. 

Nanostructured Membranes from Triblock Polymer Precursors as High Capacity Copper Adsorbents.

Controlling the nature and transfer of excited states in organic photovoltaic (OPV) devices is of critical concern due to the fact that exciton transport and separation can dictate the final performance of the system. One effective method to accomplish improved charge separation in organic electronic materials is to control the spin state of the photogenerated charge-carrying species. To this end, nanoparticles with unique iron oxide (Fe3O4) cores and zinc oxide (ZnO) shells were synthesized in a controlled manner. Then, the structural and magnetic properties of these core-shell nanoparticles (Fe3O4@ZnO) were tuned to ensure superior performance when they were incorporated into the active layers of OPV devices. Specifically, small loadings of the core-shell nanoparticles were blended with the previously well-characterized OPV active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Upon addition of the core-shell nanoparticles, the performance of the OPV devices was increased up to 25% relative to P3HT-PCBM active layer devices that contained no nanoparticles; this increase was a direct result of an increase in the short-circuit current densities of the devices. Furthermore, it was demonstrated that the increase in photocurrent was not due to enhanced absorption of the active layer due to the presence of the Fe3O4@ZnO core-shell nanoparticles. In fact, this increase in device performance occurred because of the presence of the superparamagnetic Fe3O4 in the core of the nanoparticles as incorporation of ZnO only nanoparticles did not alter the device performance. Importantly, however, the ZnO shell of the nanoparticles mitigated the negative optical effect of Fe3O4, which have been observed previously. This allowed the core-shell nanoparticles to outperform bare Fe3O4 nanoparticles when the single-layer nanoparticles were incorporated into the active layer of OPV devices. As such, the new materials described here present a tangible pathway toward the development of enhanced design schemes for inorganic nanoparticles such that magnetic and energy control pathways can be tailored for flexible electronic applications. 

Design of Super-Paramagnetic Core-Shell Nanoparticles for Enhanced Performance of Inverted Polymer Solar Cells.

Electrostatic force microscopy (EFM) is often used for nanoscale dielectric spectroscopy, the measurement of local dielectric properties of materials as a function of frequency. However, the frequency range of atomic force microscopy (AFM)-based dielectric spectroscopy has been limited to a few kilohertz by the resonance frequency and noise of soft microcantilevers used for this purpose. Here, we boost the frequency range of local dielectric spectroscopy by 3 orders of magnitude from a few kilohertz to a few megahertz by developing a technique that exploits the high resonance frequency and low thermal noise of ultrasmall cantilevers (USCs). We map the frequency response of the real and imaginary components of the capacitance gradient (∂C(ω)/∂z) by using second-harmonic EFM and a theoretical model, which relates cantilever dynamics to the complex dielectric constant. We demonstrate the method by mapping the nanoscale dielectric spectrum of polymer-based materials for organic electronic devices. Beyond offering a powerful extension to AFM-based dielectric spectroscopy, the approach also allows the identification of electrostatic excitation frequencies which affords high dielectric contrast on nanomaterials.

Nanoscale Mapping of Dielectric Properties of Nanomaterials from Kilohertz to Megahertz Using Ultrasmall Cantilevers.

Cohesive Hamaker constants of solid materials are measured via optical and dielectric properties (i.e., Lifshitz theory), inverse gas chromatography (IGC), and contact angle measurements. To date, however, a comparison across these measurement techniques for common energetic materials has not been reported. This has been due to the inability of the community to produce samples of energetic materials that are readily compatible with contact angle measurements. Here we overcome this limitation by using physical vapor deposition to produce thin films of five common energetic materials, and the contact angle measurement approach is applied to estimate the cohesive Hamaker constants and surface energy components of the materials. The cohesive Hamaker constants range from 85zJ to 135zJ across the different films. When these Hamaker constants are compared to prior work using Lifshitz theory and nonpolar probe IGC, the relative magnitudes can be ordered as follows: contact angle>Lifshitz>IGC. Furthermore, the dispersive surface energy components estimated here are in good agreement with those estimated by IGC. Due to these results, researchers and technologists will now have access to a comprehensive database of adhesion constants which describe the behavior of these energetic materials over a range of settings. 

An evaluation of complementary approaches to elucidate fundamental interfacial phenomena driving adhesion of energetic materials.

Stimuli-responsive soft materials are a highly studied field due to their wide-ranging applications; however, only a small group of these materials display hysteretic responses to stimuli. Moreover, previous reports of this behavior have typically shown it to be short-lived. In this work, poly(acrylic acid) (PAA) chains at extremely high grafting densities and confined in nanoscale pores displayed a unique long-lived hysteretic behavior caused by their ability to form a metastable hydrogen bond network. Hydraulic permeability measurements demonstrated that the conformation of the PAA chains exhibited a hysteretic dependence on pH, where different effective pore diameters arose in a pH range of 3 to 8, as determined by the pH of the previous environment. Further studies using Fourier transform infrared (FTIR) spectroscopy demonstrated that the fraction of ionized PAA moieties depended on the thin film history; this was corroborated by metal adsorption capacity, which demonstrated the same pH dependence. This hysteresis was shown to be persistent, enduring for days, in a manner unlike most other systems. The hypothesis that hydrogen bonding among PAA units contributed to the hysteretic behavior was supported by experiments with a urea solution, which disrupted the metastable hydrogen bonded state of PAA toward its ionized state. The ability of PAA to hydrogen bond within these confined pores results in a stable and tunable hysteresis not previously observed in homopolymer materials. An enhanced understanding of the polymer chemistry and physics governing this hysteresis gives insight into the design and manipulation of next-generation sensors and gating materials in nanoscale applications.

Unusually Stable Hysteresis in the pH-Response of Poly(Acrylic Acid) Brushes Confined within Nanoporous Block Polymer Thin Films.

Silver nanostructures were fabricated by femtosecond laser-induced reduction of silver ions and the impact of solution chemistry on the fabricated structures was evaluated. By investigating the exact photochemistry of the nanofabrication solutions, which contained varying amounts of diamine silver ions, trisodium citrate, and n-lauroylsarcosine sodium, and optimizing the laser processing parameters, we fabricated two-dimensional silver pads with surface roughness values of 7 nm and stable 2.5-dimensional shell structures with heights up to 10 μm and aspect ratios of 20 in a ready manner. Moreover, thermal annealing of these structures afforded materials where the average resistivity value was only a factor of 4 greater than that of bulk silver. In this way, the work presented here provides for a methodology that can be used for laser direct fabrication of metal nanostructures for applications in plasmonics and micro- and nano-electronics.

Fabrication of silver nanostructures using femtosecond laser-induced photoreduction.

Solid-state conducting polymers usually have highly conjugated macromolecular backbones and require intentional doping in order to achieve high electrical conductivities. Conversely, single-component, charge-neutral macromolecules could be synthetically simpler and have improved processibility and ambient stability. We show that poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl), a nonconjugated radical polymer with a subambient glass transition temperature, underwent rapid solid-state charge transfer reactions and had an electrical conductivity of up to 28 siemens per meter over channel lengths up to 0.6 micrometers. The charge transport through the radical polymer film was enabled with thermal annealing at 80°C, which allowed for the formation of a percolating network of open-shell sites in electronic communication with one another. The electrical conductivity was not enhanced by intentional doping, and thin films of this material showed high optical transparency.

A nonconjugated radical polymer glass with high electrical conductivity.

Although less studied than their closed-shell counterparts, materials containing stable open-shell chemistries have played a key role in many energy storage and energy conversion devices. In particular, the oxidation-reduction (redox) properties of these stable radicals have made them a substantial contributor to the progress of organic batteries. Moreover, the use of radical-based materials in photovoltaic devices and thermoelectric systems has allowed for these emerging molecules to have impacts in the energy conversion realm. Additionally, the unique doublet states of radical-based materials provide access to otherwise inaccessible spin states in optoelectronic devices, offering many new opportunities for efficient usage of energy in light-emitting devices. Here, we review the current state of the art regarding the molecular design, synthesis, and application of stable radicals in these energy-related applications. Finally, we point to fundamental and applied arenas of future promise for these designer open-shell molecules, which have only just begun to be evaluated in full.

Stable Radical Materials for Energy Applications.

A highly transparent crosslinkable thin film made of the radical polymer poly(2,2,6,6-tetramethyl-4-piperidinyloxy methacrylate)- co-(4-benzoylphenyl methacrylate) (PTMA- co-BP) has been developed as the ion storage layer in electrochromic devices (ECDs). After photo-crosslinking, the dissolution of PTMA- co-BP in electrolytes was mitigated, which results in an enhanced electrochemical stability compared with the homopolymer PTMA thin film. Moreover, the redox capacity of PTMA- co-BP increased because of the formation of a crosslinked network. By matching the redox capacity of the PTMA- co-BP thin film and bis(alkoxy)-substituted poly(propylenedioxythiophene), the ECD achieved an optical contrast of 72% in a small potential window of 2.55 V (i.e., switching between +1.2 and -1.35 V), and it was cycled up to 1800 cycles. The ECD showed an excellent optical memory as its transmittance decayed by less than 3% in both the colored and bleached states while operating for over 30 min under open-circuit conditions. Use of crosslinkable radical polymers as the transparent ion storage layer opens up a new venue for the fabrication of transmissive-mode organic ECDs.

Highly Transparent Crosslinkable Radical Copolymer Thin Film as the Ion Storage Layer in Organic Electrochromic Devices.

Adsorptive membranes offer one possible solution to the challenge of removing and recovering heavy metal ion contaminants and resources from water supplies. However, current membrane-based sorbents suffer from low binding affinities, leading to issues when contaminants are present at trace concentrations or when the source waters have a high concentration of background electrolytes that compete for open binding sites. Here, these challenges are addressed in the design of a highly permeable (i.e., permeability of ∼2.8 × 10 L m h bar) sorbent platform based on polysulfone and polystyrene--poly(acrylic acid) composite membranes. The membranes possess a fully interconnected network of poly(acrylic acid)-lined pores, which enables the surface chemistry to be tailored through sequential attachment of polyethylenimine brushes and metal-binding terpyridine ligands. The polyethylenimine brushes increase the saturation capacity, while the addition of terpyridine enables high-affinity binding to a diversity of transition metal ions (i.e., Pd, Cd, Hg, Pb, Zn, Co, Ni, Fe, Nd, and Sm). This platform removes these metal contaminants from solution with a sorbent capacity of 1.2 mmol g [based on Cu uptake]. The metal capture performance of the functionalized membranes persists in spite of high concentrations of competitive ions, with >99% removal of Pb and Cd ions from artificial groundwater and seawater solutions. Breakthrough experiments demonstrate the efficient purification of feed solutions containing multiple heavy metal ions under dynamic flow conditions. Finally, fluorescence quenching of the terpyridine moiety upon metal ion complexation offers an in situ probe to monitor the extent of sorbent saturation with a Stern-Volmer association constant of 2.9 × 10 L mol. The permeability, capacity, and affinity of these membranes, with high-density display of a metal-binding ligand, offer a chemically tailored platform to address the challenges that arise in ensuring clean water.

High-Affinity Detection and Capture of Heavy Metal Contaminants using Block Polymer Composite Membranes.

Design rules and application spaces for closed-shell conjugated polymers have been well established in the field of organic electronics, but the emerging class of open-shell stable radicals has not been evaluated in such detail. Thus, establishing the underlying physical phenomena associated with the interactions between both classes of molecules is imperative for the effective utilization of these soft materials. Here, we establish that Förster Resonance Energy Transfer (FRET) is the dominant mechanism by which energy transfer occurs from a common conjugated polymer to various radical species using a combination of experimental and computational approaches. Specifically, we determined this fact by monitoring the fluorescence quenching of poly(3-hexylthiophene) (P3HT) in the presence of three radical species: (1) the galvinoxyl; (2) the 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO); and (3) the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals. Both in solution and in the solid-state, the galvinoxyl and PTIO radicals showed quenching that was on par with that of a common fullerene electron-accepting derivative, due to the considerable overlap of their absorbance spectrum with the fluorescence spectrum of the P3HT species, which indicated that isoenergetic electronic transitions existed for both species. Conversely, TEMPO showed minimal quenching at similar concentrations due to the lack of such an overlap. Furthermore, computational studies demonstrated that FRET would occur at a significantly faster rate than other competing processes. These findings suggest that long-range energy transfer can be accomplished in applications when radicals that can act as FRET acceptors are utilized, forming a new design paradigm for future applications involving both closed- and open-shell soft materials.

Article	Year
Controlled radical polymerization and quantification of solid state electrical conductivities of macromolecules bearing pendant stable radical groups. ACS applied materials & interfaces\| PubMed ID: 24044350	2013
Tuning the Thermoelectric Properties of a Conducting Polymer through Blending with Open-Shell Molecular Dopants. ACS applied materials & interfaces\| PubMed ID: 26263124	2015
Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 26290582	2015
Nanostructured Membranes from Triblock Polymer Precursors as High Capacity Copper Adsorbents. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 26391625	2015
Design of Super-Paramagnetic Core-Shell Nanoparticles for Enhanced Performance of Inverted Polymer Solar Cells. ACS applied materials & interfaces\| PubMed ID: 26506008	2015
Nanoscale Mapping of Dielectric Properties of Nanomaterials from Kilohertz to Megahertz Using Ultrasmall Cantilevers. ACS nano\| PubMed ID: 26972782	2016
An evaluation of complementary approaches to elucidate fundamental interfacial phenomena driving adhesion of energetic materials. Journal of colloid and interface science\| PubMed ID: 27042822	2016
Unusually Stable Hysteresis in the pH-Response of Poly(Acrylic Acid) Brushes Confined within Nanoporous Block Polymer Thin Films. Journal of the American Chemical Society\| PubMed ID: 27172428	2016
Fabrication of silver nanostructures using femtosecond laser-induced photoreduction. Nanotechnology\| PubMed ID: 29091584	2017
A nonconjugated radical polymer glass with high electrical conductivity. Science (New York, N.Y.)\| PubMed ID: 29567710	2018
Stable Radical Materials for Energy Applications. Annual review of chemical and biomolecular engineering\| PubMed ID: 29579403	2018
Highly Transparent Crosslinkable Radical Copolymer Thin Film as the Ion Storage Layer in Organic Electrochromic Devices. ACS applied materials & interfaces\| PubMed ID: 29745221	2018
High-Affinity Detection and Capture of Heavy Metal Contaminants using Block Polymer Composite Membranes. ACS central science\| PubMed ID: 30648153	2018
Tuning the interfacial and energetic interactions between a photoexcited conjugated polymer and open-shell small molecules. Soft matter\| PubMed ID: 30657519	2019

Bryan W. Boudouris

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