Showing papers by "S. Michael Kilbey published in 2012"

Materials self-assembly and fabrication in confined spaces

Abstract: Molecular assemblies have been mainly researched in open spaces for a long time. However, recent research has revealed that there are many interesting aspects present regarding self-assemblies in confined spaces. Molecular association within nanospaces such as mesoporous materials provides unusual phenomena based on highly restricted molecular motions. Current research endeavors in materials science and technology are focused on developing either a new class of materials or materials with novel/multiple functionalities which is often achieved via molecular assembly in confined spaces. Template synthesis and guided assemblies are distinguishable examples for molecular assembly in confined spaces. So far, different aspects of molecular confinements are discussed separately. In this review, the focus is specifically to bring some potential developments in various aspects of confined spaces for molecular self-assembly under one roof. We arrange the sections in this review based on the nature of the confinements, accordingly the topological/geometrical confinements, chemical and biological confinements, and confinements within thin films. Following these sections, molecular confinements for practical applications are shortly described in order to show connections of these scientific aspects with possible practical uses. One of the most important facts is that the self-assembly in confined spaces stands at meeting points of top-down and bottom-up fabrications, which would be an ultimate key to push the limits of nanotechnology and nanoscience.

Ternary behavior and systematic nanoscale manipulation of domain structures in P3HT/PCBM/P3HT-b-PEO films

Abstract: Nanophase separation plays a critical role in the performance of donor–acceptor based organic photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV efficiency, the ability to exert precise control over phase separated domains and connectivity remains elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI = 1.3) is added to a binary blend of P3HT and 6,6-phenyl C61-butyric acid methyl ester (PCBM) at different concentrations (0–20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of P3HT distribution as a function of block copolymer compatibilizer concentration and thermal annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active layer domain sizes do not change substantially as compatibilizer is added; however after thermal annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer increases. The impact of compatibilizer is further rationalized through quantum density functional theory calculations. Overall, this work demonstrates the possibility of block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. If coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

Charge regulation and local dielectric function in planar polyelectrolyte brushes.

Abstract: Understanding the effect of inhomogeneity on the charge regulation and dielectric properties, and how it depends on the conformational characteristics of the macromolecules is a long-standing problem. In order to address this problem, we have developed a field-theory to study charge regulation and local dielectric function in planar polyelectrolyte brushes. The theory is used to study a polyacid brush, which is comprised of chains end-grafted at the solid-fluid interface, in equilibrium with a bulk solution containing monovalent salt ions, solvent molecules, and pH controlling acid. In particular, we focus on the effects of the concentration of added salt and pH of the bulk in determining the local charge and dielectric function. Our theoretical investigations reveal that the dipole moment of the ion-pairs formed as a result of counterion adsorption on the chain backbones play a key role in affecting the local dielectric function. For polyelectrolytes made of monomers having dipole moments lower than the solvent molecules, dielectric decrement is predicted inside the brush region. However, the formation of ion-pairs (due to adsorption of counterions coming from the dissociation of added salt) more polar than the solvent molecules is shown to increase the magnitude of the dielectric function with respect to its bulk value. Furthermore, an increase in the bulk salt concentration is shown to increase the local charge inside the brush region.

End‐group composition of poly(3‐hexylthiophene)s prepared by in situ quenching of the grignard metathesis polymerization: Influence of additives and reaction conditions

Abstract: The ability to prepare well-defined semiconducting polymers is essential for understanding the link between structure and function in organic photovoltaic devices. A general, one-pot method for altering the degree of functionality of end-functionalized poly(3-hexylthiophene)s (P3HT) prepared by Grignard metathesis (GRIM) polymerization has been developed. In the absence of additives, the degree of functionality of end-functional P3HTs prepared by quenching of the GRIM polymerization with a Grignard reagent is dependent on the Grignard reagent utilized. In this study, additives such as styrene and 1-pentene are shown to alter the end-group composition of tolyl-functionalized P3HTs as determined by Matrix-assisted Laser Desorption Ionization Time-of-flight Mass Spectrometry. In particular, when quenching the GRIM polymerization with tolylmagnesium bromide, a modest decrease in the difunctional product is observed, and the yield of the monofunctional product increases significantly. Temperature and lithium chloride (LiCl) addition also play impactful roles. Monofunctional P3HT is found to be the major product (72% abundance) when the functionalization is done in the presence of LiCl and styrene at 0 °C, whereas in the absence of additives the monofunctional product is present at only 11% abundance. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Manipulating Interfaces through Surface Confinement of Poly(glycidyl methacrylate)-block-poly(vinyldimethylazlactone), a Dually Reactive Block Copolymer

Abstract: The assembly of dually reactive, well-defined diblock copolymers incorporating the chemoselective/functional monomer, 4,4-dimethyl-2-vinylazlactone (VDMA) and the surface-reactive monomer glycidyl methacrylate (GMA) is examined to understand how competition between surface attachment and microphase segregation influences interfacial structure. Reaction of the PGMA block with surface hydroxyl groups not only anchors the copolymer to the surface, but limits chain mobility, creating brush-like structures comprising PVDMA blocks, which contain reactive azlactone groups. The block copolymers are spin coated at various solution concentrations and annealed at elevated temperature to optimize film deposition to achieve a molecularly uniform layer. The thickness and structure of the polymer thin films are investigated by ellipsometry, infrared spectroscopy, and neutron reflectometry. The results show that deposition of PGMA-b-PVDMA provides a useful route to control film thickness while preserving azlactone groups...

In Situ Formation of Pyridyl-Functionalized Poly(3-hexylthiophene)s via Quenching of the Grignard Metathesis Polymerization: Toward Ligands for Semiconductor Quantum Dots

Abstract: The synthesis of well-defined, end-functional poly(3-hexylthiophene)s (P3HTs) by in situ quenching of the Grignard metathesis (GRIM) polymerization is complicated by the extreme tendency to favor difunctional products in all but a few cases. A facile one-pot method for preparing 2-pyridyl and 3-pyridyl P3HTs with high abundance of monofunctional products is established via an examination of the kinetics of the end-functionalization quenching reaction with lithium chloride complexes of 2- and 3-pyridyl Grignard reagents. Density functional theory calculations guide the selection of pyridine as the end group, which provides the capacity to ligate cadmium selenide (CdSe) nanocrystals and arrests aggregation upon thermal annealing when dispersed in a P3HT matrix. The relative abundances of various end-functional products, as ascertained by high-resolution matrix assisted laser desorption ionization time-of-flight mass spectrometry, can be altered through the use of 1-pentene as an additive: GRIM polymerizatio...

Neutron reflectivity as a tool to understand polyelectrolyte brushes

Abstract: Because polyelectrolytes are perhaps the least understood form of soft synthetic matter, polyelectrolyte brushes are important model systems for learning how chain stretching is controlled by conditions that set the charge state of the system, as well as interactions with biological and bio-inspired molecules Motivated by this fact, we describe situations where the application of neutron reflectivity to the study of polyelectrolyte brushes is poised to deepen understanding of the complex connections between charge and nanostructure This theme is especially pertinent because the nanoscale structure dictates interactions across interfaces

Hydrodynamics of polystyrene–polyisoprene miktoarm star copolymers in a selective and a non-selective solvent

Abstract: The hydrodynamics of PSnPIn miktoarm (mixed arm) star copolymers made from polyisoprene (PI) and polystyrene (PS) arms are studied in a selective and a non-selective solvent, n-hexane and THF, respectively. It is found that in n-hexane, the number of arms affects the organization of the miktoarm copolymers: stars with 2 arms (a linear diblock for reference purposes) or 4 arms show aggregation in this selective solvent, whereas no aggregation is observed for stars with 8 and 16 arms in the concentration region studied. This behavior is due to shielding posed by the soluble blocks, which prevents the insoluble blocks from coming together. Interestingly, the contribution from aggregates observed for the two arm star (PS1PI1 diblock) at the highest concentration studied is rather small because the chains predominantly exist as single diblocks in n-hexane. This result may be due to the fact that low molecular weight PS is slightly soluble in linear hydrocarbon solvents. The hydrodynamic sizes found in THF are similar to those in n-hexane for the 2 and 4 arm stars but smaller for the 8 and 16 arms stars. We propose that this is a result of both the limited free space needed for motion of the chains and also because of an increased probability of heterocontacts between the collapsed PS blocks and the swollen PI arms near the star's core.

Charge regulation and local dielectric function in planar polyelectrolyte brushes

Abstract: Understanding the effect of inhomogeneity on the charge regulation and dielectric properties, and how it depends on the conformational characteristics of the macromolecules is a long-standing problem In order to address this problem, we have developed a field-theory to study charge regulation and local dielectric function in planar polyelectrolyte brushes The theory is used to study a polyacid brush, which is comprised of chains end-grafted at the solid-fluid interface, in equilibrium with a bulk solution containing monovalent salt ions, solvent molecules and pH controlling acid In particular, we focus on the effects of the concentration of added salt and pH of the bulk in determining the local charge and dielectric function Our theoretical investigations reveal that the dipole moment of the ion-pairs formed as a result of counterion adsorption on the chain backbones play a key role in affecting the local dielectric function For polyelectrolytes made of monomers having dipole moments lower than the solvent molecules, dielectric decrement is predicted inside the brush region However, the formation of ion-pairs (due to adsorption of counterions coming from the dissociation of added salt) more polar than the solvent molecules is shown to increase the magnitude of the dielectric function with respect to its bulk value Furthermore, an increase in the bulk salt concentration is shown to increase the local charge inside the brush region