Session L17: Focus Session: Dillon Medal Symposium

John H. Dillon Medal Talk: Solvent Annealing of Water-Dispersible Polyaniline Yields Highly Conductive Functional Components for Organic Electronics

Though highly conductive, early conducting polymers are neither melt- nor solution processable due to their extensive conjugation. To overcome this intractability, macromolecular dopants have been employed to render conducting polymers water dispersible. The gain in processability, however, has frequently come at the expense of conductivity. With a simple solvent annealing treatment, we have dramatically improved the conductivity of water-dispersible conducting polymers; the conductivities achieved through this treatment qualify water-dispersible conducting polymers as practical alternatives for metals and transparent metal oxides as electrodes in organic thin-film transistors and organic solar cells. Specifically, solvent annealing induces structural rearrangement; polyaniline transforms from a globular structure that is arrested by strong ionic interactions with its macromolecular dopants to an expanded structure that is conformationally more favorable. It is this structural transformation that increases the electrical conductivity of polyaniline by more than two orders of magnitude.   

Tuning optical properties of polymer films using nanorods and Janus particles

The optical properties of polymer films are tuned using novel nanoparticles. First, nanorods (NRs) of gold are organized and aligned within polymer films. Plasmon adsorption is investigated as a function of NP concentration as well as matrix type. NR organization and dispersion is compared with a model of a monolayer of nano-interacting rods. Second, multi-region and patchy, optically-active Janus particles were synthesized via a hierarchical self-assembly process. Gold nanoparticles were assembled on the top surfaces of nano- and sub-micron silica particles, which were selectively protected on their bottom surfaces by covalent attachment to a copolymer film. The morphologies of the gold particle layer, and the resulting optical properties of the Janus particles, were tuned by changing the surface energy between the silica and gold particles, followed by annealing.   

The effects of monomer sequence distribution and isotopic substitution on solution phase behavior of random copolymers

We report on the effect of chemical composition, co-monomer distribution and $^{1}$H/$^{2}$D isotopic substitution on the phase behavior in random copolymers of poly(styrene-co-4-bromostyrene) (PBr$_{x}$S), where x denotes the mole fraction of 4-bromostyrene (4-BrS), in cyclohexane. By adjusting the solvent quality during bromination of parent polystyrene, either random or random blocky PBr$_{x}$S, (r-PBr$_{x}$S or b-PBr$_{x}$S, respectively), were synthesized. We study the temperature dependence of phase behavior of PBr$_{x}$S with various x in cyclohexane as a function of the polymer concentration using cloud point measurements and small-angle neutron scattering. Our results reveal that for a given 4-BrS content, the cloud points of b-PBr$_{x}$S solutions are consistently higher and broader than those observed in r-PBr$_{x}$S solutions. The transition temperature has also been found to depend on the isotope substitution of $^{1}$H or $^{2}$D in either the polymer or the solvent. Small angle neutron scattering measurements indicate significant differences in the temperature dependence of the thermodynamic behavior for the random and blocky samples and the nature of the fluctuations upon approaching the phase boundaries.   

What can simulations of hydrophobic polymers teach us about cold denaturation?

Interactions between hydrophobic solutes in aqueous solution depend not only on their size and shape, but also on their van der Waals attractions with water. Molecular theories can explain these differences for idealized cases, e.g, small (high curvature) solutes versus extended (low curvature) surfaces. But do these idealized limits provide meaningful information for the hydrophobic interactions that drive the conformational transformations of proteins or polymers? In this talk, we discuss recent results that take a step toward addressing this question. Specifically, we discuss molecular simulations of hydrophobic polymers with collapse transitions that display some protein-like behavior (e.g., cold denaturation). We first show that the collapse thermodynamics reflect a competition between polymer-water attractions and a contribution due to the effect that ``shape'' of the polymer has on water, the latter of which can be described by a simple surface tension model. We also discuss the importance of attractive interactions for cold denaturation.   

Polymer-polymer interfaces in thin film transistors

Despite the importance of interfaces between semiconducting and dielectric polymers in thin film transistors, very little is known about the interplay between morphology and charge transport. Using a dry thin film transfer method we have fabricated polymer bilayers of semiconducting an insulating polymers that are inaccessible using spin coating. The transfer method allows the formation of polymer-polymer interfaces without the complication of solvent interactions during film formation and enables study of morphology and transport independently. For example, we have found that transferred layers of poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) PBTTT, have higher field effect mobility on some crosslinked dielectrics than directly spin-cast films. The microstructure of these bilayers has been examined using x-ray scattering and shows similar bulk ordering despite the difference in hole mobility.   

Correlations between Morphologies and Photovoltaic Properties of RodCoil Block Copolymers

We present results obtained using a drift-diffusion model for the structure-property correlations in photovoltaic devices based on self-assembly of rod-coil block copolymers. We use a self- consistent field theory model to generate the self-assembly morphologies of rod-coil block copolymers in confined situations. The density and orientational order parameter profiles so-obtained are then used as input to a recently proposed drift-diffusion model which predicts the photovoltaic device characteristics. The latter model allows for prescription of arbitrary morphologies of donor and acceptor phases while simultaneously incorporating the role of anisotropic charge transport of holes and excitons that arise in the ordered phases of rod-coil block copolymers. We present results elucidating the role of morphology of self-assembly, orientation of lamellar phases, domain widths, and the degree of phase separation and orientational ordering, upon the photovoltaic device characteristics.   

Nanolayer Batteries and Spray-Assembled Membranes

The electrostatic assembly method enables the construction of devices with a high degree of control of ion and electron transport, and the incorporation of inorganic and organic nanomaterials. This capability allows the generation of a broad range of reactive membranes and electrochemical devices through the use of simple aqueous processing conditions, such as salt content and solution pH, that act as tools for the manipulation of ion and electron transport characteristics in the film, as well as the morphology of these unique nano-assemblies. Ultimately, the use of layer-by-layer systems has led to a range of organic and inorganic materials systems that have incorporated metal oxide nanoparticles, carbon nanotubes, and organic and inorganic polymers to yield systems of interest for solar cells, fuel cells, capacitor/battery and electrochemical energy electrode applications. We have recently introduced an automated misting approach to multilayer assembly, spray-LbL, that enables the coating of complex surfaces and porous substrates, and greatly reduces time to assembly for multilayer devices. This approach has enabled the generation of asymmetric reactive membranes, high surface area devices, and rapidly assembled electrodes.   

Self Assembly and Interface Engineering of Organic Functional Materials for High Performance Polymer Solar Cells

The performance of polymer solar cells are strongly dependent on the efficiency of light harvesting, exciton dissociation, charge transport, and charge collection at the metal/organic, metal/metal oxide, and organic/metal oxide interfaces. To improve the device performance, two parallel approaches were used: 1) developing novel low band gap conjugated polymers with good charge-transporting properties and 2) modifying the interfaces between the organic/metal oxide and organic/metal layers with functional self-assembling monolayers to tune their energy barriers. Moreover, the molecule engineering approach was also used to tune the energy level, charge mobility, and morphology of organic semiconductors.   

Structure of thin film brush-coated nanoparticle/homopolymer systems

Nanoparticles (NPs) are incorporated within polymer hosts in order to prepare nanocomposites with ``tailored'' properties. However, understanding and controlling particle aggregation, and the structure, in these polymer nanocomposites (PNCs) remains an important challenge. We examine the phase behavior of thin film mixtures of polystyrene (PS)-grafted gold nanoparticles with different polymer hosts: PS, polymethyl methacrylate (PMMA); tetramethyl bisphenol polycarbonate (TMPC) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). We show that the phase behavior, and more importantly the nanoparticle distribution, is controlled by the enthalpic interaction parameter between the host chains and the grafted chains, the grafting density, g, the degrees of polymerization of the grafted chains and the host chains, N and P, respectively and the nanoparticle size, D. We illustrate conditions under which the entropic interactions have a more significant effect on the structure than the enthalpic interactions. Finally we illustrate conditions where the nanoparticles behave like ``hard'' spheres and conditions under which the mixture behaves like a linear chain/polymer micelle-like system.   

Universal Phase Diagrams in Rod-Coil Block Copolymers and Applications to Optoelectronics and Bioinspired Materials

The thermodynamics of self-assembly of rod-coil block copolymer systems is significantly altered from that of well-characterized coil-coil block copolymers due to the difference in chain topology and the liquid crystalline interactions between rod blocks. As a result, the phase space of rod-coil block copolymers appears to be at least four dimensional (relying on rod-coil and rod-rod interactions as well as two geometrical parameters). Here, we demonstrate that systematic tuning of the rod-rod interactions in conjugated rod-shaped polythiophenes allows for manipulation of the ratio of Maier-Saupe to the Flory-Huggins parameter. This ratio of thermodynamic parameters appears to be crucial in determining morphology and in obtaining long-range order. The fundamental self-assembly of molecules of unusual shapes as well as applications of rod-like block copolymers in optoelectronics and bioinspired materials will be discussed.   

Organosilicate Materials for High Resolution Patterning

Organosilicate glasses (OSGs) of the form [RSiO$_{1.5}$], where R is an organic functional group, have significant potential for nanoimprint lithography (NIL). We present and quantify two methods for fabricating NIL molds from OSG materials. The first utilizes conventional NIL templates, with patterns as small as 10 nm being directly imprinted into as-cast OSG films. The imprinted patterns are vitrified into hard ceramic-like materials that can then be used to imprint subsequent replica patterns with high fidelity. The second uses monomers where the R group is chosen to selectively render the OSG soluble in the polyethylene oxide (PEO) domains of a PEO-PS (PS being polystyrene) diblock copolymer. At elevated temperatures the BCP burns off while the OSG converts into a hard pattern. The resulting OSG patterns are templates of the PEO domains and can be used directly as a NIL molds to replicate the former BCP morphology into a range of materials. This approach offers incredible latitude in tuning the BCP morphology. The surface patterns can be driven through most of the morphologies of the BCP phase diagram by changing the ratio of the OSG to the BCP in the spin casting solution. These physics of these surprising results are discussed in detail.   

Quantifying the role of poly(3-hexylthiophene) and fullerene crystallinity on performance of polymer solar cells

Continued development and improvement of polymer solar cells requires an understanding of structure-function relationships which encompass the morphology of the active layer and device performance. Through a combination of grazing-incidence X-ray diffraction and X-ray rocking scans, we have quantified the crystallinity of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) in the active layer of bulk heterojunction polymer solar cells. We find that the device short-circuit is limited, to first order, by the PCBM crystallinity for a wide range of processing conditions. However, when the PCBM crystallinity is greater than 50{\%} of the maximum crystallinity achievable in our samples through thermal annealing, the extent of P3HT out-of-plane $\pi $-stacking is directly correlated with the device short-circuit current. By quantifying the relationship between the crystallinity of both organic semiconductors of the active layer and device performance, we have developed a description of the complex interplay between structure and performance in polymer solar cells.   

Block Copolymers with Composite Crystalline-Glassy Hard Domains Formed from Single-Phase Melts

Thermoplastic elastomers (TPEs) are typically ABA triblock copolymers made up of a rubbery midblock (B) and glassy endblocks (A). If instead the hard segments were crystalline, microphase separation could be driven by crystallization from a lower-viscosity single-phase melt. We use living ring-opening metathesis polymerization and subsequent hydrogenation to synthesize TPEs consisting of crystalline hydrogenated polynorbornene (hPN), rubbery hydrogenated poly(5-hexylnorbornene) (hPHN), and glassy hydrogenated polymethyltetracyclododecene (hPMTD). By suitable control of the block lengths, we achieve a single-phase melt in a symmetric pentablock TPE with the architecture hPN-hPMTD-hPHN-hPMTD-hPN, where microphase separation is driven by crystallization. The single-phase melt is confirmed by both a featureless small-angle x-ray scattering pattern, and a relatively low and Newtonian viscosity at modest shear rates. When the hPN block crystallizes, the attached hPMTD block forms a glassy layer surrounding the crystals, limiting their lateral growth. The resulting pentablock TPE shows excellent solvent resistance at room temperature due to the hPN crystallinity, while the composite crystalline/glassy hard domains impart strain-hardening behavior to the material, with only modest unrecoverable deformation.