Session L46: Dillon Medal Symposium

John H. Dillon Medal Lecture: Self-assembly of rod-coil block polymers

While the self-assembly of coil-like block copolymers into morphologies with well-defined nanometer-sized periodicities is now classic, the translation of this strategy to functional polymers with non-Gaussian chain shapes is significantly more complicated. Control and predictive understanding of nanoscale structure in rod-coil block copolymer systems are of particular importance in both conjugated polymers (for optoelectronic devices) and biomimetic polymers. Our work is targeted at gaining a fundamental understanding of the thermodynamics and kinetics of self-assembling functional rod-coil block copolymer systems, and then applying this to conjugated polymers to understand structure-property relationships in plastic electronics. We demonstrate that phase space for weakly segregated model systems is described by the Flory-Huggins block interaction parameter (\textit{$\chi $N}), the Maier-Saup\'{e} parameter (\textit{$\mu $N}) (which parameterizes rod-rod interactions and also scales with temperature), the coil volume fraction (\textit{$\varphi $}$_{coil})$, and a geometrical asymmetry term (\textit{$\nu $}) to account for aspect ratio differences in the rod and coil. Further, we also find that careful molecular design to moderate molecular interactions is essential in creating controllable systems. In particular, both sidechain substitution and sequence control can be used to control melting temperatures and liquid crystalline interactions in order to create a processing window in which self-assembly can occur.   

Wrinkles and Folds as Photonic Structures in Polymer Photovoltaics

We exploit the elastic instabilities of polymer surfaces under compressive mechanical stress to generate wrinkles and deep folds with prescribed dimensions and at pre-specified coverage over large areas. These wrinkles and deep folds act as photonic structures; they increase light coupling into and trapping within polymer photovoltaics. Devices on these surfaces show a 79{\%} increase in the external quantum efficiency (EQE) in the visible compared to analogous devices on flat surfaces. ~More significantly, we observe an exponential increase in near-infrared light absorption in these devices. In both experiments and numerical simulations, we find that these structures extend the useful range of energy conversion by $>$200 nm, corresponding to a 600{\%} increase in the EQE in the near-infrared where light is otherwise minimally absorbed.~ While we demonstrate this concept with polymer photovoltaics, the controlled introduction of compressive stress provides a straightforward and economical route to large-scale patterning of photonic structures for flexible opto-electronics.   

Molecular Exchange Dynamics in Block Copolymer Micelles

Poly(styrene-$b$-ethylene propylene) (PS-PEP) diblock copolymers were mixed with squalane (C$_{30}$H$_{62})$ at 1{\%} by weight resulting in the formation of spherical micelles. The structure and dynamics of molecular exchange were characterized by synchrotron small-angle x-ray scattering (SAXS) and time resolved small-angle neutron scattering (TR-SANS), respectively, between 100 \r{ }C and 160 \r{ }C. TR-SANS measurements were performed with solutions initially containing deuterium labeled micelle cores and normal cores dispersed in a contrast matched squalane. Monitoring the reduction in scattering intensity as a function of time at various temperatures revealed molecular exchange dynamics highly sensitive to the core molecular weight and molecular weight distribution. Time-temperature superposition of data acquired at different temperatures produced a single master curve for all the mixtures. Experiments conducted with isotopically labeled micelle cores, each formed from two different but relatively mondisperse PS blocks, confirmed a simple dynamical model based on first order kinetics and core Rouse single chain relaxation. These findings demonstrate a dramatic transition to nonergodicity with increasing micelle core molecular weight and confirm the origins of the logarithmic exchange kinetics in such systems.   

Direct Imaging of Nanoscale Ionic Clusters in a Polymer Electrolyte Membrane

One of the factors hindering development of technologies such as fuel cells that rely on polyelectrolyte membranes (PEMs) is the lack of quantitative morphological characterization. While it has been recognized that clustering of ionic groups, can impede proton transport rates, model-free methods to quantify clustering in PEMs have not been developed. We present the first electron micrographs of sulfonic acid clusters in a polymer electrolyte membranes (PEM). The clusters are spherical with an average diameter of 1.4 nm and a standard deviation of 0.25 nm. Obtaining images of densely packed clusters of this size in a soft material is non-trivial due to their overlap in projection. Imaging of the sulfur-rich clusters by dark field microscopy was facilitated by the spontaneous formation of thin, cluster-containing layers on the top and bottom surfaces of free-standing films.   

Mechanisms underlying conductivity of lamellar block copolymer electrolytes

Recent experiments have reported intriguing trends for the molecular weight (MW) dependence of the conductivity of block copolymer lamellae which are opposite that exhibited by homopolymer matrices. Using coarse-grained simulations of the transport of penetrant ions, we probe the possible mechanisms underlying such behavior. Our results indicate that the MW dependence of the conductivity of homopolymeric and block copolymeric matrices owe their origins to different mechanisms. On the one hand, the solubility of penetrants in block copolymer matrices themselves exhibit a MW dependence which arises from the MW dependence of the thickness of the conducting phase relative to the interfacial zones. Moreover, distinct mechanisms are shown to be responsible for the mobilities of ions in homopolymer and block copolymers. In the former, the mobility effects associated with the free ends of the polymers play an important role. In contrast, in block copolymer lamellae, the interfacial zone between the blocks presents a zone of hindered mobility for ions and manifests as a molecular weight dependence of the ionic mobility. Together, the preceding mechanisms are shown to provide a plausible explanation for the experimentally observed trends for the conductivity of block copolymer matrices.   

Physical Aspects of Photodynamic Corneal Collagen Crosslinking

Healthy vision depends on the stability of the shape of the cornea, which provides most of the~lens power of the optical system of the eye. ~Diseases in which the cornea progressively undergoes irregular deformation over time (e.g., keratoconus) can be treated clinically by inducing additional protein-protein crosslinks using a photosensitizing drug and a tailored dose of light. Unfortunately, the treatment moving through clinical trials is toxic to cells in and on the cornea. A path to a safer treatment is offered by the nanostructure of the corneal stroma---reminiscent of a HEX phase in block copolymers with 30nm~diameter collagen cylinders~spaced 60nm center-to-center in a hydrogel matrix of~proteoglycans and water. ~We show that using a photosensitizing drug~that sequesters itself in~the collagen fibrils can minimize the toxicity of therapeutic protein-protein cross-linking. Photorheology and transport measurements are used to quantify the parameters of a simple physical model that is useful for optimizing clinical protocols.   

Directed Assembly of Block Copolymer Cylinders: Fundamental Physical Limits to Cylinder Spacing

Understanding the fundamental 2D physics of disordering and defect generation in block copolymer films is important in setting the limits for directed assembly based block copolymer lithography. Our experiments on monolayer films of cylindrical morphology block copolymer show that the monolayer structure disorders at a lower temperature compared to the bulk order-disorder transition temperature by thermal generation of a critical density of dislocations (point defects in the monolayer). We demonstrate experimentally and theoretically how this process sets lower limits on the monolayer cylinder spacing and thus pattern spacing that can be achieved by directed assembly of a given block copolymer using graphoepitaxy. Self-consistent field theoretic simulations are used to predict the compressional elastic constant $B$ of the cylinder monolayer and cylinder spacing $a$ as a function of \textit{$\chi $N} and $f$, the minor block volume fraction. In turn these are used to estimate the formation energies $E_{d}$ ($\sim $\textit{Ba}$^{3})$ of dislocations in cylinder monolayers of various block copolymers.   

Measurement of Diffusion in Entangled Rod-Coil Triblock Copolymers

Although rod-coil block copolymers have attracted increasing attention for functional nanomaterials, their dynamics relevant to self-assembly and processing have not been widely investigated. Because the rod and coil blocks have different reptation behavior and persistence lengths, the mechanism by which block copolymers will diffuse is unclear. In order to understand the effect of the rigid block on reptation, tracer diffusion of a coil-rod-coil block copolymer through an entangled coil polymer matrix was experimentally measured. A monodisperse, high molecular weight coil-rod-coil triblock was synthesized using artificial protein engineering to prepare the helical rod and bioconjugaiton of poly(ethylene glycol) coils to produce the final triblock. Diffusion measurements were performed using Forced Rayleigh scattering (FRS), at varying ratios of the rod length to entanglement length, where genetic engineering is used to control the protein rod length and the polymer matrix concentration controls the entanglement length. As compared to PEO homopolymer tracers, the coil-rod-coil triblocks show markedly slower diffusion, suggesting that the mismatch between rod and coil reptation mechanisms results in hindered diffusion of these molecules in the entangled state.   

Taking a Hard Look at Soft X-ray Scattering

For many years neutron and X-ray scattering has been a trusted tool for polymer scientist. In the hard X-ray regime contrast for scattering has been limited to the difference in electron density between different components. At the Advanced Light Source we have been developing a techniques that combines the advantages of traditional scattering with chemical sensitive spectroscopy. Using soft X-rays at selected photon energies we can tune the contrast between different polymers based on there chemical makeup. We have applied this to a variety of polymer systems.   

Neither crystalline nor amorphous: measuring disorder in polymers and assessing its effect on charge transport

Conjugated polymers displaying high mobility are semicrystalline. Thin films of these materials are comprised of ordered regions (crystallites) and disordered regions. Because of the inherent anisotropy of polymers, the crystallites exhibit varying degrees of disorder in different directions. I will show a quantitative measurement of disorder as applied to these materials, which allows us to quantify a paracrystalline parameter g. This parameter can be used to rank polymers. I will show how g is related to the electronic structure of the polymer and with the presence of electronic traps in particular. By studying the dependence of g on molecular weight we can get to a definition of polymer behavior in an electronic transport sense.   

Shear Alignment of Perpendicular Lamellae in Block Copolymer Thin Films

Thin supported block copolymer films, containing a single layer of cylindrical microdomains lying parallel to the substrate, can be effectively aligned by applying a shear stress to the molten, ordered film. Such films have been used effectively as contact masks for pattern transfer via reactive ion etching, permitting the fabrication of in-plane nanowire arrays, where the nanowires are aligned over macroscopic (cm) distances. Such a nanowire array could also be formed from a film which contains lamellae whose interdomain interfaces lie perpendicular to the substrate; such a template film would in principle allow for the formation of nanostructures of high aspect ratio, provided that the lamellae can be aligned along a single in-plane direction while retaining their perpendicular orientation. We have generated such films of perpendicular lamellae in a polystyrene-poly(methylmethacrylate) diblock, PS-PMMA, by neutralizing the substrate with a random terpolymer brush. Shearing the film, using a moving polydimethylsiloxane (PDMS) pad in contact with the film surface, can indeed produce alignment over cm-scale distances; however, the orientational order is poorer and the defect density higher than in typical cylinder-forming systems, and a significantly higher stress is required. After peeling off the PDMS pad, both PS and PMMA blocks are exposed at the surface in thinner films, but for films thicker than one domain spacing, the lower-energy PS block tends to cap the film surface, overlaying aligned perpendicular lamellae.   

Role of nanoscale morphology on the nano and macro-scale performance of polythiophene based polymer solar cells

Maximization of the short circuit current, J$_{SC}$, the open circuit voltage, V$_{OC}$, and the fill factor (FF) to achieve highest power conversion efficiencies (PCEs) in donor/acceptor, polymer/polymer, solar cells is dependent on optimization of variables associated with the active material's chemical and morphological structure. Control of the nanoscale structure of polythiophene (P3HT)/phenyl-C61-butyric acid methyl ether (PC$_{61}$BM) active materials was achieved through use of a novel low temperature processing strategy. With the use of energy filtered transmission electron microscopy (EF-TEM), electron and X-ray diffraction, together phase contrast, deflection and photocurrent measurements at the nanoscale, we were able to tailor nanoscale morphologies to achieve increases in the J$_{SC}$ by a factor of 1.2 and the PCE by 30{\%}, beyond that using conventional heat treatments for processing.   

Semiconducting block copolymers as nano-structuring agents for high-efficiency and annealing-free bulk hetero-junction organic solar cells

Three main requirements for the industrial development of the polymer solar cells have to be addressed in order to obtain a competitive technology: a fabrication process compatible with common polymer printing technologies, an enhanced life time stability and an improved power conversion efficiency (\textit{PCE}). The active layer nano-structure in bulk heterojunction organic solar cells plays a key role on the properties of charge transfer, transport and consequently on the \textit{PCE}. Ideally a bi-continuous network of donor-acceptor domains with a length scale comparable to the exciton diffusion length is required. To obtain an optimized nano-structured active layer, an annealing process (thermal and/or solvent) is commonly performed leading to an increase of the photovoltaic performance. Currently, the implementation of common printing technologies for the fabrication of polymer solar cells on a mechanically flexible polymer substrate is impeded by this annealing step. In order to overcome the limitations above a novel approach based on an annealing-free fabrication process will be presented making use of a block copolymer as a nano-structuring agent for the polymer/fullerene derivative blend.