Conjugated polymers have been widely studied for their use in lightweight, flexible, and solution-processable electronic devices. However, the optimization of such polymer-based devices has been largely Edisonian in nature due to both a poor understanding of and an inability to control the complex hierarchical structure observed in semicrystalline polymers. In this thesis, we show that simple chemical modifications to commonly-studied conjugated polymers can have a large effect on the observed structure ranging from the unit cell to that on the order of device features. In particular, the self-assembly of block copolymers in which one of the components is optoelectronically-active is presented as a facile method to obtain nanostructured materials. For the work in this thesis, we will focus on poly(3-alkylthiophenes), a widely studied class of conjugated polymers due to their favorable optoelectronic properties, high solubility in organic solvents, and susceptibility to simple chemical modification.

Although the synthesis of conjugated block copolymers has been presented in the past, complexities arising from crystallization of the conjugated moiety have dominated the observed solid state morphologies. Specifically, the crystallization of the semicrystalline block dictates the block copolymer microphase separation, a well-known phenomenon in the literature for non-conjugated semicrystalline block copolymers, which has resulted in solid state morphologies that do not differ significantly from that of the semiconducting homopolymer. To address this, we first show that the side chain chemistry controls the thermal transitions and optoelectronic properties in poly(3-alkylthiophenes). Such control over the crystallization kinetics provides an experimentally convenient approach to investigate the importance of the crystalline structure over a wide range of length scales on the optoelectronic properties. Furthermore, the ability to control the thermal transition temperatures can be used to directly manipulate, and thereby balance, the competition between the driving forces for crystallization and self-assembly. As evidence, the nanoscale structure is shown to be directly controlled via synthesis of block copolymers in which one block is the low melting temperature semiconducting polymer, poly(3-(2-ethylhexyl)thiophene). A wide range of morphologies with curved interfaces are observed which, in the past, have been precluded by the crystallization of poly(3-alkylthiophenes) with unbranched aliphatic side chains such as poly(3-hexylthiophene). Importantly, confinement of the conjugated polymer to nanoscale domains is not detrimental to the crystallinity or to charge transport over device-scale dimensions. Additionally, this approach is shown to be effective for a number of different chemistries providing a flexible methodology for obtaining periodic, semiconducting domains on the nanoscale. Together, these simple synthetic strategies can be used to tune the morphology of various length scales of thin film active layers and provide synthetic rules for design of novel semiconducting polymer systems.

Biological molecules, such as polypeptides, form the basis for most of life's functions. These simple linear polymers made up of only 20 amino acids can fold to form complex and intricate structures that span several length scales. The structures that arise from these molecules stem from their high level of molecular control. Each molecule is an exact sequence with complete monodispersity. From this angstrom level of control, micron scale structures can be assembled. This process is one of the most studied in the history of science, but due to the diversity of amino acids and potential interactions, it is still nearly impossible for scientists to look at a peptide sequence and predict a folded structure. Likewise, it is very difficult to choose a desired structure for a particular function and reverse engineer the linear sequence that will provide that structure. Therefore, there is the need for simplified systems to understand the interactions involved in protein folding and to begin to build the knowledge necessary for engineering similar types of functional structures.

This thesis uses sequence specific biomimetic polymers, namely polypeptoids or poly N-substituted glycines, to fundamentally probe the chain conformation and assembly properties of sequence specific polymers. Polypeptoids or N-substituted glycines, are sequence specific biomimetic chains that have the same backbone as polypeptides. However, rather than the side chain being attached through the alpha carbon, it is attached to the backbone nitrogen. This chemical alteration has several implications for the system including the elimination of backbone hydrogen bonding and chirality. This chemical alteration yields a more flexible and tunable chain where intramolecular interactions can be modulated by the introduction of different side chains. In addition, the synthesis of polypeptoids is a simple two step submonomer addition that uses a primary amine as the submonomer. This results in a very high yield synthesis with virtually limitless possibilities for side chains due to the commercial availability of a wide variety of primary amines.

Using this modular system, my thesis focuses on understanding the solution self assembly of a sequence specific biomimetic polymer. Much of the work has focused on understanding the single chain conformation and collapse of polypeptoids in order to apply this information to larger self assembly systems. The persistence lengths of several polypeptoids have been measured including those containing secondary structure or ionic groups in order to understand the effect of these factors on the chain conformation. Additionally, the collapse or folding of a single polypeptoid chain into a globule structure is discussed. The impact of monomer sequence on this collapse was investigated and shown to have an important effect both on the coil to globule transition as well as the resulting globule structure. Finally, a hierarchical super helix formed through the assembly of an amphiphilic diblock copolypeptoid is discussed. Using chemical modifications coupled with x-ray scattering, the super structure was shown to include ordering stemming from the angstrom level packing of molecules all the way up to the micron scale diameter of the helix.

Biological polymers, such as polypeptides, are responsible for many of life's most sophisticated functions due to precisely evolved hierarchical structures. These protein structures are the result of monodisperse sequences of amino acids that fold into well-defined chain shapes and tertiary structures. Recently, there has been much interest in the design of such sequence-specific polymers for materials applications in fields ranging from biotechnology to separations membranes. Non-natural polymers offer the stability and robustness necessary for materials applications; however, our ability to control monomer sequence in non-natural polymers has traditionally operated on a much simpler level. In addition, the relationship between monomer sequence and self-assembly is not well understood for biological molecules, much less synthetic polymers. Thus, there is a need to explore self-assembly phase space with sequence using a model system. Polypeptoids are non-natural, sequence-specific polymers that offer the opportunity to probe the effect of sequence on self-assembly.

A variety of monomer interactions have an impact on polymer properties, such as chirality, hydrophobicity, and electrostatic interactions. Thus, a necessary starting point for this project was to investigate monomer sequence effects on the bulk properties of polypeptoid homopolymers. It was found that several polypeptoids have experimentally accessible melting transitions that are dependent on the choice of side chains, and it was shown that this transition is tuned by the incorporation of "defects" or a comonomer. The polypeptoid chain shape is also controlled with the choice of monomer and monomer sequence. By using at least 50% monomers with bulky, chiral side chains, the polypeptoid backbone is sterically twisted into a helix, and as found for the first time in this work, the persistence length is increased. However, this persistence length, which is a measure of the stiffness of the polymer, is small compared to other folded helices, indicating the conformational flexibility of polypeptoid chains.

With a firmer understanding of how monomer sequence and composition influence polypeptoid bulk properties, we designed block copolymer systems for self-assembly. Because the governing parameters of block copolymer self-assembly are well understood, this architecture provides a convenient starting point for probing the effect of changing polymer sequence. We found that polystyrene-polypeptoid block copolymers readily self-assemble into hexagonally-packed and lamellar morphologies with long range order, and furthermore, sequence control of the polypeptoid block enables us to tune the strength of segregation (and therefore the order-disorder transition) of the block copolymer. Polypeptoid chain shape also affects self-assembly. In classical synthetic block copolymers, it has typically been difficult to change chain shape without also changing polymer chemistry and therefore other factors affecting self-assembly. The advantage of the polypeptoid system is that it is modular, as the side chain chemistry (and therefore polymer properties) can easily be changed without changing the backbone chemistry. Thus, we have decoupled conformational effects from chemical composition by comparing the self-assembly of block copolymers containing either a helical peptoid block or its racemic, non-helical analog. The increase in the persistence length of the peptoid block due to helicity translates to an increase in the morphological domain spacing.

In this work, we further the understanding of the effect of monomer sequence on bulk polypeptoid properties and self-assembly. Our findings pave the way for the rational design of structured synthetic polymers with tunable, sequence-specific properties.

Polymer chain conformation underlies polymer physical properties and impacts many of polymer functionalities. Understanding chain conformation is critical for predicting and controlling the structures and properties in polymeric systems. In block copolymers, chain conformation of the consisting blocks closely impacts the thermodynamics of microphase separation and the resultant structures, which are key to block copolymers as functional materials. However, the understanding of chain conformation effects beyond coil–coil block copolymers is yet nascent, partially due to the challenge to precisely control chain conformation without introducing other complicating factors.This dissertation utilizes sequence-defined polypeptoids to install precise chain conformation control into traditional polymer systems, to examine the role of chain conformation in block copolymer self-assembly. First, the polypeptoid chain conformation is examined in terms of local stiffness, overall chain size, and response to solvent quality by comparing chemically identical helical and coil polypeptoids in dilute solution. The detailed understanding from molecular length scale reveals that the helical secondary structure, driven by steric hindrance from side chains, makes the polypeptoid chains locally stiffer but overall more compact than the coil analogues. Further, we show these helical chains are relatively insensitive to solvent conditions due to their sterically defined nature. Then, through the design of model polypeptoid-containing block copolymer systems, we are able to study the effects of the helical chain conformation, which has distinct space-filling characteristics from chemically analogous coil chains, on the melt self-assembly of block copolymers. In the lamellae-forming system, the helical chain conformation is shown to decrease the order–disorder transition temperature through a combination of decreasing the enthalpic interaction between dissimilar blocks and experiencing amplified chain stretching. Further, polypeptoids are used as a conformation tuning handle and are shown to modulate network phase formation and stability as a conformationally tunable interfacial segment, which demonstrates the importance of chain conformation at the vicinity of the interface in determining the morphology of block copolymers. The findings in this dissertation highlight the importance of chain conformation on the self-assembly thermodynamics of block copolymers, and polypeptoids as highly-controlled, precise polymers to aid the fundamental understanding of polymeric materials.

The discovery and development of conjugated polymers has led to a large and vibrant research field due to their unique semiconducting properties and possibility of offering a completely new paradigm due to their abundant, lightweight, flexible and solution processable properties. In particular, the optoelectronic properties of these materials make them very well suited to applications such as organic light emitting diodes or organic photovoltaics and their relatively high charge mobility also make them useful in organic circuits. There are several reasons why the performance of these materials is presently limited compared to inorganic alternatives. Recently, significant work has been done trying to improve the performance of these materials by synthetically tuning the electronic properties such as the band gap and energy levels. There have been many other studies trying to improve intermolecular transport by enhancing crystallinity through annealing. In all of these studies the performance organic electronics still tends to be limited because the morphology of these materials is very complex and difficult to optimize. Many different length scales must be simultaneously optimized because the structure of a single polymer chain, their interactions with other polymer chains, the orientation of these chains and their degree of mixing with other components in the device all are extremely important to the performance of these materials.

In this work, block copolymers containing conjugated polymers are used to optimize the morphology of these materials through self assembly processes. Block copolymers can be used to produce a wide variety of thermodynamically stable morphologies with long range order and tunable domain sizes. The self assembly of conjugated polymers using block copolymers is complicated because the delocalization of electrons along the backbone, which produces their interesting semiconducting properties, also makes these polymers rod-like, liquid crystalline and drastically increases their intermolecular interactions. This thesis focuses on some of the basic issues that must be understood when trying to create and optimize techniques that can be used to produce self assembled structures containing conjugated polymers using block copolymers.

The chain shape of conjugated polymers is related to their intramolecular interactions and is a fundamental property of the polymer chemistry. This contributes to their specific properties that make every conjugated polymer behave slightly differently, making it difficult to build design rules that can be used for all conjugated polymers or even classes of polymers. By understanding the chain shape of these conjugated polymers, we can begin to understand how to tune their intermolecular interactions and block copolymer phase diagram. In this work we have examined the chain shape of polythiophenes, one of the most commonly used classes of conjugated polymers. We have also explored the use of polythiophenes in block copolymers to control the morphology in polymer photovoltaics. In order to optimize the properties of these materials it is also important develop techniques to align the block copolymer structure and the conjugated polymer backbone. This work examines the use of magnetic field alignment as a method to achieve these two goals. It has been shown that high degrees of alignment have been achieved with easily accessible field strengths. Additionally, work has been done examining methods to optimize the alignment procedure for magnetic field alignment of block copolymers by examining the effect of the field strength on the block copolymer phase behavior and through detailed work focusing on the dynamics of the alignment process. By building methods to optimize the morphology of block copolymer containing conjugated polymers we hope see these techniques applied to increase the performance of polymer optoelectronics.

Incorporating an ionic liquid into one block copolymer microphase provides a platform for combining the outstanding electrochemical properties of ionic liquids with a number of favorable attributes provided by block copolymers. In particular, block copolymers thermodynamically self-assemble into well-ordered nanostructures, which can be engineered to provide a durable mechanical scaffold and template the ionic liquid into continuous ion-conducting nanochannels. Understanding how the addition of an ionic liquid affects the thermodynamic self-assembly of block copolymers, and how the confinement of ionic liquids to block copolymer nanodomains affects their ion-conducting properties is essential for predictable structure-property control.

The lyotropic phase behavior of block copolymer/ionic liquid mixtures is shown to be reminiscent of mixtures of block copolymers with selective molecular solvents. A variety of ordered microstructures corresponding to lamellae, hexagonally close-packed cylinders, body-centered cubic, and face-centered cubic oriented micelles are observed in a model system composed of mixtures of imidazolium bis(trifluoromethylsulfonyl)imide ([Im][TFSI]) and poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP). In contrast to block copolymer/molecular solvent mixtures, the interfacial area occupied by each PS-b-P2VP chain decreases upon the addition of [Im][TFSI], indicating a considerable increase in the effective segregation strength of the PS-b-P2VP copolymer with ionic liquid addition.

The relationship between membrane structure and ionic conductivity is illuminated through the development of scaling relationships that describe the ionic conductivity of block copolymer/ionic liquid mixtures as a function of membrane composition and temperature. It is shown that the dominant variable influencing conductivity is the overall volume fraction of ionic liquid in the mixture, which means there is incredible freedom in designing the block copolymer architecture in order to optimize the mechanical and other properties of the membrane without sacrificing conductivity. The derived scaling relationships are shown to be general for many block copolymer and ionic liquid chemistries.

In certain cases, the mechanism of ion conduction in the ionic liquid is affected by block copolymer nanoconfinement. The introduction of excess neutral imidazole to [Im][TFSI] leads to enhanced proton conductivity as well as a high H+ transference number due to facilitated proton hopping between imidazole molecules. We show that there is increased proton hopping when the nonstoichiometric ionic liquid is confined to lamellar block copolymer nanodomains, which we hypothesize is due to changes in the hydrogen bond structure of the ionic liquid under confinement. This, in combination with unique ion aggregation behavior, leads to a lower activation energy for macroscopic ion transport compared to that in a corresponding homopolymer/ionic liquid mixture.

Through this work, we further the understanding of the relationship between membrane composition, structure, and ion transport. The findings presented herein portend the rational design of nanostructured membranes having improved mechanical properties and conductivity.

Waste heat recovery from the human body provides opportunities to power electronics with a source that is cheap and readily available. Thermoelectrics utilize the Seebeck effect to recover useable electrical energy from this waste heat, but are limited due to material parameters being inversely coupled in the bulk. We investigate the role of novel physics at interfaces in order to develop new fundamental understanding of thermoelectrics. The goal is to discover systems where the Seebeck coefficient and the electrical conductivity are not inversely correlated. We investigate thermoelectric transport in organic-organic systems such as scanning tunneling microscope molecular break junctions on the nanoscale, gold nanocrystal arrays on the mesoscale and polymeric ion and mixed conductors at the macroscale.

The STM molecular junctions studied in this work can provide design rules to positively couple the Seebeck coefficient and the electrical conductance. Since STM molecular junctions are one-dimensional systems, by minimizing the gap between the molecular orbital energy level and the electrode Fermi energy, the power factor S2σ can be optimized. I built a toolbox of chemical structures by first understanding the role of the interface coupling to alkylthiol binding groups of thiophene-based molecules. With this understanding, I designed small molecules based on the monomer unit of donor-acceptor polymers and other conductive polymers. Molecules with very high HOMO levels or low LUMO levels were studied, and the corresponding energy levels were examined using spectroscopic techniques.

I then present our work on scaling these molecular junctions to the macroscale using ligand-exchanged gold nanocrystal arrays. Beginning with a model system of alkanethiols and alkanedithiols, I show that the electrical conductivity scales with ligand length exactly as observed in single molecule junctions, and the Seebeck coefficient follows a similar trend. By showing that gold nanocrystal arrays are a good model platform to scale molecular junctions to the macroscale, I aim to bring the same Landauer transport physics observed in single molecule junctions to macroscale devices. I further investigate the role of different binding groups, where I find that the character of the ligand binding group directly affects whether the sign of the Seebeck coefficient of the overall array.

Finally, I elaborate on our work with conductive polymers as thermoelectric materials, and specifically investigate the thermoelectric effect in polymers that are solely ion conductors or are capable of both ions and electrons transport. I demonstrate that thermoelectrics based ionic or mixed conduction can be well-described using the developed fundamental equations, and by using these rules we develop positive or negative Seebeck coefficient materials. I present our work on the newly developed thermoelectric AFM technique and how we use the thermoelectric response of different domains to understand the morphology of PEDOT:PSS. By fully understanding the thermoelectric properties of organic-inorganic hybrids from the monomer level, we can extend this same understanding to develop polymer thermoelectrics.

Recent environmental factors have triggered a strong interest towards the development of scalable technologies that can increase the share of renewable sources into our energy mix. Artificial photosynthesis systems are a promising alternative as they can simultaneously capture and store solar energy in the form of a fuel. These systems are based on photoelectrochemical (PEC) cells that can take low energy density reactants such as water and/or carbon dioxide and transform them into energy dense hydrogen or carbon containing molecules via light-driven processes. Deployable solar-fuel generators need to be able to produce fuels in a robust, scalable, and efficient manner. Despite the large number of studies focusing on this technology since its inception in the early 1970's, a system that can satisfy those three requirements does not exist. Significant innovation is required to develop cost-effective components that can perform the light-absorption, catalytic redox reaction, ion transport and product separation requirements. Additionally, understanding of component performance in integrated devices is crucial for developing high efficiency solar-fuel generators.

This dissertation focusses on several aspects of component integration in solar-hydrogen generators. The initial focus involves the development of self-assembly techniques of nanometer scale units to obtain architectures necessary for solar-fuel devices. Starting with solutions of semiconducting nanorods and polymers, this work demonstrates that by controlling the evaporation rate during solvent casting, arrays of vertically aligned nanorods embedded in polymer films can be obtained over large areas (> 1 cm2). This architecture is desirable for the integration of hydrogen generating nanorods into integrated water splitting membranes, where H2 and O2 are evolved at physically separated sites. This work also describes how the structure of proton conducting membranes (Nafion®) is affected at inorganic interfaces such as the ones present in solar-fuels devices. The effects of thin-film confinement and wetting interactions are studied in Nafion thin-film model systems using a combination of X-ray scattering and mass transport characterization techniques. These studies show how confinement of Nafion films to thicknesses below 10 nm results in significant limitations to self-assembly, disruption of phase separation in the material and ultimately decrease in ionic conductivity. Wetting interaction also play a role in the orientation of conducting domains in the material. Hydrophobic surfaces results in a parallel orientation of ionic domains while films cast on hydrophilic substrates result in an isotropic orientation of domains. The differences in domain orientation also impact the mass transport behavior of the material.

Additionally, this dissertation covers several topics related with the integration of components for the fabrication of practical hydrogen generators. Here we describe the development of a microfluidic test-bed for the incorporation of catalytic and membrane components in scales amenable for research. This tool allows for the simple exchange and quantitatively assessment of the performance of integrated electrochemical fuel generating devices as well as each of the individual components that participate in the process. Lastly, this work also describes engineering solutions that allow both membrane-separated water electrolyzers and solar hydrogen generators to operate under buffered electrolytes. This is achieved by using supporting electrolytes to carry the ionic current through the membrane while controlled convective streams around the membranes are implemented to avoid the formation of large concentration gradients between reaction sides. This development opens up the space of operating electrolytes for the incorporation of wide range of components that are not stable under strong basic or acidic conditions.

Polymeric battery binders are a ubiquitous component in composite lithium-ion cathodes, providing critical structural functionality. However, industry standard binders, such as polyvinylidene fluoride (PVDF) are insulating to both electrons and ions and are thus detrimental to performance. Mixed ion-electron conducting polymers are promising materials for next generation battery binders, as they can provide the adhesive properties of traditional binders, while also facilitating charge transport. However, simultaneously optimizing electronic, ionic, and lithium transport within a single system has proved a challenge, particularly given the need to maintain the mechanical function required of a binder. This work elucidates polymer design strategies for simultaneous lithium-electron conduction, while also considering the practical requirements of a battery binder (i.e. processability, electrochemical stability, and solubility). First, it is shown that side chain engineering can be used to control lithium transport in semiconducting polymers, emphasizing the importance of solvating ions without trapping them. This concept is further explored, studying Li+ transport and ionic/electronic conduction in a family of polythiophenes functionalized with cationic side chains. It is found that the interaction strength between the side chain and added salt is critical for ion transport, while the structure of the side chain largely governs electron transport. These fundamental insights are then bridged into real battery binders, showing that an electrostatically stabilized complex, comprising of a blend of a charged conjugated polymer with an oppositely charged polyelectrolyte, reduces kinetic limitations in LiFePO4 cathodes. The conducting binder dramatically improves both rate capability and cycle stability, compared to the industry standard, insulating PVDF binder. Finally, the method of electrostatically stabilizing conjugated polymer complexes is shown to be an effective platform for mixed conducting binders, as several polymer chemistries afford high-performing, conductive binders.

Polymers are commonly used in applications requiring long-term exposure to water and aqueous mixtures including as medical implants, marine antifouling coatings, and water purification membranes. In all of these, the chemistry and structure of the polymer surfaces determine the effectiveness of the final material via interactions with water, dispersed solutes, and larger biomolecular structures. In the past, scientific progress has been made primarily through empirical results derived from chemically distinctive coatings and membranes, but design rules are essential for improving the rate of advancement.

This work leverages a two-pronged approach for relating surface chemistry to its effects on interactions at the water–polymer surface. Design of surfaces containing sequence-defined, peptidomimetic “peptoid” side chains provides fine control over both chemistry and the relative spacing between functional groups, while self-assembled monolayers (SAMs) allow for investigation of model surfaces consisting of a single functional group with negligible surface roughness. This work also includes the development of ambient pressure X-ray photoelectron spectroscopy (APXPS) for studying the chemistry and surface interactions in hydrated polymer surfaces. First, the presence or lack of hydrogen bonding in marine antifouling coatings is found to alter both the polymer’s interactions with water and with soft algal foulers. Afterwards, the capability of side chains to present at the surface and modify polymer hydrophilicity is investigated using APXPS. Finally, the effects of functional group chemistry on the adsorption of small organic molecules are explored in APXPS experiments on model SAM surfaces. The findings of this work indicate that polymer surface chemistry can be modulated by minor changes in composition, and that these changes can have large effects on surface interactions with water, organic molecules, and organisms.