Morphology and self-assembly behavior of side chain liquid crystalline block copolymers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references.There is significant interest from both the academic and industrial communities for understanding and controlling the self-assembly behavior of complex macromolecular systems and has been an active area of research in recent years. Such systems can be designed to result in a wide range of nanoscale morphologies and greater functionality can be introduced with increasing complexity.This thesis focuses on the synthesis and characterization of a class of side chain liquid crystalline block copolymers (SCLCBCPs) that are based on a low glass transition temperature (Tg) siloxane backbone. Moieties that self-assemble into smectic liquid crystalline (LC) phases are covalently attached to the polystyrene-polyvinylmethylsiloxane (PS-PVMS) block copolymer backbone. Precise control over the functionalization of the LCs onto the functional siloxane backbone allows for unique control over the self-assembly and the resulting properties of the system. The LC content significantly affects the stability of the smectic mesophase and subsequently the interactions with the inter-material dividing surface (IMDS) with the PS domains. A strong preference for homogenous anchoring of the LC moieties relative to the IMDS is observed, and increasing the LC content intensifies the preference for this arrangement. Utilizing the effects of LC anchoring to alter the self-assembly behavior is a reoccurring theme throughout this work. Additionally, the mechanical properties of these materials can be precisely manipulated over several orders of magnitude through variations in LC content and the block copolymer backbone architecture.Several methods can be used to manipulate the morphologies of these materials once synthesized including, thermal annealing and mechanical deformation.(cont.) Thermal annealing provides additional mobility for self-assembly often resulting in morphological rearrangements. Mechanical deformation can be used to orient the self-assembled structures relative to an applied shear flow. Additionally, the self-assembled morphologies of spin cast into thin films were investigated. The presence of the substrate has significant effects upon the orientation of the morphologies; thermal annealing and variations liquid crystal content are shown to be useful tools for achieving a wide range of thin film morphologies.by Eric Anton Verploegen.Ph.D

Tuning Charge Transport in Solution-Sheared Organic Semiconductors Using Lattice Strain

Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33Å to 3.08 Å. We believe that 3.08Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04Å has been achieved through intramolecular bonding). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from $0.8 cm^2 V^{−1} s^{−1}$ for unstrained films to a high mobility of $4.6 cm^2 V^{−1} s^{−1}$ for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.Chemistry and Chemical Biolog

Effects of Odd–Even Side Chain Length of Alkyl-Substituted Diphenylbithiophenes on First Monolayer Thin Film Packing Structure

Because of their preferential two-dimensional layer-by-layer growth in thin films, 5,5′bis(4-alkylphenyl)-2,2′-bithiophenes (P2TPs) are model compounds for studying the effects of systematic chemical structure variations on thin-film structure and morphology, which in turn, impact the charge transport in organic field-effect transistors. For the first time, we observed, by grazing incidence X-ray diffraction (GIXD), a strong change in molecular tilt angle in a monolayer of P2TP, depending on whether the alkyl chain on the P2TP molecules was of odd or even length. The monolayers were deposited on densely packed ultrasmooth self-assembled alkane silane modified $SiO_2$ surfaces. Our work shows that a subtle change in molecular structure can have a significant impact on the molecular packing structure in thin film, which in turn, will have a strong impact on charge transport of organic semiconductors. This was verified by quantum-chemical calculations that predict a corresponding odd–even effect in the strength of the intermolecular electronic coupling.Chemistry and Chemical BiologyEngineering and Applied Science

Impact of regioregularity on thin-film transistor and photovoltaic cell performances of pentacene-containing polymers

Regioregular pentacene-containing polymers were synthesized with alkylated bithiophene (BT) and cyclopentadithiophene (CPDT) as comonomers. Among them, 2,9-conjugated polymers PnBT-2,9 and PnCPDT-2,9 achieved the best performance in transistor and photovoltaic devices respectively. The former achieved the most highly ordered structures in thin films, yielding ambipolar transistor behavior with hole and electron mobilities up to 0.03 and 0.02 cm(2) V-1 s(-1) on octadecylsilane-treated substrates. The latter achieved photovoltaic power conversion efficiencies up to 0.33%. The impact of regioregularity and direction of conjugation-extension (2,9 vs. 2,10), on thin-film order and device performance has been demonstrated for the pentacene-containing polymers for the first time, providing insight towards future functional material design.close101

Energy Assessment Toolkit Introduction (v1.02)

A community focused approach for identifying energy needs and market opportunitiesMalcom Standburg, Community Jameel, Paul Antony, Mercy Corps, and the Shell Foundatio

Evaporative Cooling for Improved Vegetable and Fruit Storage in Rwanda and Burkina Faso

The horticulture sector plays a vital role in supporting human nutrition and income generation for farmers in Rwanda and Burkina Faso. A lack of affordable and effective postharvest fruit and vegetable storage solutions often leads to spoilage, loss of income, reduced access to nutritious foods, and significant amounts of time spent traveling to sell and purchase fresh produce (fruits and vegetables), particularly in rural communities. Studies conducted in Rwanda and Burkina Faso indicate that postharvest losses for perishable products like tomatoes are between 50-60%. The objective of this research study is to investigate the potential for non-electric evaporative cooling devices to address challenges of postharvest fruit and vegetable storage in Rwanda and Burkina Faso. The two classes of devices evaluated in this study are commonly known as “Zero Energy Cooling Chambers” (ZECCs), which are generally used by horticulture farmers, farmer groups and cooperatives, and “clay pot coolers,” which are generally used in households. These devices rely on the evaporation of water to create a cooling effect, and their performance is significantly affected by the ambient temperature and humidity of the environment in which they operate. In this study, we used a combination of electronic sensors, fruit and vegetable shelf life measurements, and structured user interviews to gather information about users’ needs for improved postharvest storage, current methods of postharvest storage, and the performance of the evaporative cooling devices