Thermal Control of Confined Crystallization within P3EHT Block Copolymer Microdomains

The local, nanoscale organization of crystallites in conjugated polymers is often critical to determining the charge transport properties of the system. Block copolymer geometries, which offer controlled nanostructures with tethering of chains at interfaces, are an ideal platform to study the local organization of conjugated polymer crystallites. The model conjugated polymer poly­(3-(2′-ethyl)­hexyl­thiophene) (P3EHT) features a depressed melting temperature relative to the widely studied poly­(3-hexylthiophene) (P3HT), which allows it to robustly form microphase-separated domains that confine the subsequent P3EHT crystallites. Importantly, P3EHT crystallization in confinement is coupled to a rubbery second block via interfacial tethering, mechanical properties, and chain stretching. Here, the impact of thermal processing on the diblock copolymer structure is examined to elucidate the key driving forces controlling the final coupled diblock copolymer and crystalline structures. Surprisingly, the diblock copolymer domain size is significantly impacted by the temperature at which the conjugated domain is crystallized. Decreasing amounts of domain extension are observed with increasing crystallization temperatures. This temperature-dependent domain structure appears to be correlated with the crystallization processes; these processes are inferred from precise changes in the lamellar structure across melting. By carefully tracking the changes in domain structure across melting, this work identifies three distinct regimes. We suggest a structural model of the conjugated block melting processes consisting of (I) excluded-chain relaxation, followed by (II) chain interdigitation during melt-recrystallization, and finally (III) complete melting that is independent of the initial crystallization conditions. These results suggest that P3EHT crystallization processes associated with temperature-dependent chain diffusion and nucleation are primarily responsible for the unexpected temperature-dependent crystallization behavior. They also emphasize that less perfect conjugated polymer crystals may actually be associated with a poorly interdigitated structure. Furthermore, this work demonstrates the utility of leveraging a diblock copolymer structure with a rubbery second block in order to precisely track changes in the crystallite structure

Conductivity Scaling Relationships for Nanostructured Block Copolymer/Ionic Liquid Membranes

To optimize the properties of membranes composed of mixtures of block copolymers with ionic liquids, it is essential to understand universal scaling relationships between composition, structure, temperature, and ionic conductivity. In this work, we demonstrate the universality of relationships developed to describe the temperature and concentration dependence of ionic conductivity in such membranes by comparing the conductivity behavior of mixtures of ionic liquid with two block copolymer chemistries. The conductivities of all the mixtures are described by a single expression, which combines percolation theory with the Vogel–Tamman–Fulcher (VTF) equation. Percolation theory describes the power law dependence of conductivity on the overall volume fraction of ionic liquid, while the VTF equation takes into account the effect of the glass transition temperature of the conducting phase on the temperature dependence. The dominance of the overall volume fraction of ionic liquid in determining conductivity indicates that there is incredible flexibility in designing highly conductive block copolymer/ionic liquid membranes

Confined Crystallization within Cylindrical P3EHT Block Copolymer Microdomains

Confinement of crystallites within block copolymer microdomains is a promising approach to study conjugated polymer crystallization due to interfacial chain tethering and defined geometries. The nanoscale organization of crystallites is often critical to determining the charge transport properties of conjugated polymers. Here, a poly­(3-(2′-ethyl)­hexyl­thiophene)-<i>block</i>-poly­(methyl acrylate) (P3EHT-<i>b</i>-PMA) system is leveraged to study the impact of confinement within cylindrical microdomains. The crystalline P3EHT permits accessible melting temperatures and robust formation of traditional microphase-separated morphologies, while the rubbery PMA allows the local deformations required to permit P3EHT crystallization. Crystallites form with chains perpendicular to the diblock interface, causing domain expansion; TEM reveals that this is accommodated in the cylindrical geometry via local deformation. Complementary SAXS/WAXS of aligned diblocks shows preferential orientation of the alkyl chain stacks down domains. Furthermore, cylindrically confined P3EHT demonstrates a smaller window of thermal control over crystalline perfection via isothermal crystallization conditions than homopolymer P3EHT or block copolymer P3EHT in lamellar confinement. This work demonstrates that postcrystallization annealing is an alternative route to generating uniformly high quality crystallites in cylindrically confined P3EHT. These results are important for considering routes to optimizing and controlling crystallinity in nanoscale confined geometries

Melting Behavior of Poly(3-(2′-ethyl)hexylthiophene)

While polymer materials possess significant promise as components in large-area organic electronic devicessuch as thin-film transistors or photovoltaic devicesthe ability to improve the performance of these materials is critically linked to understanding and controlling the morphology, namely control of crystallinity, crystallite size, and texture. In this context, conjugated poly­(3-alkylthiophenes) are a model system for studying the structure–property relationships in conjugated polymers. Herein, we examine P3EHT as a model polymer for exploring crystallization in P3ATsas it has a final melting transition well below degradation in contrast to the more common P3HTusing differential scanning calorimetry (DSC) and wide-angle X-ray scattering. Notably, examination of the melting endotherms following isothermal crystallization of P3ATsnamely poly­(3-hexylthiophene) (P3HT) and poly­(3-(2′ethyl)­hexylthiophene) (P3EHT)reveals a bimodal final melting peak. Differential scanning calorimetry reveals a shift in the lower temperature peak to higher temperatures as the isothermal crystallization temperature is raised and convergence into a single observed endothermic peak at high crystallization temperatures. Complementary wide-angle X-ray scattering experiments reveal an increase in crystallite perfection along the π–π stack direction at higher crystallization temperatures. Thus, properties of the P3EHT crystallite populations, average size and/or perfection, can be deliberately manipulated through control of the isothermal crystallization temperature. We further determine that the bimodal nature of P3EHT’s melting behavior is a consequence of a melt-recrystallization mechanism and observe perfection of the π–π stack direction during the melt-recrystallization process. Lastly, we utilize the obtained final melting temperatures to elucidate values for Δ<i>H</i>_m⁰ and <i>T</i>_m⁰, 20 ± 4 J/g and 92 °C, respectively

Thermal Conductivity of High-Modulus Polymer Fibers

Polymers have many desirable properties for engineering systems–e.g., low mass density, chemical stability, and high strength-to-mass ratio–but applications of polymers in situations where heat transfer is critical are often limited by low thermal conductivity. Here, we leverage the enormous research and development efforts that have been invested in the production of high-modulus polymer fibers to advance understanding of the mechanisms for thermal transport in this class of materials. Time-domain thermoreflectance (TDTR) enables direct measurements of the axial thermal conductivity of a single polymer fiber over a wide temperature range, 80 < <i>T</i> < 600 K. Relaxation of thermoelastic stress in the Al film transducer has to be taken into account in the analysis of the TDTR data when the laser spot size is small because the radial modulus of the fiber is small. This stress relaxation is controlled by the velocity of the zero-order symmetric Lamb mode of a thin Al plate. We find similarly high thermal conductivities of Λ ≈ 20 W m^–1 K^–1 in crystalline polyethylene and liquid crystalline poly­(<i>p</i>-phenylene benzobisoxazole). For both fiber types, Λ­(<i>T</i>) ∝ 1/<i>T</i> near room temperature, suggesting an intrinsic limit to the thermal conductivity governed by anharmonicity, not structural disorder. Because of the high degree of elastic anisotropy, longitudinal acoustic phonons with group velocities directed along fiber axis are likely to be the dominate carriers of heat

Improving the Gas Barrier Properties of Nafion via Thermal Annealing: Evidence for Diffusion through Hydrophilic Channels and Matrix

Oxygen diffusion through commercial Nafion films is investigated using a transient electrochemical reduction method. Direct experimental evidence is found for two Fickian diffusion coefficients corresponding to oxygen permeation through the hydrophilic channels and fluorocarbon matrix. The diffusion coefficient and solubility of oxygen in each phase, which controls the overall permeability, can be tuned via thermal annealing where films annealed at 160 °C have a substantially reduced oxygen permeability relative to the as-received material. Films annealed at 200 °C show intermediate oxygen permeation to the as-received and 160 °C annealed samples. Differential scanning calorimetry and wide-angle X-ray scattering are employed to demonstrate that increasing crystallinity reduces the oxygen solubility and diffusion coefficient through the Nafion matrix. The two oxygen diffusion coefficients are discussed in the context of literature values which span more than an order of magnitude. It is found that the time scale of the experiments plays a substantial role in the measured diffusion coefficient with short time scale (<30 s) experiments sensing primarily diffusion through the hydrophilic channels. Longer time scale experiments (>300 s) are able to sense both modes of oxygen permeation

Effect of Confinement on Proton Transport Mechanisms in Block Copolymer/Ionic Liquid Membranes

Nanostructured membranes containing structural and proton-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled to high thermal stability. Understanding the effect of nanodomain confinement on proton-conducting properties in such materials is essential for designing new, improved membranes. This relationship has been investigated for a lamellae-forming mixture of poly­(styrene-<i>b</i>-2-vinyl pyridine) (PS-<i>b</i>-P2VP) with ionic liquid composed of imidazole and bis­(trifluoromethylsulfonyl)­imide, where the ionic liquid selectively resides in the P2VP domains of the block copolymer. Quasi-elastic neutron scattering and NMR diffusion measurements reveal increased prevalence of a fast proton hopping transport mechanism, 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 with that in a mixture of ionic liquid with P2VP homopolymer. The proton transference number in both samples is significantly higher than that in the neat ionic liquid, which could be taken advantage of for applications such as proton exchange membrane fuel cells and actuators. These results portend the rational design of nanostructured membranes having improved mechanical properties and conductivity

Anhydrous Proton Transport in Polymerized Ionic Liquid Block Copolymers: Roles of Block Length, Ionic Content, and Confinement

Anhydrous proton transport has been investigated in a series of proton conducting polystyrene-<i>block</i>-polymerized ionic liquid (PS-<i>b</i>-PIL) copolymers spanning a range of molecular weights and compositions. The PIL is a macromolecular analogue of imidazolium bis­(trifluoro­methane sulfonimide) (ImTFSI), a well-known proton conducting ionic liquid, and consists of imidazole linked to a polymer backbone via the 5-carbon. In contrast to prior work on nitrogen-linked imidazolium PILs, carbon-linked imidazolium has two nitrogens which can both function as proton donor/acceptors and participate in Grotthus mechanism conduction. The conductivity of the PIL block is shown to be dramatically impacted upon confinement by a PS block and can exceed the conductivity of the homopolymer in the range of 30–130 °C for PIL-rich block copolymer composition. At high temperature the conductivities track with ionic content while at room temperature the conductivities are nonmonotonic. X-ray scattering reveals a suppression of the peak associated with ionic aggregation in all block copolymers relative to the homopolymer consistent with the higher conductivities observed at room temperature. The dependence of ionic conductivity on temperature, as quantified by the VFT strength parameter <i>D</i>, decreases with decreasing PIL block length corresponding to a change in the packing efficiency of the conductive block. These changes in packing are hypothesized to lead to the different temperature dependences of conductivity which cause the nonmonotonic block copolymer conductivities observed at room temperature. Finally, we demonstrate that the fraction of PIL in the block copolymer is the main factor governing the high temperature ionic conductivity of these materials while confinement effects become important at room temperature

Tunable Phase Behavior of Polystyrene–Polypeptoid Block Copolymers

Block copolymers with tunable compositions offer the ability to directly control the interaction strength between the two blocks and therefore polymer properties. The miscibility of an A–B block copolymer can be increased by introducing B or B-like comonomers into the A block, and literature has shown that both the amount and the distribution of these comonomers affect the compatibility of the two blocks. Sequence-defined block copolymers in which one can exactly control the composition and comonomer distribution provide a unique opportunity to control the overall strength of segregation. Here, sequence-defined block copolymers have been synthesized via azide–alkyne coupling using polystyrene and sequence-specific polypeptoids (N-substituted glycines) with 2-methoxyethyl side chains. These polystyrene-polypeptoid block copolymers readily self-assemble into hexagonally packed and lamellar morphologies. <i>N</i>-(2-phenylethyl)­glycine residues, which have a styrene-like side chain, were introduced throughout the polypeptoid block to increase the compatibility with the polystyrene block. As the compatibility increased, the strength of segregation and therefore the block copolymer order–disorder transitions decreased. The polystyrene–polypeptoid block copolymers provide a tunable platform for further studies on the effect of composition and sequence design on self-assembly and block copolymer properties

Molecular Considerations for Mesophase Interaction and Alignment of Lyotropic Liquid Crystalline Semiconducting Polymers

Intermolecular interactions in conjugated polymers influence crystallinity, self-assembly, and packing motif, factors which in turn crucially impact charge transport properties such as carrier mobility in organic electronic devices. Correlated alignment of molecular and crystalline morphologies provides direct pathways for charge carriers to follow; however, the role of intermolecular interactions in achieving this is unexplored. Herein, we synthesize a series of lyotropic liquid crystalline conjugated polymers with variable side-chain structure to lend distinct steric repulsion and van der Waals attractive forces to each mesophase. We use this to investigate the role of intermolecular interactions on mesophase alignment. The strength of intermolecular interaction for each mesophase is compared by measuring melting temperature, π-stacking distance, and the Maier–Saupe interaction parameter. In general we find that side-chain structure can impact interaction strength by varying steric repulsion and backbone attractions and that the Maier–Saupe interaction parameters correlate with higher degrees of alignment after shearing, achieving a dichroic absorbance ratio of up to 2. This observation is used to develop equilibrium processing methods for fabricating macroscopically aligned polymer substrates used in transistors, improving mobility by a factor of 3 compared to spin-coated devices