Controlling Domain Spacing and Grain Size in Cylindrical Block Copolymer Thin Films by Means of Thermal and Solvent Vapor Annealing

Real-time grazing-incidence small-angle X-ray scattering (GISAXS) experiments were used to study the self-assembly of cylinder-forming block copolymers (BCPs) in thin films during thermal annealing and solvent vapor annealing. BCP thin films were annealed in near-neutral solvent vapor for solvent vapor annealing and on a hot plate under an inert gas atmosphere for thermal annealing. The initially ordered films were heated or swollen to induce an order–disorder transition (ODT) and then cooled or the solvent was removed, respectively. The domain spacings of BCPs as determined from <i>in situ</i> GISAXS measurements during solvent removal and cooling were analyzed with respect to the polymer concentration and the reciprocal temperature. Close to the ODT the domain spacing was found to be nearly identical for thermal and solvent vapor annealing. At lower solvent concentrations ϕ and lower temperatures <i>T</i>, the domain spacing was found to increase for both thermal and solvent vapor annealing until structural reorganization in the film was limited by the slow kinetics at solvent concentrations and temperatures close to the glass transition. In this regime, the domain spacing in solvent annealed films was found to be higher than that in thermally annealed films, which is likely due to a significantly smaller diffusion coefficient in the case of thermal annealing. On the basis of an <i>ex situ</i> scanning electron microscopy characterization of annealed block copolymer thin films, we show that the grain size of the cylindrical microdomains can be strongly increased by annealing films close to the ODT. Well below ϕ_ODT and <i>T</i>_ODT the formation of large grains is kinetically limited. In thermally annealed films the grain size was found to be smaller than that for the solvent annealed films, which was attributed to a smaller diffusion coefficient in the absence of solvent

Subsecond Morphological Changes in Nafion during Water Uptake Detected by Small-Angle X-ray Scattering

The ability of the Nafion membrane to absorb water rapidly and create a network of hydrated interconnected water domains provides this material with an unmatched ability to conduct ions through a chemically and mechanically robust membrane. The morphology and composition of these hydrated membranes significantly affects their transport properties and performance. This work demonstrates that differences in interfacial interactions between the membranes exposed to vapor or liquid water can cause significant changes in kinetics of water uptake. In situ small-angle X-ray scattering (SAXS) experiments captured the rapid swelling of the membrane in liquid water with a nanostructure rearrangement on the order of seconds. For membranes in contact with water vapor, morphological changes are four orders-of-magnitude slower than in liquid water, suggesting that interfacial resistance limits the penetration of water into the membrane. Also, upon water absorption from liquid water, a structural rearrangement from a distribution of spherical and cylindrical domains to exclusively cylindrical-like domains is suggested. These differences in water-uptake kinetics and morphology provide a new perspective into Schroeder's paradox, which dictates a different water content for vapor- and liquid-equilibrated ionomers at unit activity. The findings of this work provide critical insights into the fast kinetics of water absorption of the Nafion membrane, which can aid in the design of energy conversion devices that operate under frequent changes in environmental conditions

Controlling Nafion Structure and Properties via Wetting Interactions

Proton conducting ionomers are widely used for electrochemical applications including fuel-cell devices, flow batteries, and solar-fuels generators. For most applications the presence of interfacial interactions can affect the structure and properties of ionomers. Nafion is the most widely used ionomer for electrochemical applications due to their remarkable proton conductivity and stability. While Nafion membranes have been widely studied, the behavior and morphology of this ionomer under operating conditions when confined to a thin-film morphology are still not well understood. Using <i>in situ</i> grazing-incidence small-angle X-ray scattering (GISAXS) techniques, this work demonstrates that the wetting interaction in thin-film interfaces can drastically affect the internal morphology of ionomers and in turn modify its transport properties. Thin films cast on hydrophobic substrates result in parallel orientation of ionomer channels that retard the absorption of water from humidified environments; while films prepared on SiO₂ result in isotropic orientation of these domains, thus favoring water sorption and swelling of the polymer. Furthermore, the results presented in this paper demonstrate that upon thermal annealing of Nafion thin films static crystalline domains form within the polymer matrix that restrict further water uptake. The results presented in this study can aid in the rational design of functional composite materials used in fuel-cell catalyst layers and solar-fuels devices

Signatures of Multiphase Formation in the Active Layer of Organic Solar Cells from Resonant Soft X‑ray Scattering

Resonant soft X-ray scattering (RSOXS) is a complementary tool to existing reciprocal space methods, such as grazing-incidence small-angle X-ray scattering, for studying order formation in polymer thin films. In particular, RSOXS can exploit differences in absorption between multiple phases by tuning the X-ray energy to one or more resonance peaks of organic materials containing carbon, oxygen, nitrogen, or other atoms. Here, we have examined the structural evolution in poly­(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C₆₁-butyric acid methyl ester mixtures by tuning X-rays to resonant absorption energies of carbon and oxygen. Our studies reveal that the energy dependence of RSOXS profiles marks the formation of multiple phases in the active layer of organic solar cells, which is consistent with elemental maps obtained through energy-filtered transmission electron microscopy

Signatures of Intracrystallite and Intercrystallite Limitations of Charge Transport in Polythiophenes

Charge carrier mobilities in conjugated semicrystalline polymers depend on morphological parameters such as crystallinity, crystal orientation, and connectivity between ordered regions. Despite recent progress in the development of conducting polymers, the complex interplay between the aforementioned parameters and their impact on charge transport is not fully understood. By varying the casting solvents and thermal annealing, we have systematically modulated the crystallization of poly­(3-hexyl­thiophene-2,5-diyl) (P3HT) and poly­[2,5-bis­(3-hexadecyl­thiophen-2-yl)­thieno­(3,2-<i>b</i>)­thiophene] (PBTTT) thin films to examine the role of microstructure on charge mobilities. In particular, we achieve equal crystallinities through different processing routes to examine the role of structural parameters beyond the crystallinity on charge mobilities. As expected, a universal relationship does not exist between the crystallinity in either P3HT and PBTTT active layers and the charge mobility in devices. In P3HT films, higher boiling point solvents yield longer conjugation lengths, an indicator of stronger intracrystalline order, and therefore higher device mobilities. In contrast, the charge mobilities of PBTTT devices depend on the interconnectivity between crystallites and intercrystalline order in the active layer

Fluorination of Donor–Acceptor Copolymer Active Layers Enhances Charge Mobilities in Thin-Film Transistors

Several recent reports have demonstrated that fluorinated analogues of donor/acceptor copolymers surpass nonfluorinated counterparts in terms of performance in electronic devices. Using a copolymer series consisting of fluorinated, partially fluorinated, and nonfluorinated benzotriazole, we confirm that the addition of fluorine substituents beneficially impacts charge transport in polymer semiconductors. Transistor measurements demonstrated a factor of 5 increase in carrier mobilities with the degree of fluorination of the backbone. Furthermore, grazing-incidence X-ray diffraction data indicates progressively closer packing between the conjugated cores and an overall greater amount of π-stacking in the fluorinated materials. It is likely that attractive interactions between the electron-rich donor and fluorinated electron-deficient acceptor units induce very tightly stacking crystallites, which reduce the energetic barrier for charge hopping. In addition, a change in crystallite orientation was observed from primarily edge-on without fluorine substituents to mostly face-on with fluorinated benzotriazole

Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation

Organic electronic materials have the potential to impact almost every aspect of modern life including how we access information, light our homes, and power personal electronics. Nevertheless, weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. Here, we demonstrate control of donor–acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. X-ray scattering results reveal that the remarkable performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. Conjugated block copolymers thus provide a pathway to enhance performance in excitonic solar cells through control of donor–acceptor interfaces

Probing and Controlling Liquid Crystal Helical Nanofilaments

We report the first in situ measurement of the helical pitch of the helical nanofilament B4 phase of bent-core liquid crystals using linearly polarized, resonant soft X-ray scattering at the carbon K-edge. A strong, anisotropic scattering peak corresponding to the half-pitch of the twisted smectic layer structure was observed. The equilibrium helical half-pitch of NOBOW is found to be 120 nm, essentially independent of temperature. However, the helical pitch can be tuned by mixing guest organic molecules with the bent-core host, followed by thermal annealing

Toward Strong Thermoplastic Elastomers with Asymmetric Miktoarm Block Copolymer Architectures

Thermoplastic elastomers (TPEs) are designed by embedding discrete glassy or semicrystalline domains in an elastomeric matrix. Typical styrenic-based amorphous TPEs are made of linear ABA-type triblock copolymers, where the volume fraction <i>f</i> of the glassy domains A is typically less than 0.3. This limitation ultimately restricts the range of mechanical strength attainable with these materials. We had previously predicted using self-consistent field theory (SCFT) that A­(BA′)_<i>n</i> miktoarm block copolymers with an approximately 8:1 ratio of the A to A′ block molecular weights and <i>n</i> ≥ 3 should exhibit discrete A domains at considerably larger <i>f</i> and offer potential for the combination of high modulus, high recoverable elasticity, and high strength and toughness. Using transmission electron microscopy and small-angle X-ray scattering on model polystyrene-<i>b</i>-polyisoprene (PS–PI) miktoarm copolymers, we show that such polymers indeed possess discrete PS domains for <i>f</i> values considerably higher than 0.3. The hexagonal morphology with PS cylinders was achieved for <i>f</i> = 0.5 and <i>n</i> = 3. Mechanical testing indicates that these miktoarm materials are strong, tough, and elastic and thus may be potential candidates for a new generation of thermoplastic elastomers