The kinetics of UV radiation-induced fast collapse and recovery in thermally cycled and rehydrated light- and thermo- double-responsive copolymer films of poly(oligo(ethylene glycol) methyl ether ...methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated as P(OEGMA300-co-PAHA), are probed by in situ neutron reflectivity (NR). The copolymer film is exposed to a thermal treatment starting at a temperature of 60 °C, which is well above its transition temperature (TT = 53 °C) before the temperature is rapidly decreased from 60 to 23 °C. Based on the applied protocol, the initially collapsed P(OEGMA300-co-PAHA) film is rehydrated due to the switching of polymer chains from a more hydrophobic to a more hydrophilic state when the temperature falls below its TT. The whole rehydration process can be divided into 3 stages: D2O absorption, chain rearrangement, and film reswelling. After rehydration, the thermally cycled P(OEGMA300-co-PAHA) film is switched by UV irradiation via setting the UV radiation on and off. Considering the UV-induced collapse and recovery, both processes are slower than those observed in freshly hydrated films without any thermal stimulus history. Therefore, the experienced thermal history of the film should be considered in the design of sensors and detectors based on double-responsive copolymer films.
The swelling and co-nonsolvency behaviors in pure H2O and in a mixed H2O/CH3OH vapor atmosphere of two different polar, water-soluble polymers in thin film geometry are studied in situ. Films of a ...zwitterionic poly(sulfobetaine), namely, poly3-((2-(methacryloyloxy)ethyl)dimethylammonio) propane-1-sulfonate (PSPE), and a polar nonionic polymer, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), are investigated in real time by spectral reflectance (SR) measurements and Fourier transform infrared (FTIR) spectroscopy. Whereas PSPE is insoluble in methanol, PNIPMAM is soluble but exhibits co-nonsolvency behavior in water/methanol mixtures. First, the swelling of PSPE and PNIPMAM thin films in H2O vapor is followed. Subsequently, CH3OH is added to the vapor atmosphere, and its contracting effect on the water-swollen films is monitored, revealing a co-nonsolvency-type behavior for PNIPMAM and PSPE. SR measurements indicate that PSPE and PNIPMAM behave significantly different during the H2O swelling and subsequent exposure to CH3OH, not only with respect to the amounts of absorbed water and CH3OH, but also to the cosolvent-induced contraction mechanisms. While PSPE thin films exhibit an abrupt one-step contraction, the contraction of PNIPMAM thin films occurs in two steps. FTIR studies corroborate these findings on a molecular scale and reveal the role of the specific functional groups, both during the swelling and the cosolvent-induced switching of the solvation state.
The swelling and phase transition behavior upon increasing temperature of a doubly thermoresponsive diblock copolymer thin film in steps above the characteristic cloud points (CPs) of the blocks is ...studied. An upper critical solution temperature (UCST)-type zwitterionic poly(sulfobetaine), poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate) (PSPP, CPUCST = 31.5 °C), is combined with a lower critical solution temperature (LCST)-type nonionic poly(N-isopropyl-/methacrylamide) (PNIPMAM, CPLCST = 49.5 °C) block. Using time-of-flight neutron reflectivity (ToF-NR), we observe the swelling in D2O vapor at a constant temperature of 20 °C, followed by two subsequent temperature jumps, from 20 to 40 °C (above CPUCST) and from 40 to 60 °C (above CPLCST). The observed response of the diblock copolymer films deviates from the aqueous solution behavior, which is mainly attributed to the increased polymer concentration. Temperature-induced changes in the thin-film nanostructure are investigated with ToF grazing-incidence small-angle neutron scattering (GISANS). Alterations in the chain conformation and hydrogen bonding are probed by Fourier transform infrared (FTIR) spectroscopy. The ionic SO3 – groups (in the PSPP block) and the nonionic hydrophilic amide groups (in both blocks) are found to affect the mechanisms of D2O uptake and release significantly.
The ligand exchange process is a key step in fabrications of quantum dot (QD) optoelectronic devices. In this work, on the basis of grazing incidence X-ray scattering techniques, we find that the ...ligand exchange process with halide ions changes the PbS QD superlattice from face-centered-cubic to body-centered-cubic stacking, while the QD crystal lattice orientation also changes from preferentially “edge-up” to “corner-up”. Thus, the QDs’ shape is supposed to be the main factor for the alignment of QDs in close packed solids. Moreover, we tailor the alignment of the close packed solids by thermal treatments and further investigate their inner charge carrier dynamics by pump–probe transient absorption experiments. An overall better structure alignment optimizes the charge carrier hopping rate, as confirmed by the time dependence of the photon bleaching peak shift. The QD solid treated at 100 °C shows the best inner structure alignment with the best charge carrier hopping rate.
Many nanoscale biopolymer building blocks with defect-free molecular structure and exceptional mechanical properties have the potential to surpass the performance of existing fossil-based materials ...with respect to barrier properties, load-bearing substrates for advanced functionalities, as well as light-weight construction. Comprehension and control of performance variations of macroscopic biopolymer materials caused by humidity-driven structural changes at the nanoscale are imperative and challenging. A long-lasting challenge is the interaction with water molecules causing reversible changes in the intrinsic molecular structures that adversely affects the macroscale performance. Using in situ advanced X-ray and neutron scattering techniques, we reveal the structural rearrangements at the nanoscale in ultrathin nanocellulose films with humidity variations. These reversible rearrangements are then correlated with wettability that can be tuned. The results and methodology have general implications not only on the performance of cellulose-based materials but also for hierarchical materials fabricated with other organic and inorganic moisture-sensitive building blocks.
By complementing small-angle neutron scattering (SANS) with Fourier transform infrared (FTIR) spectroscopy measurements, the influence of temperature as an external stimulus and block length ...asymmetry is studied on nanoscale assemblies of thermoresponsive double hydrophilic block copolymers in aqueous solutions. Morphologies and molecular hydration characteristics of two poly(N-isopropylacrylamide)-block-poly(oligo ethylene glycol methyl ether acrylate) (PNIPAM-b-POEGA) diblock copolymers, which differ in the length of the PNIPAM block, are compared. SANS provides insights into the nanostructure of the assemblies and FTIR probes vibrations of selected functional groups at the molecular level. Upon temperature increase, the solutions undergo transformations from hierarchical assemblies to more well-defined spherical morphologies. Variations in the block lengths lead to distinct morphological transformations. The variable morphological transformations originate from differences in the strength and/or amount of hydrogen bonding and hydrophobic interactions, as well as the chain density in the clusters formed and hydration. We identify transformations from hierarchical structures to fractal core–shell morphologies in the asymmetric block copolymer with the long PNIPAM block, while spherical and core–shell spherical morphologies are formed for the symmetric diblock copolymer with the short PNIPAM block. In these assemblies of PNIPAM-b-POEGA with the short PNIPAM block, the methyl side group hydration sensitively depends on the temperature increase at temperatures even beyond the nominal volume phase transition temperature. For both blocks, the evolution of the amide I band reflects that solvent–polymer interactions are still favorable even at the highest temperatures. Morphological variations after cooling to ambient temperature are reflected in both SANS and FTIR, pointing to remnant hydrogen bond interactions.
The swelling and solvation of 100–200 nm thin films of a diblock copolymer consisting of a short poly(methyl methacrylate) (PMMA) block and a long poly(N-isopropylacrylamide) (PNIPAM) block are ...investigated in mixed water/methanol vapors. The processes are followed in real time using spectral reflectance (SR), time-of-flight neutron reflectometry (ToF-NR), and Fourier transform infrared (FT-IR) spectroscopy, applying two neutron scattering contrast variation sequences. After hydration in pure water vapor, the vapor composition (relative to a flow rate of 1 L/min ≙ 100%) is changed to 70% water (D2O/H2O) and 30% methanol (CH3OH/CD3OH). Upon the mixed vapor stimulus, a two-step response is found, in which an initially enhanced swelling of the films is followed by a contraction. Differences in the solvent exchange kinetics found in ToF-NR experiments coincide with characteristic changes in the FT-IR spectra. While the initially enhanced swelling of the films is driven by the absorption of methanol, the film contraction is related to the release of both solvents, with almost no further change in solvent composition. In analogy to the coil-to-globule transition encountered in the polymer solution, these film response characteristics are attributed to the cononsolvency behavior of PNIPAM in water/methanol mixtures.
Perovskite solar cells (PSCs) have been developed as a promising photovoltaic technology because of their excellent photovoltaic performance. However, interfacial recombination and charge carrier ...transport losses at the surface greatly limit the performance and stability of PSCs. In this work, the fabrication of high-quality PSCs based on methylammonium lead iodide with excellent ambient stability is reported. An anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), is introduced to simultaneously passivate the defect states and stabilize the cubic phase of the perovskite film. The SDBS located at grain boundaries and the surface of the active layer can effectively passivate under-coordinated lead ions and protect the perovskite components from water-induced degradation. As a result, a champion power conversion efficiency (PCE) of 19.42% is achieved with an open-circuit voltage (V OC) of 1.12 V, a short-circuit current (J SC) of 23.23 mA cm–2, and a fill factor (FF) of 74% in combination with superior moisture stability. The SDBS-passivated devices retain 80% of their initial average PCE after 2112 h of storage under ambient conditions.
The water swelling and subsequent solvent exchange including co-nonsolvency behavior of thin films of a doubly thermo-responsive diblock copolymer (DBC) are studied via spectral reflectance, ...time-of-flight neutron reflectometry, and Fourier transform infrared spectroscopy. The DBC consists of a thermo-responsive zwitterionic (poly(4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate)) (PSBP) block, featuring an upper critical solution temperature transition in aqueous media but being insoluble in acetone, and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, featuring a lower critical solution temperature transition in water, while being soluble in acetone. Homogeneous DBC films of 50–100 nm thickness are first swollen in saturated water vapor (H2O or D2O), before they are subjected to a contraction process by exposure to mixed saturated water/acetone vapor (H2O or D2O/acetone-d6 = 9:1 v/v). The affinity of the DBC film toward H2O is stronger than for D2O, as inferred from the higher film thickness in the swollen state and the higher absorbed water content, thus revealing a pronounced isotope sensitivity. During the co-solvent-induced switching by mixed water/acetone vapor, a two-step film contraction is observed, which is attributed to the delayed expulsion of water molecules and uptake of acetone molecules. The swelling kinetics are compared for both mixed vapors (H2O/acetone-d6 and D2O/acetone-d6) and with those of the related homopolymer films. Moreover, the concomitant variations of the local environment around the hydrophilic groups located in the PSBP and PNIPMAM blocks are followed. The first contraction step turns out to be dominated by the behavior of the PSBP block, whereas the second one is dominated by the PNIPMAM block. The unusual swelling and contraction behavior of the latter block is attributed to its co-nonsolvency behavior. Furthermore, we observe cooperative hydration effects in the DBC films, that is, both polymer blocks influence each other’s solvation behavior.
The swelling of thin diblock copolymer (DBC) films is investigated in situ at 22 °C in pure water vapor as well as in mixed water/methanol vapor. The DBC consists of a zwitterionic ...poly(sulfobetaine) block, poly3–((2-(methacryloyloxy)ethyl)dimethylammonio) propane-1-sulfonate (PSPE), and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block. The swelling in water vapor (either H2O or D2O) and the thin-film response to methanol vapor exchange (i.e., a part of the H2O vapor is exchanged by CD3OH vapor and a part of the D2O vapor is exchanged by CH3OH vapor) are followed with simultaneous time-of-flight neutron reflectometry (ToF-NR) and spectral reflectance (SR) measurements. In situ Fourier transform infrared (FTIR) spectroscopy complements these data. Exposure to H2O vapor leads to a slightly higher degree of swelling and amount of absorbed H2O compared to D2O. Upon methanol exchange, the PSPE-b-PNIPMAM thin film undergoes two contractions, which are assigned to the specific responses of the individual polymer blocks of the DBC. Due to its isotope sensitivity, FTIR confirms these two separate contraction processes of the blocks on a molecular level and reveals the role of each polymer block during swelling in water vapor and upon the methanol exchange. Thus, four distinct film regimes with different thicknesses dependencies on the vapor composition can be established, thereby enabling a quarternary nanoswitch.
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