Many nanoscale biopolymer building blocks with defectfree 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 cellulosebased materials but also for hierarchical materials fabricated with other organic and inorganic moisture-sensitive building blocks.
Polymer electrodes made of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) are used in many applications but are also sensitive to humidity. We study humidity-induced changes of PEDOT:PSS electrodes as monitored with in situ time-of-flight neutron reflectivity (TOF-NR) measurements under high humidity conditions. The influence of the solvent additive Zonyl and a post-treatment of PEDOT:PSS films with ethylene glycol (EG) serving as electrodes are analyzed with respect to the swelling ratio and water uptake. Depending on the applied PEDOT:PSS treatment, PEDOT and PSS enrichment layers are clearly identified with TOF-NR at the substrate-polymer and polymer-air interface, respectively. The additive Zonyl reduces the water uptake and limits film swelling. EG post-treatment further increases hydrophobicity and thereby water incorporation into the PEDOT:PSS film is strongly suppressed. The characteristic time constants and effective interaction parameters extracted from the kinetic NR data show that additive and post-treatment reduce the sensitivity of the PEDOT:PSS electrodes to humidity.
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, CP UCST = 31.5 °C), is combined with a lower critical solution temperature (LCST)-type nonionic poly(N-isopropyl-/ methacrylamide) (PNIPMAM, CP LCST = 49.5 °C) block. Using time-offlight neutron reflectivity (ToF-NR), we observe the swelling in D 2 O vapor at a constant temperature of 20 °C, followed by two subsequent temperature jumps, from 20 to 40 °C (above CP UCST ) and from 40 to 60 °C (above CP LCST ). 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 SO 3 − groups (in the PSPP block) and the nonionic hydrophilic amide groups (in both blocks) are found to affect the mechanisms of D 2 O uptake and release significantly.
The humidity-induced swelling and exchange behavior of a block copolymer thin film, which consists of a zwitterionic poly(sulfobetaine) [poly(N,N-dimethyl-N-(3-(methacrylamido)propyl)ammoniopropanesulfonate) (PSPP)] block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block, are investigated by time-of-flight neutron reflectometry (TOF-NR). We monitor in situ the swelling in the H2O atmosphere, followed by an exchange with D2O. In the reverse experiment, swelling in the D2O atmosphere and the subsequent exchange with H2O are studied. Both, static and kinetic TOF-NR measurements indicate significant differences in the interactions between the PSPP80-b-PNIPAM130 thin film and H2O or D2O, which we attribute to the different H- and D-bonds between water and the polymer. Changes in the chain conformation and hydrogen bonding are probed with Fourier transform infrared spectroscopy during the kinetics of the swelling and exchange processes, which reveals the key roles of the ionic SO3 – group in the PSPP block and of the polar amide groups of both blocks during water uptake and exchange.
The swelling and co-nonsolvency behaviors in pure H 2 O and in a mixed H 2 O/CH 3 OH vapor atmosphere of two different polar, water-soluble polymers in thin film geometry are studied in situ. Films of a zwitterionic poly(sulfobetaine), namely, poly [3-((2-(methacryloyloxy)ethyl)dimethylammonio) propane-1sulfonate] (PSPE), and a polar nonionic polymer, namely, poly(Nisopropylmethacrylamide) (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 cononsolvency behavior in water/methanol mixtures. First, the swelling of PSPE and PNIPMAM thin films in H 2 O vapor is followed. Subsequently, CH 3 OH 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 H 2 O swelling and subsequent exposure to CH 3 OH, not only with respect to the amounts of absorbed water and CH 3 OH, 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.
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