pH-induced hysteretic gating of track-etched polycarbonate membranes: swelling/deswelling behavior of polyelectrolyte multilayers in confined geometry

pH-induced hysteretic gating of track-etched polycarbonate membranes (TEPC) has been achieved by depositing layer-by-layer assembled polyelectrolyte multilayers comprising poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) at a high pH condition (pH > 9.0). Scanning electron microscopy and transmission electron microscopy showed that the average bilayer thickness of multilayers was greater within the cylindrical pores of track-etched polycarbonate membranes compared to the multilayers on planar substrates (e.g., Si wafers and the face of TEPC membranes). Swelling/deswelling properties of multilayers and gating properties of the multilayer-modified TEPC membranes were studied by measuring the flux of pH-adjusted deionized water. Large discontinuous changes in the transmembrane flux were observed, indicating that the multilayers within the cylindrical pores of TEPC membranes exhibit the discontinuous swelling/deswelling behavior observed previously for planar systems. The degree of swelling as estimated by simple models, however, showed that (PAH/PSS) multilayers in the confined geometry swelled to smaller extents compared to the same multilayers on planar substrates under the same conditions. Multilayer-modified membranes showed reversible gating properties as the pH condition of feed solution was alternated between pH 2.5 and 10.5. In situ atomic force microscopy (AFM) was used to visualize the closing of the pores as a function of time. The hysteretic gating property of the multilayer-modified TEPC membrane was utilized to achieve either a "closed" or "open" state at one pH condition depending on the pretreatment history, thereby enabling either the retention or passage of high-molecular weight polymers by varying the membrane pretreatment condition.     

Two-color visible/vacuum ultraviolet photoelectron imaging dynamics of Br2

An experimental two-color photoionization dynamics study of laser-excited Br2 molecules is presented, combining pulsed visible laser excitation and tunable vacuum ultraviolet (VUV) synchrotron radiation with photoelectron imaging. The X 1Sigmag + -B 3Pi0+u transition in Br2 is excited at 527 nm corresponding predominantly to excitation of the v' = 28 vibrational level in the B 3Pi0+u state. Tunable VUV undulator radiation in the energy range of 8.40-10.15 eV is subsequently used to ionize the excited molecules to the X 2Pi32,12 state of the ion, and the ionic ground state is probed by photoelectron imaging. Similar experiments are performed using single-photon synchrotron ionization in the photon energy range of 10.75-12.50 eV without any laser excitation. Photoelectron kinetic energy distributions are extracted from the photoelectron images. In the case of two-color photoionization using resonant excitation of the intermediate B 3Pi0+u state, a broad distribution of photoelectron kinetic energies is observed, and in some cases even a bimodal distribution, which depends on the VUV photon energy. In contrast, for single-photon ionization, a single nearly Gaussian-shaped distribution is observed, which shifts to higher energy with photon energy. Simulated spectra based on Franck-Condon factors for the transitions Br2(X 1Sigmag+, v" = 0)-Br2 +(X 2Pi12,32, v+) and Br2(B 3Pi0+u, v' = 28)-Br2 +(X 2Pi12,32, v+) are generated. Comparison of these calculated spectra with the measured images suggests that the differences in the kinetic energy distributions for the two ionization processes reflect the different extensions of the vibrational wave functions in the v" = 0 electronic ground state (X 1Sigmag+) versus the electronically and vibrationally excited state (B 3Pi0+u, v' = 28).     

Governance implications of nanomaterials companies’ inconsistent risk perceptions and safety practices

Current research on the nanotechnology industry indicates its downstream expansion at a rapid pace, while toxicological research and best practices for environmental health and safety are still being developed. Companies that use and/or produce engineered nanomaterials (ENMs) have enormous potential to influence safe-handling practices for ENMs across the product life cycle. Knowledge of both industry practices and leaders’ perceptions of risk is vital for understanding how companies will act to control potential environmental and health risks. This article reports results from a new international survey of nanomaterials companies in 14 countries. In this survey, company participants reported relatively high levels of uncertainty and/or perceived risk with regard to ENMs. However, these perspectives were not accompanied by expected risk-avoidant practices or preferences for regulatory oversight. A majority of companies indicated “lack of information” as a significant impediment to implementing nano-specific safety practices, but they also reported practices that were inconsistent with widely available guidance. Additionally, in the absence of safe-handling regulations, companies reported nano-specific health and safety programs that were narrow in scope. Taken together, these findings indicate that health and safety guidance is not reaching industry. While industry leaders’ reluctance toward regulation might be expected, their own reported unsafe practices and recognition of possible risks suggest a more top-down approach from regulators is needed to protect workers and the environment.     

Rate mechanisms of a novel thiol-ene photopolymerization reaction

A novel thiol-ene photopolymerization reaction involving copolymerization of tetrathiol monomer with vinyl silazane is experimentally characterized and is modeled successfully. The overall polymerization rate is found to be controlled by the ratio of the propagation to chain transfer kinetic parameters. The polymerization rate of this mixture, in the presence of added photoinitiator, is approximately first order in ene functionality and is independent of thiol functional group concentration. Initiation rates in this system, when cured utilizing a light centered around 365 nm light, and in the presence of no added photoinitiator, are shown to be proportional to the ene monomer concentration. When the mixture is polymerized utilizing light centered at 254 nm light, and without photoinitiator, the initiation rates are proportional to the thiol monomer concentrations. This novel reaction scheme is further utilized to form ultra rapidly polymerizable polymer derived ceramic structures with high aspect ratios.