Surface Mechanical and Rheological Behaviors of Biocompatible Poly((d,l‑lactic acid-<i>ran</i>-glycolic acid)-<i>block</i>-ethylene glycol) (PLGA–PEG) and Poly((d,l‑lactic acid-<i>ran</i>-glycolic acid-<i>ran</i>-ε-caprolactone)-<i>block</i>-ethylene glycol) (PLGACL–PEG) Block Copolymers at the Air–Water Interface

Air–water interfacial monolayers of poly­((d,l-lactic acid-<i>ran</i>-glycolic acid)-<i>block</i>-ethylene glycol) (PLGA–PEG) exhibit an exponential increase in surface pressure under high monolayer compression. In order to understand the molecular origin of this behavior, a combined experimental and theoretical investigation (including surface pressure–area isotherm, X-ray reflectivity (XR) and interfacial rheological measurements, and a self-consistent field (SCF) theoretical analysis) was performed on air–water monolayers formed by a PLGA–PEG diblock copolymer and also by a nonglassy analogue of this diblock copolymer, poly­((d,l-lactic acid-<i>ran</i>-glycolic acid-<i>ran</i>-caprolactone)-<i>block</i>-ethylene glycol) (PLGACL–PEG). The combined results of this study show that the two mechanisms, i.e., the glass transition of the collapsed PLGA film and the lateral repulsion of the PEG brush chains that occur simultaneously under lateral compression of the monolayer, are both responsible for the observed PLGA–PEG isotherm behavior. Upon cessation of compression, the high surface pressure of the PLGA–PEG monolayer typically relaxes over time with a stretched exponential decay, suggesting that in this diblock copolymer situation, the hydrophobic domain formed by the PLGA blocks undergoes glass transition in the high lateral compression state, analogously to the PLGA homopolymer monolayer. In the high PEG grafting density regime, the contribution of the PEG brush chains to the high monolayer surface pressure is significantly lower than what is predicted by the SCF model because of the many-body attraction among PEG segments (referred to in the literature as the “<i>n</i>-cluster” effects). The end-grafted PEG chains were found to be protein resistant even under the influence of the “n-cluster” effects

Mechanically Robust Magnetic Carbon Nanotube Papers Prepared with CoFe₂O₄ Nanoparticles for Electromagnetic Interference Shielding and Magnetomechanical Actuation

The introduction of inorganic nanoparticles into carbon nanotube (CNT) papers can provide a versatile route to the fabrication of CNT papers with diverse functionalities, but it may lead to a reduction in their mechanical properties. Here, we describe a simple and effective strategy for the fabrication of mechanically robust magnetic CNT papers for electromagnetic interference (EMI) shielding and magnetomechanical actuation applications. The magnetic CNT papers were produced by vacuum filtration of an aqueous suspension of CNTs, CoFe₂O₄ nanoparticles, and poly­(vinyl alcohol) (PVA). PVA plays a critical role in enhancing the mechanical strength of CNT papers. The magnetic CNT papers containing 73 wt % of CoFe₂O₄ nanoparticles exhibited high mechanical properties with Young’s modulus of 3.2 GPa and tensile strength of 30.0 MPa. This magnetic CNT paper was successfully demonstrated as EMI shielding paper with shielding effectiveness of ∼30 dB (99.9%) in 0.5–1.0 GHz, and also as a magnetomechanical actuator in an audible frequency range from 200 to 20 000 Hz

Water Is a Poor Solvent for Densely Grafted Poly(ethylene oxide) Chains: A Conclusion Drawn from a Self-Consistent Field Theory-Based Analysis of Neutron Reflectivity and Surface Pressure–Area Isotherm Data

By use of a combined experimental and theoretical approach, a model poly­(ethylene oxide) (PEO) brush system, prepared by spreading a poly­(ethylene oxide)–poly­(<i>n</i>-butyl acrylate) (PEO–PnBA) amphiphilic diblock copolymer onto an air–water interface, was investigated. The polymer segment density profiles of the PEO brush in the direction normal to the air–water interface under various grafting density conditions were determined by using the neutron reflectivity (NR) measurement technique. To achieve a theoretically sound analysis of the reflectivity data, we used a data analysis method that utilizes the self-consistent field (SCF) theoretical modeling as a tool for predicting expected reflectivity results for comparison with the experimental data. Using this data analysis technique, we discovered that the effective Flory–Huggins interaction parameter of the PEO brush chains is significantly greater than that corresponding to the θ condition in Flory–Huggins solutions (i.e., χ_PEO–water(brush chains)/χ_PEO–water(θ condition) ≈ 1.2), suggesting that contrary to what is more commonly observed for PEO in normal situations (χ_PEO–water(free chains)/χ_PEO–water(θ condition) ≈ 0.92), the PEO chains are actually not “hydrophilic” when they exist as polymer brush chains, because of the many body interactions that are forced to be effective in the brush situation. This result is further supported by the fact that the surface pressures of the PEO brush calculated on the basis of the measured χ_PEO–water value are in close agreement with the experimental surface pressure–area isotherm data. The SCF theoretical analysis of the surface pressure behavior of the PEO brush also suggests that even though the grafted PEO chains experience a poor solvent environment, the PEO brush layer exhibits positive surface pressures, because the hydrophobicity of the PEO brush chains (which favors compression) is insufficient to overcome the opposing effect of the chain conformational entropy (which resists compression)

Reduced Water Density in a Poly(ethylene oxide) Brush

A model poly­(ethylene oxide) (PEO) brush system, prepared by spreading a poly­(ethylene oxide)–poly­(<i>n</i>-butyl acrylate) (PEO–P<i>n</i>BA) amphiphilic diblock copolymer onto an air–water interface, was investigated under various grafting density conditions by using the X-ray reflectivity (XR) technique. The overall electron density profiles of the PEO–P<i>n</i>BA monolayer in the direction normal to the air–water interface were determined from the XR data. From this analysis, it was found that inside of the PEO brush, the water density is significantly lower than that of bulk water, in particular, in the region close to the P<i>n</i>BA–water interface. Separate XR measurements with a P<i>n</i>BA homopolymer monolayer confirm that the reduced water density within the PEO–P<i>n</i>BA monolayer is not due to unfavorable contacts between the P<i>n</i>BA surface and water. The above result, therefore, lends support to the notion that PEO chains provide a hydrophobic environment for the surrounding water molecules when they exist as polymer brush chains