Manipulation of Polymer/Polymer Interface Width from Nonequilibrium Deposition

We demonstrate, using neutron reflectivity, that the width of a nonequilibrium interface between an organo-soluble aromatic polyimide film and triacetate cellulose (TAC) support film created by spin-coating or solution-casting can be broadened in a controllable way using a “swelling agent” in the deposition process. In a favorable case, the adhesion, as measured by T-peel tests, can be increased by a factor of 7 by adjustment of the solvent composition. The morphologies of the TAC fractured surfaces after peeling tests measured by AFM reveal that broadening of the interfacial width causes an interconnected network in the interface, leading to a sharp increase in the interfacial adhesion. Differences in the chemistry (solubility) of the materials being deposited do make a difference in the effectiveness of this strategy of using a “swelling agent”. For one polyimide, a 3-fold increase in adhesion can be obtained by optimizing the deposition temperature, but this approach for improving adhesion is less effective than that of adding “swelling agent”. The formation of robust interfaces of this type is important because of the critical roles that multilayer films containing polymers with special properties and tailored structures play in applications as diverse as computer displays, photovoltaic devices, and polymeric electronics. The “swelling agent” strategy makes it possible to produce polymer multilayer structures in a cost-effective way with roll-to-roll mass production using direct solution coating

Molecular Weight Dependence on the Disintegration of Spin-Assisted Weak Polyelectrolyte Multilayer Films

We present the effect of molecular weight (MW) of polyelectrolytes (PEs) on the disintegration behavior of weak PE multilayer films consisting of linear poly­(ethylene imine) (LPEI) and poly­(methacrylic acid) (PMAA). The multilayer films prepared by the spin-assisted layer-by-layer deposition have well-ordered internal structures and also show the linear thickness growth behavior regardless of MWs of PMAA. The well-defined weak PE multilayer films were subject to disintegration into bulk solution when the electrostatic interactions between LPEI and PMAA layers were reduced by treatment at pH 2. However, we demonstrated the change in the disintegration mode and kinetics (i.e., from burst erosion to controlled surface erosion) as a function of MW of PMAA based on neutron reflectivity and quartz crystal microbalance with dissipation, revealing the correlation between the structural changes and the viscoelastic responses of the weak PE films upon pH treatment. Also, the unique swelling behavior as well as the significant increase in dissipation energy was monitored before the complete disintegration of the multilayer films containing high MW PMAA, which is believed to originate from their slow rearrangement kinetics within the film. We believe that the results shown in this study provide chain-level understanding as to the MW-dependence on pH-triggered disintegration mechanism of weak PE multilayer films

Controlled Release from Model Blend Multilayer Films Containing Mixtures of Strong and Weak Polyelectrolytes

We have designed the controlled release platforms based on polyelectrolyte (PE) blend multilayer films to investigate the release mode and kinetics at the nanoscale level. The model blend multilayer films are composed of positively charged layers with weak polyelectrolytes (PEs) (linear poly­(ethylenimine), LPEI) and negatively charged blend layers with mixtures of strong (poly­(sodium 4-styrenesulfonic acid), PSS) and weak (poly­(methacrylic acid), PMAA) PEs. The blend multilayer films ([LPEI/PSS:PMAA]_<i>n</i>) with well-defined internal structure were prepared by the spin-assisted layer-by-layer (LbL) deposition method. Release properties of the multilayer films were systematically studied as a function of blend ratio by neutron reflectivity (NR), ellipsometer, AFM, FT-IR spectroscopy, and quartz crystal microbalance with dissipation (QCM-D). Since PSS strong PEs serve as robust skeletons within the multilayer films independent of external pH variation, the burst disruption of pure weak PE multilayer films was dramatically suppressed, and the release kinetics could be accurately controlled by simply changing the PSS content within the blend films. These release properties of blend multilayer films form the basis for designing the controlled release of target active materials from surfaces

Scaling Behavior and Segment Concentration Profile of Densely Grafted Polymer Brushes Swollen in Vapor

The scaling of the thickness, <i>h</i>_s, of a densely grafted polymer brush of chain length <i>N</i> and grafting density σ swollen in vapor agrees quantitatively with the scaling reported by Kuhl et al. for densely grafted brushes swollen in liquid. Deep in the brush, next to the substrate, the shape of the segment concentration profile is the same whether the brush is swollen by liquid or by vapor. Differences in the segment concentration profile are manifested primarily in the swollen brush interface with the surrounding fluid. The interface of the polymer brush swollen in vapor is much more abrupt than that of the same brush swollen in liquid. This has implications for the compressibility of the swollen brush surface and for fluctuations at that surface

Loading and Distribution of a Model Small Molecule Drug in Poly(<i>N</i>‑isopropylacrylamide) Brushes: a Neutron Reflectometry and AFM Study

The structure of a hydrated poly­(<i>N</i>-isopropylacrylamide) brush loaded with 5 vol % Isoniazid is studied as a function of temperature using neutron reflectometry (NR) and atomic force microscopy (AFM). NR measurements show that Isoniazid increases the thickness of the brush before, during and after the polymer collapse, and it is retained inside the brush at all measured temperatures. The Isoniazid concentration in the expanded brush is ∼14% higher than in the bulk solution, and the concentration nearly doubles in the collapsed polymer, suggesting stronger binding between Isoniazid and the polymer compared to water, even at temperatures below the lower critical solution temperature (LCST) where the polymer is hydrophilic. Typically, additives that bind strongly to the polymer backbone and increase the hydrophilicity of the polymer will delay the onset of the LCST, which is suggested by AFM and NR measurements. The extent of small-molecule loading and distribution throughout a thermo-responsive polymer brush, such as pNIPAAm, will have important consequences for applications such as drug delivery and gating

Structure of [C₄mpyr][NTf₂] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide ([C₄mpyr]­[NTf₂]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C₄mpyr]⁺ at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C₄mpyr]⁺ onto the gold electrode

Structure of [C₄mpyr][NTf₂] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide ([C₄mpyr]­[NTf₂]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C₄mpyr]⁺ at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C₄mpyr]⁺ onto the gold electrode

Structure of [C₄mpyr][NTf₂] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide ([C₄mpyr]­[NTf₂]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C₄mpyr]⁺ at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C₄mpyr]⁺ onto the gold electrode

Structure of [C₄mpyr][NTf₂] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide ([C₄mpyr]­[NTf₂]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C₄mpyr]⁺ at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C₄mpyr]⁺ onto the gold electrode

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)