Salomon Turgman-Cohen

Simultaneous bulk- and surface-initiated controlled radical polymerization from planar substrates.

We employ Monte Carlo computer simulations to investigate the simultaneous controlled radical polymerization in solution and from a flat surface. The bulk polymers grow at faster rates and possess narrower molecular weight distribution than polymers initiated from flat, impenetrable surfaces. The rate of surface-initiated polymerization depends on the density of initiator sites. Our results provide evidence that the assumption that the molecular weight of surface-initiated polymers is equal to that of polymers grown in bulk, invoked often in determining the grafting density of surface-bound polymers, is generally invalid.

Adsorption of a Nonionic Symmetric Triblock Copolymer on Surfaces with Different Hydrophobicity

This study investigates the adsorption of a symmetric triblock nonionic polymer comprising ethylene oxide (EO) and propylene oxide (PO) blocks (Pluronic P-105, EO(37)PO(56)EO(37)) on a range of substrates including hydrophobic, i.e., polypropylene (PP), poly(ethylene terephthalate) (PET), nylon, and graphite, and hydrophilic, i.e., cellulose and silica. The adsorption process and the structure of the hydrated adsorbed layers are followed by quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR), and atomic force microscopy. The unhydrated surfaces are characterized by ellipsometry and contact angle techniques. The adsorption kinetics and the extent of adsorption are determined by monitoring the changes in resonance frequency and refractive index of sensors coated with ultrathin films of the various substrates. Langmuirian-type adsorption kinetics is observed in all cases studied. The amount of adsorbed Pluronic on hydrophobic polymer surfaces (PP, PET, and nylon) exceeds that on the hydrophilic cellulose. The hydrophobic (graphite) mineral surface adsorbs relatively low polymer mass, typical of a monolayer, while micellar structures are observed on the hydrophilic silica surface. The amount of water coupled to the adsorbed polymer layers is quantified by combining data from QCM, and SPR are found to increase with increasing polarity of the substrate. On the basis of contact angle data, the nonhydrated adsorbed structures produce modest increases in hydrophilicity of all the substrates investigated. Overall, insights are provided into the structure and stability of both hydrated and nonhydrated adsorbed triblock copolymer.

On-Demand Degrafting and the Study of Molecular Weight and Grafting Density of Poly(methyl methacrylate) Brushes on Flat Silica Substrates

We report on degrafting of surface-anchored poly(methyl methacrylate) (PMMA) brushes from flat silica-based substrates using tetrabutylammonium fluoride (TBAF) and determining their molecular weight distribution (MWD) using size exclusion chromatography (SEC). The grafted PMMA layer was synthesized using surface-initiated atom transfer radical polymerization (SI-ATRP) of MMA for polymerization times ranging from 6 to 24 h. X-ray photoelectron spectroscopy, ellipsometry, and time-of-flight secondary ion mass spectrometry were employed in tandem to characterize the degrafting process. The SEC eluograms were fit to various polymer distributions, namely Zimm-Schulz, ATRP in continuous stirred tank reactor, Wesslau, Schulz-Flory, and Smith et al. The ATRP model gives the best fit to the experimental data. The dry PMMA brush thickness and the number-average molecular weight (obtained from the MWD) suggest that the grafting density of the PMMA grafts is independent of polymerization time, indicating well-controlled/living growth of MMA. The observed polydispersity index (PDI) was higher than that generally observed in bulk grown polymers under similar conditions, indicating an effect due to chain confinement and crowding. We detect small but noticeable dependence of the polymer brush grafting density on the inhibitor/catalyst ratio. Higher inhibitor/catalyst ratio offers better control with lower early terminations, which results in a small increase in the apparent grafting density of the chains.

Molecular Dynamics of Equilibrium and Pressure-Driven Transport Properties of Water through LTA-Type Zeolites

We consider an atomistic model to investigate the flux of water through thin Linde type A (LTA) zeolite membranes with differing surface chemistries. Using molecular dynamics, we have studied the flow of water under hydrostatic pressure through a fully hydrated LTA zeolite film (~2.5 nm thick) capped with hydrophilic and hydrophobic moieties. Pressure drops in the 50-400 MPa range were applied across the membrane, and the flux of water was monitored for at least 15 ns of simulation time. For hydrophilic membranes, water molecules adsorb at the zeolite surface, creating a highly structured fluid layer. For hydrophobic membranes, a depletion of water molecules occurs near the water/zeolite interface. For both types of membranes, the water structure is independent of the pressure drop established in the system and the flux through the membranes is lower than that observed for the bulk zeolitic material; the latter allows an estimation of surface barrier effects to pressure-driven water transport. Mechanistically, it is observed that (i) bottlenecks form at the windows of the zeolite structure, preventing the free flow of water through the porous membrane, (ii) water molecules do not move through a cage in a single-file fashion but rather exhibit a broad range of residence times and pronounced mixing, and (iii) a periodic buildup of a pressure difference between inlet and outlet cages takes place which leads to the preferential flow of water molecules toward the low-pressure cages.

Asphaltene Adsorption onto Self-Assembled Monolayers of Alkyltrichlorosilanes of Varying Chain Length

The adsorption of asphaltenes onto flat silica surfaces modified with self-assembled monolayers (SAMs) of alkyltrichlorosilanes of varying thickness due to a variable number of carbon atoms (N(C)) has been studied by means of contact angle measurements, spectroscopic ellipsometry, and near-edge X-ray absorption fine structure spectroscopy. The extent of asphaltene adsorption was found to depend primarily on the ability of the SAM layer to shield the underlying silicon substrate from interacting with the asphaltenes present in solution. Specifically, asphaltene adsorption decreased with an increase in N(C) and/or an increase in SAM grafting density, sigma(SAM), (i.e., number of SAM molecules per unit area). The effect of the solvent quality on the extent of asphaltene adsorption was gauged by adsorbing asphaltenes from toluene, 1-methylnaphthalene, tetralin, decalin, and toluene-heptanes mixtures. The extent of asphaltene adsorption was found to increase proportionally with a decrease in the Hildebrand solubility parameter of the solvent.

Asphaltene Adsorption onto Self-Assembled Monolayers of Mixed Aromatic and Aliphatic Trichlorosilanes

The adsorption of asphaltenes onto flat solid surfaces modified with mixed self-assembled monolayers (SAMs) of aliphatic and aromatic trichlorosilanes with varying wettabilities, aromaticities, and thicknesses is tested. The mixed SAMs are characterized by means of contact angle to assess hydrophobicity and molecular and chemical uniformity, spectroscopic ellipsometry to measure the thickness of the films, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy to assess chemical and molecular composition. The molecular characteristics of the adsorbed asphaltene layer and the extent of asphaltene adsorption are determined using NEXAFS and spectroscopic ellipsometry, respectively. The SAMs are formed by depositing phenyl-, phenethyl-, butyl-, and octadecyl- trichlorosilanes from toluene solutions onto silica-coated substrates; the chemical composition and the wettability of the SAM surface is tuned systematically by varying the trichlorosilane composition in the deposition solutions. The adsorption of asphaltenes on the substrates does not correlate strongly with the SAM chemical composition. Instead, the extent of asphaltene adsorption decreases with increasing SAM thickness. This observation suggests that the leading interaction governing the adsorption of asphaltenes is their interaction with the polar silica substrate and that the chemical composition of the SAM is of secondary importance.

Transport Properties of Amine/Carbon Dioxide Reactive Mixtures and Implications to Carbon Capture Technologies

The structure and transport properties of physisorbed and chemisorbed CO2 in model polyamine liquids (hexamethylenediamine and diethylenetriamine) are studied via molecular dynamics simulations. Such systems are relevant to CO2 absorption processes where nonaqueous amines are used as absorbents (e.g., when impregnated or grafted onto mesoporous media or misted in the gas phase). It is shown that accounting for the ionic speciation resulting from CO2 chemisorption enabled us to capture the qualitative changes in extent of absorption and fluidity with time that are observed in thermogravimetric experiments. Simulations reveal that high enough concentration of reacted CO2 leads to strong intermolecular ionic interactions and the arrest of molecular translations. The transport properties obtained from the simulations of the ionic speciated mixtures are also used to construct an approximate continuum-level model for the CO2 absorption process that mimics thermogravimetric experiments.