Ronald C. Hedden

Rheology and Morphology of Pristine Graphene/Polyacrylamide Gels

Enhancement of toughness in nanomaterial-based hydrogels is a critical metric for many of their engineering applications. Pristine graphene-polyacrylamide (PAM) hydrogels are synthesized via in situ polymerization of acrylamide monomer in PAM-stabilized graphene dispersion. In-situ polymerization leads to the uniform dispersion of the graphene sheets in the hydrogel. The graphene sheets interact with the elastic chains of the hydrogel through physisorption and permit gelation in the absence of any chemical cross-linker. This study represents the first report of pristine graphene as a physical cross-linker in a hydrogel. The properties of the graphene-polymer hydrogel are characterized by rheological measurements and compressive tests, revealing an increase in the storage modulus and toughness of the hydrogels compared to the chemically cross-linked PAM analogues. The physically cross-linked graphene hydrogels also exhibit self-healing properties. These hydrogels prove to be efficient precursors for graphene-PAM aerogels with enhanced electrical conductivity and thermal stability.

Development of nonaqueous polymer gels that exhibit broad temperature performance

While significant work has focused on aqueous hydrogels for biotechnology applications, hydrogels suffer from a limited operating temperature range due to the moderate freezing point and high volatility of water. In this work, a nonaqueous, chemically cross-linked polybutadiene gel has been designed which exhibits stable properties over a temperature range of −60–70°C. A combination of rheology, neutron scattering, and tack adhesion testing was utilized to characterize the gel properties. The methodology employed to design the polybutadiene gel can be generalized to a variety of gel materials and applications.

Structure and Hydrogen Bonding of Water in Polyacrylate Gels: Effects of Polymer Hydrophilicity and Water Concentration.

The ability to tune the hydrophilicity of polyacrylate copolymers by altering their composition makes these materials attractive candidates for membranes used to separate alcohol-water mixtures. The separation behavior of these polyacrylate membranes is governed by a complex interplay of factors such as water and alcohol concentrations, water structure in the membrane, polymer hydrophilicity, and temperature. We use molecular dynamics simulations to investigate the effect of polymer hydrophilicity and water concentration on the structure and dynamics of water molecules in the polymer matrix. Samples of poly(n-butyl acrylate) (PBA), poly(2-hydroxyethyl acrylate) (PHEA), and a 50/50 copolymer of BA and HEA were synthesized in laboratory, and their properties were measured. Model structures of these systems were validated by comparing the simulated values of their volumetric properties with the experimental values. Molecular simulations of polyacrylate gels swollen in water and ethanol mixtures showed that water exhibits very different affinities toward the different (carbonyl, alkoxy, and hydroxyl) functional groups of the polymers. Water molecules are well dispersed in the system at low concentrations and predominantly form hydrogen bonds with the polymer. However, water forms large clusters at high concentrations along with the predominant formation of water-water hydrogen bonds and the acceleration of hydrogen bond dynamics.

Preparation of poly(diethylsiloxane) with the NaOH/12-crown-4 catalyst

Abstract A new technique for the preparation of poly(diethylsiloxane) (PDES) is described. The monomer hexaethylcyclotrisiloxane is polymerized with NaOH as a catalyst in the presence of the promoter 12-crown-4 (1,4,7,10 tetraoxacyclododecane). The effects of NaOH concentration, 12-crown-4 concentration, and polymerization time on the molecular weight distribution are reported. A mechanism for the polymerization is proposed based on current knowledge of anionic ring-opening polymerization of cyclic siloxanes. The advantages and disadvantages of this synthesis relative to previously published PDES syntheses are discussed.

Combinatorial Methodology for Screening Selectivity in Polymeric Pervaporation Membranes

Combinatorial methodology is described for rapid screening of selectivity in polymeric pervaporation membrane materials for alcohol-water separations. The screening technique is demonstrated for ethanol-water separation using a model polyacrylate system. The materials studied are cross-linked random copolymers of a hydrophobic comonomer (n-butyl acrylate, B) and a hydrophilic comonomer (2-hydroxyethyl acrylate, H). A matrix of materials is prepared that has orthogonal variations in two key variables, H:B ratio and cross-linker concentration. For mixtures of ethanol and water, equilibrium selectivities and distribution coefficients are obtained by combining swelling measurements with high-throughput HPLC analysis. Based on the screening results, two copolymers are selected for further study as pervaporation membranes to quantify permeability selectivity and the flux of ethanol. The screening methodology described has good potential to accelerate the search for new membrane materials, as it is adaptable to a broad range of polymer chemistries.

X-ray Porosimetry as a Metrology to Characterize The Pore Structure of Low-k Dielectric Films

X‐ray reflectivity porosimetry is a highly sensitive measurement method that utilizes the capillary condensation of a solvent vapor inside porous low‐k dielectric films on a silicon substrate. As the partial pressure of the solvent environment over the film increases, capillary condensation occurs in progressively larger pores. This results in an appreciable increase in the electron density of the film. By monitoring the changes in the critical angle for total X‐ray reflectance, one can directly calculate the average electron density, and therefore the solvent uptake. By invoking traditional porosimetry absorption/desorption analyses, characteristics such as porosity and the distribution of pore sizes can be extracted.

Swelling of Random Copolymer Networks in Pure and Mixed Solvents: Multi-Component Flory–Rehner Theory

A generalized extension of Flory-Rehner (FR) theory is derived to describe equilibrium swelling of polymer networks, including copolymers with two or more chemically distinct repeat units, in either pure or mixed solvents. The model is derived by equating the chemical potential of each solvent in the liquid and gel phases at equilibrium, while assuming the deformation of the network chains is affine. Simplifications of the model are derived for specific cases involving homopolymer networks, copolymer networks, pure solvents, and binary solvent mixtures. With reasonable assumptions, the number of polymer-solvent interaction parameters that must be determined by experiments can be reduced to two effective parameters (θ1 and θ2), which describe the net interactions between water/copolymer (θ1) and ethanol/copolymer (θ2), respectively. Experimental measurements of the swelling of random copolymer networks of n-butyl acrylate and 2-hydroxyethyl acrylate in water, ethanol, and a 100 g/L ethanol/water mixture are utilized to validate the model. For a random copolymer network, θ1 and θ2 can be obtained by fitting the three-component FR model to equilibrium swelling data obtained in the pure solvents. Predicted solvent volume fractions for swelling in water-ethanol mixtures obtained by inserting fitted values of θ1 and θ2 into the four-component FR model are in reasonable agreement with experimental measurements.

Determination of Pore Size Distributions in Nano-Porous Thin Films from Small Angle Scattering

Small angle neutron and x-ray scattering (SANS, SAXS) are powerful tools in determination of the pore size and content of nano-porous materials with low dielectric constants (low-k) that are being developed as interlevel dielectrics. Several models have been previously applied to fit the scattering data in order to extract information on the average pore and/or matrix size. A new method has been developed to provide information on the size distributions of the pore and matrix phases based on the “chord length distribution” introduced by Tchoubar and Mering. Examples are given of scattering from samples that have size distributions that are narrower and broader than the random distribution typical of scattering described by Debye, Anderson, and Brumberger. An example of fitting SANS data to a phase size distribution is given.

Measurement of Pore Size and Matrix Characteristics in Low‐k Dielectrics by Neutron Contrast Variation

The principle of small‐angle neutron scattering (SANS) contrast variation is applied to characterization of nanoporous low‐k thin films. Pores are filled with mixtures of the hydrogen‐ and deuterium‐containing analogs of a probe solvent by vapor adsorption. By varying the composition of the solvent in the pores, a “contrast match” solvent composition is identified for which the scattered intensity is minimized. The contrast match solvent mixture is subsequently used to conduct SANS porosimetry experiments. In combination with information from X‐ray reflectivity and ion scattering, the technique is useful for estimating the density of the matrix (wall) material and the pore size distribution. To illustrate the technique, a porous methylsilsesquioxane (MSQ) spin‐on dielectric is characterized.

The Structural Evolution of Pore Formation in Low‐k Dielectric Thin Films

Specular x‐ray reflectivity and small angle neutron scattering were used to characterize changes in the porosity, pore size and pore size distribution on processing a polymeric low‐k material filled with 21.6 volume percent of a deuterated porogen with an average radius of 56 A. Processing yielded a decrease in porosity to about 11 %, an increase in average pore radius to 83 A, and a narrower pore size distribution. A sample with an unusual pore structure could be easily identified.