Jon Samseth

Structural development of silica gels aged in TEOS

Abstract It has been shown that wet gels can be strengthened and stiffened by providing new monomers to the alcogel after gelation by aging in a tetraethoxysilane (TEOS) solution. Xerogels with properties similar to aerogels can be obtained by drying at ambient pressure. Structural changes of gels during the aging in TEOS were studied by small angle neutron scattering (SANS). The SANS measurements show only a slight increase in the volume fractal dimension of the porous gel network with increasing aging time indicating no significant change in the geometry and connectivity of the network. However, there is an increase in both primary particle size and cluster size when silica is precipitated from the aging solution. While the primary particle size stays relatively constant after only 6 h aging cluster size shows a steady increase. In addition, to obtain further information about the structural development, skeletal density of corresponding aerogels has been measured both as a function of aging time and heat treatment time. The skeletal density is higher if the wet gel is aged; however, it is independent of the aging time. A skeletal density close to the theoretical value is reached at about 350°C for both aged and unaged samples.

Nanoparticle-regulated phase behavior of ordered block copolymers

Although block copolymer motifs have received considerable attention as supramolecular templates for inorganic nanoparticles, experimental observations of a nanostructured diblock copolymer containing inorganic nanoparticles-supported by theoretical trends predicted from a hybrid self-consistent field/density functional theory-confirm that nanoparticle size and selectivity can likewise stabilize the copolymer nanostructure by increasing its order-disorder transition temperature.

Liesegang pattern formation by gas diffusion in silica aerogels

Abstract Molecular gas diffusion of HCl and NH3 has been studied both in air and in aerogel. The two gases react to form NH4Cl which precipitates when a threshold concentration is reached. When the gases diffuse from opposite ends of a tube, the precipitate appears as a white ring whose position is determined by the ratio of the gas diffusion constants and follows Grahams law. When the diffusion process takes place in an aerogel, NH4Cl precipitates in sheets that span a cross-section of the gel. As the diffusion process continues, more bands are formed. The assembly of bands is called a Liesegang pattern. This phenomenon is known to occur in diffusion reaction systems. Numerical simulations based on the Ostwald nucleation theory qualitatively agree with experiments.

Structure in cast films of poly(siloxane-imide) copolymers

Abstract The characteristics of structures induced by drying and phase separation in cast films of three poly(siloxane-imide) (PSI) multiblock copolymers are investigated by microscopy and small-angle scattering techniques. Rapid removal of solvent from thin films produces non-equilibrium Benard cells with periodicity between 10 and 20 μm, as determined from light microscopy and scanning electron microscopy. Small-angle neutron scattering (SANS) is used to discern the periodicities and sizes of microstructural elements arising from microphase separation. Scattering curves obtained from solution-cast films indicate that these elements possess periodicities between 15 and 22 nm, depending on molecular architecture, and appear similar to the curves obtained from melt-pressed films. Micrographs of one of the solution-cast materials, obtained with transmission electron microscopy, reveal a disordered biphasic morphology consisting of dispersed imide domains, whose mean size and periodicity are ca 4 and 15 nm, respectively, in excellent agreement with results from SANS.

Washing of multicomponent gels prior to drying

Abstract The sol-gel route is attractive for the preparation of multicomponent gels because homogenization on an atomic level might be possible. Prior to the drying of the wet gels it is sometimes an advantage to wash or rinse the gel using different types of solvents, i.e., for the preparation of aerogels. In this work we have studied the effect on composition of washing cordierite gels prepared using two different routes in four different solvents. Magnesium and aluminum were leached from the wet gels during washing in especially ethanol and acetone and the composition of the gel was severely altered relative to cordierite. The amount of magnesium and aluminum removed from the gels was shown to be dependent on the structure of the gels.

Nanoparticle Network Formation in Nanostructured and Disordered Block Copolymer Matrices.

Incorporation of nanoparticles composed of surface-functionalized fumed silica (FS) or native colloidal silica (CS) into a nanostructured block copolymer yields hybrid nanocomposites whose mechanical properties can be tuned by nanoparticle concentration and surface chemistry. In this work, dynamic rheology is used to probe the frequency and thermal responses of nanocomposites composed of a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer and varying in nanoparticle concentration and surface functionality. At sufficiently high loading levels, FS nanoparticle aggregates establish a load-bearing colloidal network within the copolymer matrix. Transmission electron microscopy images reveal the morphological characteristics of the nanocomposites under these conditions.

Crosslinking High Free Volume Polymers - Effect on Gas Separation Properties

Crosslinkable nanoparticle-filled poly(4-methyl-2-pentyne) [PMP] membranes were cast from carbon tetrachloride solutions containing PMP, hydrophobic fumed silica, and 4,4i¯-(hexafluoroisopropylidene)diphenyl azide [HFBAA]. The composite membranes were crosslinked by UV irradiation at room temperature. Low levels of the bis azide were effective in rendering the membranes insoluble in cyclohexane and carbon tetrachloride, both good solvents for PMP. The process is simple and effective, and thus PMP can be easily converted to mechanically stable membranes. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all tested gases due to the crosslinking. By adding nanoparticles, the permeability is again increased, crosslinking is successful in maintaining the permeability and selectivity of PMP over time.

Macromolecular self-assembly in dilute sequence-controlled block copolymer/homopolymer blends

Abstract Conventional block copolymers consist of two long contiguous monomer sequences (‘blocks’) that can, in the same fashion as low-molar-mass surfactants, self-assemble into various microstructural elements (e.g., micelles at low copolymer concentrations) to minimize repulsive contacts in the presence of a parent homopolymer. In this work, we explore the existence of segment-specific interactions, as well as the possibility of tailoring these blend morphologies (and producing altogether new ones), with novel sequence-controlled block copolymers. These copolymers are comprised of at least one block that is a random segment composed of both constituent monomer species. Transmission electron microscopy is employed here to examine the bilayered membranes and channel structures that form in two different series of such copolymers in dilute copolymer/homopolymer blends.

Configurational studies of microphase-separated diblock copolymers in the solid state

Abstract Microphase separation occurs in block copolymers due to thermodynamic incompatibility between the blocks and is responsible for the formation of periodic microstructures, or domains, which are on the same size scale as the root-mean-square end-to-end distance of the domain-forming block ( 〈r 2 k 〉 1 2 ). Experimental evidence has shown that 〈r 2 k 〉 1 2 deviates significantly from its unperturbed analogue ( 〈r 2 k 〉 1 2 0 ), implying that each block in a microphase-separated block copolymer is either expanded or contracted owing to the presence of the other block. In this work, expansion coefficients for each block and for the entire copolymer molecule are predicted as functions of both molar composition and molecular weight in poly(styrene-butadiene) diblock copolymers exhibiting lamellar morphology with modified versions of the Leary-Henderson-Williams thermodynamic theory. A comparison of predictions obtained for each of these model variations, which reflect the extent of block interdependence, is also provided.