Matthew P. Espe

Improving the electrical conductivity of polymer acid-doped polyaniline by controlling the template molecular weight

We report the synthesis and characterization of a series of polyanilines (PANI) that are templated with a polymer acid at varying molecular weights. The polymer acid template of choice, poly(2-acrylamido-2-methyl-1-propanesulfonic acid), or PAAMPSA, was synthesized via conventional free-radical polymerization with molecular weights ranging from 45 kg mol−1 to 724 kg mol−1. Subsequent aniline polymerization in the presence of PAAMPSA yielded water-dispersible, conductive PANI–PAAMPSA. The electrical conductivity of PANI–PAAMPSA increases by approximately three-fold with decreasing PAAMPSA molecular weight. This trend is accompanied by significant structural changes, including increases in PANI crystallinity and conjugation length. These conductivities (0.4–1.1 S cm−1) exceed the highest reported for polymer acid-doped PANI systems.

Reinforcement of Silica Aerogels Using Silane-End-Capped Polyurethanes

Proper selection of silane precursors and polymer reinforcements yields more durable and stronger silica aerogels. This paper focuses on the use of silane-end-capped urethane prepolymer and chain-extended polyurethane for reinforcement of silica aerogels. The silane end groups were expected to participate in silica network formation and uniquely determine the amounts of urethanes incorporated into the aerogel network as reinforcement. The aerogels were prepared by one-step sol-gel process from mixed silane precursors tetraethoxysilane, aminopropyltriethoxysilane (APTES), and APTES-end-capped polyurethanes. The morphology and mechanical and surface properties of the resultant aerogels were investigated in addition to elucidation of chemical structures by solid-state (13)C and (29)Si nuclear magnetic resonance. Modification by 10 wt % APTES-end-capped chain-extended polyurethane yielded a 5-fold increase in compressive modulus and 60% increase in density. APTES-end-capped chain-extended polyurethane was found to be more effective in enhancement of mechanical properties and reduction of polarity.

Surface Modification and Reinforcement of Silica Aerogels Using Polyhedral Oligomeric Silsesquioxanes

This study evaluated polyhedral oligomeric silsesquioxane (POSS) molecules as useful, multifunctional reinforcing agents of silica aerogels. Silica aerogels have low-density and high surface area, although their durability is often compromised by the inherent fragility and strong moisture absorption behavior of the silica networks. POSS molecules carrying phenyl, iso-butyl, and cyclohexyl organic side groups, and several Si-OH functionalities were incorporated into silica networks via reactions between Si-OH functionalities in POSS molecules and silanes. Solid state (13)C and (29)Si NMR spectra established that greater than 90% of POSS molecules grafted onto silica networks and led to an increase in fractal dimensions. An almost 6-fold increase in compressive modulus was achieved with less than 5 wt % trisilanol phenyl POSS, and a 50-fold decrease in polarity with negligible changes in density were seen in aerogels modified with less than 5 wt % trisilanol isobutyl POSS.

Synthesis and characterization of lithium hemiporphyrazines.

The hemiporphyrazines comprise a broad class of phthalocyanine analogues where one or two of the diiminoisoindolene units are replaced with alternative rings, including pyridines, benzenes, and azoles. As a means to explore the fundamental metal chemistry of these macrocycles, we have prepared the first lithium complexes of three hemiporphyrazine variants: the common bis-pyridine ring, the bis-benzene macrocycle (also known as dicarbahemiporphyrazine), and the monobenzene variant (also known as benziphthalocyanine). The metal cation can be inserted via reaction of the free bases by using lithium bis(trimethylsilyl)amide, and the resulting products all form 1:1 complexes with protonation at the meso nitrogens providing charge balance. For the two carbahemiporphyrazines studied, the internal C-H bond remains intact upon metalation. Similar structures have been observed in the transition metal complexes of the carbaporphyrins. In addition, all three complexes are characterized by (7)Li solid state NMR and by cyclic voltammetry.

New Antifouling Silica Hydrogel

In this work, a new antifouling silica hydrogel was developed for potential biomedical applications. A zwitterionic polymer, poly(carboxybetaine methacrylate) (pCBMA), was produced via atom-transfer radical polymerization and was appended to the hydrogel network in a two-step acid-base-catalyzed sol-gel process. The pCBMA silica aerogels were obtained by drying the hydrogels under supercritical conditions using CO(2). To understand the effect of pCBMA on the gel structure, pCBMA silica aerogels with different pCBMA contents were characterized using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) spectroscopy, and the surface area from Brauner-Emmet-Teller (BET) measurements. The antifouling property of pCBMA silica hydrogel to resist protein (fibrinogen) adsorption was measured using enzyme-linked immunosorbent assay (ELISA). SEM images revealed that the particle size and porosity of the silica network decreased at low pCBMA content and increased at above 33 wt % of the polymer. The presence of pCBMA increased the surface area of the material by 91% at a polymer content of 25 wt %. NMR results confirmed that pCBMA was incorporated completely into the silica structure at a polymer content below 20 wt %. A protein adsorption test revealed a reduction in fibrinogen adsorption by 83% at 25 wt % pCBMA content in the hydrogel compared to the fibrinogen adsorption in the unmodified silica hydrogel.

Self-crosslinkable poly(urethane urea)-reinforced silica aerogels

Mechanically reinforced organic–inorganic hybrid silica aerogels are produced from simultaneous hydrolysis and condensation reactions of silane precursors – tetraethoxy silane (TEOS) and aminopropyltriethoxysilane (APTES) – and silane-modified polyurethane urea molecules each carrying multiple (≥3) reactive silane groups. In this manner, the post-gelation crosslinking reactions are avoided, the amount of polymer introduced into the aerogel structures is controlled, and the chain length between two crosslink points is tailored. The long chain polymer molecules introduce a certain degree of flexibility to the hybrid aerogel structures. The morphology, compressive properties, and surface area are obtained respectively using scanning electron microscopy, Instron tensile testers, and Brunauer–Emmett–Teller (BET) surface area analysis. The data on solid state 13C and 29Si NMR spectra reveal chemical reactions of the silane-modified polymers with the silica particle networks. Small angle X-ray scattering (SAXS) data are used to determine the fractal dimension of the silica networks. It is found that the self-crosslinkable multifunctional polyurethane urea chains form coatings on the silica networks and produce large enhancements in compressive modulus although with increases in shrinkage and bulk density.

Interaction of substituted poly(phenyleneethynylene)s with ligand-stabilized CdS nanoparticles

The interfacial region between surface-modified semiconducting nanoparticles and polymers remains difficult to characterize experimentally in atomic resolution and contributes to the limited efficiency of hybrid photovoltaic cells and luminescent devices. Therefore, molecular dynamics simulation was employed to investigate the structure of cadmium sulfide nanoparticles capped with 3-mercaptopropyltrimethoxysilane (MPS) in contact with four substituted poly(phenyleneethynylene)s using a new force field for CdS and the polymer consistent force field. The results show that polymers with long alkyl side chains tend to wrap around the nanoparticles, and reduce backbone bending as well as polymer diffusion. The absence of alkyl side chains decreases the distance of conjugated backbones from the surface. Differences in the preferred location of functional groups of the polymers on the nanoparticle surface and of covalent versus non-covalent bonding were also monitored. Polymers containing terminal hydroxyl groups on alkyl side chains approach the surfactant corona and the core of the CdS-MPS nanoparticles. Close contact supports the formation of silyl ether cross-links although the interfacial structure upon bond formation remains similar to that of the non-covalently attached polymers. Ester groups bound to aromatic rings in the poly(phenyleneethynylene) backbone did not closely approach the nanoparticle surface. The results are the first step to understand nanoparticle–polymer interfaces at length scales of 10 nm and explore correlations with photovoltaic performance.

Observation of a deuteron nuclear magnetic resonance Knight shift in conductive polyaniline.

Solid state deuteron magic angle spinning nuclear magnetic resonance spectra of conductive ring-deuterated polyaniline consist of two peaks, one at the same chemical shift as the insulating form of the polymer and the second shifted by 5.8+/-1 ppm. The magnitude of the shift is field and temperature independent and is identified as a Knight shift. The deuterons undergoing a Knight shift originate from both the crystalline and amorphous regions of the sample, implying that conduction is mediated by delocalized polarons in both these regions. Spin count experiments demonstrate that in highly conductive samples, signal is lost not only by dephasing due to the proximity of localized unpaired electrons but also to high rf reflectance.

Structural Characterization of Poly(Sodium 4-Styrene Sulfonate)/CdS Semiconductor Nanoparticle Composites

The properties of polymer/semiconducting nanoparticle (NP) composites-materials used in hybrid, bulk-heterojunction photovoltaic materials-are dependent on the interaction of the NPs and polymer. Composite films of water soluble polymers and CdS NPs have been produced both by synthesizing the NPs within the polymer matrix and by adding the CdS NPs, containing a capping agent, to the polymer. The composites have been characterized by microscopy as well as 1H, 13C and 113Cd solid-state NMR. When synthesizing the NPs within the polymer, the polymer matrix plays a role in the cadmium sulfide NP nucleation, growth and structure. In the blended system, the dominant interaction between the glycerol capping agent and sulfonated polymer is observed to be hydrogen bonding.