Julie P. Harmon

Synthesis and magnetic properties of polymer nanocomposites with embedded iron nanoparticles

Magnetic nanoparticles embedded in polymer matrices have excellent potential for electromagnetic device applications like electromagnetic interference suppression. We have synthesized polymer nanocomposites of poly(methylmethacrylate) doped with varying concentrations of iron nanoparticles (∼20 nm in size). The iron nanoparticles were produced using a microwave plasma technique and have a natural oxide surface layer for passivation. These nanocomposites were processed using melt blending technique. The polymer processing conditions were optimized to achieve good uniform dispersion of the nanoparticles in the polymer matrix. The concentration and dispersion of nanoparticles were varied in a controlled way. Surface characterization with scanning electron microscopy indicates that, to a large extent, the iron nanoparticles are embedded in the bulk; the surface mainly showed features associated with the polymer surface. Static magnetic properties such as susceptibility and M–H loops were studied using a physi...

Use of hydrogel coating to improve the performance of implanted glucose sensors.

In order to protect implanted glucose sensors from biofouling, novel hydrogels (146-217% water by mass) were developed based on a copolymer of hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA). The porosity and mechanical properties of the hydrogels were improved using N-vinyl-2-pyrrolidinone (VP) and ethyleneglycol dimethacrylate (EGDMA). The results of SEM and DSC FT-IT analyses showed that the hydrogel (VP30) produced from a monomeric mixture of 34.5% HEMA, 34.5% DHPMA, 30% VP and 1% EDGMA (mol%) had an excellent pore structure, high water content at swelling equilibrium (W eq=166% by mass) and acceptable mechanical properties. Two kinds of VP30-coated sensors, Pt/GOx/VP30 and Pt/GOx/epoxy-polyurethane (EPU)/VP30 sensors were examined in glucose solutions during a period of 4 weeks. The Pt/GOx/VP30 sensors produced large response currents but the response linearity was poor. Therefore, further studies were focused on the Pt/GOx/EPU/VP30 sensors. With a diffusion-limiting epoxy-polyurethane membrane, the linearity was improved (2-30 mM) and the response time was within 5 min. Eight Pt/GOx/EPU/VP30 sensors were subcutaneously implanted in rats and tested once per week over 4 weeks. All of the implanted sensors kept functioning for at least 21 days and 3 out of 8 sensors still functioned at day 28. Histology revealed that the fibrous capsules surrounding hydrogel-coated sensors were thinner than those surrounding Pt/GOx/EPU sensors after 28 days of implantation.

Effects of gamma radiation on poly(methyl methacrylate)/single-wall nanotube composites

Single-wall carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) composites were fabricated and exposed to ionizing radiation for a total dose of 5.9 Mrads. Neat nanotube paper and pure PMMA were also exposed for comparison, and nonirradiated samples served as controls. A concentration of 0.26 wt% SWNT increased the glass transition temperature ( T g ), the Vickers hardness number, and modulus of the matrix. Irradiation of the composite did not significantly change the T g , the Vickers hardness number, or the modulus; however, the real and imaginary parts of the complex permittivity increased after irradiation. The dielectric properties were found to be more labile to radiation effects than mechanical properties.

Synthesis and performance of novel hydrogels coatings for implantable glucose sensors.

Novel hydrogel polymers were prepared, characterized, coated on implantable glucose sensors, and tested in vitro and in vivo. The effects of 2,3-dihydroxypropyl methacrylate (DHPMA) on the swelling, morphology, glass transition (T(g)), and water structure were studied. The results show that the degree of swelling increases with increasing DHPMA content. Scanning electron microscopy (SEM) studies identified uniform, porous structures in samples containing 60-90 mol % DHPMA. Glass-transition temperatures did not change significantly with DHPMA content, but the ratio of freezing to nonfreezing water tended to increase with DHPMA content. Sensors coated with different hydrogels were prepared and in vitro evaluations were performed. The 80% DHPMA hydrogels exhibited optimum sensitivity, response, and stability when coated directly onto the sensor or top of a polyurethane (PU) layer. The histology results show that 80% DHPMA samples exhibit reduced fibrosis and reduced inflammation, resulting in a longer functional life.

Symbiosis of zeolite-like metal–organic frameworks (rho-ZMOF) and hydrogels: Composites for controlled drug release

The design and synthesis of new finely tunable porous materials has spurred interest in developing novel uses in a variety of systems. Zeolites, inorganic materials with high thermal and mechanical stability, in particular, have been widely examined for use in applications such as catalysis, ion exchange and separation. A relatively new class of inorganic–organic hybrid materials known as metal–organic frameworks (MOFs) have recently surfaced, and many have exhibited their efficiency in potential applications such as ion exchange and drug delivery. A more recent development is the design and synthesis of a subclass of MOFs based on zeolite topologies (i.e. ZMOFs), which often exhibit traits of both zeolites and MOFs. Bio-compatible hydrogels already play an important role in drug delivery systems, but are often limited by stability issues. Thus, the addition of ZMOFs to hydrogel formulations is expected to enhance the hydrogel mechanical properties, and the ZMOF–hydrogel composites should present improved, symbiotic drug storage and release for delivery applications. Herein we present the novel composites of a hydrogel with a zeolite-like metal–organic framework, rho-ZMOF, using 2-hydroxyethyl methacrylate (HEMA), 2,3-dihydroxypropyl methacrylate (DHPMA), N-vinyl-2-pyrolidinone (VP) and ethylene glycol dimethacrylate (EGDMA), and the corresponding drug release. An ultraviolet (UV) polymerization method is employed to synthesize the hydrogels, VP 0, VP 15, VP 30, VP 45 and the ZMOF-VP 30 composite, by varying the VP content (mol%). The rho-ZMOF, VP 30, and ZMOF-VP 30 composite are all tested for the controlled release of procainamide (protonated, PH), an anti-arrhythmic drug, in phosphate buffer solution (PBS) using UV spectroscopy.

Anisotropic methanol transport in PMMA after mechanical deformation

Abstract Transport of methanol in compressed PMMA (23–34% strain) was found to be anisotropic. It was faster by a factor of two to three in penetration velocity and by a factor of 10 in diffusivity in the direction of compression than in the perpendicular direction. However, the penetration velocity in the slow direction is still a factor of two to five larger than that of the undeformed PMMA (two at 40°C and five at 25°C). The two-dimensional mixed diffusion and case II transport problem was solved analytically so that the diffusivity and the penetration velocity could be obtained from experimental weight gain data. The penetration front was found to correspond approximately to a front of constant concentration of methanol. When methanol was mixed with glycerol to lower the methanol concentration (glycerol does not penetrate PMMA), the penetration velocity was smaller due to reduced swelling but the diffusivity was unchanged as expected.

Intra and intermolecular relaxations 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate hydrogels

The dynamic mechanical transitions present in the 2-hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA) homopolymers and two random copolymers, with one-to-one and three-to-one HEMA-to-DHPMA molar ratios, were examined by analyzing the dry xerogels and hydrogels at various states of hydration. The temperatures of the primary a transition and the secondary b and g transitions were determined in the tension mode; the storage modulus and loss modulus as a function of temperature and frequency were recorded. The results were compared to the results obtained from dielectric analysis at low hydration using tan d. The frequency dependence of the dispersions was calculated for the dry and hydrated states, using mechanical and dielectric data. The information obtained was used to elucidate the interaction between the polymer and the sorbed water. Analysis of the low temperature secondary g transition and secondary transitions resulting from polymer –water interactions was emphasized. During the initial hydration, the temperature maxima of the dielectric and mechanical secondary g transitions, and the transitions that appear in the presence of absorbed water increased as the DHPMA content increased. The apparent activation energy associated with the secondary transitions increased as well. All of this is the result of stronger intermolecular interactions due to the increased density of hydrophilic moieties in DHPMA. q 2002 Elsevier Science Ltd. All rights reserved.

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry: determining Mark–Houwink–Sakurada parameters and analyzing the breadth of polymer molecular weight distributions

Abstract Polystyrene, poly(methyl methacrylate), and poly(ethylene glycol) standards were characterized via matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. This study develops two new contributions to the rapidly growing body of data appearing in recent literature. First, MALDI was found to be an optimum absolute molecular weight technique accompanying viscosity measurements for determining Mark–Houwink–Sakurada parameters. Second, since MALDI provides the mass spectra of polymer molecules without fragmentation, a new method of describing the breadth of the distribution of polymer standards is presented. The ratio of the distribution breadth at half-height to the molecular weight of the monomer unit, polymer spread, is shown to more clearly represent the molecular weight distribution than the customary ratio, Mw/Mn.

Molecular Relaxations in Ester-Terminated, Amide-Based Dendrimers

This study utilized Matrix Assisted Laser Desorption/Ionization Time-of- Flight Mass Spectrometry, Thermogravimetric Analysis, Differential Scanning Calo- rimetry, X-Ray Diffraction, and Dielectric Analysis to assess the viscoelastic and structural properties of three generations of tert-butyl and methyl ester, amide-based dendrimers. The effect of generation number and functionality on glass-transition temperatures and corresponding apparent activation energies, obtained via adherence to WLF behavior, were determined. Both were found to increase with increasing generation number and bulkiness of terminal functionalities. WLF constants, C1 and C2, allowed the determination of free volume, and thermal expansion coefficients, respectively. Secondary transitions, conforming to Arrhenius behavior, were also char- acterized and increased in temperature with generation number. The apparent activa- tion energy was greater when the matrix was crystalline. Dielectric relaxation re- sponses were analyzed to yield dielectric strengths of the molecular relaxations which increased with generation number and were comparable for both tert-butyl and methyl esters in the glass-transition region. Electrical properties of the dendrimers were dominated by ionic conductivity in the high temperature region. In order to unmask the glass transition, the data were treated in terms of the electric modulus.

2,3-Dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate hydrogels: gel structure and transport properties

The drying kinetics were examined in four cross-linked polymers that form hydrogels: 2,3-Dihydroxypropyl methacrylate (DHPMA) and 2-hydroxyethyl methacrylate (HEMA) and two random copolymers, with one-to-one and three-to-one HEMA-to-DHPMA molar ratios. The hydrogels were saturated with buffered, isotonic saline solution and deionized water; weight loss kinetics were monitored at temperatures from 16 to 37 °C at 30 and 60% relative humidity under air flow. While there are numerous studies of diffusion in hydrogels, this is one of few studies examining the initial evaporative drying period. The analyses of this short time data revealed that increasing the DHPMA content decreased water volatility; the percent water loss rate decreased with DHPMA content. Long time desorption data coupled with differential scanning calorimetry (DSC) results suggest that departure from Fickian desorption kinetics coincides with the onset of non-freezing water desorption. Dynamic mechanical analysis (DMA) revealed that changes in stiffness accompanying desorption are much more pronounced in HEMA containing polymers. Preliminary results indicate that the ion transport rate is greater in DMPMA containing hydrogels.