Biomedical subjects

Todd Hoare

Publications and source records attributed to Todd Hoare.

4 recordsLinked to original sources

PubMed2006

Kinetic prediction of functional group distributions in thermosensitive microgels.

A kinetic model accounting for the copolymerization of up to four comonomers is applied to predict both chain and radial functional group distributions in carboxylic-acid-functionalized poly(N-isopropylacrylamide) (NIPAM)-based microgels. The model can accurately predict the experimentally observed radial distributions of functional monomers in microgels prepared using a variety of different carboxylic-acid-functionalized monomers with significantly different hydrophobicities, copolymerization kinetics, and reactivities, without requiring the use of adjustable parameters. Multimodal distributions can both be predicted and experimentally generated by copolymerizing two -COOH-containing monomers with widely different reactivities. Chain distributions and monomer block formation can also be probed using the kinetic model, allowing for qualitative predictions of the potentiometric titration behavior of the microgels. The kinetic model reported herein therefore provides the first available analytical method for semiquantitatively predicting and controlling functional group distributions in bulk-polymerized microgel systems.

Journal Article↗

PubMed2006

Titrametric characterization of pH-induced phase transitions in functionalized microgels.

The chain and radial functional group distributions in carboxylic acid-functionalized poly(N-isopropylacrylamide)-based microgels have significant impacts on the types of swelling responses exhibited by the microgels upon the application of a temperature and/or pH stimulus. Potentiometric, conductometric, and calorimetric titration approaches are used in this work to characterize the chain distributions of -COOH groups in five microgels prepared using different -COOH-functionalized monomers. A direct correlation is observed between the kinetically predicted formation of functional monomer blocks within the microgel, the excess Gibbs free energy of ionization, and the apparent pK(a) versus degree of ionization profiles generated from potentiometric titration. Isothermal titration calorimetry (ITC) can be used to quantify the relative number of functional groups present in microgels prepared with the same functional monomer and/or identify differences between microgels with the same bulk -COOH content but different chain distributions. In particular, microgels prepared with diacid-functionalized monomers exhibit a characteristic two-step ITC profile. For microgels with the same bulk -COOH content, the heat of ionization measured via ITC increases systematically with the overall change in both the pK(a) and the excess Gibbs free energy for microgels prepared with monoacid-functionalized monomers. Diacid monomer-functionalized microgels have lower ionization enthalpies attributable to the break-up of hydrogen-bonded intramolecular ring complexes upon carboxylic acid ionization. The inferred chain functional group distributions can be used to understand differences in microgel swelling across the pH-induced phase transition.

Acrylamides↗

PubMed2006

Dimensionless plot analysis: a new way to analyze functionalized microgels.

A novel dimensionless plot strategy is developed in which multiple, independently measured macroscopic variables characterizing the same microgel phase transition are reduced onto a zero-to-one scale and simultaneously plotted. This strategy allows for direct comparisons to be made between changes in these variables over the phase transition, generating graphical "fingerprints" characteristic of specific radial and chain functional group distributions. This method is applied to study the swollen state properties of pH-induced phase transitions in five poly(N-isopropylacrylamide)-based microgels with significantly different but well-understood COOH functional group distributions. Radial functional group distributions identified based on this method match the distributions observed via electron microscopy, while the impact of functional group clustering on the ionization-driven swelling response can be directly identified. Change ratio plots in which the percentage changes of two different variables are plotted as mutual functions are also applied to generate semi-quantitative diagnostic parameters for probing radial functional group distributions and gaining insights into the mechanisms of gel phase transitions.

Journal Article↗

PubMed2004

Functional group distributions in carboxylic acid containing poly(N-isopropylacrylamide) microgels.

Control of the functional group distribution is of fundamental importance in the design of functional polymer particles, particularly in biological applications. Surface-functionalized particles are useful for bioconjugation and medical diagnostics, while internally functionalized particles may have applications in drug delivery. We have prepared a series oftemperature-sensitive poly(N-isopropylacrylamide) (PNIPAM)-based microgels containing carboxylic acid functional groups via copolymerization with methacrylic acid and acrylamide, which was selectively hydrolyzed under optimized conditions to generate the carboxylic acid functionality. The resulting microgels were analyzed using conductometric and potentiometric titration, dynamic light scattering, and electrophoresis. Acrylamide-containing microgels hydrolyzed below the volume phase transition temperature (VPTT) show broad particle size versus temperature profiles, relatively low electrophoretic mobilities at basic pH, and time-dependent base titration profiles, suggesting the presence of internal functional groups whose titration is diffusion-controlled. Methacrylic acid containing microgels show sharper particle size versus temperature profiles, higher electrophoretic mobilities at basic pH, and time-independent base titration profiles, suggesting the presence of a "core-shell" structure with primarily surface functionalization. Similar results were obtained when acrylamide-containing microgels were hydrolyzed at temperatures above the VPTT. Thus, through selection of comonomer and hydrolysis conditions, we have developed strategies to control and characterize the number and distribution ofcarboxylic acid functional groups in PNIPAM-based microgels.

Journal Article↗