Spectroscopic and Rheological Cross-Analysis of Polyester Polyol Cure Behavior: Role of Polyester Secondary Hydroxyl Content.

The sol-gel transition of a series of polyester polyol resins possessing varied secondary hydroxyl content and reacted with a polymerized aliphatic isocyanate cross-linking agent is studied to elucidate the effect of molecular architecture on cure behavior. Dynamic rheology is utilized in conjunction with time-resolved variable-temperature Fourier-transform infrared spectroscopy to examine the relationship between chemical conversion and microstructural evolution as functions of both time and temperature. The onset of a percolated microstructure is identified for all resins, and apparent activation energies extracted from Arrhenius analyses of gelation and average reaction kinetics are found to depend on the secondary hydroxyl content in the polyester polyols. The similarity between these two activation energies is explored. Gel point suppression is observed in all the resin systems examined, resulting in significant deviations from the classical gelation theory of Flory and Stockmayer. The magnitude of these deviations depends on secondary hydroxyl content, and a qualitative model is proposed to explain the observed phenomena, which are consistent with results previously reported in the literature.

Revisiting the colloidal fundamentals of water-dispersible polyesters: interactions and self-assembly of polymer nanoaggregates in water.

Water-dispersible sulfopolyesters are a major class of film-forming and solution-modifying polymers, which are routinely used in applications such as inks, adhesives, coatings, and personal care products. Since these polyesters are designed to be used as waterborne dispersions, understanding their colloidal interactions in dispersions is critical for their application. By using a range of commercially available water-dispersible sulfopolyesters as a model system, we investigated the relationship between their molecular composition, colloidal interactions, and phase equilibria. We established how these polyesters undergo different molecular configurations and nanoaggregated states, depending on the nature of the liquid medium. For example, the polyesters are in a solvated molecular form in certain organic solvents, whereas they self-assemble into compact nanoaggregates in water. We found that the interactions of these nanoaggregates follow the classical DLVO theory of critical colloidal coagulation where the stability of these nanoparticles is extremely sensitive to multivalent electrolytes (i.e., Ccrit ∝ z-6). By using static, dynamic, and electrophoretic light scattering, we correlate their nanoscale intermolecular and interparticle interactions with corresponding macroscale phase behavior in both organic medium and water, based on the theoretical framework of second virial coefficients. We present a model for nanoaggregate formation in water based on the critical surface charge density of these nanoparticles. Such fundamental understanding of colloidal interactions could be used to efficiently control and improve the colloidal stability and film-formation ability of these polyesters and may enable the design of novel high-performance surfactant-free waterborne dispersion systems.

Imidazolium-Containing ABA Triblock Copolymers as Electroactive Devices.

Two-step reversible addition-fragmentation chain transfer (RAFT) polymerization and two subsequent postpolymerization modification steps afforded well-defined ABA triblock copolymers featuring mechanically reinforcing polystyrene outer blocks and 1-methylimidazole-neutralized poly(acrylic acid)-based central blocks. Size exclusion chromatography and (1)H NMR spectroscopy confirmed predictable molecular weights and narrow distributions. The ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIm][OTf]) was incorporated at 30 wt % into polymeric films. Thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis determined the thermomechanical properties of the polymers and polymer-IL composites. Atomic force microscopy, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) determined surface and bulk morphologies, and poly(Sty-b-AA(MeIm)-b-Sty) exhibited a change from packed cylindrical to lamellar morphology in SAXS upon IL incorporation. Electrochemical impedance spectroscopy determined the in-plane ionic conductivities of the polymer-IL membranes (σ ∼ 10(-4) S/cm). A device fabricated from poly(Sty-b-AA(MeIm)-b-Sty) with 30 wt % incorporated IL demonstrated mechanical actuation under a low applied voltage of 4 V.

Single-walled carbon nanohorns decorated with semiconductor quantum dots to evaluate intracellular transport.

Single-walled carbon nanohorns (SWNHs) have great potential to enhance thermal and chemotherapeutic drug efficiencies for cancer therapies. Despite their diverse capabilities, minimal research has been conducted so far to study nanoparticle intracellular transport, which is an important step in designing efficient therapies. SWNHs, like many other carbon nanomaterials, do not have inherent fluorescence properties making intracellular transport information difficult to obtain. The goals of this project were to (1) develop a simple reaction scheme to decorate the exohedral surface of SWNHs with fluorescent quantum dots (QDs) and improve conjugate stability, and (2) evaluate SWNH-QD conjugate cellular uptake kinetics and localization in various cancer cell lines of differing origins and morphologies. In this study, SWNHs were conjugated to CdSe/ZnS core/shell QDs using a unique approach to carbodiimide chemistry. Transmission electron microscopy and electron dispersive spectroscopy verified the conjugation of SWNHs and QDs. Cellular uptake kinetics and efficiency were characterized in three malignant cell lines: U-87 MG (glioblastoma), MDA-MB-231 (breast cancer), and AY-27 (bladder transitional cell carcinoma) using flow cytometry. Cellular distribution was verified by confocal microscopy, and cytotoxicity was also evaluated using an alamarBlue assay. Results indicate that cellular uptake kinetics and efficiency are highly dependent on cell type, highlighting the significance of studying nanoparticle transport at the cellular level. Nanoparticle intracellular transport investigations may provide information to optimize treatment parameters (e.g., SWNH concentration, treatment time, etc.) depending on tumor etiology.