| Popis: |
Investigations focused on the lower critical solution temperature (LCST) exhibited by poly(N-isopropylacrylamide) (PNIPAAM) in aqueous solution. The perturbation of this transition by the addition of cosolutes and by copolymerization was examined through the application of microcalorimetry, fluorescence spectroscopy, surface tensiometry, and cloud point measurements. Cosolutes employed included polar solvents, inorganic salts, amphiphiles, and other macromolecules. Copolymerization incorporated very small amounts (less than 2 mol %) of fluorescent derivatives or N-hexadecylacrylamide (HDAM). The microcalorimetric method was established through analyzing endotherms observed upon heating various samples of PNIPAAM and other polymers that exhibit LCSTs in aqueous solution; comparisons were made with classical cloud point curves. The properties of ternary solutions were then determined. Substituting miscible polar solvents for water lead to the phenomenon of cononsolvency. Transitions present in water-methanol mixtures were quantitatively related to those found by Hirotsu in crosslinked gels of PNIPAAM. The effects of low concentrations of poly(vinyl methyl ether) (PVME) and polyacrylamide were briefly studied. The influence of PNIPAAM on the association of amphiphiles characterized by a wide range of head group and tail structures was measured. Changes in critical surfactant concentrations and bilayer transition endotherms were correlated with reciprocal effects on the LCST of PNIPAAM to establish a hierarchy of complexation strengths. Use of free probes was complemented by applying fluorescent amphiphiles and polymer-bound fluorophores. This last approach was applied not only as an environmental micropolarity sensor but also in nonradiative energy transfer experiments. Solutions of PNIPAAM/HDAM copolymers were concluded to be micellar via the above methods. Solubilization sites of probes were estimated based upon their sensitivity to the LCST. Adding amphiphiles and varying polymer concentration also aided in establishing a microheterogeneous state. |
