Abstract
Copolymers of N-(isopropylacrylamide) (NIPAM, 90 mol%) and N-L-valine acrylamide (Val, 10 mol%), as well as their hydrophobically-modified copolymers, namely a copolymer of NIPAM, Val, and N-n-octadecylacrylamide (PNIPAM-Val-C18, 1 mol%) and a copolymer of NIPAM, Val, and N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide (PNIPAM-Val-C18Py, 1 mol%) were prepared by free radical copolymerization of the respective monomers. Studies by turbidimetry, microcalorimetry, dynamic light scattering, and fluorescence spectroscopy indicated that the hydrophobically-modified copolymers form polymeric micelles in water (effective hydrodynamic diameter: 25 nm ± 5 nm). Solutions of all the copolymers underwent pH-dependent phase separation upon heating, but the pH- or temperature-stimulated coil-to-globule collapse/chain expansion did not result in complete disruption of the hydrophobic microdomains.
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Note that fluorescence measurements were performed on solutions far less concentrated than those used to monitor changes in turbidity (0.02 g L−1vs. 1.0 g L−1) in order to avert artifacts due to inner filter effects associated with excessive pyrene concentration and undesired scattering of the excitation light by turbid samples. Solutions for fluorescence experiments remained clear to the eye even above the cloud point.
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Poncet-Legrand, C., Winnik, F. Solution Properties of Hydrophobically-Modified Copolymers of N-Isopropylacrylamide and N-L-Valine Acrylamide. A Study by Fluorescence Spectroscopy and Microcalorimetry. Polym J 33, 277–283 (2001). https://doi.org/10.1295/polymj.33.277
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DOI: https://doi.org/10.1295/polymj.33.277