Solution properties of hydrophobically modified copolymers of N-isopropylacrylamide and N-glycine acrylamide:: A study by microcalorimetry and fluorescence spectroscopy

The temperature- and pH-induced coil-globule transition has been studied in dilute aqueous solutions for different copolymers of N-isopropylacrylamide (NIPAM) and N-glycine acrylamide (Gly) using turbidimetry, scanning microcalorimetry, fluorescence spectroscopy, and dynamic light scattering. The four different samples prepared are a copolymer of NIPAM and Gly (PNIPAM-Gly), a copolymer of NIPAM, Gly, and N-(1-pyrenyl)methylacrylamide (PNIPAM-Gly-Py), and their hydrophobically modified (HM) derivatives, namely a copolymer of NIPAM, Gly, and N-(n-octadecylacrylamide) (PNIPAM-Gly-C-18) and a copolymer of NIPAM, Gly, and N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamde (PNIPAM-Gly-C18Py). Polymeric micelles 16 +/- 2 nm in diameter were detected in cold solutions of the hydrophobically modified polymers. All polymers underwent pH-dependent phase separation upon heating. Endotherms with enthalpies on the order of the strength of hydrogen bonds were observed at temperatures concurring, in the case of PNIPAM-Gly and PNUPAM-Gly-Py, with the transition temperatures detected by classical cloud-point measurements. Discrepancies between the two values were detected in the case of the hydrophobically modified polymers. Evidence from fluorescence spectroscopy, corroborated by dynamic light scattering and microcalorimetry data, suggests that the pH- or temperature-stimulated coil to globule collapse of the polymer main chain does not trigger the disruption of the hydrophobic core of HM-polymer micelles.