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 microc alorimetry, fluorescence spectroscopy, and dynamic light scattering. The fo ur 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 copolyme r of NIPAM, Gly, and N-(n-octadecylacrylamide) (PNIPAM-Gly-C-18) and a copo lymer of NIPAM, Gly, and N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamde (PNI PAM-Gly-C18Py). Polymeric micelles 16 +/- 2 nm in diameter were detected in cold solutions of the hydrophobically modified polymers. All polymers unde rwent pH-dependent phase separation upon heating. Endotherms with enthalpie s on the order of the strength of hydrogen bonds were observed at temperatu res concurring, in the case of PNIPAM-Gly and PNUPAM-Gly-Py, with the trans ition temperatures detected by classical cloud-point measurements. Discrepa ncies between the two values were detected in the case of the hydrophobical ly modified polymers. Evidence from fluorescence spectroscopy, corroborated by dynamic light scattering and microcalorimetry data, suggests that the p H- or temperature-stimulated coil to globule collapse of the polymer main c hain does not trigger the disruption of the hydrophobic core of HM-polymer micelles.