Kinetics of Demixing and Remixing Transitions in Aqueous Solutions of Poly(N-isopropylacrylamide): A Temperature-Jump 1H NMR Study

✍ Scribed by Pavel V. Yushmanov; István Furó; Ilias Iliopoulos

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 141 KB
• Volume: 207
• Category: Article
• ISSN: 1022-1352
• DOI: 10.1002/macp.200600394

✦ Synopsis

Abstract

Summary: The time course of the coil‐to‐globule collapse and intermolecular aggregation of poly(N‐isopropylacrylamide) in aqueous solution upon exceeding the lower critical solution temperature (LCST) are investigated by temperature‐jump ^1^H NMR spectroscopy. After the temperature jump, we record the time dependences of (i) the mobile fraction of the polymer chain as revealed by the intensity of the liquid‐like NMR signal, (ii) the local mobility of those chains as revealed by the transverse relaxation time T~2~, and (iii) their self‐diffusion coefficient D. The same data are also reported at their temperature‐dependent long‐time limits. The results suggest a sudden, faster than one second, collapse and intermolecular aggregation into globules and a slower reorganization/redistribution of the individual chains among and within the globular and mobile states. We found that all molecular changes are reversible if the temperature remains less than ca. 6–8 K above the LCST for less than a few minutes; under those conditions, experiments upon sudden temperature quench below the LCST show that the aggregates disintegrate and swell into coils in less than a few seconds.

^1^H NMR signal intensity of the methyl groups of 1 wt.‐% PNIPAM dissolved in 0.1 M NaCl solution in D~2~O in a temperature‐jump experiment from 300 to 312 K. The data were recorded by the 90°−τ–(180°−2__τ__‐)~n~‐detection of the CPMG pulse sequence.

magnified image^1^H NMR signal intensity of the methyl groups of 1 wt.‐% PNIPAM dissolved in 0.1 M NaCl solution in D~2~O in a temperature‐jump experiment from 300 to 312 K. The data were recorded by the 90°−τ–(180°−2__τ__‐)~n~‐detection of the CPMG pulse sequence.