Effect of concentration and ionic strength on the lower critical solution temperature of poly(N-isopropylacrylamide) investigated by small-angle X-ray scattering

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ABSTRACT

Thermoresponsive polymers, with special emphasis on poly(N-isopropylacrylamide) (PNIPAAM) have been the focus of several investigations due to its potential applications in many fields of physical and polymer chemistry. PNIPAAM has a “lower critical solution temperature” at 32°C. The LCST can be finely tuned by copolymerization with hydrophobic or hydrophilic comonomers, and with a change of physical chemical parameters, such as ionic strength of the solution. We investigated the effect of polymer concentration on the LCST in two different solution environment, in pure water and in 50 mM NaCl solution with small-angle X-ray scattering (SAXS) of relatively low molecular mass polymers. We showed that the radius of gyration of the pNIPAAM chains increases with the addition of NaCl to the system due to the low level of specific adsorption of chloride ions on the polymer, whilst increasing the temperature causes a shrinkage of the polymer chains. Furthermore, by increasing the temperature the attractive interaction between the individual polymer chains is enhanced which turns into aggregation at the LCST.

KEYWORDS:

• PNIPAAM
• SAXS
• LCST

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This publication was produced under the operation program called Research and Innovation for the project: “Support of research and development capacities in the area of nanochemical and supramolecular systems,” code ITMS2014+ 313011T583, funded from the resources of the European Regional Development Fund. Support from Independent Research Fund Denmark (Natural Sciences, grant no 4002-00479 and grant no 8021-00133B), Independent Research Fund Denmark (Technology and Production, grant no 4184-00218) and from the Hungarian National Research, Development and Innovation Office (NKFIH K116629) are gratefully acknowledged for funding.