Short CommunicationPoly(N-isopropylacrylamide) (PNIPAM) is never hydrophobic
Graphical Abstract
Phase separated PNIPAM contains too much water to be considered hydrophobic.
Introduction
The last decade has seen the publication of hundreds of papers involving poly(N-isopropylacrylamide), (PNIPAM) as homopolymers and as constituents of copolymers, gels, microgels and surface layers. This interest is driven by the famous lower critical solution temperature (LCST) behavior, first described by Heskins and Guillet [1], where heating an aqueous solution of PNIPAM above 32 °C induces phase separation. The goal of this short communication is to address what I believe to be a misconception in many recent publications (including a few of my own). A Google Scholar search of “hydrophobic PNIPAM” reveals many publications claiming that PNIPAM becomes hydrophobic when aqueous solutions are heated above the LCST. Authors discuss “switching” or “intelligent”, implying that a hydrophilic material becomes a hydrophobic material when the temperature is elevated a few degrees. I believe that this view is incorrect and misleading. In the following paragraphs I make the case that PNIPAM chains have hydrophobic and hydrophilic domains below and above the LCST. Temperature dependent interactions between PNIPAM and solutes arise because of changes in the local environment around the hydrophobic isopropyl domains. Below the LCST the isopropyl groups are surrounded by water, whereas above the LCST the hydrophobic groups are in contact with both water and polymer segments. None of the ideas or results herein are new; however, judging by the number of papers and manuscripts discussing hydrophobic PNIPAM, more realistic descriptions need to be reiterated.
Section snippets
Discussion
Hydrophobicity is a qualitative concept. Evans and Wennerström state that “hydrophobicity has meant different things to different people at different times” [2] and van Oss comments “there is no absolute measure of hydrophobicity” [3]. Nevertheless, there are generally accepted concepts. When describing bulk material properties, hydrophobic is used to describe materials with very low water solubility and very low equilibrium water content. Polystyrene and poly(tetrafluoroethylene) are
Conclusions
In summary, PNIPAM does not satisfy the normal criteria for being either a bulk hydrophobic material or a hydrophobic surface except when a deswollen PNIPAM is in air. When explaining the rich range of behaviors with PNIPAM based materials, I believe it is more useful to consider the local environments of the hydrophobic domains, the isopropyl groups and the polymethylene backbone. As an example, consider the interaction of small molecules with PNIPAM.
Solutes undergoing hydrophobic interactions
Acknowledgment
Professor Todd Hoare is acknowledged for useful discussions.
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