Search for the Origin of Discrepancies in Osmotic Measurements of the PNIPAM - Water System

Enikő Manek,Erik Geissler,Etelka Tombácz,Krisztina László

doi:10.3311/ppch.10273

Abstract

Major, still unelucidated, inconsistencies exist in the literature among measurements of the thermodynamic properties of poly(N-isopropylacrylamide) (PNIPAM) solutions and gels. This paper looks for evidence of intrinsic ionic behaviour in cross-linked PNIPAM homopolymer hydrogels synthesized in water under standard conditions. Systematic measurements are made of the swelling and osmotic properties of lightly cross-linked PNIPAM hydrogels, as well as of their potentiometric titration and DSC response, over a wide range of pH and ionic strength conditions, in order to distinguish the effects of the latter two parameters on putative intrinsic ions. The intrinsic ion content of the gel is found to be vanishingly small, and consequently unlikely to be the source of the divergences among past measurements. By contrast, a major finding of this study is that comparison of the present results with the literature reveals that frustrated equilibrium can be a source of substantial discrepancies.

Highlights

Poly(N-isopropyl-acrylamide) (PNIPAM) is a temperature sensitive water soluble polymer that displays lower critical solution behaviour: above a certain temperature it phase separates
Major, still unelucidated, inconsistencies exist in the literature among measurements of the thermodynamic properties of poly(N-isopropylacrylamide) (PNIPAM) solutions and gels
This paper looks for evidence of intrinsic ionic behaviour in cross-linked PNIPAM homopolymer hydrogels synthesized in water under standard conditions

Summary

Introduction

Poly(N-isopropyl-acrylamide) (PNIPAM) is a temperature sensitive water soluble polymer that displays lower critical solution behaviour: above a certain temperature it phase separates. Hydrogels made from this material correspondingly exhibit a volume phase transition (VPT), in which the gel collapses at a temperature TVPT ~34 °C, expelling most of the solvent. Measurements of the osmotic pressure, another thermodynamic parameter, are, by contrast, rare [5, 6] In this case agreement is merely qualitative, the numerical results being mutually incompatible. Such disorder, which should be a legitimate source of concern for the scientific discipline, suggests that the PNIPAM system may be more complex than previously believed

Methods

Results

Conclusion