Temperature-Responsive Polymers

Chapter 9Application of NMR Spectroscopy to Study Thermoresponsive Polymers

Jiří Spěváček

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

9.1 Introduction

It is well known that most thermoresponsive homopolymers show in aqueous solutions a lower critical solution temperature (LCST). They are soluble at lower temperatures, but heating above the LCST results in phase separation, which especially at polymer concentrations c ≥ 1 wt% makes solutions milk‐white turbid [1, 2]. At temperatures above the LCST, the system is therefore rather a colloidal suspension than the solution. For dilute solutions of poly(N‐isopropylacrylamide) (PNIPAm), which is a most often investigated thermoresponsive polymer, a coil–globule transition was revealed by light scattering at the LCST [3], while for semidilute and concentrated solutions, aggregation results in formation of larger globular‐like structures called mesoglobules [2]. When these polymers are chemically crosslinked, their hydrogels undergo a volume phase transition (collapse) around this temperature, when the volume of the collapsed hydrogel is reduced substantially. Of various methods used in investigations of thermoresponsive polymers in aqueous solutions and hydrogels, NMR spectroscopy can also provide important information on phase transition and interactions in these systems [4]. In this chapter, in addition to basic principles, application of NMR methods in the studies of ...