Manipulating the thermoresponsive behaviour of poly(N-isopropylacrylamide) 3. On the conformational behaviour of N-isopropylacrylamide graft copolymers

The thermally triggered conformational change of poly(N-isopropyl acrylamide), PNIPAM, in aqueous media occurs at the lower critical solution temperature, LCST. Manipulation of the switch can be achieved via simple free radical copolymerisation, for example. However, the magnitude of the transition is reduced which has a detrimental effect on the solubilisation and controlled release properties of the polymer. In an attempt to over come these limitations the effect of architecture on the thermal response has been examined through syntheses of a range of fluorescently labelled graft copolymers. To examine the effect of topography, samples containing a NIPAM-based backbone and NIPAM branches have been prepared. Simultaneous variation of the entropic and enthalpic contribution to the thermal response has been achieved through syntheses of macromolecules containing a dimethylacrylamide-based backbone and NIPAM grafts. Time-resolved fluorescence anisotropy (TRFA) measurements have been successful in determining the onset, magnitude and dispersion of the LCST of these samples and, by selective labelling of sites, have provided information concerning the role of backbone and graft on the resultant thermorespsonive behaviour. TRFA measurements confirm that the temperature of the conformational switch can be varied through simultaneous manipulation of the entropic and enthalpic contribution to the thermal response of the graft copolymers, whilst maintaining the magnitude of the transition.