Studies of the smart thermoresponsive behavior of copolymers of N-isopropylacrylamide and N,N-dimethylacrylamide in dilute aqueous solution

Various fluorescence techniques and cloud point measurements have been used to study the effects of altering the hydrophilic/hydrophobic balance in a series of N-isopropylacrylamide (NIPAM)/ N,N-dimethylacrylamide (DMAC) statistical copolymers upon the smart thermal responses of these systems in dilute aqueous solution. As expected, incorporation of DMAC into the polymer structure raises its lower critical solution temperature to an extent dependent upon DMAC content. However, use of such a hydrophilic modifier reduces the magnitude of the collapse transition that characterizes the macromolecule's thermal response. In PNIPAM, the LCST is associated with a conformational transition between a coil and a globule. However, introduction of DMAC derivatives into the polymer expands its globular form into a much more open structure that progressively loses its capacity for solubilization of organic guests. Consequently, although copolymerization with more polar monomers can be used to raise the LCST of NIPAM-based thermoresponsive polymers, the value of this approach will be limited in applications requiring switchable carrier/release properties.