Stimuli-responsive Polymers in Solution and on Grafted Surfaces

Thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM) have lower critical solution temperature (LCST) in aqueous solutions. Below the LCST, these polymers are hydrophilic with an extended coil conformation. Above the LCST, they undergo a sharp phase transition to form a collapsed hydrophobic conformation. The LCSTs are also affected by cosolutes and the effects of anions on LCSTs follow the Hofmeister series. We successfully used a simple digital melting point apparatus to study the effects of heating rates, solvent compositions, cosolutes, and redox state, on the LCSTs of thermoresponsive polymers. Moreover, the temperature range of the apparatus allowed for analyses at much higher temperatures and provides a simple way to examine irregular clouding behavior in more complex systems. Meanwhile, stimuli-responsive surfaces grafted with thermoresponsive polymers can switch from hydrophilic to hydrophobic thermally. As the LCST can be subsequently changed with the addition of salts, the salt effects on the wettability of these thermoresponsive surfaces will dramatically impact the surface performance. In this dissertation, I prepared PNIPAM/SiO2 nanocomposite surfaces by a covalent layer-by- layer assembly procedure and such surfaces were then used in studies of salts effects on surface wettability. Both the effects of anions and cations on the changes of advancing angles (Delta Theta a) of the PNIPAM/SiO2 nanocomposite surfaces were significant (Delta Theta a up to 90 degrees). The anion effects on the surface wettability followed the Hofmeister effect as expected. Parallel studies on solution showed that variation of cations had a large effect on the LCST of PNIPAM too. Moreover, analyses of the Theta a and LCST data using activity instead of using concentration showed different orders for the cation effects which were readily grouped by the cation charge numbers. No difference was seen for the anion effects in similar studies. AFM studies showed that surface morphology changes were correlated with the Delta Theta a.