| Summary: | 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.
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