|M.Sc Student||Shehter-Garkavyk Inna|
|Subject||Molecular Aspects of Cosolvent Effects on Volume Phase|
Transition in Poly(N-isopropylacrylamide)
|Department||Department of Chemical Engineering||Supervisors||Professor Yaron Paz|
|Professor Yoav Livney|
|Full Thesis text|
PNIPA is insoluble in water above the lower critical solution temperature, and is reversibly solubilized below that temperature. The driving force for this coil-to-globule transition is associated with the temperature-dependent molecular interactions. It was found that presence of co-solutes in the Poly (N-isopropylacryamide) (PNIPA) aqueous solutions changes drastically the Lower Critical Solution Temperature (LCST). According to their ability to affect the LCST all co-solutes can be divided on two subgroups: 1) decreasing LCST known also as salting-out agents and 2) increasing LCST or salting-in agent. In this work KSCN was used as a salting-in salt and K2SO4 was used as a salting-out salt. Molecular aspects of phase separation behavior were studied by microcalorimetry and Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) techniques.
The main results of the work were:
1. As the concentration of KSCN is increased the LCST initially increased showing salting-in behavior, and then decreased showing an apparent salting-out behavior. According to microcalorimetric measurements PNIPA-KSCN 0.9M-water solution have the same LCST as binary solution. However, microcalorimetric thermograms and the FTIR measurements revealed distinct differences between the solutions. The addition of thyocianate caused a redshift of all hydrophobic peaks of the polymer and moderated the changes in polymer at LCST point. At low temperatures amide II amide-water H-bonded population was lower in PNIPA ternary solutions than in binary.
2. FTIR measurements revealed that ternary solution containing 0.1M K2SO4 and the solution containing 1.5M of KSCN have an equal LCST, however the mechanisms by which these salts change the LCST were completely different. Adding of KSCN to the PNIPA binary solution brought closer the initial and the final state of the polymer, thus causing to more moderate Coil-to-Globule Transition (CGT) process. In opposite to this, K2SO4 increased the shift in wavenumbers of the peaks between the PNIPA solution below and above the LCST causing steeper phase transition process.
3. It was suggested that anion SCN- are present in the hydration shell of the PNIPA at the higher concentration than in the bulk. Such an effect can be result of exclusion of SCN- as a chaotrope from the bulk water. Being in the hydration shell of the PNIPA thyocianate disrupts the hydrogen bonds between the N-H and water molecules, and the hydrogen bonds between the water molecules in “water cages” near the hydrophobic groups. Such dehydration decreases the cooperativity of the transition causing more gradual CGT.