The effects of additives and chemical modification on the solution properties of thermo-sensitive polymers

This thesis focused on the phase separation of two well-known thermoresponsive polymers, namely PNIPAM (poly(N-isopropylacrylamide)) and PIPOZ (poly(2-isopropyl-2-oxazoline).  Despite various studies of the two polymers, two aspects of their thermal properties remained unclear and needed to be investigated.  One is the cononsolvency effect of PNIPAM in water and a second water miscible solvent. The other is the effect of the end group properties on the phase separation of PIPOZ.   With this in mind, we first studied the effect of the chain architecture on the cononsolvency of PNIPAM in water/methanol mixture, employing a 4-arm star shape PNIPAM and a cyclic PNIPAM as model.  Tethering PNIPAM arms to a hydrophobic core resulted in a reduced Tc (cloud point temperature) and a lower phase transition enthalpy change.  The Tc of the star shape PNIPAM was inversely dependent on the polymer molecular weight.  The dehydration cooperativity was depressed for the star PNIPAM and cyclic PNIPAM due to topological constraints.  A study of the effect of polymer concentration on the cononsolvency of PNIPAM in water/methanol mixture revealed a macroscopic liquid-liquid phase separation (MLLPS) for PNIPAM in water/methanol mixtures of methanol molar fraction ranging from 0.127 to 0.421 at a polymer concentration of 10 g·L-1.  The turbid suspension of PNIPAM/water/methanol separated into a polymer rich phase coexisting with a polymer poor solution phase after equilibration for two days at room temperature.  The phase diagram showing the MLLPS for the PNIPAM/water/methanol mixtures was constructed based on experimental data.  The droplets in the condensed polymer rich phase showed a dependence on the methanol molar fraction.  Methanol affects the surface tension of the liquid droplets.  The slow equilibrium kinetics of PNIPAM phase separation was sped up and a macroscopic liquid-liquid phase separation realized. In order to study the effect of end groups on the solution properties of PIPOZ, two telechelic PIPOZ end capped with perfluorodecanyl groups (FPIPOZ) and octadecyl groups  (C18PIPOZ), respectively, were synthesized.  The Tc values of the telechelic polymers were greatly reduced after end-functionalization.  Stable micelles formed in aqueous solutions of the telechelic polymers. The micellization and phase separation of the telechelic polymers in water were studied.  The phase separation of the telechelic PIPOZs in water followed a liquid-liquid phase separation mechanism. Differences in the sizes of droplets formed inside of the two polymer solutions were observed.  To further investigate the differences in the association behaviour between the two telechelic polymer, NMR signal intensities and T1 and T2 relaxation times were examined. Higher 1H T2 values were obtained for the IPOZ unit in FPIPOZ than that in C18PIPOZ, indicating a higher mobility of the main chain in the FPIPOZ micelles than that in the C18PIPOZ micelles.  Together with the 13C NMR and 19F NMR relaxation studies, we obtained better knowledge of the association properties of the telechelic PIPOZ in water.  NMR relaxation studies proved to be efficient way of probing the solution behaviour of the polymers.

Keyphrases: Phase transition, Liquid-liquid phase separation, Telechelic, NMR relaxation, Relaxation RMN