The Research On Mechanism Of The Conformational Transition Of Poly (N, N-diethylacrylamide) In Extremely Dilute Aqueous Solution

In this thesis, three different polymers, acenaphthylene labeled poly (N,N-diethylacrylamide) (PDEA/ACE), poly(N,N-diethylacrylamide-co-styrene) (P(DEA-co-St)) copolymers, P(N,N-diethylacrylamide-co-N-hydroxymethylacryl-amide) (P(DEA-co-NHMAA)) copolymers, were synthesized by radical solution polymerization. In dilute aqueous solutions, the effects of salts, urea and methanol on the conformation of PDEA/ACE were investigated systematically. The thermo-sensitive properties of hydrophilic or hydrophobic modified PDEA were also investigated by UV-vis and fluorescence spectroscopy techniques. The results are summarized as following:1. The effects of salts on the conformational behavior of PDEA/ACE were investigated in dilute aqueous solution. It has been demonstrated that the effectiveness of various salts on changing the lower critical solution temperature (7lcs) of the PDEA/ACE solution followed Hoffmeister series. The addition of NaCl linearly lowered the T_LCS. In dilute aqueous solution, the PDEA/ACE adopted a loosely coil conformation below its T_LCS. PDEA/ACE adopted a compact globular conformation above the temperature at which the coil to globular transition (CGT) was completed. Fluorescence anisotropy investigation suggested a molten globular conformation existed during the CGT of PDEA/ACE solution. In all the above three conformations, the PDEA/ACE was more compact in presence of NaCl than that in absence of any salt. It was demonstrated via ACE label studies that introduction of NaCl only change the T_LCS of PDEA/ACE not the process of CGT.2. The effects of urea on the conformational behavior of PDEA/ACE were investigated in dilute aqueous solution. It was demonstrated that the addition of urea increased the openness of conformation and increased the segmental mobility of macromolecules of PDEA/ACE in CGT, resulted in increasing the average diameter of particles. At high temperature, the macromolecules might adopt a simple compactcoil conformation. With addition of urea, the compact coils tended to break into several sub-molecular hydrophobic microdomains, the process of CGT slowed and the ?lcs was not changed. The data did not support a model in which urea destroyed hydrogen bonds between PDEA and water by directly binding to the amide units and decreasing the hydrophobic effect. Urea formed nearly ideal mixtures with water. The results supported an "indirect mechanism" where urea altered the "structure" of water in a way that facilitated salvation of hydrocarbon chains.3. The phase transition behavior of PDEA and P(DEA-co-NHMAA) aqueous solution were studied by UV/Vis spectrophotometer. The results suggested that when the mass fraction of NHMAA was below 5.25%, the Jlcs decreased with the increase of mass fraction of NHMAA. In contrast, Tics increased with the increase of mass fraction of NHMAA while it was higher than 5.25%. Having stronger hydrogen bonds between side chains of copolymer and water, as well as between two side chains of copolymer in aqueous solution, the phase transition of P(DEA-co-NHMAA) aqueous solution showed less dependence on the concentration when compared with that of PDEA aqueous solution. For PDEA and P(DEA-co-NHMAA) aqueous solution, the abrupt transition from coil to globular and the gradual transition from globular to coil may involve different mechanisms. A model has been proposed for the structure of P(DEA-co-NHMAA) formed in dilute aqueous solution.4. In a slow heating and cooling circular, the conformational change behavior of PDEA/ACE was investigated in dilute aqueous solution. In the absence and presence of salts (urea), the UV-vis curves of transitions between globular to coil and coil to globular were not superposed. For PDEA/ACE aqueous solution, the abrupt transition from coil to globular due to hydrophobic interaction and the gradual transition from globular to coil may involve different mechanisms. The experimental results were consistent with the view of irreversible process which mainly results in entwisting between the segment of PDEA/ACE except for the stronger hydrogen bonds and hydrophobic interaction.5. The effects on the conformational behavior of hydrophilic or hydrophobic modified PDEA/ACE were investigated in dilute solution as a function of solvent composition in mixed solvent of water and methanol. The results suggested that there existed the complex between water and methanol molecules in PDEA/ACE mixed solution. As the volume fraction of methanol increase, the hydrophilic modified PDEA/ACE and PDEA/ACE always adopted a loose coil conformation but the hydrophobic modified PDEA/ACE underwent transition from coil to globular. The content of methanol in mixed solvent when the transition began to take place increased with the increase of hydrophobic segments. The change of conformation of thermo-sensitivity polymer was clearly associated to the balance between the hydrophobic effect and hydrophilic effect. The "structure" of solvent around the macromolecules affected the CGT. The results also showed that the fluorescence techniques could be more suitable to study the details of polymer conformation change especially in the late transition stage. Meanwhile, the intensity of the second harmonic Reyleigh Scatter Peak could be effectively used as a new method for determining the 7lcs-...