NMR Studies On The Response Of Poly (N-substituted Acrylamides) To Multiple Environments

N-substituted polyacrylamides have attracted tremendous attention for decades due to their unique thermal sensitivity in aqueous medium. Among them, poly(N-isopropylacrylamide) (PNIPAM) is the most representative thermo-sensitive polymer, that exhibits a sharp and reversible coli-globule phase transition in water at a lower critical solution temperature (LCST) of～32℃. This transition temperature is slightly lower than body temperature, which makes PNIPAM a promising intelligent material for the application of medicine and biotechnology. The specific transition temperature is highly related to delicate hydrophilic/hydrophobic balance of polymer chains and their interactions with solvents. That’s why the N-substituted polyacrylamide derivatives with different side chain chemical structure possess different phase transition temperatures. The substituent group with more hydrophilicity will lead to a higher LCST and vice versa.Both PNIPAM linear chain and its cross-linked hydrogel show the sharp temperature response. Besides that, the phase behaviors of PNIPAM can be influenced by the solvent environments. PNIPAM presents the cononsolvency in some binary solvent mixtures, although each solvent can individually dissolve PNIPAM well. Because of the same amide moieties and similar thermo-induced phase behavior, PNIPAM has always been requisitioned as a simple model for protein study.Poly(N,N-diethylacrylamide) (PDEA) is another well known thermosensitive polymer which has nearly the same LCST as PNIPAM in water. Owing to the absence of NH in the amide group, PDEA can only act as hydrogen bond acceptor. Therefore, PDEA is a suitable candidate for comparative study with PNIPAM to check the role of NH moiety. In this dissertation, the phase transition of poly(N-substituted acrylamides) gels has been studied in multiple environments by HRMAS NMR.The effect of urea on LCST transition of thermosensitive polymer hydrogels have been investigated for both PNIPAM and PDEA. It is found that Urea lowers the LCST of PNIPAM and serves as a protector of the globule structure, but increases the LCST of PDEA and acts as a destructor of its globule structure. Through comparing the compositions of confined solvents and free solvents inside and outside polymer network respectively, we find that the urea molecules homogeneously distribute inside and outside polymer network, and no obvious urea enrichment occurs inside the PNIPAM or PDEA network. The PGSE diffusion and NOES Y experiments reveal that the urea molecules can form the direct hydrogen bond with two polymers, but the urea molecules are more strongly hydrogen bonded to the PNIPAM than PDEA. It clearly show that the NH group is crucial for determining stability of the globular polymer structure.Preferential interactions of protic solvents (methanol and ethanol) with PNIPAM hydrogel have been reported to account for the cononsolvency. It is worth knowing whether the same preferential interactions exist for aprotic solvents, like acetone or DMSO, since similar cononsolvency appears for PNIPAM in water/acetone and water/DMSO mixtures too. Both 1D quantitative 1H spectra and NOE results show that PNIPAM preferentially interacts with acetone molecules rather than DMSO molecules. Different molecular mechanisms have been proposed for their decrease effect of LCST.The phase behaviors of thermo-sensitive polymers in binary solvent mixtures are complex because of the specific interactions between the polymers and cosolvents. The preferential interactions often exist but not the only reason for the transition temperature depression of PNIPAM. It seems that PNIPAM exhibits more favorable preferential interactions with molecule who has higher hydrophobicity. More properties of cosolvent should be considered for their effects on thermo-sensitive polymers, like chemical structure, size, dipole moment, etc.