NMR studies of the structure and dynamics of polymer gels based on N-isopropylacrylamide (NiPAAm) and methacrylic acid (MAA)

Full Paper: A series of copolymer gels and interpenetrating networks (IPN) based on N-isopropylacrylamide (NiPAAm) and methacrylic acid (MAA) were synthesized and subsequently investigated by magic angle spinning H-1 MAS, cross polarization C-13 CP-MAS and pulse-saturation transfer C-13 PST-MAS NMR. The structure of the interpenetrating networks is compared to that of the copolymer gels with the same composition. The study of the proton lines reveals low mobility of the chains in the copolymer gels and IPN with higher concentration of MAA, whereas those which are predominantly NiPAAm show higher mobility and therefore narrower proton lines. C-13 CP-MAS revealed an interaction between the components of the IPNs. The PST-MAS technique was utilized to study the structure of the polymers in the gel phase. Copolymer gels exhibit some additional peaks in the carbonyl region due to the new chemical environment in the stereoregular sequences. In order to assure assignment, uncrosslinked copolymers with similar compositions as the copolymer gels were synthesized for comparative purposes and examined by conventional solution- state C-13 NMR. A linear homopolymer of NiPAAm was synthesized inside of a P(MAA) gel. The polymer was extracted after the completion of the reaction and characterized by solution C-13 NMR. About 24% of this P(NiPAAm) homopolymer could not be extracted presumably because of a strong hydrogen bonding or partial topological constrains. In the C-13 NMR spectrum a new signal appeared in the carbonyl region due to MAA monomeric units. It may correspond to the methacrylic carbonyls interacting by hydrogen bonds with the amide groups