Microstructure of hydrophobically modified alkyl acrylamides

Date of Completion

January 2004

Keywords

Chemistry, Polymer|Engineering, Chemical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Physically crosslinked, 2-(N-ethylfluorooctanesulfonamido)ethyl acrylate, FOSA, modified N-isopropylacrylamide (NIPA) copolymer (NF) hydrogels exhibited a volume phase transition (VPT) similar to what is observed for covalently crosslinked NIPA hydrogels. The temperature of the VPT (TVPT) decreased and the transition broadened as the FOSA concentration in the copolymer increased. No VPT has been reported for covalently crosslinked N,N-dimethylacrylamide (DMA) hydrogels, and the introduction of as much as 21.5 mol% of the hydrophobic FOSA did not produce a VPT in FOSA/DMA copolymer (DF) hydrogels. ^ Hydrophobic interactions promoted nanophase separation of the FOSA in both the DF and NF gels. The microstructure of these physical gels was significantly different from covalently crosslinked hydrogels in that the crosslink junctions of the physical gels consisted of FOSA nanodomains. The structure of the nanodomain junctions as determined from neutron scattering was consistent with a FOSA core surrounded by a water-depleted layer of the alkyl acrylamide. The nanodomain junctions are distributed in a matrix of water-swollen alkyl acrylamides that exhibit chain-size heterogeneities due to solution-like thermal fluctuations. In addition, the hydrogels exhibited larger, solid-like inhomogeneities due to clustering of the polymer as a consequence of nonuniform swelling of the alkyl acrylamide. ^ Weak segregation of a FOSA-rich phase from an alkyl acrylamide-rich matrix was observed in DF and NF copolymers. A microphase separation transition was not determined from either the temperature dependent SAXS or rheological measurements. Dynamic shear measurements indicated that all the copolymers obey the principle of TTS, and they all exhibited a terminal behavior at higher temperatures or lower frequencies. ^ Two modes of diffusive relaxations were observed in acetone solutions of NF or DF copolymers. The fast mode was due to the diffusion of the polymer chain, and the slow mode was due to that of the multichain aggregates formed by fluorophilic associations. The hydrodynamic radius of the single polymer chain ranged from 7 to 11 nm, however, the size of the multichain aggregates increased with concentration considerably by one order of magnitude for all the copolymer acetone solutions. ^

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