Physically Cross-Linked Alkylacrylamide Hydrogels: A SANS Analysis of the Microstructure

Abstract

The microstructure of copolymer hydrogels composed of either N,N-dimethylacrylamide (DMA) or N-isopropylacrylamide(NIPA) and 2-(N-ethylfluorooctanesulfonamido)ethyl acrylate, FOSA, was studied by small-angle neutron scattering (SANS). The FOSA/NIPA gels exhibited a thermally reversible volume phase transition and a temperature-sensitive microstructure, while the FOSA/DMA gels did not. A modified interacting core-shell microstructure model was used to explain the SANS data for both gels. The cross-link junctions of the gel consisted of nanophase-separated FOSA domains composed of a FOSA core surrounded by a water-poor layer of the alkylacrylamide. The average spherical core radius ranged from 1 to 3 nm, and the average shell thickness ranged from 0 to 1.5 nm; the FOSA aggregation number ranged from 10 to 180. The hydrogels exhibited chain-sized heterogeneities due to solutionlike thermal fluctuations and larger sized heterogeneities due to a solidlike cluster structure. The chain correlation and the large heterogeneities were similar to those observed in covalent DMA and NIPA hydrogels. The chain correlation length ranged from 0.5 to 3.5 nm, and the large heterogeneities ranged from 50 to 90 nm. A previous paper 1 described the phase behavior and small-angle scattering characterization of the micro- structure of hydrophobically modified hydrogels com- posed of 2-(N-ethylperfluorooctanesulfonamido)ethyl acry- late (FOSA) and N-isopropylacrylamide (NIPA) or N,N- dimethylacrylamide (DMA). Aggregation of the FOSA moieties produced a physically cross-linked microstruc- ture. The FOSA/NIPA copolymer hydrogels exhibited a thermally reversible volume phase transition (VPT) 2,3 between a swollen and a collapsed phase, similar to that observed for covalently cross-linked NIPA gels. Increas- ing the FOSA content decreased the temperature of the VPT (TVPT) and broadened the transition. In contrast, covalently cross-linked DMA hydrogels did not exhibit a VPT, and the introduction of FOSA did not produce a VPT for those copolymers. This paper describes the