title: Microrheology and microstructure of Fmoc-derivative hydrogels.
abstract: The viscoelasticity of hydrogel networks formed from the low-molecular-weight hydrogelator Fmoc-tyrosine (Fmoc-Y) is probed using particle-tracking microrheology. Gelation is initiated by adding glucono-δ-lactone (GdL), which gradually lowers the pH with time, allowing the dynamic properties of gelation to be examined. Consecutive plots of probe particle mean square displacement (MSD) versus lag time τ are shown to be superimposable, demonstrating the formation of a self-similar hydrogel network through a percolation transition. The analysis of this superposition yields a gel time t(gel) = 43.4 ± 0.05 min and a critical relaxation exponent n(c) = 0.782 ± 0.007, which is close to the predicted value of 3/4 for semiflexible polymer networks. The generalized Stokes-Einstein relation is applied to the master curves to find the viscoelastic moduli of the critical gel over a wide frequency range, showing that the critical gel is structurally and rheologically fragile. The scaling of G'/G″ as ω(0.795±0.099) ≈ ω(3/4) at high frequencies provides further evidence for semiflexible behavior. Cryogenic scanning electron micrographs depict a loosely connected network close to the gel point with a fibrillar persistence length that is longer than the network mesh size, further indications of semiflexible behavior. The system reported here is one of a number of synthetic systems shown to exhibit semiflexible behavior and indicates the opportunity for further rheological study of other Fmoc derivatives.
authors.affiliation: Cavendish Laboratory, Department of Physics, University of Cambridge , JJ Thomson Avenue, Cambridge, CB3 0HE U.K.
authors.forename: Anders
authors.initials: A
authors.lastname: Aufderhorst-Roberts
authors.forename: William J
authors.initials: WJ
authors.lastname: Frith
authors.forename: Mark
authors.initials: M
authors.lastname: Kirkland
authors.forename: Athene M
authors.initials: AM
authors.lastname: Donald
mesh_terms: D020100:Hydrogels / Q000737:chemistry*; D012212:Rheology; D014783:Viscosity
chemical_list: D020100:Hydrogels