LAUSR

Abstract We present a diffusing-wave spectroscopy (DWS)-based microrheological study of hydrophobically modified poly(acrylic acid) physical hydrogels which are typically “living” networks. The modified polymer was synthetized with 3% randomly grafted C14 alkyl groups on PAA precursor chains (50 kg/mol), called HMPAA3C14. With increasing polymer concentration C P , intra-chain hydrophobic interactions are replaced by inter-chain hydrophobic ones at concentrations above a critical aggregation concentration, CAC. Then, non-relaxed aggregates percolate at a polymer concentration C P * (gel point) leading to a physical network connected, first, by unrelaxed strands (the Rubenstein and Semenov (R–S) regime), and then by relaxed ones. At observation times longer than the lifetime of the network connection or non-covalent reversible crosslink (hydrophobic groups belonging to neighboring chains form micelles) the system flows but exhibits elastic character at shorter time scales. We determined the gel point C P * at ~ 25 °C using viscoelastic loss tangent uniqueness method. We confirmed the gel point by performing time-cure superposition for the mean-squared displacement (MSD) curves for pregel and postgel regimes, and found that time and probe displacement shift factors increase when approaching C P 14 *. The viscosity plateau which characterizes the R–S regime around the gel point was larger than that of HMPAA3C18 with longer alkyl grafted chains (C18), in agreement with the fact that the lifetime of the physical crosslink for HMPAA3C14 is lower than that of HMPAA3C18. This behavior is confirmed even in the pregel zone where the size of the longest relaxing cluster for HMPAA3C14 (32 nm) is almost half that of HMPAA3C18, meaning that the latter lives longer. The variation of the plateau modulus (G0) in the low-frequency regime, compared to that of the relaxation modulus G z determined in the high-frequency one, for the postgel regime suggests that the crosslinking micelles possibly change in size and shape as the polymer is added.