(704b) Structural Rearrangements during Stress Relaxation of a Tough, Physically Crosslinked Hydrogel

Synthetic hydrogels have typically been limited by their poor mechanical behavior, which limits their applications as compared to natural tissues. The seminar work of Gong and co-workers identified the use of double networks as a route to signfiicantly toughen hydrogels [1]. The enhanced toughness is attributed to a sacrificial network that dissipates energy on deformation. However, the energy dissipation is not reversible, but similar concepts of energy dissipation with healable crosslinks have been identified [2]. One simple route is through assembly of hydrophobic components of a amphiphilic copolymer, where these hydrophobic aggregates act as crosslinks to prevent the copolymer from dissolving in water [3]. However in this case, the energy required to break up the aggregates is less than that to break covalent bonds; under sufficient deformation, the aggregates will break up, dissipating energy, but the driving force to re-aggregate after the force is removed leads to re-formation of the network to generate extendable and tough hydrogels. Perfluorinated hydrophobes present to advantages: first, the energy of the association is larger, which leads to increased mechanical strength of the hydrogel over simple hydrocarbons. Second, the fluorine provide contrast in both x-rays and neutrons in order to examine the nanoscale structure with scattering [4]. Here we exploit the contrast difference between the aggregates and the water-rich hydrophilic segments in the copolymer to examine how the crosslink structure recovers from a large deformation with small angle neutron scattering (SANS). Use of different ratios of D2O and H2O allow the recovery of the hydrophilic chains and the aggregates to be probed independently. The extension of the hydrogel leads to anisotropic scattering that slowly recovers over several hours. We will describe methods to quantify the temporal evolution in the scattering profiles in order to relate the nanoscale structure to macroscopic stress relaxation.

[1] J.P. Gong, Y. Katsuyama, T. Kurokawa, Y. Osada. Adv. Mater. 2003, 15, 1155.

[2] J.-Y. Sun, X. Zhao, W. R. K. Illeperuma, O. Chaudhuri, K. H. Oh, D. J. Mooney, J. J. Vlassak, Z. Suo, Nature 2012, 489, 133.

[3] J. Hao, R. A. Weiss, Macromolecules 2011, 44, 9390.

[4] J. Tian, T. A. P. Seery, D. L. Ho, R. A. Weiss, Macromolecules 2004, 37, 10001.