SANS study of highly resilient poly(ethylene glycol) hydrogels

Polymer networks are critically important for numerous applications including soft biomaterials, adhesives, coatings, elastomers, and gel-based materials for energy storage. One long-standing challenge these materials present lies in understanding the role of network defects, such as dangling ends and loops, developed during cross-linking. These defects can negatively impact the physical, mechanical, and transport properties of the gel. Here we report chemically cross-linked poly(ethylene glycol) (PEG) gels formed through a unique cross-linking scheme designed to minimize defects in the network. The highly resilient mechanical properties of these systems (discussed in a previous publication) [J. Cui, M. A. Lackey, A. E. Madkour, E. M. Saffer, D. M. Griffin, S. R. Bhatia, A. J. Crosby and G. N. Tew, Biomacromolecules, 2012, 13, 584-588], suggests that this cross-linking technique yields more homogeneous network structures. Four series of gels were formed based on chains of 35000 g mol ^-1, (35k), 12000 g mol^-1 (12k) g mol^-1, 8000 g mol^-1 (8k) and 4000 g mol^-1 (4k) PEG. Gels were synthesized at five initial polymer concentrations ranging from 0.077 g mL ^-1 to 0.50 g mL^-1. Small-angle neutron scattering (SANS) was utilized to investigate the network structures of gels in both D ₂O and d-DMF. SANS results show the resulting network structure is dependent on PEG length, transitioning from a more homogeneous network structure at high molecular weight PEG to a two phase structure at the lowest molecular weight PEG. Further investigation of the transport properties inherent to these systems, such as diffusion, will aid to further confirm the network structures.