Nanoparticle Diffusion During Network Formation of Tetra-poly(ethylene glycol) Hydrogels*

Single particle tracking (SPT) of PEG grafted NPs was used to examine the gelation of succinimidyl glutarate (TPEG-SG) and amine (TPEG-A) terminated four arm stars, with mesh sizes of 3 to 6 nm for polymer concentration of 40 to 20 mg/mL. As concentration decreased from 40 to 20 mg/mL, gelation time, (i.e. sol-gel transition) increased. From SPT during gelation, NP mobility and NP spatial coverage increased as polymer concentration decreased in the sol state. Once in the gel state, NP mobility decreased, and NP motion became sub-diffusive and eventually localized in all concentrations. The mean square displacement and displacement distributions were investigated as a function of increasing time. Unexpectedly, in these relatively homogeneous gels, the onset of sub-diffusivity was marked by a rapid increase in dynamic heterogeneity followed by a decrease consistent with a homogeneous network. We propose a gelation mechanism in which clusters initially form a heterogeneous structure which fills in to form a fully gelled homogenous network. This work examines the kinetics of TPEG gelation and the homogeneity on the nanometer scale, which will aid in the implementation of these gels in biomedical or filtration applications.