Nanoparticle diffusion during gelation of tetra poly(ethylene glycol) provides insight into nanoscale structural evolution

Single particle tracking (SPT) of PEG grafted nanoparticles (NPs) was used to examine the gelation of tetra poly(ethylene glycol) (TPEG) succinimidyl glutarate (TPEG-SG) and amine (TPEG-A) terminated 4-armed stars. As concentration was decreased from 40 to 20 mg mL−1, the onset of network formation, tgel, determined from rheometry increased from less than 2 to 44 minutes. NP mobility increased as polymer concentration decreased in the sol state, but remained diffusive at times past the tgel determined from rheometry. Once in the gel state, NP mobility decreased, became sub-diffusive, and eventually localized in all concentrations. The NP displacement distributions were investigated to gain insight into the nanoscale environment. 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 relatively homogenous network. This work aims to examine the kinetics of TPEG gelation and the homogeneity of these novel gels on the nanometer scale, which will aid in the implementation of these gels in biomedical or filtration applications.