Tracer Diffusivity in Amphiphilic Polymer Model Co-Networks

Amphiphilic polymer conetworks (APCNs) are highly interesting material for membranes, drug delivery, or tissue engineering since their heterogeneous structure and interactions allow for the control of the diffusion of molecules differing by architecture, size, and interactions. We investigate the diffusion of hydrophilic and hydrophobic star polymers in model APCNs formed by heterocomplementary end-linking of tetra-poly-(ethylene glycol) (t-PEG) and tetra-poly-(ε-caprolactone) (t-PCL). Using Fluorescence Recovery After Photobleaching (FRAP) and Forced Rayleigh Scattering (FRS), we gain complementary insights into star polymer transport across different length and time scales. We compare the diffusion of hydrophilic t-PEG and hydrophobic t-PCL of various molecular weights across a wide range of APCN polymer volume fractions, swollen in a cosolvent (toluene) and a selective solvent (water). FRS reveals Fickian diffusion for all tracers in APCNs swollen in toluene. In the unentangled regime, the diffusivity of the tracer follows approximately the expected Rouse scaling for semidilute solutions. Corrections arise for increasing polymer content due to enforcing contacts with the other type of polymer in the APCN. At larger concentrations, the PEG tracers develop a diffusion behavior, as expected for entangled star polymers. Since the transition occurs below the expected entanglement concentration, an additional impact of the strangulation regime is likely. Partial swelling in a selective solvent leads to an enhanced diffusion behavior as compared to a homogeneously swollen network at the same polymer volume fraction; however, the concentration dependence of diffusion agrees best with the strangulation regime, despite an overall enhanced diffusion. At swelling equilibrium in the selective solvent water, the equilibrium degree of swelling, the network morphology, and the diffusion behavior become independent of the preparation conditions. These findings provide insights into the diffusion mechanism of star polymers within APCNs and contribute to the development of polymer-based drug delivery systems for biomedical applications.