Impact of swelling on macroscopic and nanoscopic mechanical properties of amphiphilic polymer co-networks in non-selective and selective solvents

Abstract

Amphiphilic polymer gels show environmentally sensitive mechanical properties depending on the solvent polarity, which makes them useful for applications in soft contact lenses, membranes, drug delivery systems, and tissue engineering. To rationally design the material properties for such applications, a sound knowledge about the mechanical properties at different solvency states is necessary. To acquire such knowledge, amphiphilic networks are prepared by hetero-complementary coupling of amine-terminated tetra-poly(ethylene glycol) (t-PEG-NH2) with 2-(4-nitrophenyl)-benzoxazinone terminated tetra-poly(ε-caprolactone) (t-PCL-Ox). The mechanical properties are investigated on different length-scales and under non-selective and selective solvent conditions using shear rheometry and atomic force microscopy (AFM). The swelling as well as the modulus in good solvent are in accord with scaling laws found for other four-arm star-shaped polymer networks and theoretical predictions. The swelling in selective solvent reveals a concentration-independent volume swelling degree and a nearly linear scaling of the modulus with concentration. The surface topography probed by AFM reveals microphase-separated structures in the range of 20 nm. Similar modulus values are obtained for bulk films in water using the complementary methods of atomic force microscopy and rheometry. The data are compared with pure hydrophilic networks to identify the effect of amphiphilicity on the material properties.