Amphiphilic PEG Multiblock Copolymers Form Highly Ordered Transient Network Hydrogels
Abstract
Amphiphilic PEG multiblock copolymers can be readily synthesized by polycondensation reactions. Their multiblock structure leads to the formation of hydrogels consisting of highly interconnected micelles. Here, we investigate as model systems copolyesters consisting of alternating hydrophobic alkyldicarboxylic acids (Cm) and hydrophilic polyethylene glycols (EGn). We investigate the structure of the formed micelles and hydrogels over a wide range of concentrations, temperatures, and block lengths using small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), cryo-TEM, differential scanning calorimetry (DSC), and rheology. We find that the multiblock copolymers form hydrogels already at low concentrations. With increasing concentration, we observe for all copolymers the formation of a highly ordered body-centered cubic (BCC) phase and a crystalline lamellar phase. The increase of the alkyl and PEG chain lengths systematically increases the BCC unit cell size. We observe that the PEG-blocks significantly penetrate into the hydrophobic micellar cores, thereby increasing micellar exchange rates. This leads to the formation of transient networks with high mechanical moduli, which are of interest for self-healing, robotics and medical applications.