Photoreconfigurable Physically Cross-Linked Triblock Copolymer Hydrogels: Photodisintegration Kinetics and Structure–Property Relationships

Photoreconfigurable physically cross-linked hydrogel networks are prepared by self-assembly from amphiphilic ABA triblock polymers with photolabile poly(o-nitrobenzyl methacrylate) (PNBMA) A blocks and poly(ethylene glycol) (PEG) B blocks. Generalizable structure–property relationships of this class of photosensitive compounds have yet to be reported. Here, a library of amphiphilic linear PNBMA-b-PEG-b-PNBMA triblock copolymers is synthesized, and the physical properties of subsequent hydrogel networks are characterized across two parameters: the degree of polymerization of PNBMA segments and the concentration of PNBMA-b-PEG-b-PNBMA in precursor solutions. The storage modulus, photodisintegration kinetics, and swelling ratio are reported. A quantitative model to correlate molecular scale photolysis of NBMA groups with macroscopic mechanical properties is proposed and validated. Hydrogel network parameters including cross-link density and mesh size are also included and compared to covalently cross-linked PEG-diacrylate analogues. The concept of reduced swelling ratio is introduced to map the physical properties of self-assembled physically cross-linked photolabile networks with covalently cross-linked hydrogels. This revised parameter permits direct comparisons of macroscopic network properties between PEG-based gels with either physical or covalent cross-links.