Relating Monomer Sequence, Self-Assembly and Mechanical Response in Dual Associative Protein Hydrogels

The nanostructure and rheological response of block copolymer hydrogels are critical to engineering them for a wide variety of different applications. In our lab, we have developed a system of well-defined triblock copolymer gels based on associative protein midblocks and thermoresponsive endblocks that responsively transition from a shear-thinning state at low temperature to a reinforced state at high temperature. Small changes in the amino acid sequence in the thermoresponsive endblock can create large, qualitative changes in the mechanical response of the materials, such as the observation of tackiness. Here, a combination of scattering and mechanical testing is used to understand the relationships between molecular structure, self-assembly, and mechanical response. Thixotropic responses in shear deformation correspond to the formation of more highly ordered structures during deformation. The kinetics of structure formation as a function of temperature and concentration depend upon the relative mobilities of the mid and endblocks. Relaxation after the cessation of shear is characterized by two timescales corresponding to distinct relaxation processes in the materials. The substitution of glycine by alanine in specific positions within the protein endblocks leads to dramatic slowing of the endblock relaxation, which substantially changes the way that the structures deform under shear, leading to higher modulus and much higher toughness in the materials as a result of a minor chemical change in the polymer sequence. This shows how endblock engineering can be used as a tool to tune hydrogel properties.