(721a) Nanostructure-Driven Fatigue Resistance and Dynamic Recovery in Thermoplastic Elastomer Hydrogel Networks
Here, we report a new paradigm in hydrogel design based on prefabrication of an efficient nanoscale network architecture using the melt-state self-assembly of amphiphilic block copolymers. Rigorous characterization and mechanical testing reveal that swelling of these preformed networks produces hydrogels with physiologically relevant moduli and water compositions, negligible hysteresis, sub-second elastic recovery rates, and unprecedented resistance to fatigue over hundreds of thousands of compression cycles. Furthermore, by relying only on simple thermoplastic processing to form these nanostructured networks, the synthetic complexities common to most solution-based hydrogel fabrication strategies are completely avoided. We use melt blends of sphere morphology forming AB and ABA diblock copolymers to form a network of tethered micelle units. During melt-state self-assembly, A blocks form spherical aggregates while the B blocks form the corona or matrix in which the spherical A domains sit. The triblock copolymer acts as a bridge, mechanically linking adjacent spherical domains. Hydrogel formation relies on two important phenomena that dictate the choice of A and B blocks. One, the A block must be able to become a vitrified solid upon cooling. We use polystyrene (PS) with a glass transition temperature near 80 ËC. Two, the B block must be selectively soluble in the swelling media, while the A block remains impervious to it. We use poly(ethylene oxide) (PEO) as the water soluble B block.
In terms of network structure this method of hydrogel fabrication produces key features we believe are critical for eliminating fatigue, producing a high modulus, and ensuring rapid recovery. First we counter the effects of strand length inhomogeneity by keeping the density of fixed junction points (Ïx-link, glassy spherical domains) low, and the strand molecular weight adjoining them high (Mx-link,180kDa) and narrowly distributed (PDI < 1.06). This would normally favor soft, weak materials, but we use extremely high junction point functionalities (f ~ 230, block copolymer chains per spherical aggregate) in the 200 â 300 range. Thus even moderate triblock copolymer compositions produce a high numbers of strands emanating from each sphere. Finally, because assembly takes place in the melt, the density of topologically trapped entanglements among these bridging (and looping) triblock copolymers is intrinsically quite high. In the model PS-PEO systems used to develop our preliminary systems, we estimate the number of entanglements per fixed junction point numbers in the thousands. These entanglements act much like the sliding junction points introduced by Ito in his slide ring-gels, and provide the network with both a significant modulus and an ability to rapidly redistribute stress, the latter being the keystone to the unusual fatigue resistance exhibited by these systems.
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