Hyperelastic swelling of tough hydrogels
arxiv(2024)
摘要
Hydrogels are biphasic, swollen polymer networks where elastic deformation is
coupled to nanoscale fluid flow. As a consequence, hydrogels can withstand
large strains and exhibit nonlinear, hyperelastic properties. For low-modulus
hydrogel and semiflexible biopolymer networks, previous studies have shown that
these materials universally contract when sheared on timescales much longer
than the poroelastic relaxation timescale. Using rheological and tribological
measurements, we find that tough polyacrylamide and polyacrylic acid hydrogels,
with moduli of order 10-100 kPa, exclusively exhibit dilatancy when sheared.
The poroelastic relaxation process was examined using strain-controlled
compression, indicating a diffusion constant of order 10^-9 m^2/s. On short
timescales (minutes), an applied shear stress induced an increase in normal
stress that varied quadratically with shear strain. At long timescales (hours),
creep experiments revealed that tough hydrogels can “remember” the initial
direction of applied shear, suggesting an evolution of the polymer network.
Moreover, we show that this dilatant behavior manifests as swelling during
tribological sliding, imbibing the hydrogel with fluid. We suggest that this
inherent, hyperelastic dilatancy is an important feature in all tough
hydrogels, and may explain rehydration and mechanical rejuvenation in
biological tissues such as cartilage.
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