The dynamics of a hydrogel strip.

Area of Science:

• Polymer Science
• Materials Science
• Soft Matter Physics

Background:

• Hydrogels exhibit complex mechanical behaviors, including stress relaxation after deformation.
• Understanding the underlying mechanisms of stress decay in hydrogels is crucial for their application in various fields.

Purpose of the Study:

• To investigate the phenomenon of stress relaxation in hydrogel strips.
• To develop and validate a continuum model explaining the decay in tensile stress.
• To determine the relationship between sample thickness and the time constant of stress relaxation.

Main Methods:

• A continuum model of the gel-solvent system was developed, incorporating an equation of motion and a linear constitutive law for the polymer network.
• Darcy's law and conservation of mass for the network and interstitial fluid were solved under appropriate boundary and initial conditions for stress-relaxation experiments.
• Non-linear regression was used to estimate material properties such as shear modulus, bulk modulus, and hydraulic permeability.

Main Results:

• The primary cause of tensile stress decay in stretched hydrogels is identified as strain-induced swelling of the polymer network.
• The developed model predicts that the stress relaxation time constant is inversely proportional to the square of the sample thickness.
• Experimental results confirmed the model's prediction regarding the thickness dependence of the time constant.

Conclusions:

• The continuum model accurately describes stress relaxation in hydrogels, attributing it to strain-induced swelling.
• The study provides experimental validation for the model's prediction of the inverse square relationship between relaxation time and sample thickness.
• Estimated material properties (shear modulus, bulk modulus, hydraulic permeability) align with independent measurements, confirming the model's predictive power.