Superabsorbent polymers undergoing large three-dimensional deformation: flory-rehner and beyond

Superabsorbent polymers (SAPs) are defined as ionised hydrogels with the ability to absorb a large quantity of fluid with respect to their original volume (in the orders of 10,000%). This material is applied widely across a variety of fields, equating to a predicted total market size of almost $10 Billion by 2025. Its prime uses are fluid absorption in personal hygiene products and in medical procedures. Unlike conventional materials, SAP particles in this application are rarely mechanically loaded, but rather chemically loaded through exposure to a solution. As a result, mechanical characterisation of the swelling process is challenging without the use of uni-axial/bi-axial test methods. This issue leads to in-silico modelling to help characterise the material properties. Therefore, the following thesis focuses on advancing the current work of SAP swelling models.

Initially, this thesis adds the mixing energy portion of Flory-Rehner theory into an existing Mixed Hybrid Finite Element Method (MHFEM) swelling model. This energy, associated with the attraction of water molecules to the hydrophilic polymer chains, has been experimentally shown to have negligible contribution to ionised hydrogel swelling pressure at equilibrium states. However, its effect during swelling transience was not considered. By adding it to the dynamic numerical model, its true effect on swelling transience could be quantified. For standard SAP porosities, the mixing energy contributed almost 50% of the swelling pressure at the beginning of the swelling process.

The constitutive relation used in any numerical model to represent the stress-strain behaviour of the material is essential for the accuracy of the simulation. When materials experience such high deformations, a change in the elastic modulus is not uncommon and was found to be present in sodium polyacrylate. Therefore, a material specific strain softening and hardening constitutive model was developed. It was found that the new constitutive model significantly affected swelling dynamics and magnitude, especially with increasing cross-link density. Also, as fluid permeability in these materials is highly dependent on water content, a core-shell setup developed during transient swelling and hence an increase in compressive surface stress, linked with surface instability formation. As well as a strain dependent modulus, ionised gels have experimentally shown a coupling of the ionic and elastic energies of Flory-Rehner theory at constant strain. This phenomenon was tested in neutralized polyhydroxyethylmethacrylate (pHEMA) gels, leading to the development of a new mechano-electrochemical model. This model provides more representative initial conditions and transience of the MHFEM swelling model. The hydrogel now shows mechano-sensing capabilities by having the electrochemical potentials of the ions directly coupled to strain.

Finally, experimental validation was performed to ensure the models efficacy. As standard mechanical testing techniques are difficult to use for fluid induced deformation, imaging must be implemented. However, as these SAP particles deform across several length scales, standard microscopy is not effective, failing to the trade off between field of view and resolution. To overcome this limitation, lensfree imaging was developed, which uses LEDs and a CMOS image sensor to capture the shadows of the object under investigation. As the system is lensless, a high spatial resolution over a large field of view and frame rate is possible. This technique allowed the statistical quantification of the efficacy of the numerical model at each time point throughout transient swelling.

Overall, this thesis progresses the material modelling of superabsorbent polymers through the development and validation of an in-silico swelling model for use in a variety of applications.