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Modeling of Thermotropic Swelling of Smart Microgels by Finite Element Method

Ghasemian, Hossein | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57414 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Naghdabadi, Reza
  7. Abstract:
  8. Microgels, three-dimensional networks belonging to the family of smart materials, exhibit significant structural changes in response to external stimuli such as temperature. The rapid deformation of microgels upon thermal stimulation makes them a promising alternative for various mechanisms. Despite the similarities between hydrogels and microgels, many of their properties differ. Moreover, the Flory-Rehner model, typically used to describe the swelling behavior of temperature-sensitive hydrogels, only provides a qualitative description of microgel swelling. Nevertheless, previous numerical models of microgels have largely been based on the assumption that their behavior mirrors that of hydrogels. In such models, the polymer-solvent interaction is often represented by relations that lack physical meaning, and the effect of cooperativity, which reflects the polymer's tendency to absorb water, is overlooked. The Hill-like equation, which incorporates cooperativity and physical parameters for the polymer-solvent interaction variable, refines the Flory-Rehner model. This project aims to present a finite element model based on the modified Flory-Rehner model, incorporating the Hill-like equation to quantitatively simulate the thermotropic swelling of smart microgels. Using energy equations, a nonlinear finite element relationship was derived from the modified Flory-Rehner model and implemented in Abaqus via the UHYPER subroutine. The modeling results were validated against experimental data and existing analytical methods for PNNPAM, PNIPAM, and PNIPMAM microgels. Additionally, copolymers composed of these microgels were modeled and validated. A parametric study was conducted to examine the sensitivity of microgels to various parameters, such as the initial polymer volume fraction, average degree of polymerization, and all parameters of the Hill-like equation. A comparison between the original Flory-Rehner model and the modified model with the Hill-like equation was also performed. The original Flory-Rehner model demonstrated considerable discrepancies when predicting the volume phase transition temperature and the radius of the microgel post-transition. In contrast, the modified model successfully resolved these differences. The average error percentage for the original Flory-Rehner model across different crosslinking densities for PNNPAM, PNIPAM, and PNIPMAM microgels was 5.8%, 5.24%, and 4.44%, respectively. For the modified model, the error percentages were reduced to 1.88%, 2.25%, and 2.26%, respectively. Due to its near-room-temperature volume phase transition and minimal volume change before the phase transition, PNNPAM microgel was selected for microvalve design. Various parameters, such as wall contact temperature and stress distribution, were investigated. A comparison between traditionally designed microvalves and those designed using the modified model with the Hill-like equation for PNIPAM microgel revealed that traditional designs made contact with the microvalve wall two degrees later. The final volume change of the microgel in traditional designs was 1.8 times greater than in the modified model. The effect of loading sequence—thermal and shear—on the microgel was also examined, with results showing less than a 1% impact. Furthermore, it was observed that applying shear stress reduced the microgel radius by 3.4%
  9. Keywords:
  10. Temperature Sensitivity ; Microgel ; Swelling Behavior ; Smart Materials ; Cooperativity Effect ; Temperature-Sensitive Microgels ; Flory–Rehner Model ; Phase Transition Temperature

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