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Morphological control in the design of bio-inspired, stimuli-responsive, bilayer hydrogels
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
The development of next-generation responsive polymeric systems requires materials with built-in practical functionality within the structure. Nature has provided many muses for the design of these “smart” stimuli-responsive materials, particularly hydrogels. For example, the pinecone has inspired bilayer hydrogel systems that curve upon application of a stimulus due to a contraction mismatch between interfaced responsive, “active” and non-responsive, “passive” hydrogels. However, as the stimuli-responsive polymer field has developed, there is a need for understanding the impact of hydrogel morphology on the actuation of bilayer hydrogels. In this thesis, the thermal response of bilayer hydrogels is linked to the morphological changes caused by nanofillers and the synthetic solvent in the component active, stimuli-responsive hydrogels. ☐ First, to study the effect of morphological changes induced by synthetic solvent, the thermal response of hydrogel bilayers fabricated via digital light processing (DLP) 3D printing is tuned using a cosolvent mixture. These bilayers are comprised of a thermally-responsive hydrogel, poly(N-isopropyl acrylamide) (pNIPAAm), and a non-responsive hydrogel, poly(2-hydroxyethyl acrylate) (pHEA). By printing these hydrogels from either ethanol or an ethanol-water solution, morphological changes including pore size are enacted in the pNIPAAm hydrogels, while the pHEA hydrogels are relatively unaffected. These morphological changes are correlated to a decreased thermal response with increased presence of water during printing due to the formation of a hydrophobic “skin” layer upon heating of the hydrogel. This skin layer is also observed in bilayer hydrogels formed by interfacing pNIPAAm and pHEA hydrogels. A comparison to an existing theoretical model is shown to have poor agreement with the experimental data, which is quantified using a “correction factor.” This research showcases precursor solution solvent as a handle to tune the morphology of bilayer hydrogels and presents limitations with existing theory. ☐ Extending these research findings, montmorillonite (MMT) clay is explored as a filler to potentially improve water transport to accelerate the thermal response of the bilayers fabricated in Chapter 1 using the solvent mixture. The introduction of surface hydrophilicity by the clay is hypothesized to mitigate the formation of the skin layer within the active pNIPAAm layer of the bilayers printed from the ethanol-water cosolvent mixture. Through exploration of single-layer active hydrogels, the addition of MMT is shown to cause faster actuation than the clay-free control, which is attributed to a decrease in skin layer formation. However, at higher MMT loading, the size of clay-rich domain structures is shown to correlate to a decreased initial rate of actuation and final magnitude of actuation. By interfacing these pNIPAAm/MMT hydrogels with pHEA hydrogels, bilayers are formed and shown to actuate considerably faster and to a greater curvature than the clay-free control. The increased bilayer actuation is related to the ability of the active layer to exert a contractile force on the passive layer during contraction, which is evidenced by improved prediction by the existing theoretical model which had shown poor predictive capability in Chapter 2. This research demonstrates the impact of inorganic, hydrophilic nanofillers for accelerating the response of bilayers through inhibition of skin layer formation and improvement of contractile force. ☐ Finally, the impacts of the loading and the structure of non-isocyanate polyurethanes (NIPUs) on the melt rheology and mechanical properties of blends with poly(lactic acid) (PLA) are investigated for potential use in 3D printing. Pendent methoxy groups on the lignin-derivable NIPU are hypothesized to improve blend melt strength and viscosity compared to the methoxy-free, petroleum-derived NIPU due to intermolecular interactions. The potential for plasticization or rubber toughening by the NIPUs is investigated through thermal and rheological characterization of the phase morphology. The inclusion of NIPU within PLA is shown to decrease the relaxation time and viscosity of the blend compared to the neat PLA due to the NIPU likely being incorporated into a singular phase with the PLA. The Young’s moduli, yield strength, strain-at-break, and toughness of the blends are found to be generally comparable to the properties of the neat PLA, demonstrating a lack of plasticization. However, in both the melt and the solid state, the properties of the lignin-derivable NIPU are shown to slightly exceed the petroleum-derived NIPU, potentially illustrating the increased intermolecular interaction by the pendent methoxy groups. This research serves as a preliminary study for potentially 3D-printable materials as well as how polymer processing aid structure and loading may affect polymer blend rheology and mechanical properties.
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Keywords
Bilayer, Hydrogel systems, Polymer blends, Stimuli-responsive, Polymeric systems, Non-isocyanate polyurethanes