Tuning Structure and Properties of Porous Hydrogels Exhibiting Properties Reminiscent of Natural Tissues from the Self-Assembly of Amphiphilic Triblock Copolymers
Restricted (Penn State Only)
- Author:
- Lloyd, Elisabeth
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 26, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Robert Hickey, Chair & Dissertation Advisor
Hee Jeung Oh, Outside Unit & Field Member
Urara Hasegawa, Major Field Member
Ralph Colby, Major Field Member - Keywords:
- Hydrogels
Porous Hydrogels
Biomimicry
Amphiphilic Block Copolymers
Self-Assembly of Block Copolymers - Abstract:
- Natural tissues possess diverse and unique properties derived from highly complex arrangements of biomolecules over multiple length scales. While bulk materials replicating these properties have been researched extensively, there has been comparatively little consideration or success regarding mimicking the structures or their impact on the properties of these materials. We demonstrate a bottom-up process by which porous microstructures are introduced into hydrogels via rapid spinodal decomposition of amphiphilic triblock copolymer solutions in water. At the nanoscale, the hydrophobic chain-ends of the block copolymer self-assemble into micelle cores bridged by the hydrophilic mid-blocks. At the microscale, the diffusion of the solvent from the polymer solution produces rapid spinodal decomposition, leaving water-filled voids within the hydrogels. The hydrogels exhibit extreme softness yet high elasticity, and reversibly deform over many cycles In the first research aim, we investigate the influence of the initial solution conditions—namely, solvent and concentration—on the resulting structure and properties. We show how the micelle network appears to be insensitive to shear, with no change in the network domain spacing at each tested condition. In contrast, the porous microstructure is significantly affected by solvent and concentration, with hydrogels possessing either a coaxial morphology or a monomodal distribution of pore sizes. The deformation behavior of the hydrogels is initially due to the buckle and collapse of the pores, followed by chain stretching and finally, fracture. This work demonstrates how the introduction of a porous microstructure can produce properties reminiscent of natural tissue in what would otherwise be a stiff and brittle gel. In the second research aim, we investigate the role of block copolymer molecular weight and composition in determining the structure and properties of the hydrogels. We synthesize seven triblock copolymers, varying the relative volume fraction and block length of the hydrophilic and hydrophobic species. We find that composition relates to the mechanical properties in a foam-like manner: the composition of the polymers comprising the pore walls primarily influences the initial deformation behavior, i.e., the Young’s Modulus. The composition has a lesser degree of influence on ultimate tensile strength and elongation at break, as is expected, though deviations in these trends are likely due to variations in pore morphology. Most strikingly, we find two samples demonstrate mechanical properties very near to those of cartilage and adipose tissues. Thus, we conclude that the Young’s Modulus of these hydrogels may be tailored via block copolymer composition, but the ultimate tensile strength and elongation at break of the hydrogels will depend significantly on the pore morphology. The third research aim attempts to quantify the impact of diblock impurities on the strength of the hydrogel. By doping the triblock copolymer with a diblock of equivalent relative composition, we can produce hydrogels that are weakly bridged, with triblock content ranging from 25-72% by weight. We find the addition of diblock increases the average cross-sectional areas and swelling ratios of the hydrogel fibers, and leads to significant decreases in Young’s Modulus, ultimate tensile strength, and elongation at break. Lastly, we investigate the toughening of the hydrogel fibers via crystallization induced by swelling the hydrogels in a poor solvent. The porous morphology allows a solvent that is poor for both blocks to diffuse throughout the hydrogel. In this work, we reswell the hydrogels in ethylene glycol, which induces crystallization in the semicrystalline hydrophilic mid-block. This crystallization produces a dramatic toughening of over three orders of magnitude in the gels, but the effect is near-completely reversed when the hydrogels are returned to water. If the hydrogels are swollen in mixtures of ethylene glycol and water, crystallization does not occur, but a toughening effect is still present, which demonstrates how the properties of these hydrogels may be potentially modified by their environments. This research demonstrates how hydrogels possessing porous microstructures may be formed by the self-assembly of triblock copolymers in water. The hydrogels possess radically different properties that are reminiscent of natural tissues in comparison with their bulk, homogenous counterparts, demonstrating the need for microstructure to be incorporated in the mimicry of natural tissues. This work presents a set of design rules for an exciting class of new materials, demonstrating how the initial processing conditions, block copolymer composition, and reswelling can all be used to tailor the properties of the hydrogels, providing a wide range of accessible properties. Future work will further investigate the structure-process-property relationships for this system, with diverse potential applications and an opportunity to further expand block copolymer self-assembly principles that simultaneously harness nano and macroscale organization.