| dc.description.abstract | Hydrogels are three-dimensional, cross-linked, polymeric networks that are typically soft materials that contain more than 90% water. Many technologies require hydrogels with improved mechanical properties (modulus, failure properties and toughness). Drawing inspiration from biological systems that are complex and highly ordered, yet constructed efficiently, this dissertation advances understanding of multi-component hydrogels. This work also develops correlating relationships of composition, water content, microstructure network properties and mechanical properties. This dissertation investigates three multi-component hydrogel systems which fall into the category of interpenetrating network (IPN): two or more networks which are interlaced, independent of each other and each network is covalently cross-linked. (1) Semi-IPN hydrogels: A subcategory of IPNs in which two or more networks are interlaced and independent of each other, but one is chemically cross-linked and one is an entangled polymer. A systematic study of the formulations of single-network (SN) and semi-IPNs of agarose and poly(ethylene glycol) diacrylate (PEGDA) showed that these gels typically exhibited an effect somewhat greater than the sum of the two component SNs, in moduli, fracture stress and toughness. The semi-IPNs of agarose/PEGDA also behaved as ideal elastomers. Imaging hydrated semi-IPNs of agarose/PEGDA using scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that the semi-IPNs had pores sizes that are between the two SNs, 1-4 micron pores. The pore size decreased as the PEGDA concentration was increased. (2) Double-network (DN) hydrogels: A subcategory of IPNs in which two independent, covalently cross-linked networks display the DN effect: substantial yielding and significant improvement in mechanical properties compared to the SNs, notably toughness and modulus. The most widely accepted view for the improvement of the mechanical properties in DNs is that sacrificial covalent bonds breaking in the brittle first network dissipates strain energy while the ductile second network hold the gel together. This work explored the molecular mechanisms produced in DNs such as the sacrificial covalent bonds breaking leading to irreversible network damage. When this work began, the first synthesized DN, poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS)/polyacrylamide (PAAm) was the only composition that produced the DN effect. This led to the question of the generality of the DN effect. Here, the generality of the DN effect was demonstrated by synthesizing the first biopolymer-based DN. This was accomplished by replacing the PAMPS network with a biopolymer, methacrylated chondroitin sulfate (MCS), which forms a brittle network similar to PAMPS but has a fundamentally different structure. The generality was further emphasized by replacing the second network PAAm with poly(N, N-dimethylacrylamide) (PDMAAm) to synthesize a DN of MCS/PDMAAm. Detailed analysis of mechanical properties in tension and compression were completed to understand why DNs are so tough. In tension, the MCS/PAAm DN formulations were manipulated by adjusting the concentration and cross-linking of the two networks to achieve mechanical properties (failure stress, failure strain, Young's modulus, and yielding behavior) that cover a broad range (more than five times in most cases). Increasing cross-linking in the first network by (1) increasing the MCS concentration from 13 to 20% in MCS/PAAm DNs or (2) adding PEGDA as a cross-linker increases the modulus and failure stress, but decreases the yielding region. Since the increased modulus in DN gels is believed to be the result of strain energy dissipation by fracturing of the first network, the primary effect of cross-linking appears to be the reduction of chain extensibility resulting in failure at lower strains.In compression, the mechanisms for toughening were believed to be different than tension. Although energy dissipation mechanisms from the fracture of the first network may increase the toughness of DN, based on this work the improved failure properties under compression are believed to be from the entanglement and rearrangement of the two networks. (3) IPN hydrogel with high density charge complexation that are engineered with the non-covalent sacrificial bonds, which was hypothesized to improve toughness. IPNs of poly(N-vinyl formamide) (PNVF) and polyacrylamide (PAAm) were hypothesized to have intimate molecular mixing of the two networks (monomers are isomers of each other, and polymers have similar interaction with water) to help minimize molecular phase separation. First, synthesis of a new hydrogel, SN PNVF, is significant because PNVF gels provide an alternative to commercially important PAAm and provide a simple route to poly(vinylamine) (PVAm), high density cationic network. Then creating an IPN from the high charge density of both of the hydrolyzed counterparts poly(acrylic acid) (PAAc) and poly(vinylamine) (PVAm) maximizes interactions between the two networks which improved charge complexation, as evidenced by deswelling of the IPNs at intermediate pHs. Furthermore, comparing the toughness and failure stress of the hydrolyzed IPN immediately after hydrolysis to the same IPN at intermediate pHs showed 15-fold increases in the complexed state. In developing these multi-component hydrogels, a deeper understanding of the complexity of the interactions of the networks requires the visualization of the hydrogels in the hydrated state. Common techniques such as AFM and SEM have limitations when imaging in the swollen state. Introductory work on applying a super resolution microscopy technique, direct stochastic optical reconstruction microscopy (dSTORM), allows for imaging hydrated materials. The work in this dissertation is important for advancing basic science of hydrogels for potential applications in fields such as tissue engineering, drug delivery, and gel-based separation techniques. This work focused on identifying and correlating the mechanisms between the materials, formulations and network properties (water content, mechanical properties, structure) on tough multi-component hydrogels. In order to educate and communicate to a broader audience, a summarized research statement in the form AND, BUT, THEREFORE is stated below: Hydrogels are water-loving three-dimensional materials. AND they can be manipulated to have unique properties. BUT manipulating hydrogels requires understanding of the inner mechanisms in order to improve properties (toughness, fracture properties, moduli). THEREFORE I am doing research to understand the molecular mechanisms of hydrogels. | |
