| dc.description.abstract | Despite widespread use of hydrogels in biomedical devices like soft contact lenses, absorbent products, and other applications, low moduli and brittleness of hydrogels has hindered applications with load bearing requirements. In this dissertation, different molecular network design strategies (homopolymer, copolymer, and double network [two interlocked networks with synergistic improvement in mechanical properties]) were used to control the arrangement of macromers to tune the mechanical properties of hydrogels. Changes in swelling ratio, shear (G) and elastic (E) moduli, and fracture properties of the gels were then investigated. Chondroitin sulfate (CS) and hyaluronic acid (HA) as macromers, oligo(ethylene glycol) diacrylates (OEGDA) as comonomers, and acrylamide to form double networks (DN) were the focus of this work. The two main hypotheses for tuning the properties of homopolymer and copolymer gels were based on the structure of glycosaminoglycans (GAGs) in solution. The idea was to improve the crosslink effectiveness of photopolymerized GAG hydrogels to improve moduli and fracture properties of the gels. First, it was hypothesized that changing the degree of substitution of the macromer carboxyl groups (DM) would improve the crosslink effectiveness of photopolymerization by providing a higher ratio of crosslinkable groups. Second, considering the relatively rigid structure of glycosaminoglycans, it was hypothesized that copolymerization of GAGs with small amount of oligo(ethylene glycol) diacrylates would overcome the steric hindrances of the macromer in solution and would increase the crosslink density of the gels. By changing the degree of methacrlyation from 24 to 34 mol%, the swelling degree and G of MCS 13 wt% gel was changed from 210 to 44 g/g and from 3 to 45 kPa respectively. Copolymerization of MCS 13 wt% with 0.5 to 2.0 wt% OEGDA (molar ratio of OEGDA to CS disaccharide unit equal to 0.12) increased the shear modulus of MCS homopolymer (45 kPa) up to five times and lowered the swelling degree (44 g/g) to a third. The dependence of moduli and swelling on the crosslinker (OEGDA) length and the fact that mono-acrylated monomers equally increased the gels' moduli suggested that crosslinking mainly happens via the formation of kinetic chains. With this mechanism, macromer methacrylate groups and crosslinker acrylates covalently polymerize into a chain rather than connection of each methacrylate to one acrylate or vice versa. Despite improved moduli and tuned swelling (which are both correlated with crosslink density), the fracture strain of the homo- and copolymer gels remained fairly low (15 to 25% strain). Low fracture strain despite improved crosslink density suggested that fracture strain is a function of polymer chains conformation rather than crosslinking reaction. The polyelectrolyte nature of CS and HA was used to understand the fracture mechanism of single network (homopolymer) gels. The core hypothesis here was that polymer chains conformation in solution controls the fracture in polyelectrolyte gels. Therefore, introducing counterions would change the polyelectrolyte persistence length and change the fracture properties as well as moduli and swelling of the gels. The fracture strain of MCS 13 wt% was increased from 18% to about 60% in 1M NaCl solution. At the same time, Young's modulus and swelling ratio of the gel decreased from 142 kPa and 44 g/g to 4kPa and 6 g/g respectively. Increased fracture strain and lowered moduli and swelling were then associated with the polyelectrolyte chain distribution shifting from non-Gaussian toward Gaussian regime. Lowering the persistence length of the polyelectrolytes, highly extended chains in water would have a higher number of conformations in salt (Gaussian). Gaussian chains could then be stretched farther before rupturing which would increase the fracture strain of the gel. This interpretation was consistent with lower moduli and swelling ratio and higher fracture strain of the gels in salt. The toughening mechanism of double network (DN) gels was studied using the findings of homopolymer gels in salt. It was hypothesized that lowered persistence length of polyelectrolytes (i.e. Gaussian chains) in salt would suppress the bond fracture of the first network. Therefore, the proposed necessity of brittleness of the first network would be examined. The DNs moduli were lowered 3 to 6 times (depending on type and concentration) in the presence of counterions. The lowered moduli reflected the transition of the chains distribution as previously observed in single networks. However, the improved toughness of DNs compared to single networks even at higher ionic strengths was consistent with the significance of first and second network chains entanglements rather than bond breakage as a toughening mechanism in DN gels. This dissertation establishes copolymerization of MCS with OEGDA as a simple strategy to form hydrogels with tunable stiffness for desired applications. This work is a clear demonstration that crosslinking through kinetic chains is the dominating reaction mechanism in this system. This approach limits the required extent of macromer functionalization to form stable gels while low ratio of OEGDA to MCS would not change the chemical composition of the gels significantly. Furthermore, it was hypothesized and shown that OEGDA cross-linked MHA has similar behavior to MCS's. This proved the generality of the cross-linking effect and its applicability to other macromer based hydrogels, opening new areas of investigation to advance the methodology. Investigating mechanical behavior of DN gels in salt was a novel approach to better understand the toughening mechanism of these hydrogels. Recognizing chain entanglements as a major mechanism in toughening the DNs will be inspiring for other researchers in the field and will advance the understanding of the DNs microstructure. | |
