|M.Sc Student||Shreiber Livne Inbar|
|Subject||Self-crosslinked, High Porosity Hydrogels through|
|Department||Department of Materials Science and Engineering||Supervisor||PROF. Michael Silverstein|
|Full Thesis text|
Polymeric hydrogels are three-dimensional polymer network structures which are capable of absorbing water or physiological fluids and swelling due to the hydrophilic nature of the polymer. The resistance of hydrogels to dissolution arises from the network’s crosslinked structure. Hydrogels are used in a variety of applications, ranging from consumer products, such as disposable diapers to biomedical applications, such as tissue engineering scaffolds and drug delivery carriers. Hydrogels are usually crosslinked either chemically, using crosslinking comonomers, or physically through weaker bonds. Recently, hydrogels based on N,N-dimethylacrylamide (DMAA) have been shown to undergo self-crosslinking through the formation and reaction of methyl radicals during polymerization.
PolyHIPEs are highly porous, emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs), usually through free radical polymerization. PolyHIPEs are almost always crosslinked through the use of crosslinking comonomers. Hydrogel polyHIPEs (HG-PHs) are typically synthesized within oil-in-water HIPEs. The high pore volumes in HG-PHs have proven highly advantageous for water absorption applications.
The objectives of this research were to develop innovative HG-PHs, through self-crosslinking of DMAA and to determine the effects of the synthesis parameters (initiator content, polymerization temperature, comonomers) upon the porous structures and properties. This study compares self-crosslinked (SC) HG-PHs with those crosslinked using N,N'-methylene(bis)acrylamide (BIS) as a crosslinking comonomer (CV crosslinking) and the combination of both types of crosslinking (CM crosslinking). A further comparison was made to non-polyHIPE poly(DMAA) references hydrogels (HG‑Rs). In addition, HG-PHs were also synthesized successfully through copolymerization of DMAA with acrylamide (AAm) and N‑isopropylacrylamide (NiPAAm). The distinguishing differences in the HG-PHs synthesized using different crosslink mechanisms were investigated. pH stimuli self-crosslinking HG-PH based DMAA-co-acrylic acid (AA) was synthesized also.
The highly interconnected porous structures were similar to those of typical polyHIPEs, with ~80 µm voids and ~18 µm interconnecting holes. The water absorption capacities of the self-crosslinked HG-PHs were around 49 g/g, about twice those of the CV and CM crosslinked HG-PHs, and more than 4 times that of the HG-Rs. The moduli of the hydrated, self-crosslinked copolymer HG-PHs, around 0.65 kPa, significantly lower than those of the hydrated, BIS-crosslinked HG-PHs (1.3 and 4 kPa for CV and CM crosslinking, respectively). The HG-PHs moduli were more than an order of magnitude lower than those of the HG-Rs. Significantly, the HG-PHs could reach compressive strains of over 60 % and exhibited shape recovery once the stress was removed, while the HG-Rs were brittle.
“Smart” stimuli responsive, self-crosslinked HG-PHs were synthesised successfully. Copolymerization with NiPAAm exhibited a dramatic response to temperature, with 12 g/g at 97 °C compared to 52 g/g at 37 °C. The SC HG-PH copolymer with AA exhibited a pronounced dependence of properties upon pH. At pH 10 the water absorption was 71 g/g and the modulus was 17 kPa, while at pH 2 the water absorption and modulus were significantly smaller at 21 g/g and 2.9 kPa, respectively.
This work generated novel polyHIPEs, using an innovative synthesis. Self-crosslinking has been conclusively demonstrated to be a viable route to producing flexible HG-PHs with superior water absorption capacities.