Abstract
Stiff thermosensitive hydrogels (HG) were synthesized by self-crosslinking free radical polymerization of N,N-dimethylacrylamide (DMAA) and N-isopropylacrylamide (NIPAAm), adjusting the degree of swelling by carboxylate-containing sodium acrylate (NaAc) or a 2-oxazoline macromonomer (MM). The formation of hydrogels was possible due to the self-crosslinking property of DMAA when polymerized with peroxodisulfate initiator type. The MM was synthetized by the ring-opening cationic polymerization of 2-methyl-2-oxazoline (MeOxa) and methyl-3-(oxazol-2-yl)-propionate (EsterOxa), and contained a polymerizable styryl endgroup. After ester hydrolysis of EsterOxa units, a carboxylate-containing MM was obtained. The structure of the hydrogels was confirmed by 1H high-resolution (HR)-MAS NMR spectroscopy. Suitable conditions and compositions of the comonomers have been found, which allowed efficient self-crosslinking as well as a thermoresponsive swelling in water. Incorporation of both the polar comonomer and the macromonomer, in small amounts furthermore allowed the adjustment of the degree of swelling. However, the macromonomer was better suited to retain the thermoresponsive behavior of the poly (NIPAAm) due to a phase separation of the tangling polyoxazoline side chains. Thermogravimetric analysis determined that the hydrogels were stable up to ~ 350 °C, and dynamic mechanical analysis characterized a viscoelastic behavior of the hydrogels, properties that are required, for example, for possible use as an actuator material.
Highlights
In recent years, the field of smart hydrogels has generated great technological interest due to the variety of applications in which they can be used, since these materials are able to respond in a reversible way to external stimuli such as pH value, temperature, light, etc., [1,2]
Thermogravimetric analysis determined that the hydrogels were stable up to ~ 350 ◦ C, and dynamic mechanical analysis characterized a viscoelastic behavior of the hydrogels, properties that are required, for example, for possible use as an actuator material
We report on the synthesis of temperature sensitive PNIPAAm hydrogels, using, in addition, DMAA simultaneously as a monomer favoring water absorption and as a self-crosslinking agent
Summary
The field of smart hydrogels has generated great technological interest due to the variety of applications in which they can be used, since these materials are able to respond in a reversible way to external stimuli such as pH value, temperature, light, etc., [1,2]. Smart hydrogels can be synthesized based on one or several monomers, according to the type of properties that one wants to assign to the material. N-isopropylacrylamide (NIPAAm) is commonly used when obtaining a temperature-sensitive hydrogel with a transition temperature close to human body temperature is desired. Other factors that determine both the properties of these hydrogels, and their applicability are the functional groups of the monomers and the degree of crosslinking. Geckeler et al reported the synthesis of hydrogels based on NIPAAm copolymerized with sodium acrylate [11], demonstrating
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