Reversible gelation, thermoresponsiveness and formation of double networks by N-alkylated poly(N-vinylamide)s

Poly(N-vinylamide)s are attractive building blocks for hydrogels due to their good biocompatibility and versatile substitution at the amide position or after hydrolysis via the amine function. Gels of poly(N-alkyl-N-vinylamides) with short N-alkyl side chains exhibit a volume phase transition in water, similar to which is observed for poly(N-isopropylacrylamide). This has been rarely investigated so far. In this work, firstly, poly(N-alkyl-N-vinylamide)s with different alkyl chain length (n) and degree of substitution (DS) were prepared by N-alkylation of poly(N-vinylamide)s. Inverse molecular weight dependence of the cloud points of poly(N-alkyl-N-vinylamide)s (n = 3 or 4) polymers were observed meaning that the lower molecular weight samples have lower cloud points. Next, three types of hydrogels were prepared from the aforementioned polymers: (i) reversibly crosslinked hydrogels prepared from poly(N-alkyl-N-vinylamide)s (n = 10 to 18, DS = 3 mol%); (ii) covalently crosslinked thermoresponsive hydrogels prepared from sparsely allylated poly(N-alkyl-N-vinylamide)s (n = 3 or 4) crosslinked by photo-initiated crosslinking via thiol-ene reaction; (iii) thermoresponsive double network hydrogels prepared from the combination of polymers used for Type (i) and Type (ii) hydrogels. Swelling, thermoresponsiveness and rheological properties of the aforementioned hydrogels were investigated. The strain- and stress-controlled rheological experiments of Type (i) hydrogels demonstrated no crystalline ordering even in the case of longer side chains, the network will be connected by amorphous and thus liquid hydrophobic clusters, i.e., the side chains are sticky attachments to the hydrophilic polymer backbone, whose stickiness is systematically increased by their length. Furthermore, their rheological properties follow a time-temperature-stickiness superposition principle indicating that the side chains serve as sticky substituents which extend the terminal relaxation time according to their hydrophobicity. Furthermore, these hydrogels demonstrated low cytotoxicity. Type (ii) hydrogels were characterized for their temperature-dependent swelling and swelling kinetics. It was observed that once the hydrogels collapsed at elevated temperature, swelling would recover below the volume phase transition temperature in short time but only to reduced degree of swelling. Further swelling to the original degree of swelling appeared to be extremely slow and was difficult to be observed because of the long times needed. For each network of Type (iii) hydrogels, polymers with different alkyl chain lengths and molecular weights were employed to study the effects on the temperature-dependent swelling behavior and mechanical properties of the double network hydrogels. Enhanced mechanical strength and tissue-like stress-strain mechanical response were achieved for selected double network hydrogels.