No abstract

A fundamental understanding of the effect of a crosslinker on gel properties is important for the design of novel soft materials because a crosslinking is a key component of polymer gels. We focused on post-polymerization crosslinking (PPC) system utilizing activated ester chemistry, which is a powerful tool due to structural diversity of diamine crosslinkers and less susceptibility to solvent effect compared to conventional divinyl crosslinking system, to systematically evaluate the crosslinker effect on the gel properties. A variety of alkyldiamine crosslinkers was employed for the synthesis of poly(N-isopropylacrylamide) (PNIPAAm) gels and it was clarified that the length of alkyl chains of diamine crosslinkers strongly affected the gelation reaction and the swelling behavior. The longer crosslinker induced faster gelation and decreased the swelling degree and the response temperature in water, while the crosslinking density did not significantly change. In addition, we were able to modify the polymer chains in parallel with crosslinking by using a monoamine modifier along with a diamine crosslinker. This simultaneous chain modification during crosslinking (SMC) was demonstrated to be useful for the regulation of the crosslinking density and the swelling behavior of PNIPAAm gels.Gels 2020, 6, 2 2 of 10 of a crosslinker, a divinyl compound. Free radical polymerization of a monomer in the presence of a crosslinker is a simple and important method for the preparation of functional gels but, when focusing on a crosslinker, the design of a divinyl compound often requires a bothersome synthetic procedure. In addition, divinyl crosslinking reaction is susceptible to the reaction solvents and produce the different network structure depending on the solvent even if using a same monomer/crosslinker combination [20]. This strong solvent effect on the reaction leads to the difficulty in the understanding the effect of a crosslinkers on gel properties even at simply changing the alkyl chain length of a crosslinker, because the solubility of a crosslinker also affects the reaction particularly in the case of a hydrogel prepared in water. As a result, the influence of the crosslinking agent on the gel has been difficult to be investigated so far in spite of the importance in gel properties [21].We are focusing on a gel synthesis by post-polymerization crosslinking (PPC) system in which prepolymers having activated ester moieties are reacted with a diamine compound as a crosslinker (Scheme 1a) [22][23][24][25][26]. This method is performed by simply mixing a prepolymer with a crosslinker, and it is possible to diversely change the gel structure by an appropriate design of the prepolymer. For example, simple mixing two kinds of poly(acrylamide derivative) prepolymers with different solubility afforded an amphiphilic co-network structure showing unique thermoresponsive swelling behavior [24]. In addition, a designed amphiphilic structure with crosslinked domains can be obtained by using the triblock prepolymer having reactive s...