Supramolecular Hydrogels with Tunable Swelling by Host Complexation with Cyclobis(paraquat-p-phenylene)

Abstract

Controlling the swelling properties of hydrogels is of primary importance for many applications ranging from actuators and valves to tissue engineering and drug delivery.Herein, we report the use of cyclobis(paraquat-p-phenylene) (CBPQT 4+ , 4X -) as a versatile host 2 to finely tune the swelling behavior of 1,5-dialkyloxynaphthalene guest containing poly(N,Ndimethylacrylamide) hydrogels (NaphtGelz) through supramolecular host-guest complexation.While the equilibrium swelling of NaphtGelz in water decreases with increasing amount of hydrophobic naphthalene groups, the opposite behavior is observed with superabsorbing behavior (up to 180 times their initial dry mass) upon immersion in aqueous solutions containing the macrocyclic CBPQT 4+ ,4Xdue to formation of tetracationic host-guest complexes. In this case, the swelling amplitude could be conveniently programmed either by variation of the naphthalene content of the hydrogels or by controlling the stoichiometry of the host-guest binding events. Furthermore, by modifying the nature of the counter-ions (X = Cl -, Br -, I -) of the tetracationic CBPQT 4+ macrocyclic host, the swelling of the hydrogels could be tuned in line with Pearson's absolute hardness scale of X -. The swelling behavior of these supramolecular hydrogels could be successfully described by a theoretical model taken into account the hydrophobic association of the naphthalene groups and their screening by host-guest complexation. Finally, addition of SDS (Sodium Dodecyl Sulfate) as surfactant to the supramolecularly swollen hydrogels led to a large decrease in hydrogel size due to dissociation of the host-guest complexes and the formation of CBPQT 4+ ,4DSwithin the hydrogel.

Effect of Host–Guest Complexation on the Thermoresponsive Behavior of Poly(oligo ethylene glycol acrylate)s Functionalized with Dialkoxynapththalene Guest Side Chains

“…These novel insights into thermoresponsive polymers will be important for future of design of responsive polymer sensors and actuators, whereby it is anticipated that the thermoresponsive behavior can be further tuned by the content of NPA and by variation of the counterions of CBPQT 4+ . [ 28,29 ]…”

Covalent Hydrogels with Dual Temperature and Time Memory Function Based on Supramolecular Host–Guest Complexation

“…The unfunctionalized PNIPAmGel hydrogel showed no change in its swelling capability when immersed in an aqueous solution containing CBPQT 4+ ,4Cl – . In contrast, immersion of the Napht- co -PNIPAmGel in a solution containing CBPQT 4+ ,4Cl – led to immediate purple coloration, indicative of host–guest complexation, as well as significant swelling of the hydrogel due to the incorporation of the charged CBPQT 4+ ,4Cl – hosts that masks the hydrophobic naphthalene units …”

“…Hence, variations in both the temperature and the duration of the heat treatment would lead to a different amount of CBPQT 4+ ,4Cl – host retained in the hydrogels. To record these different levels of the retained host in the hydrogels, they can be swollen in fresh water as various degrees of complexation of the Napht- co -PNIPAmGel lead to different swelling degrees, as recently shown on naphthalene guests containing poly­( N , N -dimethylacrylamide) hydrogels . Consequently, the difference in swelling between the original complexed hydrogel and the hydrogel that is collected and reswollen in fresh water after heat treatment can be used as an output signal to record the thermal treatment temperature and time, providing information about how long the hydrogel has been exposed to a certain temperature.…”

“…In contrast, immersion of the Napht-co-PNIPAmGel in a solution containing CBPQT 4+ ,4Cl − led to immediate purple coloration, indicative of host−guest complexation, as well as significant swelling of the hydrogel due to the incorporation of the charged CBPQT 4+ ,4Cl − hosts that masks the hydrophobic naphthalene units. 36 PNIPAm based hydrogels are well-known to exhibit LCSTtype thermoresponsive behavior inducing a volume phase transition temperature (VPTT) upon heating, which is characterized by dehydration-induced shrinking of the 3D polymer network upon heating above the VPTT. The VPTT of both PNIPAmGel and Napht-co-PNIPAmGel hydrogels were determined in the absence and in the presence of CBPQT 4+ ,4Cl − via differential scanning calorimetry (DSC) (see Supporting Information, Figure S2).…”

“…To record these different levels of the retained host in the hydrogels, they can be swollen in fresh water as various degrees of complexation of the Naphtco-PNIPAmGel lead to different swelling degrees, as recently shown on naphthalene guests containing poly(N,N-dimethylacrylamide) hydrogels. 36 Consequently, the difference in swelling between the original complexed hydrogel and the hydrogel that is collected and reswollen in fresh water after heat treatment can be used as an output signal to record the thermal treatment temperature and time, providing information about how long the hydrogel has been exposed to a certain temperature. Even though reswelling in fresh water might not be the most straightforward method for recording an output signal, it represents the first fundamental example of a molecular system that allows monitoring the time and temperature of a heat treatment.…”