Extremely stretchable thermosensitive hydrogels by introducing slide-ring polyrotaxane cross-linkers and ionic groups into the polymer network

Abstract

Stimuli-sensitive hydrogels changing their volumes and shapes in response to various stimulations have potential applications in multiple fields. However, these hydrogels have not yet been commercialized due to some problems that need to be overcome. One of the most significant problems is that conventional stimuli-sensitive hydrogels are usually brittle. Here we prepare extremely stretchable thermosensitive hydrogels with good toughness by using polyrotaxane derivatives composed of α-cyclodextrin and polyethylene glycol as cross-linkers and introducing ionic groups into the polymer network. The ionic groups help the polyrotaxane cross-linkers to become well extended in the polymer network. The resulting hydrogels are surprisingly stretchable and tough because the cross-linked α-cyclodextrin molecules can move along the polyethylene glycol chains. In addition, the polyrotaxane cross-linkers can be used with a variety of vinyl monomers; the mechanical properties of the wide variety of polymer gels can be improved by using these cross-linkers.

Figure 1. Preparation of the polyelectrolyte hydrogels using nonionic PR cross-linker.

(a) Preparation of HPR-C from HPR, 2-acryloyloxyethyl isocyanate, DBTDL (catalyst) and BHT (polymerization inhibitor) in DMSO. (b) Preparation of the NIPA–AAcNa–HPR-C hydrogel from HPR-C (cross-linker), NIPA (main monomer), AAcNa (comonomer), APS (initiator) and TEMED (accelerator) in water.

Figure 2. Properties of the polyelectrolyte hydrogels using nonionic PR cross-linker.

(a) Elongated state of the NIPA–AAcNa–HPR-C hydrogel. (b) Compressed state of the NIPA–AAcNa–HPR-C hydrogel. (c) Coiled and knotted states of the NIPA–AAcNa–HPR-C hydrogel. (d) The NIPA–AAcNa–HPR-C hydrogel could not be easily cut with a knife. (e) Swelling of the NIPA–AAcNa–HPR-C hydrogel in water. Left: the dry gel (129 mg); right: the water-swollen gel (80 g). The NIPA–AAcNa–HPR-C hydrogel with 0.65 wt% of HPR-C absorbs up to ca. 62,000 wt% of water in its dry state. (f) Stress–strain curves of hydrogels: (i) NIPA–AAcNa–BIS (0.65 wt%), (ii) NIPA–AAcNa–BIS (0.065 wt%), (iii) NIPA–AAcNa–HPR-C (2.00 wt%), (iv) NIPA–AAcNa–HPR-C (1.21 wt%) and (v) NIPA–AAcNa–HPR-C (0.65 wt%). (g) Schematic of swollen HPR-C in the NIPA–AAcNa–HPR-C hydrogel.

Figure 3. SAXS results of the polyelectrolyte hydrogels using nonionic PR cross-linker.

(a) SAXS isointensity patterns of the NIPA–AAcNa–HPR-C hydrogel with 0.65 wt% of HPR-C for different elongations in the vertical direction. (b) Sector-averaged I(q) of the NIPA–AAcNa–HPR-C hydrogel with 0.65 wt% of HPR-C for different elongations in the parallel (open circles) and perpendicular (filled circles) directions. The solid lines are the equation (1) fitting results. (c) Stretching ratio dependence of ξ and Ξ for the NIPA–AAcNa–HPR-C hydrogel with 0.65 wt% of HPR-C in parallel to the elongation direction, and (d) stretching ratio dependence of ξ and Ξ for the NIPA–AAcNa–HPR-C hydrogel with 0.65 wt% of HPR-C in perpendicular to the elongation direction.

Figure 4. Preparation of the hydrogels using ionic PR cross-linker.

(a) Preparation of iPR-C from iPR, 2-acryloyloxyethyl isocyanate, DBTDL (catalyst) and BHT (polymerization inhibitor) in DMSO. (b) Schematic of swollen iPR-C in the NIPA–iPR-C hydrogel. (c) Elongated state of the NIPA–iPR-C hydrogel. (d) Stress–strain curves for NIPA–iPR-C hydrogels with different amounts of iPR-C.

Figure 5. Swelling behaviours of the polyelectrolyte hydrogels using nonionic PR cross-linker and the hydrogels using ionic PR cross-linker.

The degree of swelling D/D₀ for the (a) NIPA–AAcNa–HPR-C hydrogel with 0.65 wt% of HPR-C during both heating (red) and cooling (blue) processes, and (b) NIPA–iPR-C hydrogel with 0.80 wt% of iPR-C during heating process in aqueous solutions with different pHs as a function of temperature. D and D₀ denote the gel diameters at equilibrium and on synthesis, respectively.