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

Bin Imran A, Esaki K, Gotoh H, Seki T, Ito K, Sakai Y, Takeoka Y - Nat Commun (2014)

Bottom Line: One of the most significant problems is that conventional stimuli-sensitive hydrogels are usually brittle.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.

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.

No MeSH data available.


Related in: MedlinePlus

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.
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f2: 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.

Mentions: If polyelectrolyte hydrogels are prepared using NIPA, the ionic monomer sodium acrylic acid (AAcNa), and HPR-C (Fig. 1b), the obtained hydrogels (NIPA–AAcNa–HPR-C hydrogels) are highly stretchable, flexible and durable when compressed (Fig. 2). Thus, the mechanical properties of these hydrogels are completely different from those of all existing chemical poly(NIPA) hydrogels and chemical poly(NIPA–AAcNa) hydrogels (Supplementary Movies 1 and 2). These hydrogels are so strong that they cannot be easily cut with a knife (Fig. 2d). Interestingly, the NIPA–AAcNa–HPR-C hydrogels rapidly shrink isotropically without undergoing any deformation at the gel surface (Supplementary Fig. 4). Furthermore, these hydrogels can absorb up to several tens of thousands percentage weight of water compared with their dried state. For example, the weight of a hydrogel with 0.65 wt% of HPR-C increases by up to 620-fold when it is placed in water (Fig. 2e). Figure 2f shows the stress–strain curves of NIPA–AAcNa–HPR-C hydrogels with different amounts of HPR-C and chemical poly(NIPA–AAcNa) hydrogels with different amounts of BIS. Young’s moduli, maximum elongation ratios and tensile strengths are summarized in Supplementary Table 2. For hydrogels with similar cross-linker concentrations (0.63–0.65 wt%), the maximum elongation of the hydrogels with the HPR-C cross-linker (912%) is substantially greater than that of the hydrogels with the BIS cross-linker (29%); the extensibility and toughness of the NIPA–AAcNa–HPR-C hydrogels are improved greatly by replacing the BIS cross-linker with HPR-C. In addition, NIPA–AAcNa–BIS (0.063 wt%) and NIPA–AAcNa–HPR-C (0.65 wt%) hydrogels have similar cross-linking density due to the presence of equal number of active vinyl groups in the cross-linkers BIS and HPR-C used for gelation (Supplementary Table 1). However, NIPA–AAcNa–BIS (0.063 wt%) hydrogel exhibits high stiffness and very poor tensile strength compared with NIPA–AAcNa–HPR-C (0.65 wt%) hydrogel. When the amount of HPR-C increases to 1.21 and 2.00 wt%, Young’s modulus and tensile strength are enhanced. In the NIPA–AAcNa–HPR-C hydrogels, the AAcNa component is ionized. The Na+ counter ions cannot stay near the ionized poly(NIPA–AAc−) polymer chains and thus move long distances away from them, making them stretch, to achieve a large gain in entropy22. The aggregation of the α-CD molecules at the HPR-C cross-linker can be prevented by stretching the poly(NIPA–AAc−) chains (Fig. 2g). As a result, the α-CD molecules can move along the PR main chains, and the pulley effect might occur.


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

Bin Imran A, Esaki K, Gotoh H, Seki T, Ito K, Sakai Y, Takeoka Y - Nat Commun (2014)

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.
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f2: 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.
Mentions: If polyelectrolyte hydrogels are prepared using NIPA, the ionic monomer sodium acrylic acid (AAcNa), and HPR-C (Fig. 1b), the obtained hydrogels (NIPA–AAcNa–HPR-C hydrogels) are highly stretchable, flexible and durable when compressed (Fig. 2). Thus, the mechanical properties of these hydrogels are completely different from those of all existing chemical poly(NIPA) hydrogels and chemical poly(NIPA–AAcNa) hydrogels (Supplementary Movies 1 and 2). These hydrogels are so strong that they cannot be easily cut with a knife (Fig. 2d). Interestingly, the NIPA–AAcNa–HPR-C hydrogels rapidly shrink isotropically without undergoing any deformation at the gel surface (Supplementary Fig. 4). Furthermore, these hydrogels can absorb up to several tens of thousands percentage weight of water compared with their dried state. For example, the weight of a hydrogel with 0.65 wt% of HPR-C increases by up to 620-fold when it is placed in water (Fig. 2e). Figure 2f shows the stress–strain curves of NIPA–AAcNa–HPR-C hydrogels with different amounts of HPR-C and chemical poly(NIPA–AAcNa) hydrogels with different amounts of BIS. Young’s moduli, maximum elongation ratios and tensile strengths are summarized in Supplementary Table 2. For hydrogels with similar cross-linker concentrations (0.63–0.65 wt%), the maximum elongation of the hydrogels with the HPR-C cross-linker (912%) is substantially greater than that of the hydrogels with the BIS cross-linker (29%); the extensibility and toughness of the NIPA–AAcNa–HPR-C hydrogels are improved greatly by replacing the BIS cross-linker with HPR-C. In addition, NIPA–AAcNa–BIS (0.063 wt%) and NIPA–AAcNa–HPR-C (0.65 wt%) hydrogels have similar cross-linking density due to the presence of equal number of active vinyl groups in the cross-linkers BIS and HPR-C used for gelation (Supplementary Table 1). However, NIPA–AAcNa–BIS (0.063 wt%) hydrogel exhibits high stiffness and very poor tensile strength compared with NIPA–AAcNa–HPR-C (0.65 wt%) hydrogel. When the amount of HPR-C increases to 1.21 and 2.00 wt%, Young’s modulus and tensile strength are enhanced. In the NIPA–AAcNa–HPR-C hydrogels, the AAcNa component is ionized. The Na+ counter ions cannot stay near the ionized poly(NIPA–AAc−) polymer chains and thus move long distances away from them, making them stretch, to achieve a large gain in entropy22. The aggregation of the α-CD molecules at the HPR-C cross-linker can be prevented by stretching the poly(NIPA–AAc−) chains (Fig. 2g). As a result, the α-CD molecules can move along the PR main chains, and the pulley effect might occur.

Bottom Line: One of the most significant problems is that conventional stimuli-sensitive hydrogels are usually brittle.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.

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.

No MeSH data available.


Related in: MedlinePlus