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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.


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.
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f1: 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.

Mentions: Several previous attempts have been made to prepare stretchable hydrogels using PRs as the cross-linker. We first prepared a NIPA-based hydrogel using a PR modified by 2-acryloyloxyethyl isocyanate, which contains both isocyanate and vinyl groups, as the cross-linker (PR-C)23. The isocyanate groups form stable carbamate bonds with the α-CD hydroxyl groups in the PR to generate the cross-linking structures. It was expected that the NIPA–PR-C hydrogel would exhibit high extensibility, similar to the slide-ring gel composed of only the PR; however, the mechanical properties of the hydrogel were only slightly improved compared with those of chemical poly(NIPA) hydrogels prepared using conventional cross-linkers such as BIS. Although the NIPA–PR-C gel prepared in dimethyl sulphoxide (DMSO) was transparent, the gel became slightly opaque after replacing DMSO with water, even at 20 °C. These characteristics were attributed to the possibility that PR-C was in a contracted state in the hydrogel. The unmodified PR, which consists of α-CDs and PEG, is soluble only in a few solvents, such as DMSO, some ionic liquids, mixtures of organic amides and lithium salts and high-pH aqueous solutions. It is insoluble in pure water because the α-CD molecules aggregate due to hydrogen bonding between the hydroxyl groups on α-CDs24. Thus, α-CDs could not move along the PEG chains, and the pulley effect was restricted in water. As a result, the PR slide-ring gel appeared opaque and did not exhibit high extensibility. In high-pH aqueous solutions, such as a sodium hydroxide (NaOH) aqueous solution, the α-CDs do not aggregate through hydrogen bonding due to the ionization of the α-CD hydroxyl groups by NaOH; as a result, the slide-ring gel could swell19. However, a stimuli-sensitive hydrogel that is only useful in high-pH aqueous solutions does not have broad utility. Even when a nonionic, water-soluble hydroxypropylated PR cross-linker (HPR-C; Fig. 1a) was used, the resulting NIPA-based hydrogel did not exhibit greater extensibility than chemical poly(NIPA) hydrogels25. Although this hydrogel was nearly transparent, the pulley effect might not have worked well in this hydrogel because the solubility of HPR-C in water might be insufficient.


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)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4214411&req=5

f1: 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.
Mentions: Several previous attempts have been made to prepare stretchable hydrogels using PRs as the cross-linker. We first prepared a NIPA-based hydrogel using a PR modified by 2-acryloyloxyethyl isocyanate, which contains both isocyanate and vinyl groups, as the cross-linker (PR-C)23. The isocyanate groups form stable carbamate bonds with the α-CD hydroxyl groups in the PR to generate the cross-linking structures. It was expected that the NIPA–PR-C hydrogel would exhibit high extensibility, similar to the slide-ring gel composed of only the PR; however, the mechanical properties of the hydrogel were only slightly improved compared with those of chemical poly(NIPA) hydrogels prepared using conventional cross-linkers such as BIS. Although the NIPA–PR-C gel prepared in dimethyl sulphoxide (DMSO) was transparent, the gel became slightly opaque after replacing DMSO with water, even at 20 °C. These characteristics were attributed to the possibility that PR-C was in a contracted state in the hydrogel. The unmodified PR, which consists of α-CDs and PEG, is soluble only in a few solvents, such as DMSO, some ionic liquids, mixtures of organic amides and lithium salts and high-pH aqueous solutions. It is insoluble in pure water because the α-CD molecules aggregate due to hydrogen bonding between the hydroxyl groups on α-CDs24. Thus, α-CDs could not move along the PEG chains, and the pulley effect was restricted in water. As a result, the PR slide-ring gel appeared opaque and did not exhibit high extensibility. In high-pH aqueous solutions, such as a sodium hydroxide (NaOH) aqueous solution, the α-CDs do not aggregate through hydrogen bonding due to the ionization of the α-CD hydroxyl groups by NaOH; as a result, the slide-ring gel could swell19. However, a stimuli-sensitive hydrogel that is only useful in high-pH aqueous solutions does not have broad utility. Even when a nonionic, water-soluble hydroxypropylated PR cross-linker (HPR-C; Fig. 1a) was used, the resulting NIPA-based hydrogel did not exhibit greater extensibility than chemical poly(NIPA) hydrogels25. Although this hydrogel was nearly transparent, the pulley effect might not have worked well in this hydrogel because the solubility of HPR-C in water might be insufficient.

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.