Article pubs.acs.org/Macromolecules
Multifunctional Stimuli-Responsive Supramolecular Materials with Stretching, Coloring, and Self-Healing Properties Functionalized via Host−Guest Interactions Yoshinori Takashima,† Koki Yonekura,† Kohei Koyanagi,† Kazuhisa Iwaso,† Masaki Nakahata,† Hiroyasu Yamaguchi,† and Akira Harada*,†,‡ †
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan JST-ImPACT, 5-7, Chiyoda-ku, Tokyo 100-8914, Japan
‡
S Supporting Information *
ABSTRACT: The visualization of changes in the stress and bonding state inside polymeric materials is an attractive function in materials science. In this study, phenolphthalein (PP) was selected to prepare stimuli-responsive coloring materials. PP becomes purple under basic conditions in aqueous solutions; however, PP becomes colorless under basic conditions when it forms a complex with β-cyclodextrin (βCD). To exploit this property of PP, we prepared a colorchanging hydrogel (βCD-PP AAm hydrogel) based on acrylamide (AAm) as the main chain and βCD and PP moieties as the side chains. The βCD-PP AAm hydrogel exhibits a color change when heat or a competing molecule is applied at a pH less than 8. This color change was confirmed by ultraviolet−visible (UV−vis) spectroscopy, and the mechanical properties were determined via compression and tensile measurements. The βCD-PP AAm hydrogel also exhibits a rapid, reversible color change upon Joule heating produced by an electric current passing through the gel.
1. INTRODUCTION Recently, multistimuli-responsive and functional polymeric materials have become emerging areas of research. 1−6 Representative functions of such materials include selfhealing,7−13 actuation,14−32 sensors,33−39 and drug delivery.40−44 However, fabricating polymeric materials with multistimuli responsiveness or multiple functions, such as biological materials based on simple molecular design, remains difficult. Indeed, a complicated molecular design is necessary to achieve multiple functions. To prepare multifunctional materials using a simple molecular design, reversible/dynamic covalent bonds combined with supramolecular/polymer chemistry is an effective approach. In fact, dynamic covalent bonds have been introduced into materials to obtain mechanochromic functional materials that can detect various stimuli (for example, stress, heat, and light)45−57 and have self-healing properties.58−62 We hypothesize that combining two or more types of noncovalent bonds will enable the preparation of multifunctional supramolecular materials via polymeric design. We chose host−guest interactions as noncovalent bonds on polymer side chains to realize stimuli-responsive coloring supramolecular materials. We have previously reported on the preparation of various stimuli-responsive supramolecular materials, such as macroscopic self-assemblies,63,64 self-healing materials,65,66 and artificial muscles,67,68 based on host−guest interactions between cyclodextrins (CDs) and various guest molecules on the side chain of the polymer. These materials are © XXXX American Chemical Society
relatively easy to endow with functions through the association and dissociation of an inclusion complex with CDs and guest molecules. Herein, we selected phenolphthalein (PP)69,70 as a guest molecule because an aqueous solution of an inclusion complex between βCD and PP does not exhibit a purple color at a pH of 10. However, the addition of competitive guest molecules colors the solution (Figure 1a).71,72 Ritter and his coworkers studied the interaction of βCD derivatives with pHsensitive polymers bearing covalently attached PP.73−75 They successfully prepared the PP polymer and observed the colorchange behavior in response to thermo- and pH-stimuli in the presence of βCD. We chose to polymerize inclusion complexes with βCD and PP guest monomers to effectively introduce the complexes into supramolecular materials. On the basis of the formation of complexes between βCD and PP, we obtained supramolecular materials with βCD and PP and characterized the coloring and self-healing properties of the materials.
2. RESULTS AND DISCUSSION 2.1. Preparation of CD-PP Gels Using Inclusion Complexes of βCD and Phenolphthalein Monomers. Figure 1b shows the chemical structure of the βCD-PP acrylamide hydrogel (βCD-PP AAm hydrogel) and the Received: April 27, 2017 Revised: May 22, 2017
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DOI: 10.1021/acs.macromol.7b00875 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) pH-dependent color reaction of phenolphthalein (PP) in the presence or absence of β-cyclodextrin (βCD). (b) Chemical structures of acrylamide (AAm)-based βCD-PP hydrogel (βCD-PP AAm hydrogel(m, n)) and chemically cross-linked AAm hydrogel. (c) Chemical structures of hydroxylethyl acrylate (HEA)-based βCD-PP xerogel (βCD-PP HEA xerogel(m, n)) and chemically cross-linked HEA xerogel.
poly(acrylamide) hydrogel (AAm hydrogel) with methylenebis(acrylamide) (MBAAm, 2 mol %), which is a chemically crosslinked hydrogel. To realize self-healing properties in the dry state, we prepared the βCD-PP hydroxyethyl acrylate (HEA) xerogel (βCD-PP HEA xerogel, Figure 1c) because the glass transition point (Tg) of poly(HEA) is lower than that of poly(AAm). Prior to radical copolymerization, the PP monomer was mixed in aqueous solutions of the corresponding βCD monomer to form inclusion complexes in water. Then, the βCD-PP AAm hydrogel was prepared by homogeneous radical copolymerization of the inclusion complex with the main chain monomer at a concentration of 2 mol/kg using ammonium peroxodisulfate (APS) as an initiator and N,N,N′,N′-tetramethylethane-1,2-diamine (TEMED) as a cocatalyst (Supporting Information, Scheme S2). The βCD-PP HEA xerogel was dried naturally for 3 days. The AAm hydrogel was prepared via the
radical copolymerization of AAm with MBAAm (2 mol %) under the same conditions. In the composition ratio of the βCD-PP hydrogels (m, n), m and n represent the mole percentages of the CD monomer and guest monomer, respectively. These hydrogels lack chemical cross-linking molecules. Therefore, the host−guest interactions act as noncovalent cross-linkers and stabilize the morphology of the βCD-PP hydrogels without relaxation. 2.2. Toughness of the βCD-PP AAm Hydrogel(2, 2) Compared to the AAm Hydrogel. We compared the mechanical properties of the host−guest gel with those of the chemically cross-linked gel. Figure 2 shows the stress−strain curves of the βCD-PP and AAm hydrogels (test sample size: 2 × 12 × 1 mm3), which were analyzed using a creep meter at a tensile speed of 1 mm/s. The rupture stress and strain of the βCD-PP hydrogel increased with the increasing content of βCD and PP units. The βCD-PP hydrogel(2, 2) and AAm B
DOI: 10.1021/acs.macromol.7b00875 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. Stress−strain curves and fracture energy of the βCD-PP AAm hydrogel(2, 2) and AAm hydrogel(2).
hydrogel(2) exhibited maximum rupture strains. The chemically cross-linked hydrogel, AAm hydrogel(2), exhibited a significantly lower rupture strain (
