Article pubs.acs.org/Macromolecules
Supramolecular Materials Cross-Linked by Host−Guest Inclusion Complexes: The Effect of Side Chain Molecules on Mechanical Properties Yoshinori Takashima,† Yuki Sawa,† 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, Tokyo 102-0076, Japan
‡
S Supporting Information *
ABSTRACT: Functional polymeric materials constructed by noncovalent bonds have attracted considerable attention due to their beneficial stretching and self-healing properties. We chose host−guest interactions using cyclodextrins (CDs) as host molecules to realize supramolecular materials with stretching and self-healing properties. Notably, an inclusion complex of a CD and a guest molecule functions as a reversible bond in a material. Herein, we studied the relationship between the mechanical properties of the materials and host−guest interactions based on the association constants of CDs with guest molecules and molecular structures of the guest molecules. A chemically cross-linked poly(acrylamide) gel showed high rupture stress, although the rupture strain was noticeably low. However, the host−guest hydrogels (CDAAmMe-R hydrogels) exhibited a higher rupture stress and strain of approximately 1000%. These rupture stress and strain values were related to the association constants of the CDs with guest units on the polymer side chain and the structure of the guest molecules. In particular, the αCDAAmMe-Dod hydrogel with a dodecyl group with a long, rod-like structure showed better rupture stress and strain (1250%). The βCDAAmMe-AdAAm hydrogel with a spherical adamantyl acrylamide (AdAAm) group showed better self-healing properties. To realize a practical self-healing process under dry conditions, a poly(methyl triethylene glycol acrylate) xerogels with βCDAAmMe and AdAAm (βCDAAmMe-AdAAm TEGA xerogel) was prepared. The βCDAAmMe-AdAAm TEGA xerogel exhibited selfhealing properties, regaining 61% of its initial material strength at 100 °C. (a) from linear supramolecular polymers,43−46 (b) from a mixture of host and guest polymers,47−49 and (c) from the polymerization of host and guest monomers50−55 (Figure 1). We have prepared supramolecular materials with CDs using methods a−c.56 In our previous studies using method c, the mechanical properties (stress and strain) of the resulting supramolecular materials were higher than those prepared by methods a and b. These reports indicated that method c not only effectively introduces the host−guest complexes but also functionally employs host−guest interactions to enhance the mechanical properties. On the basis of these results, we hypothesized that the mechanical and self-healing properties of supramolecular materials consisting of CDs and guest molecules without chemical cross-linkers would be affected by the structure of the guest molecules and the association constants of CDs with the guest units. Herein, we studied the relationship between the functions of supramolecular materials and association constants of CDs with guest units.
1. INTRODUCTION Supramolecular materials1−3 based on noncovalent interactions (i.e., hydrogen bonds,4 π−π stacking,5−7 ions,8 hydrophobic interactions,9 and metal-coordination bonds10,11) have recently attracted considerable attention due to their functional properties (stimuli-responsiveness,12−21 toughness,22−27 selfhealing properties,28−31 etc.). Host−guest interactions are noncovalent interactions that can easily introduce target functions into materials.32−34 Furthermore, by choosing host−guest interactions, researchers can apply various designs of supramolecular materials to produce the desired functions. Macrocyclic molecules, including crown ethers,35 calixarenes,36 cucurbiturils,37,38 pillararenes,39 and cyclodextrins (CDs),40−42 and functional guest molecules are incorporated into supramolecular materials based on host−guest interactions. The control of the association and dissociation of the inclusion complexes enables the ON/OFF switching of functions. The incorporation of these macrocyclic molecules into polymers can change cross-linking density to realize unique mechanical properties. There are three basic approaches to preparing supramolecular polymeric materials through host−guest interactions: © XXXX American Chemical Society
Received: February 9, 2017 Revised: March 31, 2017
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DOI: 10.1021/acs.macromol.7b00266 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
CDAAm-R hydrogel(m,n)), which represent the mol % of the CD monomer and guest monomer, respectively. These hydrogels lacked chemical cross-linking molecules, indicating that host−guest interactions acting as noncovalent cross-linkers to stabilize the morphology of the supramolecular hydrogels without permitting relaxation. The polyacrylamide hydrogel (AAm hydrogel) was prepared by the radical copolymerization of acrylamide (AAm) with methylenebis(acrylamide) (MBAAm, 1−6 mol %) under the same conditions. 1 H solid-state field-gradient magic-angle-spinning (FGMAS), nuclear magnetic resonance (NMR) spectroscopy (Supporting Information, Figures S5−S15) and Fourier transform infrared (FT-IR) spectroscopy (Supporting Information, Figures S16 and S17) were used to characterize the chemical structure and CD/guest unit ratio of the hydrogels. The desired ratio of the guest monomers to the CD monomer was introduced into the hydrogels. 2.2. Effect of the Formation of the Inclusion Complex To Prepare the CD−Guest Hydrogels(m,n). We investigated the influence of solvent polarity and the presence of competitive molecules on the mechanical properties of the hydrogels. Homogeneous radical copolymerization in DMSO did not produce the hydrogels, even if the DMSO solution of the polymers was replaced by water, because CD derivatives and hydrophobic guest molecules do not form inclusion complexes in aprotic polar solvents due to decreased hydrophobic interactions. These results indicated that the preorganized inclusion complexes were essential to obtaining the supramolecular hydrogel. Similarly, prior to the radical copolymerization, native cyclodextrins (αCD, βCD, and γCD) as competitive CD molecules were added to the polymerization solution. The polymerization solutions of αCDAAmMe/DodA with αCD or βCD did not yield hydrogels but that with γCD changed to a hydrogel. The difference between the hydrogelation with αCD, βCD, and γCD was related to the association constant (Ka) of CDs with Dod. The Ka of αCD and βCD with 1,10dodecanediol are 7100 and 2200 M−1, but the Ka of γCD with the same moiety is much smaller (Ka < 10 M−1).57 Even when adding γCD into the polymerization solution, γCD did not inhibit the complex formation between the αCD and the Dod units. These results indicate that the formation of inclusion complexes facilitated stabilization of the morphology of the supramolecular hydrogels. 2.3. Toughness of the αCDAAmMe-Dod Hydrogel and the αCDAAm-Dod Hydrogel vs the AAm Gel. We compared the mechanical properties of the host−guest gel with those of the chemically cross-linked gel. Furthermore, we investigated the effect of the distance between the polymer main chain and the CD unit on the mechanical properties. Parts a and b of Figure 3 show the stress−strain curves of the αCDAAmMe-Dod and αCDAAm-Dod hydrogels (test sample size: 2 × 12 × 1 mm3), which were analyzed by a creep meter at a tensile speed of 1 mm/s. The rupture stress of the αCDAAmMe-Dod hydrogel and the αCDAAm-Dod hydrogel increased with the increased content of the αCD and the Dod units, whereas the rupture strain decreased. The chemically cross-linked hydrogel, AAm hydrogel(1.0), showed a significantly lower rupture strain (
