Movable Cross-Linked Polymeric Materials from Bulk Polymerization

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Movable Cross-Linked Polymeric Materials from Bulk Polymerization of Reactive Polyrotaxane Cross-Linker with Acrylate Monomers Kohei Koyanagi,† Yoshinori Takashima,† 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: Topological cross-linked polymers attract much attention from their unique mechanical properties derived from their cross-linking structure. Here, we have fabricated a polymeric material (Acryl-AcO-PRx) incorporating topological cross-links into various universal acrylate polymers by using a modified polyrotaxane as a polymerizable cross-linking agent. Acryl-AcO-PRx obtained by a simple photocuring technique under bulk conditions showed a distinctly higher extensibility compared with the chemical cross-linked polymers. In addition, Acryl-AcO-PRx materials showed high stress relaxation and deformation hysteresis. These results indicate that the topologically cross-linked structure composed of polyrotaxane plays an important role in mechanical properties even in bulk state.

1. INTRODUCTION The improvement of toughness and durability remains a critical issue in polymeric materials. Generally, conventional polymeric materials have upper limit of stress and strain in the large deformation. Lake and Thomas predicted the threshold minimum energy to break the polymeric materials, which depend on the number of cross-linkers.1,2 To consistent with increasing the materials strength and strain, there are some materials design approaches to achieve toughness of polymeric materials. The one approach is the addition of fillers to polymeric materials, which effectively improve elastic modulus and strain at break.3 Another is molecular design of cross-linked structure, which improves mechanical properties at the molecular level. The cross-linked structures in polymeric materials fall into three broad categories: chemical cross-linking (permanent cross-linking), reversible cross-linking, and movable cross-linking.4 Recently, supramolecular materials crosslinked by reversible bonds have attracted much attention due to their functional properties (stimuli-responsiveness,5−14 toughness,15−20 self-healing properties,21−24 etc.). In our previous studies with cyclodextrins (CDs) as host molecules, host−guest interactions as noncovalent interactions can easily introduce target functions into supramolecular materials.25−34 Polymeric materials with movable cross-linkers (topological cross-linker) indirectly connect polymers with mechanically interlocked © XXXX American Chemical Society

architecture such as polyrotaxanes (PRxs). PRx is a macromolecule composed of ring molecules threaded by an axle polymer, with bulky stoppers at both ends. There have been numerous PRx depending on the type of cyclic compound (crown ethers,35 cyclobis(paraquat-p-phenylene),36 and cucurbituriles,37 and cyclodextrins,38) and other axis molecules.39−41 The movable cross-linking points along the axis realized unique mechanical properties such as high flexibility, entropic elasticity, and physical self-healing ability of materials by dispersing stress at deformation process.42−58 PRx materials based on αCD and poly(ethylene glycol) (PEG) have a good swelling property arising from topological cross-linkers. The preparation of PRx elastomer prepared from bulk polymerization is difficult because PRx with αCD and PEG was not dissolved in vinyl monomers. Previously, we reported polymeric materials with movable cross-linker to prepare hydrogels with PRx59,61 or a [c2] daisy chain molecule.60 The material with the [c2] daisy chain molecule exhibited the light-induced actuation arisen from the sliding motion and high rupture strain after ultraviolet irradiation. The polymeric gel composed of PRx and polyReceived: April 17, 2017 Revised: June 12, 2017

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DOI: 10.1021/acs.macromol.7b00797 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Synthetic scheme of Acryl-AcO-PRx.

groups of αCD were converted to the acetyl group, and 4% of the hydroxyl groups were acrylated in Acryl-AcO-PRx, indicating that the single molecule of Acryl-AcO-PRx has 54 units of the acrylate group. When the acetylation substitutional ratio was less than 50%, Acryl-AcO-PRx did not dissolve in liquid acrylate monomers. 2.2. Preparation of Movable Cross-Linked and Chemically Cross-Linked Materials. Movable cross-linked polymers were prepared by radical copolymerization of Acryl-AcOPRx with EA or nBA in bulk conditions (Figure 2). As

(acrylamide) cross-linked with boronate linkages showed effective self-healing properties combined with chemical and physical properties. However, we have not prepared PRx elastomer from bulk polymerization. To expand the range of adaptation of attractive features of topological cross-linkage, it is more preferable to use a mixed system with other polymer that uses only a small amount of polyrotaxane. It is useful if it can improve stretchability and toughness while maintaining physical properties of main polymer. As a method to satisfy these requirements, we decided to prepare a new type of acrylated PRx with αCD and PEG (Acryl-AcO-PRx) as a polymerizable cross-linker. AcrylAcO-PRx was actually copolymerized with some abundant acrylates without polymerization solvent. Here we investigated the effects of movable cross-linking structure on mechanical properties for the obtained cross-linked polymer.

2. RESULTS AND DISCUSSION 2.1. Preparation of Hydrophobic Acrylated Poly(rotaxane). Previously, some of the movable cross-linked polymers (PRx materials) have been prepared from organic media40−50 or aqueous51−54 and reactive blend between PRx and polymers.55 Bulk polymerization with PRx monomer is interesting method to apply constructional materials; however, it is difficult to prepare PRx materials by bulk polymerization because PRx based on αCD and PEG (αCD−PEG PRx) did not dissolve in organic media due to a hydrophilic property. The modification of suitable functional groups to αCD−PEG PRx is essential to dissolve in organic media. We focus primarily on solubilization of αCD−PEG PRx in acrylate monomers such as ethyl acrylate (EA) and butyl acrylate (nBA). Prior to polymerization of acrylate monomers with αCD−PEG PRx, we prepared an acrylated PRx with αCD and PEG (Acryl-AcO-PRx) from a hydroxylpropyl group modified αCD−PEG PRx (HPPRx).55 The adamantyl group at both ends prevented the dethreading αCD units of HPPRx. The preparation of Acryl-AcO-PRx proceeded in two stages: acrylation and acetylation. Acryloyl chloride reacted with HPPRx to give an acrylated HPPRx. The acrylated HPPRx converted to Acryl-AcO-PRx by reacting with the acetylation of HPPRx with acetic anhydride. Acryl-AcO-PRx dissolved in liquid acryl monomers. The solubility of Acryl-AcO-PRx depends on the modification ratio of the acetyl group. AcrylAcO-PRx has Mw = 20K of PEG (450 units of ethylene glycol) and 74 units of αCD threaded onto PEG (penetration ratio: 33% per the diethylene glycol unit). 66% of the hydroxyl

Figure 2. Chemical structures of movable cross-linked materials with Acryl-AcO-PRx (PEA-AcO-PRx and PBA-AcO-PRx) and chemical cross-linked materials (PEA-BDA and PBA-BDA).

described above, Acryl-AcO-PRx dissolved in EA or nBA with a shaking apparatus. A mixture of 3 wt % of Acryl-AcO-PRx with acrylates and a photoinitiator (Irgacure 184) was poured into the dumbbell-shaped Teflon mold. UV irradiation (λ = 365 nm) for an hour cured the mixed solution to afford colorless transparent solid materials. Acryl-AcO-PRx with poly(EA) as a main chain is called PEA-AcO-PRx PEA-AcO-PRx, and B

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Macromolecules similarly that with poly(BA) is called PBA-AcO-PRx. PEA-AcOPRx and PBA-AcO-PRx have 0.10 mol % of cross-linking unit from Acryl-AcO-PRx and 99.90 mol % of poly(EA) or poly(BA) units. As a control sample, chemically cross-linked polymers were prepared in a similar manner replacing AcrylAcO-PRx to butane diacrylate (BDA) with the same molar cross-linking ratio corresponding to the topologically crosslinked one, which is called PEA-BDA and PBA-BDA. Without Acryl-AcO-PRx or BDA, photopolymerization of EA or nBA with Irgacure 184 gave Mn = 568K g/mol of polymers, which were characterized by gel permeation chromatography using DMSO as an eluent. Mn = 570K g/mol of the linear polymers will be cross-linked by Acryl-AcO-PRx or BDA in these materials. 2.3. Swelling Test. To investigate the effect of the physical property of the materials with the movable cross-linking or the chemical cross-linking (permanent cross-linking) units, swelling properties of PEA-AcO-PRx, PBA-AcO-PRx, PEA-BDA, and PBA-BDA were evaluated by an immersion treatment with DMF. The weight changes of PEA-AcO-PRx, PBA-AcO-PRx, PEA-BDA, and PBA-BDA were measured as the swelling ratio Q defined as follows:

Figure 4. Tensile test of cross-linked PEAs and PBAs. Stress−strain curves for PEAs (a) and PBAs (b). Dashed lines represent the predictions of the neo-Hookean model with the calculated Young’s modulus.

test sample: see in Figure S6), which were analyzed by a creep meter at a tensile speed of 1 mm/s. The initial sample length was set to 20 mm. The rupture stress of the movable cross-linked materials, PEA-AcO-PRx and PBA-AcO-PRx, was higher than that of chemical-cross-linked materials, PEA-BDA and PBA-BDA. The rupture strain of the movable cross-linked materials reached to about 600%, which was much higher than that of chemicalcross-linked materials (about 200%). Although both polymers have almost same Young’s modulus, movable cross-linked materials showed significantly larger rupture stress and strain than chemically cross-linked materials. Mechanical properties are summarized in Table 1. When applying the load, the

Q = (Wswollen − Winitial) × 100/Winitial [%]

Winitial and Wswollen are the weights of test pieces before and after immersion in dimethylformamide (DMF). Figure 3 shows the swelling ratio of PEA-AcO-PRx, PBAAcO-PRx, PEA-BDA, and PBA-BDA. The movable cross-linked

Table 1. Summary of the Tensile Test polymer name PEA-AcO-PRx PEA-BDA PBA-AcO-PRx PBA-BDA

rupture strain (%) 580 230 720 140

± ± ± ±

160 10 60 10

rupture stress (kPa) 1380 470 180 150

± ± ± ±

420 80 20 30

Young’s modulus (kPa) 580 540 140 210

± ± ± ±

30 50 10 20

movable-cross-linked materials demonstrated high rupture stress and strain due to the dispersion of applying stress, not focusing stress inside the materials. These results indicate that the introduction of the movable cross-linking unit, Acryl-AcOPRx, plays an important role to improve mechanical property of PEA- or PBA-based materials. 2.5. Stress Relaxation Properties of Movable CrossLinked Materials. To clarify the effect of the movable crosslinking and anchored chemical cross-linking points, a stress relaxation test was conducted. Figure 5 shows the process of relaxing the stress of the movable cross-linked materials (PEA-

Figure 3. Swelling ratio of movable cross-linked materials with AcrylAcO-PRx (PEA-AcO-PRx and PBA-AcO-PRx) and chemical crosslinked materials (PEA-BDA and PBA-BDA) with DMF.

materials, PEA-AcO-PRx and PBA-AcO-PRx, showed larger swelling ratio as compared with the chemically cross-linked materials, PEA-BDA and PBA-BDA, respectively. Especially, the difference of the swelling ratio between PBA-AcO-PRx and PBA-BDA was larger than that between PEA-AcO-PRx and PEA-BDA. The anchored cross-linking point in the polymer network caused the small swelling property of chemical crosslinked materials. On the other hand, the movable cross-linked materials demonstrated larger swelling property because the movable cross-linking point relaxes the stress concentration in the increase in swelling pressure inside the materials. These results suggest that movability of cross-link points was exhibited in the swelled state. 2.4. Characterization of Mechanical Properties of Movable Cross-Linked Materials. Mechanical strength of PEA-AcO-PRx and PBA-AcO-PRx were evaluated by tensile test comparing with chemically cross-linked PEA-BDA and PBA-BDA. Figures 4a and 4b show the stress−strain curves of the PEA-AcO-PRx and PBA-AcO-PRx (size of dumbbell shape

Figure 5. Stress relaxation of PEAs at 150% strain (a) and PBAs at 100% strain (b). C

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relaxation and deformation hysteresis seems to suggest sliding motion of cross-linking points at deformation.

AcO-PRx and PBA-AcO-PRx) and chemical cross-linked materials (PEA-BDA and PBA-BDA) while keeping the strain at 150% at PEA materials or 100% at PBA materials. For comparison, measured stress was normalized by initial stress. PEA-AcO-PRx showed a large stress relaxation over a longer period of time. At equilibrium in 1800 s, stress relaxation reached to about 50%. PEA-BDA relaxed less than 20% stress and reached equilibrium in shorter time. In PBA materials, these differences were exhibited more clearly. Although PBABDA shows similar relaxation process to PEA-BDA, stress relaxation of PBA-AcO-PRx reached to more than 80% at equilibrium. The difference of stress relaxation between PEAAcO-PRx and PBA-AcO-PRx is dependent on glass transition point of the base materials. More importantly, the stress relaxation showing PEA-AcO-PRx and PBA-AcO-PRx derived from the movable cross-linked point. 2.6. Cyclic Tensile Test. To obtain further insight into the movable cross-linking and anchored chemical cross-linking points, we investigated the deformation hysteresis by cyclic tensile tests. Maximum strain was set to 100%, 200%, 300%, 400%, and 500% at deformation rate of 1 mm/s. Test pieces were continuously stretched and recovered without interval. Figures 6a and 6c show that both PEA-AcO-PRx and PBAAcO-PRx clearly exhibited hysteresis even at deformation with

3. CONCLUSION We successfully prepared the polymerizable polyrotaxane crosslinker (Acryl-AcO-PRx) and obtained movable cross-linked polymeric materials by radical copolymerization of Acryl-AcOPRx with various vinyl monomers in the bulk state. Previously, the bulk polymerization of vinyl monomers with hydrophilic αCD−PEG PRx or HPPRx does not succeed because αCD− PEG PRx and HPPRx did not dissolve in liquid vinyl monomer. This bulk polymerization of Acryl-AcO-PRx with vinyl monomer effectively introduces into the polymeric materials. Evaluation of the mechanical properties of the obtained polymeric materials revealed greatly improved stretchability and toughness, although it contains only 3 wt % of polyrotaxane. Acryl-AcO-PRx works as cross-linker to give unique mechanical properties to polymeric materials. Using polyrotaxane as cross-linker for connecting universal polymers enables introducing the feature of slide ring materials into abundantly used polymeric materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00797. Materials and methods, detailed polymerization procedures, association constants, mechanical property of hydrogels, and self-healing efficiency in supplemental data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.H.). ORCID

Hiroyasu Yamaguchi: 0000-0002-4801-5071 Akira Harada: 0000-0002-9309-5939 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan), a Grant-in-Aid for Scientific Research on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206), a Grant-in-Aid for Scientific Research (B) (No. 26288062) from MEXT of Japan, and a Research Grant Program of the Asahi Glass Foundation.

Figure 6. Cyclic tensile test for PEA-AcO-PRx (a), PEA-BDA (b), PBA-AcO-PRx (c), and PBA-BDA (d).

low strain. In particular, large energy loss was observed at the deformation of PBA-AcO-PRx. On the other hand, chemical cross-linked materials, PEA-BDA and PBA-BDA, exhibited significantly small hysteresis and elastic deformation due to fixing cross-linking points. PEA-BDA and PBA-BDA broke before reaching to plastic deformation. Therefore, chemical cross-linked materials cannot disperse the loading stress inside the materials. However, the movable cross-linked materials can disperse it by sliding motion of αCD on the PEG chain. The recovery of stress and residual strain of PEA-AcO-PRx was confirmed by a cycle tensile test. The recovery ratio of PEAAcO-PRx showed an average 74% with healing interval (results shown in Figure S9). Even with a cross-linked structure composed of covalent bonds, confirmation of such a clear stress

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DOI: 10.1021/acs.macromol.7b00797 Macromolecules XXXX, XXX, XXX−XXX