Self-healing and shape-memory properties of polymeric materials cross-linked by hydrogen bonding and metal–ligand interactions

Yuichiro Kobayashi a, Tomohiro Hirase a, Yoshinori Takashima ab, Akira Harada *c and Hiroyasu Yamaguchi *a
aDepartment of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan. E-mail: hiroyasu@chem.sci.osaka-u.ac.jp
bInstitute for Advanced Co-Creation Studies, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
cProject Research Center for Fundamental Sciences, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan. E-mail: harada@chem.sci.osaka-u.ac.jp

Received 25th March 2019 , Accepted 16th July 2019

First published on 17th July 2019


Polymeric materials were prepared by cross-linking them with two independent non-covalent interactions, namely hydrogen bonding and metal–ligand interactions. The fracture energy of the resultant polymeric material is higher than that of the polymeric material without non-covalent interactions as well as that with one non-covalent interaction. Moreover, the prepared polymeric materials show self-healing and shape memory properties.


Functional materials with self-healing and stimuli-responsive properties have attracted much attention to create a sustainable society. One of the most promising methods for producing functional materials is the supramolecular polymerization of low-molecular weight monomeric units by non-covalent interactions.1–5 These supramolecular materials show unique functions due to their reversibility and molecular selectivity. Supramolecular polymeric materials utilizing two non-covalent interactions have also been reported for their functionalization.6–14 In another promising method, the supramolecular polymerization of high-molecular weight monomeric units is developed and various excellent supramolecular polymeric materials consisting of one non-covalent interaction such as hydrogen bonding15 or metal–ligand interaction16 have been reported; however, there are few reports on supramolecular materials consisting of two non-covalent interactions.17,18 If these features can be integrated into one polymer, further functionalization based on multivalent interactions and multiple non-covalent bonds can be expected.

In this paper, we prepared supramolecular polymeric materials based on the self-assembly of a tritopic polymer using the combination of hydrogen bonding and metal–ligand interactions. We synthesized poly(δ-valerolactone)–polylactic acid (PVL-PLA) copolymer containing 2-ureido-4(1H)-pyrimidinone (UPy) and 2,2′-bipyridine (bpy) ([M(bpy)n]2+-UPy film, Fig. 1) as polymer parts. The fracture energy of the [M(bpy)n]2+-UPy film is higher than that of the polymeric material without non-covalent interactions prepared by bpy-PVL-PLA (bpy film). The fracture energy of the [M(bpy)n]2+-UPy film depends on the type of metal ion added. Note that the [Fe(bpy)3]2+-UPy film showed both self-healing and shape memory properties.


image file: c9py00450e-f1.tif
Fig. 1 (a) Preparation of bpy-PVL-PLA-UPy. (b) Schematic illustration of the bpy-UPy film and the [M(bpy)n]2+-UPy film.

bpy-PVL-PLA was prepared by the bulk polymerization of δ-valerolactone (δ-VL) and DL-lactide (DL-LA) using 4,4′-bis(hydroxymethyl)-2,2′-bipyridine (bpy) as an initiation group of polymerization (Fig. 1a and Scheme S1). bpy-PVL-PLA-UPy was obtained by the reaction of 2-[(6-isocyanatohexyl)amino]-6-methyl-4(1H)-pyrimidinone (UPy-NCO)19 with bpy-PVL-PLA. The molecular weight (MW) of bpy-PVL-PLA-UPy was determined from the 1H NMR spectra by comparing the integration values of the protons of the bpy part and the main chain of PVL and PLA (MW = 21[thin space (1/6-em)]000, Fig. S1). Films were prepared by solution casting. A MeOH solution containing the metal ion (M2+) and CH2Cl2 solution of bpy-PVL-PLA-UPy were mixed. The resultant solution was dried to obtain the [M(bpy)n]2+-UPy film (Fig. S2a). Reference samples such as the bpy-UPy film and the bpy film were prepared by the similar method as described above using a CH2Cl2 solution of bpy-PVL-PLA-UPy or bpy-PVL-PLA, respectively (Fig. S2b).

To investigate the formation of hydrogen bonding and metal–ligand interactions in the films, we carried out swelling tests. The bpy-UPy film was dissolved by immersing it into CH2Cl2. In contrast, the bpy-UPy film in the presence of Fe2+ {[Fe(bpy)3]2+-UPy film} was swelled without dissolving (Fig. 2a). The swelling ratio of the [Fe(bpy)3]2+-UPy film decreased by increasing the amount of Fe(BF4)2 added during film preparation (Fig. S3). These results indicate that the metal–ligand complex between bpy and Fe2+ acts as a cross-linking point. In fact, the formation of a 1[thin space (1/6-em)]:[thin space (1/6-em)]3 metal–ligand complex between bpy and Fe2+ was confirmed by UV-Vis measurements, where the addition of Fe(BH4)2 into the bpy-PVL-PLA-UPy solution (Fig. S4) produced a new peak. To demonstrate that the [Fe(bpy)3]2+-UPy film was cross-linked by hydrogen bonding, we performed a competitive experiment. The swelling ratio of the [Fe(bpy)3]2+-UPy film increased on the addition of N,N′-(1,8-naphthyridine-2,7-diyl)didodecanamide (NaPy) as a competitive molecule (Fig. 2a) because hydrogen bonding between UPys in the film dissociated to give hydrogen bonding of UPy with the added NaPy instead of the UPy in the film (Fig. 2b). These results support that the [Fe(bpy)3]2+-UPy film is cross-linked by both hydrogen bonding and metal–ligand interactions.


image file: c9py00450e-f2.tif
Fig. 2 (a) Swelling ratio of the bpy-UPy film and [Fe(bpy)3]2+-UPy film immersed in CH2Cl2 or CH2Cl2 containing NaPy. The bpy-UPy film was dissolved in CH2Cl2, thus there is no swelling ratio. (b) Schematic illustration of swelling of the [Fe(bpy)3]2+-UPy film when the film was immersed in a solution containing NaPy.

To investigate the mechanical properties of the [Fe(bpy)3]2+-UPy film, we carried out tensile tests. Due to brittleness, the tensile test of the bpy film could not be performed (Fig. 3 inset). In contrast, the bpy-UPy film formed a stable film with a fracture energy of 24 MJ m−3 (Fig. 3). These results indicate that introducing hydrogen bonding improves the mechanical properties of the bpy-UPy film. Interestingly, the fracture energy of the [Fe(bpy)3]2+-UPy film was higher than that of the bpy-UPy film (Fig. 3). This is because the loading stress inside the materials was more effectively dispersed by introducing both hydrogen bonding and metal–ligand interactions. The introduction of two independent non-covalent interactions into a material is an effective method to improve the mechanical properties.


image file: c9py00450e-f3.tif
Fig. 3 Fracture energies of the bpy, bpy-UPy, and [Cu(bpy)2]2+-UPy film, and [M(bpy)3]2+-UPy films. The insets show the photographs of the as-prepared bpy and bpy-UPy films. Fracture energy of the bpy film was not obtained because the bpy film was too brittle to withstand the tensile test.

The bpy part formed a 1[thin space (1/6-em)]:[thin space (1/6-em)]3 complex with not only Fe2+ but also other divalent metal ions such as Zn2+ and Co2+. Thus, the fracture energy of [M(bpy)3]2+-UPy should depend on the metal ion. The bpy-UPy film was immersed in a MeOH solution containing Zn(NO3)2·6H2O or Co(NO3)2·6H2O to obtain the [Zn(bpy)3]2+-UPy film and [Co(bpy)3]2+-UPy film, respectively. The order of the fracture energies of the [M(bpy)3]2+-UPy film was Fe2+ > Co2+ > Zn2+ (Fig. 3), which is related to the order of the complexation constant between the bpy and metal ion (log[thin space (1/6-em)]β3) [Fe2+ (17.45) > Co2+ (16.02) > Zn2+ (13.63)].20

image file: c9py00450e-t1.tif

The fracture energy of the [Fe(bpy)3]2+-UPy film is higher than that of the [Co(bpy)3]2+-UPy film, although the log[thin space (1/6-em)]β3 values of Fe2+ and Co2+ are close. The complexation constant on the coordination of bpy to [Fe(bpy)2]2+ (log[thin space (1/6-em)]K3) (9.55) is higher than that to [Co(bpy)2]2+ (4.70).20 This difference is thought to affect the mechanical properties of the films. Moreover, interestingly, the fracture energy of the [Zn(bpy)3]2+-UPy film is higher than that of the [Cu(bpy)2]2+-UPy film in spite of the same complexation constant between the bpy and metal ion [Zn2+ (13.63) and Cu2+ (13.60)]. This is due to the difference in the number of ligands that can be coordinated to the metal. Zn2+ can coordinate three ligands and Cu2+ can coordinate two ligands. Since the structure of the cross-linked point is different between the [Cu(bpy)2]2+-UPy film and [Zn(bpy)3]2+-UPy film, the mechanical properties of the [Cu(bpy)2]2+-UPy film and the [Zn(bpy)3]2+-UPy film are different. The metal ion can control the mechanical properties.

As the [Fe(bpy)3]2+-UPy film is cross-linked by reversible bonds, self-healing properties are expected. The film was scratched with a razor to create a scar, and it was incubated at 50 °C for 30 min. No obvious change was observed in the scar on the bpy film (Fig. 4a and S5b). In contrast, the scars on the bpy-UPy film and the [Fe(bpy)3]2+-UPy film were closed (Fig. 4b and c). These results indicate that the self-healing properties are derived from hydrogen bonding and metal–ligand interactions. To investigate the self-healing properties quantitatively, we determined the self-healing ratio by using the cross-sectional height profiles of 3D laser micrographs. The scar areas of the film were measured immediately after scratching (S0) and incubation (S1). The recovery ratio was determined as S1/(S0 + S1) × 100 (%) (Fig. S5a). The self-healing ratio of the [Fe(bpy)3]2+-UPy film (77%) was higher than that of the bpy-UPy film (51%) (Fig. 4d, S5c, and 5d), confirming that the introduction of the metal–ligand interactions enhances the self-healing properties. In addition, the glass transition temperature (Tg) of the [Fe(bpy)3]2+-UPy film (2.3 °C) is lower than that of the bpy-UPy film (12 °C) due to the introduction of metal–ligand interactions (Fig. S6 and Table S1). This result shows that the mobility of the polymer chain in the [Fe(bpy)3]2+-UPy film is higher than that in the bpy-UPy film. Self-healing occurred by the formation of hydrogen bonding and metal-ligand interaction in polymeric materials. [Fe(bpy)3]2+-UPy had a higher mobility of the polymer chain and a larger number of sites responsible for self-healing than bpy-UPy due to its low Tg and two kinds of non-covalent interactions. Consequently, the [Fe(bpy)3]2+-UPy film showed better self-healing properties than the bpy-UPy film. These results demonstrate that two independent non-covalent interactions inside a material play an important role in realizing effective self-healing.


image file: c9py00450e-f4.tif
Fig. 4 Optical microscopy images of the damaged (a) bpy, (b) bpy-UPy, and (c) [Fe(bpy)3]2+-UPy films measured with a laser microscope before and after heating at 50 °C for 30 min. (d) Self-healing ratio of the cross-sectional area of the scar for the bpy, bpy-UPy, and [Fe(bpy)3]2+-UPy films.

Fig. 5 shows the shape memory properties of our film with the size of 5 mm × 40 mm × 0.1 mm. The bpy-UPy film was bent at 70 °C and then cooled to 25 °C while maintaining its shape to obtain a second temporary bending shape (Fig. 5a and b). The crystal blocks in the bpy-UPy film were melted by heating above the melting temperature (Tm) of the PVL part. Then the PVL part was crystallized by cooling below Tm. As a result, the shape was memorized (Fig. 5f). The bent bpy-UPy film was immersed in a 1 M Fe(BF)4 EtOH solution for 3 h. The film color changed to red and the sample memorized a third temporary chair shape (Fig. 5b–c), indicating the formation of a metal–ligand complex of [Fe(bpy)3]2+ (Fig. 5f). The chair shaped film was immersed in a 1 M bpy EtOH solution for 3 h. The film color became lighter, and the shape returned to the previous one (Fig. 5c and d) because [Fe(bpy)3]2+ in the film dissociated (Fig. 5f). In fact, the color of the solution changed from clear to [Fe(bpy)3]2+-derived red. The bent film returned to its original shape by heating at 70 °C due to the melting of the crystal part of PVL (Fig. 5d5e). This shape memory experiment could be repeated for at least two cycles (Fig. S8). Consequently, our polymeric material shows triple shape memory properties.


image file: c9py00450e-f5.tif
Fig. 5 (a–e) Shape memory experiment. (f) Schematic illustration of the shape-memory mechanism.

This study demonstrates that the introduction of two independent non-covalent interactions into a polymer is a useful strategy to prepare multi-functional and multi-responsive materials. The fracture energy of the resultant polymeric material is higher than those of polymeric materials without non-covalent interactions and with one non-covalent interaction. In addition, the polymeric materials cross-linked by both hydrogen bonding and metal–ligand interactions show both self-healing and shape memory properties. We believe that our methodology will contribute to the development of functionalized materials with two or more non-covalent interactions such as not only hydrogen bonding and metal–ligand interactions but also other interactions such as host–guest interactions.21–24

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research 17H03416 and 15H05807.

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Footnote

Electronic supplementary information (ESI) available: Experimental details, 1H and 13C NMR and mechanical properties. See DOI: 10.1039/c9py00450e

This journal is © The Royal Society of Chemistry 2019