Hard, tough and fast self-healing thermoplastic polyurethane

https://doi.org/10.1016/j.porgcoat.2021.106409Get rights and content

Highlights

  • Hard and tough thermoplastic polyurethane.

  • Thermoplastic polyurethane was prepared from mixed isocyanates.

  • Young's modulus, tensile strength and elongation at break of PCL-HM can reach 76 MPa, 11.8 MPa and 841%, respectively.

  • PCL-HM can quickly recover to 92% of its original tensile strength within 1 h at 50 °C.

Abstract

It's necessary to develop self-healing polymers, which will significantly improve the safety and service life of artificial materials. Although extensive research efforts have been devoted to the field, it is still a challenge to synthesize hard and tough polymers, which own self-healing ability under mild conditions. In this work, we fabricated a hard and tough thermoplastic polyurethane (TPU, PCL-HM) and the wound of PCL-HM can be quickly healed under mild conditions (50 °C). Comprehensive characterization results suggest that the Young's modulus, tensile strength and elongation at break of PCL-HM can reach 76 MPa, 11.8 MPa and 841%, respectively, which is better than most recently reported TPUs. PCL-HM prepared with aromatic disulfide bonds as dynamic covalent bonds, polycaprolactone diol (PCL-2000) as soft segment, isophorone diisocyanate (IPDI) and dicyclohexylmethane-4,4′-diisocyanate (HMDI) as hard segment, can quickly recover to 92% of its original tensile strength within 1 h at 50 °C. Due to the better segment movement and looser packing of aromatic disulfide in PCL-HM, the PCL-HM displays better healing properties compared with other types of TPUs (PCL-I, PCL-H, PCL-M, PCL-HM′) prepared in this work. In addition, the fast healing properties of PCL-HM under mild conditions endow it with great application prospects in the field of protective coatings. The successful preparation of PCL-HM also provides a new idea for the synthesis of hard and tough TPU.

Graphical abstract

PCL-HM prepared by two different kinds of alicyclic isocyanates, which is hard and tough and can be quickly repaired at 50 °C.

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Introduction

In nature, animals and plants can spontaneously heal their physical damage, thereby increasing their survivability and longevity, while man-made polymer materials always fail and become useless after being damaged or broken. For decades, scientists have dreamed of developing self-healing polymer materials to improve their safety, service life and reduce maintenance costs, which have potential applications in many fields, such as aviation, biomedical materials, surface coatings, and smart sensors [[1], [2], [3], [4]]. However, it is still a challenge to synthesize polymer materials that are hard and tough and can heal its wounds under mild conditions [5,6].

Dynamic reversible covalent bonds (disulfide bond [7,8], Diels-Alder reaction [9,10], ester bond [11,12], transcarbamoylation reaction [13,14], etc.) or dynamic reversible non-covalent bonds (hydrogen bond [15,16], host-guest interaction [17,18], coordination bond [19,20], π-π stacking [21], etc.) are usually introduced into the polymer chain to fabricate self-healing material. These chemical bonds can be reversibly broken and reorganized when heated to promote self-healing. Besides, scientists have also prepared self-healing polymers triggered by light [22], electricity [23], and microwave [24] and so on.

As we all know, disulfide bond is an interesting dynamic covalent bond that can initiate dynamic exchange under heat, light or reducing agent [25]. In particular, the bond dissociation energy of disulfide bond is low, which makes it possible to achieve dynamic exchange under mild conditions. There are many reports on the use of disulfide bond as dynamic covalent bonds to prepare self-healing polyurethane in recent years. Wang et al. [26] synthesized thermoplastic polyurethane (TPU) using polytetramethylene ether glycol (PTMEG) as soft segment, bis(2-hydroxyethyl)disulfide (HEDS) and m-xylylene diisocyanate (XDI) as hard segment. The Young's modulus of the TPU can reach 19.94 MPa and its mechanical strength can be restored 37% after healing at 80 °C for 24 h. Nevejansa et al. [27] prepared a series of waterborne polyurethanes (PU) by modifying bis(4-aminophenyl) disulfide. The ultimate tensile strength of these polyurethanes can achieve 10–23 MPa, and they have good healing efficiency at 80 °C. Chang et al. [28] used 4-aminophenyl disulfide, poly (ethylene glycol) (PEG) and PTMEG to prepare a transparent and highly stretchable PU, which can be restored 93% of its original mechanical strength at 80 °C for 24 h. Among the TPU currently prepared, the Young's modulus of the TPU with a lower healing temperature is relatively low, and the TPU with a higher Young's modulus requires a higher healing temperature. Therefore, it is still a huge challenge to prepare high-modulus TPU that can be healed quickly under mild conditions [29,30].

Polycaprolactone (PCL) is a semi-crystalline polymer with biodegradability and biocompatibility. Moreover, PCL has better flexibility and process-ability due to the existence of abundant ester bonds, and is widely used in environmental protection materials and medical fields [31]. Besides, employing PCL as soft segment could impart TPU with higher Young's modulus and a certain shape memory performance owing to its good crystallization property. A large number of studies also have found that shape memory can promote the healing of polymer wounds to a certain extent [32]. Hence, employing PCL as soft segment can not only endow TPU with excellent mechanical properties, but also make TPU have outstanding healing properties under mild conditions.

In this work, TPU (PCL-HM) with higher Young's modulus, breaking strength and breaking elongation (76 MPa, 11.8 MPa and 841%, respectively) was prepared through structural design. This TPU can recover 92% of its original breaking strength in 1 h at 50 °C. In PCL-HM, the aromatic disulfide bond is directly connected to the asymmetric alicyclic diisocyanate (IPDI), which benefits for the exchange of the disulfide bond. Introduction of the semi-crystalline soft segment (PCL) ensures that TPU has better segment movement and mechanical strength. Addition of symmetrical alicyclic diisocyanate (HMDI) enhances the mechanical properties of PCL-HM while increasing the molecular weight. Through the coating healing experiment, PCL-HM has potential application prospects in protective coating filed.

Section snippets

Experimental materials

4,4′-diphenylmethane diisocyanate (MDI), isophorone diisocyanate (IPDI), 4,4′-ethylenedianiline (4-ED), dibutyltin dilaurate (DBTDL), hexamethylene diisocyanate (HDI), dicyclohexylmethane-4,4′-diisocyanate (HMDI), and 4-aminophenyl disulfide (4-AD) were all acquired from Aladdin and used as received without further purification. Polycaprolactone diol (PCL-2000, Jining Huakai) vacuumed for 2 h at 100 °C before use. Anhydrous dimethylformamide (DMF) was obtained from the Saen Chemical Technology

Structure characterization

Firstly, the structure of synthesized intermediate and final products was characterized by ATR-FTIR (Fig. 2). In Fig. 2a, I-4-I shows characteristic peaks of amino-group (Nsingle bondH), isocyanate (-NCO) and carbonyl (C=O) at 3356 cm−1, 2261 cm−1 and 1620–1700 cm−1, respectively. Among them, the Cdouble bondO belongs to the urea bond, which means that IPDI-terminated I-4-I was successfully synthesized [33]. In I-4-PCL, because excessive PCL was used to cap I-4-I, the characteristic peaks of hydroxyl (-OH, belongs

Conclusion

In summary, we have developed a hard and tough TPU (PCL-HM), which used crystalline PCL-2000 as soft segment and mixed isocyanates (IPDI and HMDI) as hard segment. Young's modulus, tensile strength and elongation at break of PCL-HM can reach 76 MPa, 11.8 MPa and 841%, respectively. Because of the better polymer chain movement of PCL-HM, it can quickly recover to 92% of its original tensile strength within 1 h at 50 °C. The study also found that the mechanical properties of TPU prepared with

CRediT authorship contribution statement

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgments

The authors gratefully acknowledge the National Key Research and Development Program of China under Grant 2018 [Grant No. YFB1802300], the Strategic Priority Research Program of Chinese Academy of Sciences [Grant No. XDA17020302] and 2019229 Youth Innovation Promotion Association, CAS.

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