Design and development of trivalent Fe ion-induced novel urushi organogels

doi:10.1016/j.polymer.2020.122835

Polymer

Volume 205, 28 September 2020, 122835

https://doi.org/10.1016/j.polymer.2020.122835 Get rights and content

Highlights

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Organogel based on natural urushiol was prepared in the presence of Fe³⁺ ions.
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Multi-network structure was fabricated via the formation of chemical and coordination bonds.
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Coordination bonds and flexible long side chains of urushiol provide the urushi organogel with high mechanical strength.
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Highly stretchable and durable urushi film was obtained due to the multi-network structure.

Abstract

An organogel based on natural urushiol with a high mechanical strength was prepared in the presence of Fe³⁺ ions at 40 °C. Urushiol obtained from lacquer sap of the lacquer tree, which is a mixture of catechol derivatives having long alkyl side chains, was used as a monomer. The urushi organogels were formed through the polymerization of urushiol and coordination bonds in the presence of Fe³⁺ ions in the urushiol solution. The Fe³⁺ ions acted as catalysts for the polymerization of urushiol, which solves the problem of the bio-enzyme's activity (laccase) being uncontrollable in lacquer sap. The fabrication and properties of urushi organogels with different concentrations of Fe³⁺ ions were investigated. Coordination bonds between Fe³⁺ ion and urushiol acted as additional crosslinking sites to form a multi-network structure, which was confirmed by Raman spectroscopy. The formation of the coordination bonds improved the mechanical strength of the chemically crosslinked urushi organogel, in addition to the entanglement of the long alkyl side chains of the urushiol. A highly stretchable and durable urushi film was obtained owing to the multi-network system of the urushi organogel, which expands the applicability of environmentally friendly natural urushi.

Graphical abstract

Introduction

Organogels are composed of an organic liquid phase in a three-dimensional crosslinked network, and the crosslinking includes physical entanglement, ionic interactions, and chemical crosslinking, etc. Although gels have been investigated for various applications, such as artificial muscles, absorbents, and sensors [1], their poor mechanical properties limit their applications. The mechanical strength of gels (particularly hydrogels) has been improved by controlling the network topologies and/or adding crosslinkers. Organic crosslinkers [2,3] and nanoparticles [[4], [5], [6]], have been used as a crosslinker to improve the mechanical strength of gels. Here, the careful selection of additional components during the formulation of the gels is crucial. Recently, double-network gels exhibited a remarkably high mechanical strength with high stretchability [7]. Therefore, additional crosslinking, e.g., ion interactions and coordination bonds, was suggested for increasing the toughness and fracture energy of hydrogels [1]. For example, a tough polyacrylamide hydrogel was obtained by mixing long-chain polyacrylamide and alginate ionically crosslinked by Ca²⁺, and covalently crosslinked long-chain polyacrylamide improved the mechanical strength [1]. These approaches for fabricating tough and stretchable double-network gels have motivated researchers to investigate other applications of gels.

Recently, Sun et al. reported a porous double-network gel having high toughness and high stretchability [7], which inspired us to prepare a multi-network organogel with excellent mechanical properties using natural urushiol from the lacquer sap of the lacquer tree (Rhus vernicifera). This tree grows in Korea, Japan, and China and provides materials what is known as an “oriental lacquer.” Because oriental lacquer has a superhigh durability, toughness, thermal stability, anticorrosion, and beautiful brilliance (in addition to being an ecofriendly natural material) [8], it has been widely used for centuries as a coating material in the orient. Because of its high durability, oriental lacquerware >1000 years old is still in existence in the orient.

Urushiol—the main component and the basic coating (or film forming) constituent of oriental lacquer—is a mixture of catechol derivatives with an alkyl side chain, which itself has zero to three double bonds. Under the traditional drying (or curing) process at room temperature and high humidity, urushiol is polymerized to form urushi by the bio-enzyme (laccase) in the lacquer sap. This process is complicated by the oxidation polymerization of urushiol which is followed by a coupling reaction and an autoxidation reaction on the unsaturated double bond in the side chain [9]. Although this is an ecofriendly process, the long reaction time and the limited penetration of O₂ into the bulk of the urushiol to participate in the polymerization (which starts on the surface of the urushiol) restrict the use of highly durable urushi in applications other than coating.

The main structure of urushiol is catechol, which is considered to have great potential for various applications owing to its useful properties. However, the investigation and exploitation of urushi for applications other than coating is restricted by the tricky properties of the bio-enzyme laccase. Therefore, studies have been performed to find a new catalyst and/or curing mechanism using thermal treatment, ultraviolet irradiation, etc. for large-scale applications, as urushi has high thermal stability and high durability even under harsh acidic/basic conditions [8,[10], [11], [12], [13], [14], [15]].

Recently, it is reported that Fe³⁺ cations in seawater play a significant role in the adhesion of blue mussels (Mytilus edulis, common mussel) having a catechol functional group in 3,4-dihydroxyphenylalanine (DOPA) [16,17]. Adhesion to rocks is achieved through noncovalent coordination bonds of the catechol in DOPA with inorganic oxides [16,17]. The Fe³⁺ ions trigger the oxidation of catechol to quinone, and the active quinone reacts to form a polymer [18]. A comprehensive review of catechol crosslinking mechanism has been given elsewhere [18]. This inspired us to use Fe³⁺ ion as not only a catalyst for the urushiol polymerization but also a noncovalent coordination bonding site with urushi (ol), to replace the liable to loss the activity of laccase catalyst and form the urushi polymer network, as urushiol is a catechol derivative with a long alkyl side chain.

The objective of this study was to prepare a chemically and physically multi-crosslinked urushi organogel via a simple procedure. The primary urushi organogels chemically crosslinked via the polymerization of urushiol were added with secondary crosslinking points formed by physical coordination bonds with Fe³⁺ ions, as shown in Scheme 1. It is expected that noncovalent interactions such as coordination bonds, as well as entanglement of long side chains of the urushiol, can synergistically help the supramolecular gel maintain a three-dimensional network structure with a high mechanical strength.

Additionally, Fe³⁺/urushi-based supramolecular organogels can be used as a structural material, as urushi has high durability, and chemical inactivity.

Section snippets

Materials and reagents

Analytically pure ferric chloride hexahydrate (FeCl₃·6H₂O, 97%) and ethylenediaminetetraacetic acid disodium salt dihydrate (99.0%–101.0%, Sigma–Aldrich) were purchased from Sigma–Aldrich, Korea and used without further purification. Acetone (99.9%) was purchased from Samchun, Korea. Lacquer was purchased from Fujii Urushi Kogei Co. Ltd., Japan. Urushiol was extracted from the lacquer using acetone; a 20 wt% lacquer/acetone solution was prepared and stirred for at least 3 h. The upper

Results and discussion

Urushiol dissolved in acetone yielded a gel, whose urushiol content varied in the range of 50–80 wt% and different ratio of urushiol to Fe³⁺ ions. A homogeneous urushi organogel was fabricated at 40 °C using a Fe³⁺ catalyst. The gelation phenomena suggested that the crosslinking polymerization reaction of urushiol was triggered by the Fe³⁺ ion, leading to the growth of covalent and coordination bonds, which is also reported [21]. The gelation time was determined by the visual observation of

Conclusion

For the first time, we fabricated a multi-network urushi organogel having excellent mechanical performance by forming chemical and physical bonds via a simple procedure.

Natural urushiol, which is a catechol derivative having long alkyl side chains, was selected owing to the unusual and useful properties of catechol. The urushiol was polymerized and formed a gel phase directly with Fe³⁺ ions, without any catalyst or crosslinkers. Multi-crosslinking was formed by the chemically crosslinked urushi

CRediT authorship contribution statement

Myeongjin Ko: Investigation, Methodology, Data curation, Visualization, Investigation. Jiyoon Jung: Methodology, Data curation, Validation, Investigation. Seung Sang Hwang: Writing - review & editing, Supervision, Resources. Jongok Won: Conceptualization, Writing - original draft, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning [grant numbers NRF-2017R1A2B4004737 and NRF-2020R1A2C1003562].

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Design and development of trivalent Fe ion-induced novel urushi organogels

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and reagents

Results and discussion

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Science

Phys. Solids

Prog. Org. Coating

Prog. Org. Coating

Chem. Eng. J.

J. Membr. Sci.

Prog. Org. Coating

Prog. Org. Coating

Mater. Lett.

The polyrotaxane gel: a topological gel by figure‐of‐eight cross‐links

Adv. Mater.

Extremely stretchable thermosensitive hydrogels by introducing slide-ring polyrotaxane cross-linkers and ionic groups into the polymer network

Nat. Commun.

Nanocomposite hydrogels: a unique organic–inorganic network structure with extraordinary mechanical, optical, and swelling/deswelling properties

Adv. Mater.

Mechanical Properties and Structure of polymer−clay nanocomposite gels with high clay content

Macromolecules

Highly stretchable and tough hydrogels

Nature

On the UV-induced polymeric behavior of Chinese lacquer

ACS Appl. Mater. Interfaces