Catalyst-free self-healing fully bio-based vitrimers derived from tung oil: Strong mechanical properties, shape memory, and recyclability
Introduction
Vitrimers are a class of resins that can change their topology via dynamic bond exchange, resulting in their self-healing and reprocessable molding (Capelot et al., 2012b; Liu et al., 2017a; Pei et al., 2014). At high temperatures, vitrimers, like thermoplastic resins, are processable while at low temperatures they have a stable structure like traditional thermoset resins (Demongeot et al., 2017; Dhers et al., 2019; Snyder et al., 2018). It is because of the combination of their thermosetting and thermoplastic properties that vitrimers are deemed to be useful materials (Chen et al., 2019; Wang et al., 2018; Wu et al., 2017), and for this reason they have received increasing research attention.
To comply with sustainable development strategies, increasingly more bio-based vitrimers are being developed and widely used in coatings (Hao et al., 2019; Wang et al., 2020), adhesives (Wu et al., 2020a; Zhao and Abu-Omar, 2019), sensors (Niu et al., 2021; Xiong et al., 2020), and as materials for three-dimensional (3D) printing (Shi et al., 2017). Vegetable oil is a class of biomass raw material with a wide source and good biocompatibility. In the last decade, it has been widely used for the preparation of bio-based vitrimers (Auvergne et al., 2014; Gandini et al., 2016). Altuna et al. (Altuna et al., 2013) detailed the curing of citric acid (CA) and epoxidized soybean oil (ESO) to obtain fully bio-based vitrimers that exhibit self-healing and thermally-activated stress relaxation properties without the use of a catalyst. However, the glass transition temperatures (Tg) of the prepared vitrimers were only around 25 °C and their tensile strength values were less than 1 MPa. Wu and colleagues (Wu et al., 2020a) described the curing of ESO with glycyrrhetinic acid to prepare fully bio-based vitrimers that exhibit self-healing, shape memory, and degradable properties, however these materials have tensile strength values of only 3.10 MPa. Liu et al. (Liu et al., 2020b) reported the curing of ESO with 4,4′-dithiodiphenylamine to prepare dynamic disulfide bond-based self-healing vitrimers. However, since the epoxy groups in ESO are intramolecular epoxy groups, which have greater steric hindrance relative to the terminal epoxy groups, resulting in low crosslinking density, the tensile strength values of these materials were only 3.50 MPa. Even under high temperature, high pressure, and catalyst conditions, the low activity of the intramolecular epoxy groups leads to uneven curing, so that the cross-linked network cannot be completely cured (Altuna et al., 2011). This process results in the poor mechanical properties and low Tg of this type of vitrimer, thus limiting their application.
The introduction of terminal epoxy groups into polymers effectively reduces their steric hindrance and increases reactivity in these materials, resulting in a higher density of crosslinks, better mechanical properties, and higher Tg values of the resulting materials (Huang et al., 2013; Zhang et al., 2018). Huang et al. (Huang et al., 2013) demonstrated the introduction of terminal epoxy groups in dimer and acrylpimaric acids, which were then cured with methylnadic anhydride to prepare an epoxy resin with a Tg of 132 °C. Furthermore, Zhang et al. (Zhang et al., 2018) introduced terminal epoxy groups in castor oil derived sebacic acid and solidified it with ozone-treated kraft lignin to synthesize fully bio-based vitrimers. Due to the introduction of terminal epoxy groups, vitrimers with a high density of crosslinks were obtained after the starting materials were cured for only 3 h. The resulting bio-based vitrimers exhibited tensile strength values of up to 12.91 MPa and a Tg of 133 °C. These results confirm the possibility of preparing vegetable oil-based vitrimers that have good mechanical properties from vegetable oils that have terminal epoxy groups as raw materials. On the other hand, the toxicity, poor compatibility, and corrosiveness of the dynamic transesterification reaction (DTER) catalysts in vitrimers has presented a challenge in past research studies (Hao et al., 2020; He et al., 2019; Liu et al., 2019). The introduction of a large number of free −OH groups into such polymers is an effective way of solving this issue (Chakma and Konkolewicz, 2019; Han et al., 2018; Liu et al., 2020a). Natural product-derived CA has three carboxyl groups and one hydroxyl group, and the resulting polymers derived from the use of CA as a curing agent can undergo autocatalytic DTER due to the large number of −OH groups introduced into the materials by CA (Wang et al., 2020).
Tung oil (TO) is a renewable resource extracted from the seeds of the tung tree. It is widely used in coatings (Cao et al., 2015), epoxy resins (Huang et al., 2013) and toughening agents (Xiao et al., 2019) due to its easy modification and good flexibility. Similar to most vegetable oils, the fatty acids in tung oil are in the form of triglycerides and therefore tung oil is readily hydrolyzed. Tung oil contains about 80 % eleostearic acid (9, 11 13–octadecatrienoic acid), which has three conjugated double bonds and can easily undergo Diels–Alder reaction with dienophiles (Hilditch and Mendelowitz, 1951). Furthermore, Diels-Alder reaction adducts of tung oil with acrylic acid, maleic anhydride or fumaric acid are readily epoxidized to obtain epoxy resins with terminal epoxy groups.
In this work, tung maleic triacid (TMTA) derived from tung oil was epoxidized, and TO–based triglycidyl ester TOTGE with terminal epoxy groups was obtained. Catalyst-free self-healing fully bio-based TOTGE–CA vitrimers were synthesized by curing TOTGE with CA that contains −OH groups. Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), tensile testing, and dynamic mechanical analysis (DMA) were used to study the effects that different ratios of carboxyl and epoxy groups and H–bonds have on TOTGE–CA vitrimers. Furthermore, stress relaxation experiments were used to show that the TOTGE–CA vitrimers can achieve effective DTER without the use of any additional catalyst. Thus, the vitrimers show self-healing, shape memory, and reprocessability properties. Moreover, they can be used as repairable and recyclable adhesives.
Section snippets
Materials
TO, containing 80 % eleostearic acid, was obtained from the Institute of Chemical Industry of Forestry Products. Methanol, epichlorohydrin, benzyl triethyl ammonium chloride, and CA were purchased from Aladdin Reagent, China.
Preparation of fatty acid methyl esters
Briefly, TO (260.00 g), methanol (70.00 g), and sodium hydroxide (NaOH, 1.80 g) were added to a four-necked flask and reacted for 2 h at 70 °C. After completion of the reaction, the upper layer of bright yellow liquid in the flask was taken up and dissolved in ethyl acetate
FTIR characterization of TO, TMTA, and TOTGE
Fig. 1A shows an overview of the synthesis route of TOTGE. Methyl esterified TO (A) undergoes a Diels–Alder reaction with maleic anhydride, B is obtained. After the addition of aqueous sodium hydroxide, the maleic anhydride ring is opened and C is obtained, and D is obtained after acidification. Next, D is hydrolyzed under alkaline conditions to give E. TMTA is obtained after E is acidified. Next, TMTA undergoes epoxidation to form TOTGE. Fig. 2 shows the FTIR spectra of TO, TMTA, and TOTGE. In
Conclusion
In this study, the renewable tung oil underwent the methyl esterification reaction, Diels-Alder reaction, and epoxidation reaction to obtain TOTGE with terminal epoxy groups. Furthermore, catalyst-free self-healing fully bio-based vitrimers were prepared via the reaction of TOTGE with CA. Due to the vitrimer networks featuring a high density of crosslinks that form due to the interactions between the terminal epoxy groups and the large number of H–bonds in their structure, these materials
CRediT authorship contribution statement
Ya-zhou Xu: Conceptualization, Methodology, Investigation, Writing - original draft. Pan Fu: Investigation, Analysis. Song-lin Dai: Investigation, Analysis. Hai-bo Zhang: Writing - review & editing. Liang-wu Bi: Data curation. Jian-xin Jiang: Resources, Software. Yu-xiang Chen: Conceptualization, Supervision, Project administration.
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgements
The authors would like to thank the National key research and development program of China (2016YFD0600804) and the National Key Technology R&D Program of China (2015BAD15B08) for financial support.
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