Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Catechol-Mediated Glycidylation toward Epoxy Vitrimers/Polymers with Tunable Properties Shou Zhao† and Mahdi M. Abu-Omar*,†,‡ †
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Department of Chemistry & Biochemistry, University of California, Santa Barbara, Building 232, Santa Barbara, California 93106, United States ‡ Department of Chemical Engineering, University of California, Santa Barbara, Engineering II Building, Santa Barbara, California 93106, United States S Supporting Information *
ABSTRACT: This work demonstrates a series of epoxy vitrimers/polymers whose properties can be tuned by incorporating a catechol moiety as a part of the starting bisphenol motif. Glycidylation of the catechol containing bisphenol yielded a single-epoxide derivative (E1) and a triepoxide (E3) with tunable ratios through a two-step synthetic approach. E1 creates a dangling group (benzodioxane) within the network. By comparison, E3 with higher functionality (n = 3) affords networks with improved crosslinks. By controlling the E1/E3 ratio of epoxy prepolymers, the properties of the resulting vitrimers/polymers can vary significantly. Polymers with the E1/E3 ratio of ca. 6.1 (86:14) exhibited high strain, excellent self-healing properties, and fast stress relaxation that made them readily reconfigurable and could be used as hot-melt adhesives that permitted debonding on demand. Networks with higher content of E3 (e.g., E1/E3 ratio of ca. 0.6 (36:64)), by comparison, exhibited significantly improved thermomechanical properties like tensile strength and modulus.
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INTRODUCTION Mechanical properties, thermal stability, and solvent resistance of thermosets have several advantages over thermoplastics because of thermosets’ cross-linked structure.1 However, the downside that comes with these advantages is irreversible covalent cross-links, which renders thermosets nonprocessable once synthesized. In an attempt to convert conventional thermosets into less inert and more responsive materials, Leibler and co-workers developed the concept of vitrimers.2 A typical vitrimer macromolecule can be prepared by reacting multifunctional carboxylic acid with an epoxy prepolymer to create a repeating unit that contains both hydroxyl and ester groups. Within the network, free hydroxyls and esters can undergo a transesterification reaction at an elevated temperature with the aid of a preloaded catalyst. At sufficient temperature, the topology of the network can be rearranged, leading to thermal malleability. The rearrangement of a vitrimer makes use of the associative exchange mechanism, with cross-link density within the network remaining constant.3−5 As a result, vitrimers, irrespective of whether they utilize the classic ester function2,6−13 or other functions like vinylogous urethane,14−16 alkylated poly(thioether),17 dioxaborolanes,18 polyhydroxyurethanes,19 and disulfide,20,21 are insoluble in most common organic solvents but exhibit sufficient ability to be reshaped, self-healed, recycled, and reprocessed after curing. © XXXX American Chemical Society
The preparation of vitrimers with tunable mechanical, malleable, and self-healing properties is of major interest in materials science. Generally, these properties are related to cross-link density, the exchange reaction kinetics, density of exchangeable groups, and the intrinsic rigidity of monomers.3 Several methods have been employed for tuning the properties of epoxy-acid/anhydride vitrimers through (1) adjusting the hardeners (e.g., short-chain anhydrides vs long-chain fatty acids),2 (2) varying the content and nature of catalysts,22,23 (3) regulating the stoichiometry of epoxy vs acid/anhydride,13 and (4) modifying the properties of epoxy prepolymers,24 which significantly affect the properties of the resulting network. However, the development of novel epoxy vitrimer monomers, especially those that can yield vitrimers with tunable properties, is often neglected. This is supported by the fact that the petroleum-based diglycidyl ether of bisphenol A (DGEBA) is still predominately employed for vitrimers syntheses.2,6−8,11,12 It is noteworthy that the glycidylation reaction between a phenol moiety and epichlorohydrin (ECH) yields only one major product, phenyl glycidyl ether. Thus, the synthesis of a property-tunable epoxy prepolymer using only one starting phenol is rarely reported. The conventional Received: February 15, 2019 Revised: April 4, 2019
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DOI: 10.1021/acs.macromol.9b00334 Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
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Article
RESULTS AND DISCUSSION Catechol-Incorporated Bisphenol. Catechol-incorporated bisphenol was synthesized through the condensation of VA and MC. As seen in Figure 1, the methylene bridge of
approach requires at least two starting phenols with varying rigidity and functionality. For example, in our previous work, a triphenylmethane-type polyphenol with high rigidity and functionality (n = 5) was employed in the synthesis of epoxy thermoset with high glass-transition temperature (Tg = 167 °C).25 To tune the network toward more facile processing properties, Tg was decreased to 82 °C by introducing another fatty acid-based flexible epoxy co-monomer. In another example, Zhang and co-workers developed a high Tg (187 °C) vitrimer from triguaiacolmethane-based epoxy prepolymer (TEP) with high rigidity and functionality (n = 3).24 Using a 1:1, by weight, mixture of TEP and DGEBA (lower rigidity and functionality, n = 2), the Tg was decreased to 172 °C. Complementary to the conventional approach described above, which requires at least two starting phenols, we report herein a series of epoxy prepolymers whose properties can be tuned by using the same starting phenol in a single epoxidation step. A bisphenolic compound (VAMC) based on reacting vanillyl alcohol (VA) with 4-methylcatechol (MC) (Scheme 1), both can be obtained from renewable sources, is Scheme 1. Chemical Structure of Catechol-Incorporated Bisphenol (VAMC), Epoxy Prepolymers (Monoepoxy Monobenzodioxane-Substituted, E1, and TriepoxySubstituted, E3), and Carboxylic Acid Hardeners
Figure 1. (a) Proton and (b) carbon NMR spectra of VAMC. Solvent: acetone-d6.
VAMC was observed at 3.72 ppm in the 1H NMR spectra and at 38.8 ppm in the 13C spectrum, which suggested the successful linkage between VA and MC. According to our previous study, the phenolic para site of MC is significantly more reactive than the ortho sites for condensation with hydroxymethyl.25 As a result, para-substituted bisphenol was the main product. The X-ray structure of VAMC was determined to confirm the structure (Figure 2).
described.25,26 VAMC possesses a unique structure with one phenol and one catechol moiety. Unlike phenol that always yields predominant glycidyl ether when reacted with epichlorohydrin, glycidylation of catechol with epichlorohydrin results in two products: benzodioxane and diglycidyl ether (Scheme 1). These two products are significantly different in terms of functionality. Benzodioxane should not be considered as a reactive functional group for epoxy network synthesis since its hydroxyl group is inert under common curing conditions. This is in agreement with the fact that β-OH formed by ring opening of epoxy is seldom considered cross-linkable. By comparison, diglycidyl ether possesses two epoxides, which make the network more constrained and cross-linked. In this study, factors that determine the ratio of benzodioxane and diglycidyl ether are first investigated using a two-step glycidylation method. By adjusting these ratios through a controlled synthesis, a series of vitrimers are prepared, whose reprocessing properties can be readily tuned on demand.
Figure 2. X-ray structure of VAMC. Crystal was obtained from the slow evaporation of an acetone solution at room temperature. The phenolic para site of 4-methylcatechol is the preferred site for condensation with vanillyl alcohol.
Glycidylation of VAMC To Make Epoxy Prepolymers with Tunable Functionality. VAMC was then glycidylated with epichlorohydrin to prepare epoxy prepolymers. Two products were obtained: monoepoxide (E1) and triepoxide (E3) (Schemes 1 and 2). Using preparative thin layer chromatography, E1 and E3 were isolated, and their structures were confirmed by 1H, 13C, and 2D-HSQC NMR spectra (Supporting Information, Figures S1−S4), mass spectroscopy (Figures S5 and S6), infrared (IR) spectra (Figure S7), and high-performance liquid chromatography (HPLC, Figure S8). According to previous studies, the benzodioxane moiety was B
DOI: 10.1021/acs.macromol.9b00334 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 2. Glycidylation of Catechol Moiety with Epichlorohydrin Yields Two Products: Benzodioxane and Diglycidyl Ethera
a
Ratio of benzodioxane and diglycidyl ether products is determined by the monochlorohydrin, dichlorohydrin, and monoglycidyl ether intermediates.
Table 1. Effects of Reaction Conditions on the Molar Ratio of E1/E3
step 1 a
step 2
entry
VAMC (equiv)
ECH (equiv)
TBAB (equiv)
t1 (h)
NaOH (equiv)
t2 (h)
Tb (°C)
molar ratioc (E1/E3)
1 2 3 4 5 6
1 1 1 1 1 1
28 28 28 28 28 28
0.085 0.085 0.085 0.085 0.085 0.085
3.5 0.5 24 0.5 0.5 3.5
3.0 3.0 3.0 1.5 6.0 1.5
1.5 1.5 1.5 1.5 1.5 1.5
60 60 60 60 60 60
49:51 86:14 36:64 60:40 95:5 42:58
a
Weight ratio of VAMC and epichlorohydrin was set to 1:10. bTemperature was kept same for both steps. cMeasured by HPLC.
The reaction time of step 1 (t1) had a significant impact on E1/E3 ratio. For example, in entry 2, t1 was reduced to 0.5 h and the E1/E3 ratio was increased to ca. 6.1 (86:14). Increasing t1 to 24 h (entry 3), however, decreased the E1/E3 ratio to ca. 0.6 (36:64). Variation of E1/E3 ratio is related to the relative content of chlorohydrin intermediates (Scheme 2): when t1 was relatively short (i.e., 0.5 h, entry 2), catechol was mainly monosubstituted due to steric hindrance, and monochlorohydrin was the major intermediate; increasing t1 to 24 h, on the other hand, promoted the disubstitution of catechol and rendered dichlorohydrin as the major intermediate. Besides t1, alkaline/aromatic OH ratio in step 2 also affected the E1/E3 ratio. For entries 2, 4, and 5 (in which t1 was short and thus monochlorohydrin was the dominant intermediate), as NaOH equivalent increased from 1.5 to 6.0, the E1/E3 ratio increased from 60:40 to 95:5. It is noteworthy that monochlorohydrin could simultaneously undergo two reactions after the addition of NaOH (Scheme 2): (1) base-
converted from monochlorohydrin and monoglycidyl ether intermediates in the presence of alkaline (Scheme 2).27,28 By comparison, the diglycidyl ether moiety was mainly derived from a dichlorohydrin intermediate (Scheme 2).27 Thus, the molar ratio of E1 and E3 can be adjusted by controlling the ratio of these intermediates. A two-step glycidylation method was, thus, employed for controlling the ratios of E1 and E3: in the first step, VAMC was reacted with epichlorohydrin to form the chlorohydrin intermediates; in the second step, an alkaline solution was introduced for ring-closing reactions. Experimental conditions of each step are listed in Table 1. In all reactions, the molar ratio of VAMC/epichlorohydrin was set to 1:28 (corresponding weight ratio of 1:10). Excess epichlorohydrin helped reduce the viscosity and hydrolyzable chlorine content of epoxy prepolymers.26,29 In entry 1, the first step was kept at 60 °C for 3.5 h, followed by the second step in which the molar ratio of NaOH to aromatic OH was set to 1:1, and the mixture was kept at 60 °C for another 1.5 h. The molar ratio of E1 to E3 was measured to be ca. 1 (49:51) by HPLC. C
DOI: 10.1021/acs.macromol.9b00334 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 2. Thermal and Mechanical Properties of VAMC-Derived Epoxy Vitrimers/Polymers vitrimer/polymer
stress (MPa)
strain (%)
Tga (°C)
Tαb (°C)
Tonsetd (°C)
Td30e (°C)
self-healing efficiency (%)f
adhesive strength (MPa)
activation energy (kJ/mol)
36 60 86 (1:1) 86 (1.15:1) 86 (1:1.15)
25.1 10.3 2.7 3.0 0.18
97 187 620 565 2250
23 18 16 12 11
38 34 NAc NAc NAc
354 339 313 309 300
446 442 439 440 440
NAg NAg 92 95 97
NAh NAh 1.6 1.4 0.2
59.5 24.3 NAi NAi NAi
a Measured by differential scanning calorimetry (DSC). bMeasured by dynamic mechanical analysis (DMA). c86-derived samples underwent fast moduli drop. Irregular and fluctuated tan δ curves were observed, which made the data unreliable (Figure S22). dMeasured by thermogravimetric analysis (TGA), onset degradation temperature, which is the temperature at 5% weight loss. eTemperature at 30% weight loss. fSelf-healing efficiency was calculated to be 100% × strain (healed sample)/strain (original sample). gSelf−healing efficiency was not applicable for comparing flat original samples and lap−welded samples. h36 and 60 could not be used as solid adhesives for binding aluminum sheets. i86-derived polymers underwent sudden moduli drop, which did not follow the Arrhenius law.
indicated a significant conversion. The resulting polymers were also resistant to organic solvents like toluene, tetrahydrofuran (THF), and dimethylformamide (DMF). 36 and 60 exhibited good solvent resistance with
