Effect of monomer composition on thermal, mechanical, and self-healing properties of thermosets containing Diels-Alder adduct moieties and disulfide bonds

Highlights

• Networks having Diels-Alder (DA) adduct moieties and disulfide bonds were prepared.

• The networks were healable by the DA/retro-DA and disulfide metathesis reactions.

• The scratches of the highest-disulfide network were completely disappeared at 60 °C.

• The retro-DA reaction at 120 °C was effective to heal a higher-DA-fraction network.

• The polymer networks had excellent thermal and mechanical properties.

Abstract

The Diels-Alder (DA) and thiol-maleimide reactions of difurfuryl disulfide (DFDS), 1,4-bismaleimidodiphenylmethane (BMI), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), and thiol-terminated disulfide-containing polyether (LP) at various feed monomer ratios produced polymer networks containing both DA adduct moieties and disulfide bonds. Fourier transform infrared spectral analysis revealed that the DA and thiol-maleimide reactions occurred for all the cured products. A differential thermal analysis revealed that all the cured products exhibited an endothermal peak due to the retro-DA reaction at 115–124 °C. The tensile moduli and strengths of the cured products increased with increasing DFDS (or DA) and PETMP fractions. The elongations at break increased with increasing LP (or disulfide) fraction. Most of the cured products exhibited self-healing ability by annealing at 60 °C for 24 h, and/or at 120 °C for 1 h followed by additional annealing at 60 °C for 24 h. The cured product with a higher LP (or disulfide) fraction exhibited a better healing ability at 60 °C for 24 h by the disulfide metathesis. Also, the cured product with a higher DFDS fraction exhibited a better healing ability at 120 °C for 1 h and 60 °C for 24 h by the DA/retro-DA reaction.

Introduction

Thermosetting polymers (thermosets), which are obtained by thermally hardening (or curing) soft solid or viscous liquid prepolymers (resins), are generally stiffer and more thermo-stable than thermoplastic polymers due to their three-dimensional network structure [1]. However, infusible/insoluble properties arising from the highly crosslinked structure of the cured products make them difficult to recycle and reprocess. From this viewpoint, self-healing thermosets have attracted considerable attention due to their ability to repair physical damages and cracks, thereby extending the service time [2,3]. Utilization of dynamic covalent bonds as the cross-linkages is one of the most desirable methods of providing self-healing abilities in thermoset materials [4,5]. The dynamic covalent bonds undergo reactions including the reversible Diels-Alder (DA) reaction [[5], [6], [7], [8], [9], [10], [11]], disulfide metathesis [[12], [13], [14], [15], [16], [17], [18], [19]], transesterification [20,21], and imine exchange reaction [[22], [23], [24], [25], [26], [27]]. Especially, thermosetting systems using the DA reaction and disulfide metathesis have been actively investigated in past studies. However, it was difficult to attain both high healing efficiency and excellent mechanical properties. It is expected that the self-healing and mechanical properties can be improved by introducing a dual healing mechanism into a single thermosetting system. Regarding self-healing polymers combining both the DA and disulfide metathesis reactions, C.-M. Yeh et al. reported a self-healing polyurethane (PU-CYS) bearing both DA adduct moieties and disulfide bonds by the DA reaction of a telechelic furan-terminated poly(propylene glycol)-based polyurethane and bis(2-maleimidoethyl) disulfide [28]. PU-CYS is a linear polymer, and not a crosslinked thermoset polymer. The scratches of PU-CYS, whose glass transition temperature (T_g), tensile strength, and modulus are −23 °C, 4.63 MPa, and 1.2 MPa, respectively, were healed by annealing at 60–100 °C for 5 min. P.K. Behera et al. reported a self-healable polyurethane elastomer (DFS-PU-BM) bearing both DA adduct moieties and disulfide bonds by the DA reaction of a furan-grafted polyurethane (DFS-PU) and BMI [29]. The scratches of DFS-PU-BM, whose T_g, tensile strength, and modulus are −20 °C, 39.4 MPa, and 131 MPa, respectively, were healed by annealing at 135 °C for 10 min, then subsequently at 70 °C for 24 h. As the cross-linkages of DFS-PU-BM are generated only by the DA reaction of the linear DFS-PU and BMI, DFS-PU-BMI would be decomposed to the linear DFS-PU with the T_g of −31 °C, and BMI after the rDA reaction of DFS-PU-BM at 135 °C. It is desirable to maintain the crosslinked structure during the healing process at a high temperature in order to maintain the molded shape.

Herein, we prepared the polymer networks (FMTxyz-PLmn), bearing both DA adduct moieties and disulfide bonds, by the DA and thiol-maleimide reactions of DFDS, BMI, PETMP, and LP at the furan: maleimide: thiol molar ratio of (x:y:z), and the thiol ratio of PETMP/LP m/n at 50 °C produced (FMTxyz-PLmn; x:y:z = 1:2:1, 1:3:2, 1:5:4, m/n = 1/3, 1/2, 1/1, 2/1, 3/1) (Scheme 1). As the cross-linkages are formed by both the DA and thiol-maleimide dual “click” reactions, the shape of the cure films is expected to be maintained during the high-temperature healing process. In this study, we analyzed the influence of the different feed reactant ratios (i.e., the DA/disulfide ratios) on the thermal, mechanical, and self-healing properties of FMTxyz-PLmn films.