Recycling and Self-Healing of Polybenzoxazines with Dynamic Sulfide Linkages

Abstract

In this work, a recycling and self-healing strategy for polybenzoxazines through both S-S bond cleavage-reformation reaction and supramolecular attractions is described. Both recyclable and selfhealable polybenzoxazines can be prepared from low cost chemicals with a simple procedure in only 30 minutes. For this purpose, inverse vulcanization of poly(propylene oxide)benzoxazine (PPOB) and diallybenzoxazine (B-al) with elemental sulfur was performed at 185 °C. The obtained cross-linked polymer films exhibited thermally driven recycling ability up to 5 cycles. Moreover, the self-healing ability of a test specimen was shown. Spectral characterizations, thermal stability and fracture toughness of the films were investigated after each recycling.In the past few decades, polybenzoxazines (PBZs), also known as benzoxazine resins, emerged as a superior alternative to classical resol and novolac type phenolic resins. The main structural difference of PBZs from classical phenolics is the tertiary amine groups in each repeating unit, which generates immense effect especially on hydrogen bonding types and strength. Due to these unique structural characteristics, PBZs exhibit high tensile strength and modulus (100-125 MPa, and 3.8-4.5 GPa, respectively), and high glass transition temperatures (T g ) (170-340 °C). Accordingly, these materials, particularly rigid ones or those admixed with transition metals, have high service temperatures, char yields, and are stable under acidic/alkali conditions. Unlike the general nature of phenolics, PBZs have low water adsorption stemming from strong intra-molecular hydrogen bonding. Another important aspect is related to their dimensional stability during synthesis. These resins are obtained by ring-opening polymerization of benzoxazine monomers at temperatures between 160-250 °C, sometimes higher, without any additive (Scheme 1) [1][2][3][4][5][6] . It should be emphasized that 1,3-isomers are the only active benzoxazines to undergo such polymerization. According to mechanistic studies, polymerization proceeds via the cationic pathway as oxazine rings have N and O atoms, which are capable of stabilizing cations during polymerization 7-9 .The other appealing side of PBZs is the synthesis of corresponding benzoxazine monomers, which can be accomplished using any suitable phenol, a primary amine, and formaldehyde; therefore, the number of possible benzoxazines is large (Fig. 1) [10][11][12][13][14][15][16] . Apparently, this design flexibility brings about a huge molecular diversity and control of structure and properties of PBZs. For example, crosslinking density of the resins can be increased by introducing dimerizable or polymerizable functionalities on benzoxazines 17,18 . The toughness of PBZs can be manipulated by using long chain amines and other soft or conversely rigid groups [19][20][21][22] . Gas forming moieties can be attached to benzoxazines causing macroporosity to occur during curing; thus, sponge-like materials can be obtained, etc 23,24 . Such suppleness in the properties ...

Sustainable inverse-vulcanised sulfur polymers

“…Similarly, the reversibility of sulfur bonds in inverse vulcanised polymers has been shown to allow them to "heal" scratches, 6 and even be fully re-processed. 12 However, so far no inverse vulcanised polymers from renewable crosslinkers have been tested for this vitrimer behaviour, and we therefore tested both squalene and perillyl alcohol polymers. A block each of sulfur-squalene and sulfur-perillyl alcohol copolymers, both with 50 wt% sulfur, were smashed with a hammer and then placed back in moulds.…”

“…6,7 However, DIB is a niche synthetic chemical relative to sulfur, and it would be preferable to couple the readily available waste sulfur with sustainable crosslinkers were possible. While crosslinker sustainability will impact less on 'high-end' applications of sulfur polymers such as LiS batteries, 5,8 and optical devices, 6,9 for applications with potential for wide distribution and use, such as heavy metal remediation 10,11 or self-healing 12 and antimicrobial materials, the sustainability and green credentials of the crosslinker may have more signicance. Recently there has been a surge of further reports of other inverse vulcanised polymers using a variety of crosslinkers.…”

Sulfur Radical Transfer and Coupling Reaction to Benzoxazine Groups: A New Reaction Route for Preparation of Polymeric Materials Using Elemental Sulfur as a Feedstock

“…As inverse vulcanization usually occurs via radical addition to unsaturated CC bonds, glyceride‐based vegetable oil and plant oils, both possessing unsaturated units in the alkyl tails, are especially suitable for this process to prepare corresponding functional materials exhibiting sustainable and nonhazardous characteristics . Moreover, inverse vulcanization has also been applied to preparation of sulfur‐containing polymers possessing various functional units including polyhedral oligomeric silsesquioxane, benzoxazine, oleylamine, and triisopropenylbenzene . PbS nanoparticles have also been introduced to polymer composites through inverse vulcanization to tune the optical properties of the materials …”

“…In this work, we demonstrate a new reaction of sulfur for the synthesis of sulfur‐containing polymeric materials. Rather than the radical addition reaction to vinyl groups, a recent study reports that chain transfer reactions of sulfur radicals also involve in the inverse vulcanization process . The newly generated radicals with the chain transfer reactions might react with sulfur radicals through radical coupling reactions.…”

“…Benzoxazine compounds possess a six‐membered heterocyclic benzoxazine structure consisting of an oxazine ring fused to a benzene ring . Inverse vulcanization reactions involving vinyl group‐containing benzoxazine compounds and S 8 have been reported to occur via the addition of sulfur radicals to the vinyl groups . It is noteworthy that the acidic medium associated with the formation of the H 2 S byproduct has been shown to catalyze benzoxazine polymerization .…”