Thermally Reversible Diels–Alder Bond‐Containing Acrylate Networks Showing Improved Lifetime

Abstract

printing). [5] A key requirement for all these applications is spatiotemporal control of the polymerization reaction. [6] Commonly used monomers for photopolymerization are (meth)acrylates, which polymerize quickly, [7,8] and form a polymeric network with high dimensional stability and good resistance to heat and solvents. However, polymerization of multifunctional (meth)acrylates into a highly crosslinked network is often accompanied by stress development due to volumetric shrinkage, [9-11] which is the largest shortcoming in bulk applications such as stereolithography. [12] In the liquid monomer resin, the molecules interact weakly through van der Waals forces which define the intermolecular distance. [13] During the polymerization, covalent bonds are formed, which are shorter than the van der Waals distances between monomers in the initial mixture. The density will increase and volumetric shrinkage will occur; [7,12] the conversion of CC to CC bonds results in a volumetric shrinkage of 23 cm 3 mol −1. [14] Furthermore, energy dissipation through material flow is inhibited by cross-links, causing stress build-up in the material. [15,16] The shrinkage-induced stress can cause creep distortions or crack formation, leading to premature material failure. [17] Several methods toward reducing the volumetric shrinkage in (meth)acrylic systems have been investigated, e.g., the use of composite materials, [18-20] adjusting the bulkiness of the monomers, [21-23] the use of a volumetric expansion, [24-26] and covalent adaptable networks (CANs). [27-30] In CANs, rearrangement of a dynamic network is triggered by a specific stimulus, [27] where the chemistry of dynamic bonds determines the nature of the stimulus. [31,32] Bowman and co-workers developed photochemically controlled reversible addition-fragmentation monomers which show efficient stress relaxation when incorporated in (meth)acrylate networks. [30,33,34] Inspired by the studies on CANs, here we explore the use of reversible bond formation to counteract the results of volumetric shrinkage. A well-known example of thermally reversible covalent bond formation is the Diels-Alder reaction. While reversible Diels-Alder reactions have been used to create self-healing materials [35-37] and recyclable thermosets, [38,39] they have, to the best of our knowledge, not been explored for the current purpose. Since reversible Diels-Alder reactions have a low activation energy, these reactions can be used in the photopolymerization of (meth)acrylates without adding a catalyst; the Diels-Alder and retro Diels-Alder Photopolymerization of (meth)acrylates into highly crosslinked networks is in general accompanied by volumetric shrinkage, often leading to premature material failure. In this work, thermoreversible Diels-Alder groups are investigated to explore their effect on material lifetime. A difunctional acrylic monomer containing Diels-Alder moieties is synthesized and successfully incorporated in a polymer network by photopolymerization. It is demonstrated that upon heating ...

Chemically recyclable crosslinked thiol‐ene photopolymers via thiol‐disulfide exchange reactions

“…[22][23][24][25][26] When exposed to designated external stimuli (e.g., heat), however, these reversible or dynamic linkages can cleave or undergo bond exchange, imparting these novel network materials with self-healing and reprocessing/recycling capabilities. [22][23][24][25][26] Various types of reversible/dynamic bonds have been exploited for such network formation, including exchangeable disulfide [27][28][29][30] and ester linkages, [31][32][33][34] reversible Diels-Alder linkages, [35][36][37][38] and many others. [39][40][41][42] Inspired by these exciting advancements, research has also begun to incorporate such dynamic bonds into crosslinked photopolymers to make them reprocessable or recyclable.…”

“…When exposed to designated external stimuli (e.g., heat), however, these reversible or dynamic linkages can cleave or undergo bond exchange, imparting these novel network materials with self‐healing and reprocessing/recycling capabilities 22–26 . Various types of reversible/dynamic bonds have been exploited for such network formation, including exchangeable disulfide 27–30 and ester linkages, 31–34 reversible Diels‐Alder linkages, 35–38 and many others 39–42 …”

Strain‐induced post‐curing of acrylate networks

“…Films with a thickness of 80 μm were prepared by spin coating BPAda 512 on a silicon wafer, followed by UV curing for 200 s (Samples A), after which a maximum double bond conversion of 74% was reached. 20 In order to study the evolution of the glass-transition temperature during thermo-mechanical tests, two-cycle DMTA tests were performed. A small oscillatory strain was applied to the sample while it was heated at a constant rate.…”

“…In the course of a study on dynamic covalent bonds in photocured thermosets, the thermo‐mechanical properties of bisphenol‐A‐ethoxylate diacrylate samples were investigated using standard DMTA measurements 20 . Samples were subjected to multiple heating cycles under oscillatory strain.…”

“…In the course of a study on dynamic covalent bonds in photocured thermosets, the thermo-mechanical properties of bisphenol-A-ethoxylate diacrylate samples were investigated using standard DMTA measurements. 20 Samples were subjected to multiple heating cycles under oscillatory strain. In these experiments, an increase in T g of 17 C was observed between the first and second heating cycle and this effect was not found by thermal treatment alone.…”