Elsevier

Polymer

Volume 221, 14 April 2021, 123631
Polymer

High resolution additive manufacturing with acrylate based vitrimers using organic phosphates as transesterification catalyst

https://doi.org/10.1016/j.polymer.2021.123631Get rights and content

Highlights

  • Digital light processing 3D printing with acrylate based vitrimers is demonstrated.

  • An organic phosphate is introduced as a new transesterification catalyst.

  • Vitrimer networks with varying glass transition temperature are prepared.

  • Complex 3D objects with feature sizes below 50 μm are manufactured.

  • Structures feature triple-shape memory and thermo-activated mendability.

Abstract

The present study highlights the high resolution additive manufacturing of covalent adaptable acrylate photopolymers, which undergo catalyzed transesterification reactions at elevated temperature. A methacrylate phosphate is introduced as a new transesterification catalyst, which considerably extends the toolbox of acrylate monomers for 3D printing of vitrimers, as it is easily soluble in a wide range of acrylate monomers and does not affect cure kinetics or storage stability of the resins. By appropriate design of monomer composition and catalyst, a series of acrylate-based vitrimers was prepared, whose glass transition temperature was conveniently adjusted by the chemical structure and functionality of the acrylate monomers. Rheometer studies revealed that the stress relaxation rate slows down with increasing crosslink density and lower amount of –OH moieties. In contrast, increasing the catalyst concentration in the photopolymer network from 5 to 15 wt% significantly accelerated the relaxation rate, with 63% of the initial stress being relaxed within 102 min. Complex 3D objects with feature sizes below 50 μm were manufactured by bottom-up digital light processing (DLP) and the dynamic nature of the covalent crosslinks endowed the printed structures with triple-shape memory and thermo-activated mendability. As shown by tensile tests, up to 99% of the initial strength could be recovered after the first healing step, showing the potential to improve functionality and lifetime of additively manufactured duromer networks. Moreover, the fast response time (60 s) of the shape recovery and the high resolution of the 3D printed structures pave the way towards a customized fabrication and miniaturization of soft robotic applications.

Introduction

Additive manufacturing technologies (AMTs) have become a popular field of research in the last years as they allow for the fabrication of complex and individually shaped architectures from different materials including metals, [1], ceramics [2] and polymers [3]. Going beyond materials produced by conventional processes such as injection molding or extrusion, AMT enables the design and the fabrication of complex structures with new functionalities and/or improved performance [4,5]. One exciting approach towards 3D objects with additional functionality is the additive manufacturing of covalent adaptable networks (CANs), which undergo exchange reactions upon an external stimulus [6,7]. The dynamic nature of the bonds endows the polymer networks with unique properties such as recyclability, weldability, malleability and the ability for self-healing [[8], [9], [10], [11]]. The majority of the reported studies use filament extrusion techniques to fabricate 3D objects from CANs. In particular, Voit and co-workers exploited thermo-reversible Diels-Alder chemistry to create a thermoset with dynamic furan–maleimide links, which was processable with a syringe extrusion technique [12]. Thermo-reversible Diels-Alder networks belong to the class of dissociative CANs, which undergo a temporary decrease in crosslink density due to thermally triggered bond cleavage reactions [13]. Reformation of the cleaved bonds occurs at lower temperatures due to their entropically favored state. The decrease in both crosslink density and viscosity leads to a loss of mechanical integrity and elicits a softening process, reminiscent of thermoplastic reprocessing [14]. Thus, the printed 3D objects could be conveniently melted and re-printed again at temperatures >90 °C. Recently, Vanderborght and co-workers successfully fabricated furan–maleimide networks with fused filament fabrication (FFF) techniques and printed adaptive grippers for soft robotics, which were able to self-repair cuts and punctures [15].

Along with dissociative CANs, research is also geared to the additive manufacturing with thermosets comprising dynamic bonds, which are associative in nature. Whilst the former suffers from a loss of network integrity during the exchange reactions, the latter maintains its network integrity due to a simultaneous breakage and reformation of bonds [16]. Leibler et al. transferred the concept of associative bond exchange to classic epoxy/acid or epoxy/anhydride thermosets by adding appropriate transesterification catalysts [17]. The rate of the transesterification reactions is affected by several parameters including type and amount of catalyst, availability of –OH and ester groups or network structure. Typically used transesterification catalysts for vitrimeric networks are Brønsted acids, organo-metallic complexes and organic bases [18]. Above the so-called topology freezing transition temperature (Tv), the bond exchange reactions become significantly fast and result in a macroscopic flow of the polymer network. The thermally triggered gradual decrease in viscosity follows an Arrhenius trend analogous to silica-based glasses. Thus, Leibler and co-workers coined this new class of CANs vitrimers [17].

Shi et al. employed the concept of vitrimers to additively manufacture recyclable polymer parts [19]. They prepared an epoxy-based thermoset with hydroxyl ester moieties that undergo catalyzed transesterifications. By slightly pre-curing the formulation, they obtained a highly viscous ink, which was 3D printable with a syringe extrusion technique. Recycling was accomplished by dissolving the networks via solvent-assisted transesterification in the presence of ethylene glycol. Repeated printability of the solution could be shown without a significant loss of the mechanical performance of the printed parts.

Advancing from filament extrusion, Zhang et al. reported the 3D printing of vitrimers with digital light processing (DLP) technique [20]. DLP relies on a layer-by-layer polymerization of a photo-curable resin formulation, which is locally solidified in a vat [3]. It benefits from a high resolution and surface quality and comparably fast throughput rates [21]. Their network design relied on a photo-curable acrylate resin formulations, which contained hydroxyl-functional mono- and diacrylates, a Norrish Type I photoinitator and an organic zinc salt for catalyzing the bond exchange reactions. During the printing process, the 3D objects were fabricated by the radical induced chain growth polymerization of the acrylates. Solid networks with hydroxyl ester moieties were formed, which underwent catalyzed transesterifications upon elevated temperature and rendered the networks self-healable and reprocessable. However, in this approach the network design is limited by the poor solubility of the zinc salt in the acrylic monomers. A high amount of 2-hydroxy-3-phenoxypropyl acrylate is required for adequately dissolving the catalyst, which only allows the preparation of photopolymers with low crosslink density.

Recently, we introduced a mono-functional methacrylic phosphate as new catalyst for thermo-activated transesterification in thiol-click photopolymers [22]. Compared to commonly used catalysts, it is liquid, easily soluble in a wide range of acrylate monomers, covalently incorporated into the network across its methacrylate group and does not compromise on cure kinetics of radically induced photopolymerization reactions. We successfully 3D printed thiol-acrylate photopolymers with DLP and the functional test structures were able to undergo shape memory and intrinsically heal damages at elevated temperature.

Herein, we expand the concept towards DLP printable acrylate networks and highlight the versatility of this new catalyst, which enables the fabrication of a wide range of vitrimeric acrylate photopolymers. Due to its superior solubility properties, the catalyst can be applied in numerous acrylate monomers and we prepared a series of photopolymer networks with varying network structure, mobility and related mechanical properties. Structure-property relationships with the corresponding stress relaxation were established, giving evidence on the influence of network mobility, number of functional groups and catalyst content on the kinetics of the stress relaxation. The high photoreactivity of the functional resins was exploited to additively manufacture functional 3D objects with a feature size of 50 μm by DLP printing. Due to the dynamic nature of the hydroxyl ester groups, the printed structures could be conveniently mended and underwent triple-shape memory at elevated temperature, demonstrating the improved functionality and the potential to enhance the lifetime of additively manufactured duromer networks.

Section snippets

Materials and chemicals

Miramer A99 serving as transesterification catalyst was purchased from Miwon Specialty Chemical (Korea). Trimethylolpropane triacrylate was supplied by TCI (Belgium). All other transesterification catalysts, acrylate monomers and chemicals were obtained from Sigma-Aldrich (USA) and were used without further purification.

Preparation of resin formulations

For the preparation of vitrimeric photopolymers with Zn(acac)2 as catalyst, Zn(acac)2 was dissolved in 2-hydroxy-3-phenoxypropyl acrylate (HPPA) at 70 °C. After cooling the

Results and discussion

The design of covalent adaptable acrylate photopolymers combines the advantages of photo-triggered curing reactions, which enable a localized solidification of the material, with thermo-activated exchange reactions, to impart the network with advanced functions such as triple-shape memory and self-healability (Fig. 1). The thermo-activated exchange reactions are based on catalyzed transesterifications of hydroxyl ester links, which are introduced in the photopolymer network by using acrylate

Conclusions

DLP 3D printable acrylate photopolymers were developed undergoing thermally triggered exchange reactions at elevated temperature. Miramer A99 was introduced as new transesterification catalyst, which is superior to traditionally applied organic zinc salts or amine bases when it comes to radically cured photopolymer networks. On the one hand, it does not compromise on the cure kinetics enabling a fast printing process. On the other hand, the catalyst enables an efficient stress relaxation of the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research work was performed within the COMET-Module project “Chemitecture“ (project-no.: 21647048) at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and the Federal Ministry for Digital and Economic Affairs with contributions by Montanuniversitaet Leoben (Institute of Chemistry of Polymeric Materials). The PCCL is funded by the Austrian Government and the State Governments

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