Multimaterial 3D Printing in Activating Bath Enables In Situ Polymerization of Thermosets with Intricate Geometries and Diverse Elastic Behaviors

Abstract

Polydicyclopentadiene, p(DCPD), is a high-performance thermoset valued for its exceptional toughness, strength, and stiffness. When copolymerized with 1,5-cyclooctadiene (COD), its mechanical properties can be tuned from glassy to rubbery at room temperature. While frontal polymerization enables a rapid and energy-efficient route to 3D print DCPD-based materials, challenges such as ink shelf life and gravitational distortion, especially in direct ink writing of soft COD-rich formulations, must be considered. Here, a complementary chemical strategy is presented, embedded 3D printing, that enables localized in situ polymerization of printed DCPD/COD inks within a reactive support matrix. The matrix provides both physical support and a reservoir of chemical activator, which diffuses into the ink, activates a latent bis(N-heterocyclic carbene) Ru precatalyst, and initiates ring-opening metathesis polymerization. Curing begins at the ink-matrix interface and propagates inward via diffusion, stabilizing the interface and preventing capillary-driven deformation regardless of the matrix yield stress. This approach eliminates the need for cold storage, external curing, or photoinitiation, significantly expanding the processing window. Using this method, diverse thermosetting and elastomeric architectures are fabricated with features as small as 5 µm and aspect ratios of 100, including interlinked chains, shallow spherical shells exhibiting snap-through buckling, and hair-like fin arrays inaccessible through traditional techniques.

Keywords: chemical activation; elastomer; embedded 3D printing; ring opening metathesis polymerization; thermoset.

Figure 1

EMB3D in chemical curing matrix. a) Printing and in situ curing process in a matrix containing a chemical activator. b) Printed octahedral truss structures spontaneously cured in the matrix. c) Diffusion of activator ions from the matrix to the ink, chemically curing the prepolymer ink. d) Four sequentially printed rectangles (20 mm × 7 mm), each requiring 5 min to print (left), and optical microscope images of the filament curing process (right). e) Polymerization of DCPD/COD ink through the activation of D899 by a Cu(I) ion.

Figure 2

Effect of matrix chemistry on EMB3D. a) Illustration of the effect of miscibility on surface topology. b) Solubility of DCPD in different glycol solutions: ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), polyethylene glycol (PEG), and propylene glycol (PG). c) Surface topology of threads printed in 1) immiscible (PG), 2) partially miscible (PEG), and 3) fully miscible (PO) matrices. d) Sample images printed in PG and PEG matrix solutions (FS is fumed silica). e,f) Rheological properties of an immiscible matrix solution containing FS using (e) large amplitude oscillatory shear (LAOS) test and (f) shear rate sweep test.

Figure 3

Chemical curing matrix formulation and its effect on EMB3D. a) Illustration showing the curing process of the printed ink in a chemical curing matrix. b) Activation of D899 with a Cu(I) ion yields a highly active ROMP catalyst via NHC transmetalation for polymerizing DCPD and COD. c,d) Effect of the activator on the printing geometry. Printed features of liquid inks in the matrix with activator (c) and without activator (d). Yield stress (σ_y) was controlled by varying the concentration of FS. e) Polymer layer thickness as a function of time at the interface for various activator concentrations. f) Effect of catalyst in the ink and activator in the matrix stoichiometry.

Figure 4

Tuning rheological properties of ink via partial polymerization. a) Schematic of partial polymerization for DCPD or COD monomers at various G2 concentrations. b) Zero‐shear viscosity of DCPD and COD solutions after viscosity modification. c) Viscosity and shear‐thinning behavior of DCPD prepolymer inks. d) Reaction scheme for partial co‐polymerization of DCPD and COD monomer mixtures. e) Proportions of COD monomer and G2 loadings for fabricating printable co‐prepolymer inks. f) Relative viscosity and shear‐thinning properties of the co‐prepolymer inks.

Figure 5

Printed and cured copolymers and their properties. a) Chain structures demonstrating the weight‐bearing capacity of DCPD‐rich formulations (scale bars 2 mm). b,c) Printed complex geometries including (b) shallow spherical shell structure exhibiting snap‐through buckling behavior and c) hair‐like structures at different printing speeds. d) Multi‐material printing within a single matrix solution (under UV light). e) Stress–strain curves of p(DCPD‐co‐COD). f) Glass transition temperatures and g) thermomechanical properties of copolymers with varying COD ratios.