Room temperature 3D printing of super-soft and solvent-free elastomers

Abstract

Super-soft elastomers derived from bottlebrush polymers show promise as advanced materials for biomimetic tissue and device applications, but current processing strategies are restricted to simple molding. Here, we introduce a design concept that enables the three-dimensional (3D) printing of super-soft and solvent-free bottlebrush elastomers at room temperature. The key advance is a class of inks comprising statistical bottlebrush polymers that self-assemble into well-ordered body-centered cubic sphere phases. These soft solids undergo sharp and reversible yielding at 20°C in response to shear with a yield stress that can be tuned by manipulating the length scale of microphase separation. The addition of a soluble photocrosslinker allows complete ultraviolet curing after extrusion to form super-soft elastomers with near-perfect recoverable elasticity well beyond the yield strain. These structure-property design rules create exciting opportunities to tailor the performance of 3D-printed elastomers in ways that are not possible with current materials and processes.

Fig. 1. Bottlebrush polymer design for solvent-free DIW at room temperature.

(A) Chemical structure and self-assembly of PDMS-stat-PEO statistical bottlebrush polymers into BCC spheres at minority PEO volume fractions (f_PEO = 0.04 to 0.06). (B) These soft materials undergo sharp yielding at room temperature in response to shear corresponding with the lattice disordering of PEO micelles, which enables facile extrusion-based 3D printing without solvent or other thixotropy-inducing additives.

Fig. 2. Morphology and yield-stress behavior of PDMS-stat-PEO bottlebrush polymers at 20°C.

(A) SAXS patterns of samples with N_SC = 68 and 136 are consistent with well-ordered BCC unit cells; triangles mark the expected Bragg reflections. The domain spacing (d) depends primarily on N_SC (c.f., fig. S15). (B) Dynamic frequency sweeps in the linear viscoelastic regime indicate robust, soft solids (G⁰ ≈ G′ ≫ G″) across five decades in frequency; solid lines are fast Fourier transforms of stress relaxation data (62). G⁰ depends on d through the as-shown scaling relationship. (C) Dynamic stress sweeps (oscillatory frequency = 1.0 rad/s, strain amplitude increases from 0.01 to 1.00) reveal sharp yielding at a critical stress (τ_y) defined as the crossover of G′ and G″. The value of τ_y is correlated with N_SC, d, and G⁰. (D) Cyclic dynamic time sweeps between two stresses, 1.4 kPa (τ < τ_y) and 8.2 kPa (τ > τ_y), demonstrate reversible yielding, fast reordering kinetics, and excellent mechanical stability. Data were collected at 1.0 rad/s with the sample N_SC = 68.

Fig. 3. Room temperature, solvent-free 3D printing of bottlebrush inks.

(A to C) Neat PDMS-stat-PEO bottlebrush copolymers (no additives). (A) Log pile (8 mm by 8 mm by 1.5 mm) printed with a 0.80-mm gap and layer thickness of 0.36 mm. (B) Magnified top-down perspective. (C) Hollow pyramid with a layer thickness of 0.36 mm. (D to F) Formulations of PDMS-stat-PEO bottlebrush copolymers that include a photocrosslinker (see Fig. 4 for details). (D) Lower resolution bowl with a layer thickness of 0.28 mm. (E) Higher resolution bowl with a layer thickness of 0.10 mm. Movies are available in the Supplementary Materials. (F) Dog bone with a layer thickness of 0.10 mm. Scale bars, 1 mm. Photo credit: Veronica Reynolds, University of California, Santa Barbara.

Fig. 4. Photocrosslinking to form super-soft elastomers at room temperature.

(A) Photocrosslinkable formulations include PDMS-stat-PEO bottlebrush copolymers mixed with a bis-benzophenone–based PDMS photocrosslinker. (B) UV curing kinetics of PDMS-stat-PEO (N_SC = 136) with eight cross-linkers per bottlebrush molecule (n_cl = 8), as measured by oscillatory rheology (frequency = 1.0 rad/s, strain = 0.01). Sample dimensions: thickness = 0.4 mm, diameter = 20 mm; UV source: irradiance = 150 mW/cm², wavelength = 365 nm; see Materials and Methods for details. The illustration highlights the transformation from an as-printed yield stress fluid with discrete micelles to a cross-linked bottlebrush elastomer; note that other possible products of the benzophenone photochemistry (e.g., intramolecular bonds) have been omitted for clarity. (C) Photographs capture the development and gradual decrease of phosphorescence intensity, characteristic of benzophenone reactivity, while UV curing the 3D-printed bowl from Fig. 3D. Photo credit: Renxuan Xie, University of California, Santa Barbara.

Fig. 5. Structure-property relationships of super-soft elastomers derived from photocrosslinked PDMS-stat-PEO bottlebrush copolymers.

(A) Cyclic uniaxial tensile experiments (engineering stress σ_E versus extension ratio λ) on an elastomer with N_SC = 136 and n_cl = 8. Note that this sample was processed by molding (see Fig. 6 for 3D printing). Two distinct regions of mechanical response are evident with super-soft shear moduli G_p = 32 kPa and G_x = 7.7 kPa. Near-perfect recovery is possible even after applying strains well above the yield point (σ_y = 10 kPa). Data were collected with 10 min of rest in between cycles and the same 1.0 mm/min loading and unloading rate. (B) SAXS experiments demonstrate the unique yielding behavior in the cross-linked elastomeric state is connected to a reversible structural transition between a BCC (σ < σ_y) and lattice-disordered (DIS) (σ > σ_y) arrangement of spheres, as illustrated in (C). Triangles mark the expected Bragg reflections of a BCC unit cell.

Fig. 6. Mechanical properties of 3D-printed bottlebrush elastomers.

(A) Uniaxial tensile response of 3D-printed and molded samples (true stress σ_T versus extension ratio λ). Values of the post-yield moduli (G_x,molded = 7.7 kPa and G_x,printed = 8.6 kPa) were extracted from fits (black lines) to Dobrinyn’s model [see Supplementary Text, (5, 51), and table S2]. (B) The photocrosslinked, 3D-printed bowl from Fig. 3D is elastic, as evidenced by a series of snapshots taken from movie S4. Photo credit: Renxuan Xie, University of California, Santa Barbara.