3D printable soft and solvent-free thermoplastic elastomer containing dangling bottlebrush chains

Polymer networks containing bottlebrush chains are emerging materials with exceptionally soft and highly tunable mechanical properties. However, such materials have not been extensively implemented in functional processing techniques such as three-dimensional (3D) printing. Here, we introduce a new design of soft and solvent-free polydimethylsiloxane (PDMS)-based thermoplastic elastomer which contains dangling and space-filling bottlebrush chains, featuring a yield stress and a rapid recovery after stress removal; both required for high spatial fidelity 3D printing. The developed material is composed of two copolymers; the main building block is a diblock copolymer with linear polystyrene (PS) block and bottlebrush PDMS block (PS-b-bbPDMS) while the second component is PS-b-PDMS-b-PS triblock, self-assembling to a physical network. This design provides independent tunability of each structural parameter on the molecular level, hence, macroscopic control of the materials' mechanical properties. Multiple self-supportive 3D structures with spanning elements are 3D printed at elevated temperatures using a developed material with a low shear modulus of G′ = 3.3 kPa containing 3 : 1 molar ratio of diblock to triblock copolymers without the need for volatile solvent, or post-treatment. This 3D printing compatible design opens new opportunities to utilize the distinctive mechanical properties of bottlebrush materials for applications such as soft tissue scaffolds, sensors, actuators, and soft robots.

The PDF file includes: • Molecular characterization of the synthesized PS-b-bbPDMS diblock • Figure S1: Schematic representation of the structural parameters of PS-b-bbPDMS diblock.
• Figure S2: 1 H NMR spectra of the PDMS-H chains with different degree of polymerization.
• Figure S4: 1 H NMR spectra for synthesis of the PS-b-bbPDMS diblock copolymer with different length of grafted side chains and 10% grafting density.
• Figure S5: 1 H NMR spectra for synthesis of the PS-b-bbPDMS diblock copolymer with different length of grafted side chains and 20% grafting density.
• Figure S6: 1 H NMR spectra for synthesis of the PS-b-bbPDMS diblock copolymer with different length of grafted side chains and 30% grafting density.
• Figure S8: PS domain size distribution obtained from TEM image.
• Figure S9: Custom-made 3D printing setup used for DIW printing of bottlebrush thermoplastic elastomer.
• Figure S10: Calculation of the percentage of die swell.
• Table S1: Overview of the 3D printing parameters. 1 Electronic Supplementary Material (ESI) for Materials Advances.This journal is © The Royal Society of Chemistry 2023 Other Supplementary Material for this manuscript includes the following: • Movie S1 (.avi format, duration: 00:01:17): Printed lines deposited with a constant Q of 15 µL/hr and h of 0.2 mm but varied V t ranging from 0.2 to 0.03 mm/s.The movie is 40 times real-time.
• Movie S3 (.avi format, duration: 00:00:35): log-pile structure composed of four layers 3D printed with Q/V t = 0.125 mm 2 , L = 1 mm, and with h = 0.25 mm for the first layer and with h = 0.4mm for the remaining layers.The movie is 100 times real-time.
1 Molecular characterization of PS-b-bbPDMS diblock Calculation of the graft number for bottlebrush block: By knowing the grafting density and also the DP of PMVS block, the graft number is calculated as follows: Graf t number = graf ting density × DP P M V S = 0.24 × 234 ≈ 56 2 Preliminary synthesis of PS-b-bbPDMS diblock

Structural parameters
PS-b-bbPDMS diblock copolymer features multiple topological parameters including the degrees of polymerization of linear PS block (N L ), bottlebrush backbone (N bb ), grafted side chains (N sc ), and the grafting density (Gr), as shown schematically in Figure S1.These structural parameters are independently tunable but collectively affect the self-assembly and mechanical properties.In this study a library of samples are prepared by altering two of these parameters, i.e.N sc and Gr, while keeping other parameters constant in order to achieve a target viscoelastic behavior.

Synthesis of precursors
Monohydride-terminated polydimethylsiloxane (PDMS-H) with three different degree of polymerization are synthesized through the same synthesis protocol mentioned in the main manuscript by only changing the monomer to initiator ratio. 1 H NMR spectra of the synthesized PDMS-H with N sc of 15, 33, and 45 are represented in Figure S2.PS-b-PMVS diblock copolymer is also separately synthesized through the same synthesis route as described in the main manuscript, leading to a diblock with N L of 115 and N bb of 148. 1 H NMR spectrum and the SEC curves of the diblock is shown in Figure S3.

Grafting step
Prepared PDMS-H side chains with three different lengths and the PS-b-PMVS diblock are dissolved in anhydrous toluene with specific weight ratios, aiming for three different grafting densities of 10, 20, and 30.The grafting reaction is carried out with the same route as described in the main manuscript but scale downed to a smaller batch size.The obtained PS-b-bbPDMS diblocks which differ in N sc and Gr are labeled as S Nsc Gr .For instance, sample S 15 10 represent sample with aimed grafting density of 10% with side chains length of 15.The grafting reaction are monitored by taking an aliqout from the reaction and running 1 H NMR measurement at days 3, 8, 12, 16 after the grafting has started.All these 1 H NMR spectra and the calculated grafting density are shown in Figures S4 -S6. ) with M n = 26200, M w = 30000 g.mol −1 , and Ð = 1.14.

Figure S1 :
Figure S1: Schematic representation of the structural parameters of PS-b-bbPDMS diblock.N L : degrees of polymerization of linear PS block, N bb : degrees of polymerization of bottlebrush backbone, N sc : degrees of polymerization of grafted side chains, and Gr: grafting density.

Figure
Figure S4: 1 H NMR spectra for synthesis of the PS-b-bbPDMS diblock copolymer for 10% aimed grafting density with side chains length of (a) N sc = 15, (b) N sc = 30, and (c) N sc = 45 at different reaction stages and after purification.(d) Evolution of the grafting density during the grafting reaction calculated based on reduction of vinyl groups from 1 H NMR spectra.Dashed line represents the aimed grafting density considered to determine the stoichiometric mixing ratio between the side chains and the backbone.

Figure S5 :
Figure S5: 1 H NMR spectra for synthesis of the PS-b-bbPDMS diblock copolymer for 20% aimed grafting density with side chains length of (a) N sc = 15, (b) N sc = 30, and (c) N sc = 45 at different reaction stages and after purification.(d) Evolution of the grafting density during the grafting reaction calculated based on reduction of vinyl groups from 1 H NMR spectra.Dashed line represents the aimed grafting density considered to determine the stoichiometric mixing ratio between the side chains and the backbone.

Figure
Figure S6: 1 H NMR spectra for synthesis of the PS-b-bbPDMS diblock copolymer for 30% aimed grafting density with side chains length of (a) N sc = 15, (b) N sc = 30, and (c) N sc = 45 at different reaction stages and after purification.(d) Evolution of the grafting density during the grafting reaction calculated based on reduction of vinyl groups from 1 H NMR spectra.Dashed line represents the aimed grafting density considered to determine the stoichiometric mixing ratio between the side chains and the backbone.

Figure S7 :
Figure S7: Pictures of vials containing the prepared samples in the upside down position at different times.Based on whether the sample flows under the gravitational force a qualitative phase diagram is plotted showing the grafting density as a function of side chain length.Solid symbols represent samples being intact after two hours of being upside-down, however, open symbols represent samples that flow either immediately or at any point within the two hours.Some combinations of grafting density and side chain length are impossible to experimentally synthesize through grafting onto approach due to the steric hindrance effects, shown as the grey area in this phase plot.

Figure S9 :
Figure S9: Custom-made 3D printing setup used for DIW printing of bottlebrush thermoplastic elastomer consisting mixture of PS-b-bbPDMS diblock and PS-b-PDMS-b-PS (15-b-60-b-15 kg.mol −1 ) triblock copolymer with 3:1 molar ratio.The thermoplastic elastomer is loaded into a glass syringe which has been wrapped with a heating rope and sealed by aluminum foil.The Temperature of the material is read by a temperature sensor which has been taped on the outer side of the syringe and controlled by using a temperature controller.The rate of the injection and the motion trajectory are controlled through a MATLAB manuscript which converts G-Codes to motional commands for the translational stages.
Calculation of DP of PMVS block:DP of PMVS block is determined based on the ratio of the ring protons of styrene and vinyl protons (H styrenic /H vinyl = 1.27, shown in Fig.2bin the main manuscript while DP of PS block is determined by the molecular weight values obtained from SEC for PS shown in Fig.2a.