Wet/Dry Bottlebrush Pressure Sensitive Adhesives via a Dangling Defect-Driven Design

Abstract

A series of bottlebrush pressure sensitive adhesives (PSAs) containing progressively greater numbers of large dangling ends were synthesized using ring-opening metathesis polymerization. These defects were engineered into PSAs by promoting the formation of loop defects and by manipulating the kinetic chain length such that large dangling defects are covalently bound to the bulk. The unique structure of these samples is shown to promote adhesion at interfaces by maximizing surface area contact and increasing local van der Waals forces. This design philosophy (termed defect-driven design, D³) provides an easy route to synthesize bottlebrush PSAs ~6 times stronger than commercial VHB1000 tape. Furthermore, the inherent tendency of water to dewet on poly(dimethylsiloxane) PSAs is exacerbated in these samples, resulting in PSAs capable of indefinite cycles of wet–dry-wet adhesion (tested up to 50 cycles, 7 months apart). Further development of the D³ concept is expected to result in increased applications (e.g., dielectric actuators, wearable electronics, and especially soft robotics) as the unique design parameters are further explored.

Graphical Abstract

INTRODUCTION

Polymeric networks with low T_g side chains densely grafted along the backbone form the basis of an emerging class of materials known as bottlebrush elastomers (BBEs).^1,2 BBEs have recently seen a surge in popularity due to their “supersoft” (10²–10⁶ Pa) elastic moduli (E), allowing them to mimic a wide variety of biological materials while remaining solventless.^3–5 These properties are derived from the unique architecture of BBEs, where the sterics between side chains force network strands to extend locally into Rouse-like chains.⁶ Such strands are substantially less likely to entangle with adjacent strands, increasing the molecular weight between entanglements and reducing the plateau modulus. Additionally, the toolbox of tunable parameters available to BBEs (e.g., side chain length, grafting density, and cross-link density) make the architecture ideal for designer applications in additive manufacturing, wearable sensors, and soft robotics.^7–12

One area which has seen only limited growth in this regard is bottlebrush pressure sensitive adhesives (PSAs).^13–17 Generally speaking, PSAs are a class of adhesive that are tacky when dry and can easily form interfaces with substrates when pressure is applied.¹⁸ Bottlebrush PSAs are particularly intriguing options because the BBE architecture allows for high surface area interfaces to be formed (1) without the time-dependent properties typically incurred from small molecule adhesives (e.g., molecular glues) and (2) without leaving residue behind (no tackifiers).^13,14 While some bottlebrush PSAs have been demonstrated to have tunable adhesive properties, their practical usefulness is considerably limited by either weak adhesive strength, insufficient architectural control, or relatively inaccessible synthetic routes for nonchemists (such as atom-transfer radical polymerization).^13–16

More recently, we demonstrated that ring-opening metathesis polymerization (ROMP) provides synthetically simple routes to high conversion bottlebrush networks with controllable molecular architectures.^19–23 In particular, we demonstrated that (1) ROMP BBEs have inherently higher levels of defects (e.g., loop defects, dangling ends, sequential crosslinks) than those generated via radical polymerization²² and (2) that ROMP allows for control over the relative size of defects via constitutional isomerism (identical chemical compositions but varied covalent connectivity).^19,23 This level of control over network defects inspired us to develop a “defect-driven design” (D³) philosophy, where these unique features of ROMP BBEs were employed to build ultralarge dangling defects within the network (Figure 1). This strategy is implemented by simultaneously (1) keeping n_x ≈ R_K (kinetic chain length) and (2) promoting loop defect formation by keeping the concentration of polymerizations low ([0.11 M]).^24,25 These two points strongly encourage the formation of large dangling ends by (1) producing strands with very few cross-links per chain and (2) reducing the number of effective cross-links even further through loop defect formation, respectively. These engineered defects were expected to elicit strong increases in the viscous component of samples compared to constitutional isomers synthesized without D³ (directly measurable via dynamic mechanical analysis [DMA]).

Figure 1.

Illustration of a bottlebrush network (BBN), highlighting the most important network parameters for this study, n_x: the degree of polymerization between cross-links and R_K: the kinetic chain length. These parameters are controlled by the inverse of mol % cross-linker and the monomer to initiator (M/I ratio). Other notable bottlebrush parameters (side chain length, cross-linker length, and grafting density) remain consistent throughout this study and are not highlighted here. The backbones here are depicted as straight “rods” solely for ease of presentation of concepts—they are Rouse-like chains.

We showcase here how the increased viscous components observed in DMA correlate strongly to increases in adhesive strength, where samples with higher magnitude Tan(δ) exhibit greater critical energy release rates (G_c), resulting in samples with G_c values ~6 times larger than commercial VHB1000 tape (3M).²⁶ The vital role that defects play in this design philosophy is then confirmed by increasing the concentration of the polymerization and directly observing how the increased number of stress-supporting strands reduces the viscous contribution and G_c of samples. We conclude by illustrating both the efficacy and reusability of our D³ PSAs in a series of practical demonstrations showcasing (1) shear-induced release (failure) mechanisms, (2) soft robotics tasks, and (3) underwater adhesion. Further development of the D³ concept (and living polymerization chemistries in general) is expected to result in expanded applications for bottlebrush materials as a result of the unique design parameters, especially in soft robotics.

RESULTS AND DISCUSSION

Defect-Driven Design.

All BBEs in this study were synthesized via ring-opening metathesis polymerization (ROMP) of mono- and bis-norbornene functionalized poly(dimethylsiloxane), [PDMS]. The use of bis-norbornene PDMS allows for in situ cross-linking to occur during the polymerization, producing insoluble networks rather than soluble bottlebrush polymers. Incredibly, all polymerizations were performed under near ambient conditions, owing to the extreme tolerance of Grubbs second Generation catalyst toward ambient conditions (dried solvent and a cover are still used). Despite the relatively painless synthesis, such networks retain all the features BBEs have become famous for: high conversion of macromonomer into high molecular weight strands (Figure 2a) and “super-soft” elastic moduli (E, Figure 2b).³ This approach offers a more accessible route to supersoft BBEs than radical polymerization methods, where extreme care must be taken to synthesize analogous samples with high gel fractions (ours are >90%).^5,27–29

Figure 2.

(a) Gel-permeation chromatography (GPC) chromatograms illustrating the high conversion of macromonomer starting materials into ultralong (DP = 1840, M_n = 2.02 MDa) bottlebrush polymer controls (soluble, non-networks). The absolute molecular weight of these bottlebrush polymers was determined via multiangle light scattering and converted to M_n via dispersity values found via GPC (M_n = M_w (Đ)⁻¹). This polymerization serves as a control for the length of the backbones polymerized in BBEs with equivalent M/I = 1000. The right side of the macromonomer GPC chromatogram is distorted because it occurs outside the lower bounds of our calibration standards. (b) Plot of the elastic modulus (E) determined via indentation for a series of bottlebrush elastomers with progressively larger n_x values. The large range of E available to these samples (E ∈ [0.5,88] kPa) illustrates the significance of the large kinetic chain lengths (R_K) available via living polymerization methods such as ROMP. (c) Synthetic schematic illustrating how the polymerization of mono- and bis-norbornene functionalized PDMS (M_w = 1 and 9 kDa, respectively) can produce PSAs with large viscous components due to their unique structural composition. Critical to this defect-driven design (D³) philosophy are the ratio of M/I and n_x and low polymerization concentrations (as low as 0.11 M in this study). The PSAs synthesized in this manner have large, dangling defects which behave globally as viscous-contributing defects. These engineered defects contribute strongly to the adhesive properties of synthesized samples. The backbones here are depicted as straight “rods” solely for ease of presentation of concepts—they are Rouse-like chains.

ROMP is further distinguished from radical polymerization methods by its ease of access to network architectures with large amounts of synthetic defects.²² Following from a previous study,¹⁹ it was theorized that BBEs with numerous, large defects would maximize surface areas at interfaces by allowing for increased surface area contact with the cracks and crevices of substrates. The dramatically increased local van der Waals forces at the interface would cause such materials to behave as strong, reversible pressure-sensitive adhesives (PSAs) due to their “fluid-like” dangling defects.³⁰ This principle is at the core of this design philosophy, termed “defect-driven design” (D³).

In D³, the number of defects in our systems is maximized by purposefully building large dangling defects into synthesized networks (Figure 2c). This is accomplished primarily by (1) keeping the polymerization concentration low—which maximizes the formation of loop defects—and (2) by keeping the degree of polymerization between cross-links approximately equal to the degree of polymerization of the primary network backbone (n_x ≈ M/I). The latter point serves to introduce ultralarge dangling ends into the network, dramatically increasing the adhesive strength at the interface. Furthermore, the use of PDMS side chains endowed our PSAs with the ability to perform underwater adhesion, owing to their dewetting behavior displacing water at interfaces. We emphasize that increasing the level of loop defects directly increases the size and quantity of dangling ends by keeping monomers out of stress-supporting strands.

Programming Large Defects into Bottlebrush PSAs.

While networks with low n_x relative to backbone M/I form multiple stress-supporting strands per backbone chain (Figure 3a), networks designed to have n_x = M/I will contain a population of backbones with only 1–2 cross-links per backbone chain (Figure 3b). This population of backbones results in large portions of the network not being interconnected, with any noninterconnected structures behaving as massive dangling defects. In these D³ PSAs, the distribution of macromonomers and cross-linkers are different from those with n_x < M/I, but their chemical compositions remain identical (equal amounts of material being used), making them network constitutional isomers (NCIs) of each other.¹⁹ The dangling ends present in D³ PSAs are essentially viscous liquids which are forced to behave as solids due to their covalent constraints, being predicted to contribute significantly to the viscous component of networks while simultaneously providing long-term stability to their adhesive properties. Importantly, the elastic moduli of NCI pairs remain the same (as previously reported¹⁹), highlighting the alteration of dangling-defect size as the root cause of any differences in material properties.

Figure 3.

Representative schematic illustrating the effects of network constitutional isomerization in high n_x BBEs with (a) n_x < M/I and (b) n_x = M/I. While network constitutional isomers (NCIs) with n_x < M/I have larger numbers of cross-links for each backbone which comprise the network, NCIs where n_x → M/I have so few cross-links that a proportion of backbone chains will have only one or two cross-links per backbone, with the low polymerization concentration resulting in an increased probability for intramolecular cyclization. Samples with n_x → M/I will thus contain dangling fractals which contribute highly to the viscous component of samples, while also containing the same number of stress-supporting strands as their isomeric counterparts. The backbones here are depicted as straight “rods” solely for ease of presentation of concepts—they are Rouse-like chains. (c) Plot of Tan(δ) for high n_x samples with n_x = M/I (red data) and n_x < M/I (black data), as determined via dynamic mechanical analysis (DMA). The viscous component of the n_x = M/I NCI is considerably larger than that of the n_x < M/I NCI. (d) Plot of the storage (E′) and loss moduli (E″) for the n_x = 1000, M/I NCI. Note the similarities between this profile and that of gels near the criticality point.

To examine this hypothesis, an n_x = 1000 NCI pair with M/I = 1000 and 2000 were synthesized and tested via dynamic mechanical analysis (DMA) to directly measure increases in the samples’ viscous components (Tan(δ)). As hypothesized, NCIs with n_x = M/I exhibited Tan(δ) values considerably higher than NCIs with n_x < M/I (Figure 3b), with storage (E′) and loss modulus (E″) profiles similar to those of gels near the criticality point (Figure 3c)—where the network is only just interconnected enough to be one continuous molecule.³¹ As correlated in our previous studies, the increased viscous contributions observed via DMA experiments should indicate the potential for stronger PSAs.¹⁹ See Section S8 of the Supporting Information for a more thorough description of how loss tangent behavior is special for NCI pairs synthesized in this manner.³²

To characterize the adhesive strength of D³ PSAs, we adopted the recently established pressurized interfacial failure (PIF) technique. This newer technique was adopted instead of the contact adhesion testing (CAT) method because the adhesive strength and elasticity of these samples cause probe contact area (a) to remain essentially constant during displacement (δ) until failure, complicating calculations because $\frac{\Delta a}{\Delta \delta }=0$ and CAT requires $\frac{\Delta a}{\Delta \delta }>0$.³³ In PIF, an interface is formed between an annular probe and the flat surface of a sample before being separated by pushing pressurized air through the probe. The pressure required to separate the interface is then correlated to the critical energy release rate (G_c) using established geometric equations—providing a characterization method which does not rely upon Δa for accurate determination of G_c.²⁶ The G_c of BBEs produced without D³ scale inversely with E up to 16.37 J/m² (for n_x = 1000 with E = 1.16 ± 0.60 kPa), as expected for PSAs (black data, Figure 4a).⁷ These increases in G_c are derived from increasingly greater contact at the interface as the samples become more deformable (as E decreases).

Figure 4.

(a) Plot of G_c vs n_x for a series of n_x < M/I BBEs with progressively larger n_x values (black data). NCIs with dramatically improved G_c (red data) illustrate the power of the D³ strategy in designing more effective PSAs. These G_c values were determined via PIF of synthesized samples using a glass annular probe (see Experimental Section for more details). (b) Plot depicting the results of contact angle testing measurements performed on synthesized samples. Importantly, the values for both the advancing and receding angles are independent of the architectural changes being made (e.g., cross-link density or isomerization).

Samples with shorter kinetic chain lengths (recall that R_K ∝ M/I) were then synthesized such that n_x approaches M/I, introducing to the network the dangling defects discussed prior. PIF was then performed on NCI pairs synthesized with n_x = M/I (red data, Figure 4a), revealing that the G_c of NCIs engineered with D³ were twice as large as those without. Importantly, increasing the M/I of samples while maintaining n_x = M/I resulted in larger dangling ends and dramatically increased G_c (red data at n_x = 500 to red data at n_x = 1000) without meaningfully impacting sample E (Figure S.6). This directly relates the increased size of the dangling defects to observed increases in G_c, with the n_x = 1000, M/I = 1000 NCI having G_c = 33.8 ± 3.7 J/m². For reference, commercial VHB1000 tape (3M) has a G_c value of ≈6 J/m² (as determined via PIF).²⁶ These measurements demonstrate the strong efficacy of D³ despite its relatively simple premise. Importantly, because these samples retain the same base architecture (that of PDMS bottlebrushes), the contact and receding angles of networks observed during water contact angle goniometry remain relatively constant across all compositions—regardless of n_x or isomerization (Figure 4b). Furthermore, the >90° contact angles (white bars, ≈120°) and <90° receding angles (shaded bars, ≈63°) reveal that waterwetting behaviors are thermodynamically unfavorable in these BBEs.³⁴ This indicates that when the D³ PSAs adhere to water-wetted substrates, they should displace water at the interface as pressure is applied.

Reduction of Defects.

Polymerization concentration plays a vital role in D³ through encouraging the formation of intramolecular loop defects—and therefore increasing the number of dangling ends. This dependency is easily confirmed by increasing the concentration of polymerizations and monitoring the level of defects in synthesized samples. By concentrating the polymerization, more stress-supporting strands are formed as intramolecular cross-linking is suppressed (Figure 5a).^24,25 The increased number of stress-supporting strands can be directly observed via tensile testing of samples (Figure 5b), where increasing the concentration from 0.11 to 0.22 M doubles sample E. Doubling the concentration once more (to 0.45 M) increases E even further. Incidentally, these higher concentration polymerizations result in BBEs with higher extensions-at-breaks than ever previously reported (the previous theoretical limit was given as λ_max = 5).⁵

Figure 5.

(a) Schematic illustration of how increasing the concentration of the polymerization results in the suppression of intramolecular cyclization and the formation of stress-supporting strands. (b) Plot of uniaxial tensile testing for NCIs synthesized at polymerization concentrations of 0.11 M (red), 0.22 M (cyan), and 0.45 M (magenta) samples. Testing was performed at a loading rate of 10 μm/s. (c) Tan(δ) profiles for NCIs synthesized at 0.11 M (red) and 0.45 M (magenta) polymerization concentrations. A 0.11 M sample synthesized at n_x = 10 (black) is included to emphasize the strong role that polymerization concentration plays in D³. (d) Plot of G_c values determined via PIF of PSAs synthesized at varied polymerization concentrations. The strong dependence of G_c on polymerization concentration confirms the rationale of the D³ concept.

This increase in stress-supporting strands also translates to decreases in Tan(δ) values through reductions in the viscous components of networks (Figure 5c). For example, increasing the polymerization concentration from 0.11 to 0.45 M results in more than a 10-fold decrease in the low frequency Tan(δ). This effect is so pronounced that the Tan(δ) of the 0.45 M samples is nearly identical to the Tan(δ) of samples synthesized with 100 times the cross-linker content (n_x = 10) at 0.11 M. As with previously produced samples, these variations in Tan(δ) translate directly to observed changes in the G_c values as determined by PIF (Figure 5d). Specifically, increasing the concentration from 0.11 to 0.45 M resulted in a near 8-fold decrease in G_c. Note again that there are no differences in the contact angle measurements performed at these various concentrations (Figure 5d), despite the large disparities in observed mechanical properties.

Underwater Adhesion and Soft Robotics.

Using the knowledge gleaned from this study, a series of D³ PSAs were synthesized for use in soft robotics and underwater adhesion. We take advantage of the supersoft moduli of these PSAs to utilize shear induced failure to deliver adhered items during our demonstrations (Figure 6a, Videos S1, S2, S3 and S4).^35,36 A 3D printer was then programmed to perform a series of demonstrations using our PSAs as the attached adhesive: the most complex demonstration is having the arm consecutively lift a series of weights before depositing them into a glass jar and placing its lid (phenol-resin) atop the container (Figure 6b). Critically, while the [0.22 M] and [0.11 M] n_x = 1000, M/I = 1000 PSAs can perform all these functions, both the [0.45 M] samples and commercial double-sided tape fail to perform them—further illustrating the impact of the D³ concept.

Figure 6.

(a) Illustration of how the weak tolerance of soft PSAs toward shear forces allows for easy separation of the interface when release is desired. (b) Using our D³ PSA ([0.22 M] n_x 1000, M/I = 1000) the robot can lift a series of weights into a glass jar, dropping each using the shear failure mechanism, and then close the jar itself via the same mechanism. Importantly, the same adhesive was used for each of the demonstrations in the Videos S1, S2, S3 and S4 (more than 50 contact events), without losing any of its tack or leaving any residue upon substrates when delaminated. These demonstrations provide a strong proof of concept for this PSA as a component for soft robotics applications. (c) Illustration of the wet–dry-wet adhesion cycling performed using the D³ PSA. This demonstration was performed 25 times in a row, each time holding the Petri dishes up for 15 s before manually delaminating the probe from the interface. The probe was then stored with the sample in a freezer for one month before performing the same demonstration once again.

The D³ PSAs were additionally used for a series of wet–dry-wet adhesion demonstrations, where the sample was pressed into a glass Petri dish full of water before lifting the dish. Samples were then delaminated and pressed into dry Petri dishes before lifting once more (Figure 6c). This cycle of wet–dry adhesion can be performed indefinitely if the dry Petri dish does not contain any water, never damaging or deforming the sample. If the PSAs are adequately stored (cool, dark places), they can be used for all the illustrated demonstrations even seven months later.

CONCLUSIONS

Using a novel strategy termed D³, bottlebrush PSAs with ~6 times greater G_c than heavy bonding commercial tape were synthesized for the first time. The role of engineered defects in improving adhesion was highlighted by illustrating differences in sample performance as the level of defects is controlled. This was accomplished via (1) introduction of large dangling ends to BBEs via constitutional isomerization, (2) increasing the size of those dangling defects, and finally (3) through the suppression of intramolecular defects due to increased polymerization concentrations. It was further demonstrated that the water wetting/dewetting properties of PDMS D³ PSAs are relatively independent of cross-link density/isomerization/concentration and strongly promote dewetting of water at interfaces. These points guided us to illustrate the usefulness of D³ PSAs in a series of soft robotics and wet–dry-wet adhesion demonstrations. It is expected that further evolution of the D³ concept via living polymerization chemistry will result in materials of even greater utility for applications in advanced materials.

EXPERIMENTAL SECTION

Synthesis of Monofunctional PDMS.

Norbornene-functionalized PDMS was synthesized via the Mitsunobu coupling reaction. Monobutyl PDMS (5.944 g, 5.40 mmol, Gelest) was placed in a dry round-bottom flask with a stir bar and dried overnight in a vacuum oven. The dried PDMS was dissolved in 30 mL of dried THF and placed under N₂ before adding exo-5-norbornene-2-carboxylic acid (1.490 g, 10.78 mmol, Sigma) and TPP (2.835 g, 10.81 mmol, Alfa Aesar) to the reaction mixture. The stirring reaction mixture was chilled in ice water for 15 min before adding DIAD (2.120 mL, 10.77 mmol, Alfa Aesar) dropwise. The reaction was allowed to run 24 h before being concentrated, doused in hexanes, and refrigerated at −8 °C to precipitate triphenylphosphine oxide (TPPO). The TPPO was removed using gravity filtration and the resulting mixture was washed four times with methanol. The resulting mixture was concentrated and dried in a vacuum oven to remove excess hexanes. Product obtained was a clear liquid obtained at a yield of 86%. ¹H NMR (500 MHz, CDCl₃, δ): 6.09–5.99 (m, 2H), 4.21–4.10 (m, 4H), 3.60–3.50 (T, 2H), 3.40–3.32 (T, 2H), 3.00–2.95 (s, 1H), 2.91–2.82 (s, 1H), 2.23–2.15 (q, 1H), 1.90–1.80 (m, 1H), 1.60–1.44 (m, 4H), 1.35–1.15 (m, 7H), 0.03-(–)0.02 (m, 93.22H) (THF GPC in Figure S1).

Synthesis of Difunctional PDMS.

Norbornene-difunctionalized PDMS was synthesized via the Mitsunobu reaction in a manner identical to the monofunctional PDMS. To dried PDMS diol (5.023 g, 0.50 mmol, Gelest), exo-5-norbornene-2-carboxylic acid (0.278 g, 2.01 mmol, Sigma) and TPP (0.528 g, 2.01 mmol, Alfa Aesar) were added under N₂. DIAD (0.396 mL, 2.02 mmol, Alfa Aesar) was added dropwise to the reaction. Product was a clear, viscous liquid obtained at a yield of 82%. ¹H NMR (500 MHz, CDCl₃, δ): 6.19–6.09 (m, 4H), 4.25–4.10 (t, 4H), 3.60–3.50 (T, 4H), 3.40–3.34 (T, 4H), 3.00–2.95 (s, 2H), 2.91–2.82 (s, 2H), 2.23–2.15 (q, 2H), 1.90–1.80 (m, 2H), 1.60–1.44 (m, 12H), 1.35–1.15 (m, 4H), 0.15-(–)0.15 (m, 985.02H) (THF GPC in Figure S1).

ROMP Bottlebrush Network Synthesis.

ROMP BBNs were synthesized by polymerizing norbornene functionalized macromonomers (250 mg) with varied amounts of cross-linker. BBNs were all polymerized using Grubbs’ second-generation catalyst (Sigma) at varied M/Is. For a typical reaction, the macromonomer and cross-linker were first dissolved in 1 mL of dried dichloromethane (DCM) in a 20 mL vial before adding 1 mL of catalyst/DCM solution. The reaction was allowed to run overnight before being quenched with 3 drops of ethyl vinyl ether (EVE, Sigma). Samples were molded into desired geometries for PIF, contact angle goniometry, tensile testing, and indentation using a variety of petri dishes, teflon molds, and glass vials.

ROMP Bottlebrush Polymer Synthesis.

Bottlebrush polymerizations were performed in round-bottom flasks containing 20 mL of still-dried DCM with 125 mg of macromonomer and no cross-linker added to the reaction (Scheme S2). The synthesized bottlebrush polymers were characterized by size exclusion chromatography multiangle light scattering (SEC MALS) and gel permeation chromatography (GPC).

Nuclear Magnetic Resonance (¹H NMR).

¹H NMR was used to determine the successful synthesis of macromonomer materials. NMR spectroscopy was performed using a Bruker Advance 500 MHz NMR spectrometer with CDCl₃ as a solvent.

Gel Permeation Chromatography (GPC).

GPC was performed using an Agilent Technologies 1260 Infinity series system with two 5 μm mixed-D columns, a 5 μm guard column, a PL Gel 5 μm analytical Mixed-D column, and a RI detector (HP1047A); dried tetrahydrofuran (THF) was used as the eluent with a flow rate of 1.0 mL/min; polystyrene standards were used for the calibration. The dispersities (Đ) and molecular weights (M_w) of each macromonomer, cross-linker, and bottlebrush were measured using THF-GPC (Figure S1).

Size Exclusion Chromatography Multi-Angle Light Scattering (SEC MALS).

MALS was performed in THF + 1 vol % triethylamine (TEA) using two Polymer Laboratories 10 μm mixed-B LS columns connected in series with a Wyatt Technologies DAWN EOS MALLS detector and RI detector at a flow rate of 1.0 mL/min. MALS was used to determine the M_w of synthesized bottlebrushes, with an eluent of THF and 1% TEA.

Indentation and Tensile Testing with a Texture Analyzer.

Force/displacement data were collected using a TA.XT Plus Texture Analyzer from Texture Technologies. Networks were indented with a 2 mm diameter flat cylindrical stainless steel probe at a loading rate of 0.01 mm/s to a force of 20 mN, whereupon the probe retracted at an unloading rate of 0.01 mm/s. The probe was cleaned with acetone between runs. Force/displacement data was analyzed using eq 1³⁷

$$\mathit{E}=\frac{3}{\mathbf{8}\mathit{a}}\frac{\mathbf{\Delta }\mathit{P}}{\mathbf{\Delta }\mathit{\delta }}{\left\{\mathbf{1}+\mathbf{1.33}\frac{a}{h}+\mathbf{1.33}{\left(\frac{a}{h}\right)}^3\right\}}^{-1}$$ (1)

where E is the elastic modulus, P is the load applied, a is the contact radius of the indentation probe at the interface, δ is the displacement of the probe (0.01 mm/s), and h is the thickness of the sample (between 1–2 mm in our case). A Poisson’s ratio of 0.5 is assumed for these calculations. Tensile testing was performed using tension clamps at a rate of 0.01 mm/s until tensile failure.

Dynamic Mechanical Analysis (DMA).

Networks were cut into 8 mm circles with a circular punch and placed within the compression clamps of a Discovery DMA 850 from TA Instruments. Samples were tested at room temperature, sweeping a frequency range of 0.1–10 Hz at a strain amplitude of 1% with a preload force of 0.1 N. The sample stage was cleaned with isopropyl alcohol between samples. Additional DMA storage and loss modulus data are presented in Figure S.5.

Pressurized Interfacial Failure (PIF).

The pressurized interfacial failure (PIF) tests were run on a customized instrument (Figures S3 and S4) to characterize each—sample’s critical energy release rate (G_c). The annular probe was made of borosilicate glass supplied by Hilgenberg GmbH, Germany featuring an outer radius of 1 mm and an inner pore radius of 0.1 mm. An actuator (Burleigh Inchworm, Exfo) was utilized to control the probe displacement, and the cantilever-based load cell measured the contact force. The pressure was increased by a syringe pump (Chemyx Fusion 6000) equipped with a metal syringe (20 mL, Harvard Apparatus) and measured by a pressure-sensor (PX409-100GUSBH, Omega). In a typical test, the probe was brought into contact with the sample at 10 μm/s until the target load, 50 mN, was attained. Subsequently, the pressure in the system was elevated by 100 mL/min to separate the sample/probe interface. The peak load and pressure were recorded and used to calculate G_c, following established protocols (see Supporting Information—Adhesion Mechanics).²⁶

Contact Angle Goniometry.

Contact angle (CA) measurements with ultrapure water (Thermo Scientific MicroPure UV/UF, 18.2 ΩM·cm) were conducted with a Biolin Scientific Attension Theta Optical Tensiometer, fitted with a C301 motorized up/down movement, a C201 automatic single liquid dispenser, a Hamilton 1001 LT 1.0 mL syringe, and Hamilton 30 gauge blunt-tipped needle. One Attention software was used to analyze video capture (6.9 FPS) of water drops automatically dispensed at a rate of 0.5 μL/s to a maximum volume of 10 μL and determine the CA. Measurements were collected at ambient temperature and humidity. Advancing (ACA or CA) and receding contact angle (RCA) were determined by statistical analysis (R 4.3.2,³⁸ the tidyverse package,³⁹ and previously described protocols⁴⁰). Reported ACA and RCA values were determined by averaging five water drops per sample.

Supplementary Material

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.5c05934.

Weights in a Jar_Crop_LowRes-converted.avi (AVI)

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Details regarding synthesis (macromonomer, cross-linker, bottlebrushes, and networks), GPC, analysis of force/displacement curves to calculate the elastic modulus, PIF, additional DMA data, additional tensile data, and a description of how loss tangent is used to measure defects in NCIs (PDF)