Mechanically Diverse Gels with Equal Solvent Content.

Mechanically diverse polymer gels are commonly integrated into biomedical devices, soft robots, and tissue engineering scaffolds to perform distinct yet coordinated functions in wet environments. Such multigel systems are prone to volume fluctuations and shape distortions due to differential swelling driven by osmotic solvent redistribution. Living systems evade these issues by varying proximal tissue stiffness at nearly equal water concentration. However, this feature is challenging to replicate with synthetic gels: any alteration of cross-link density affects both the gel's swellability and mechanical properties. In contrast to the conventional coupling of physical properties, we report a strategy to tune the gel modulus independent of swelling ratio by regulating network strand flexibility with brushlike polymers. Chemically identical gels were constructed with a broad elastic modulus range at a constant solvent fraction by utilizing multidimensional network architectures. The general design-by-architecture framework is universally applicable to both organogels and hydrogels and can be further adapted to different practical applications.

Introduction

The human body comprises a diverse family of mechanical phenotypes ranging from supersoft brain to supertough tendons (modulus in wet state, Ggel ∼ 102–107 Pa) while maintaining their water fraction within the narrow range 60–80 wt % (Figure a).[1,2] Different experimental parameters, including preparation concentrations, pH, and temperature, have been explored to control the equilibrium swelling ratio, Qeq, and modulus, Ggel, of synthetic gels.[3−6] However, no method exists that independently varies gel stiffness and swelling at constant chemical composition. In stark contrast with tissues, Qeq and Ggel of traditional linear polymer gels are intimately coupled through the degree of polymerization of network strands, nx (Figure b).[7−10] This mechano-compositional interdependence drives shape distortions, interfacial stress, and mechanical property drift in devices that rely on proximal gels, e.g., a bionic arm, where fatlike shock-absorbing paddings, musclelike actuators, and skinlike protective membranes adjoin to perform distinct yet coordinated functions.[11−22] Furthermore, extensive swelling is required to achieve soft gels (Ggel ∼ 1 kPa), which is detrimental to mechanical stability yet can be prevented by decoupling Qeq and Ggel.

Figure 1

Diverse gel mechanics at equal solvent concentration. (a) Five-order variation of the tissue shear modulus, G, at nearly the same water fraction of ca. 70 wt %. (b) Augmenting the strand length, ∼nx, in linear polymer networks leads to concurrently increasing the gel’s equilibrium swelling ratio as Qeq = Veq/V0 ∼ nxα and decreasing the shear modulus as Ggel ∼ nx–1−(α/3), where nx is the degree of polymerization (DP) of the network strand.[10] (c) In brush networks, the modulus reduces with side chain length (∼nsc) and grafting density (∼ng–1), while the swelling is hindered by rapid strain-stiffening of the architecturally pre-extended network strands with nsc/ng, where nsc and ng are the DPs of side chains and backbone spacers between neighboring side chains. The tuning of distinct brush triplets [nsc, ng, nx ] allows Ggel variation at constant Qeq and vice versa as shown by the iso-Q and iso-G dashed lines, respectively.

We endeavor to invert these conventional relationships by incorporating side chains onto network strands, which imparts two distinct effects on network deformation (Figure c). First, side chains dilute the cross-link density to promote softening.[22−27] Second, steric repulsion between densely grafted side chains pre-extend network strands, which obstructs swelling.[28,29] Unlike the one-dimensional Qeq (Ggel) correlation in linear chain networks (Figure b), a variation of the architectural triplet [nsc, ng, nx] in brushlike gels covers a two-dimensional [Qeq × Ggel] domain of independently varied swelling and stiffness. This not only broadens the range of accessible (Qeq, Ggel) combinations but also allows an adjustment of the modulus at a constant solvent fraction, and vice versa (Figure c).

Results and Discussion

To validate the concept of Ggel variation at constant Qeq, we synthesize a library of brush elastomers with systematically varied [nsc, ng, nx] combinations as well as chemical compositions of side chains (Figure a, the Experimental Section, and Figures S1–S5). Respectively, the architectural parameters are varied within nsc = 0–41, ng = 1–10, and nx = 50–1200, while the chemical compositions include poly(n-butyl acrylate) (PBA), polyethylene glycol (PEG), polyisobutylene (PIB), and poly(dimethylsiloxane) (PDMS) as well as PBA–PEG copolymers (Table , Tables S1–S3). The most comprehensive series is composed entirely of PBA, including side chains, backbone, and cross-linkers, which is vital for the swelling consistency, strongly depending on monomer–solvent interactions. Samples are polymerized with stoichiometric ratios of side chains, spacers, and cross-links corresponding to the appropriate [nsc, ng, nx], where H1 NMR verifies the precise nsc (Figures S1, S2, and S4). The nx values are assessed by measuring the shear modulus of dry elastomers (Table S1 and Figures S6–S9).[30] The network uniformity is validated by the agreement between the experimental and theoretical elongations-at-break, λmax,exp ≅ λmax,theo for different [nsc, ng, nx] triplets (Figure S10).[30]

Figure 2

Brush network synthesis and deformation response. (a) A stoichiometric mixture of macromonomers, cross-linkers, and backbone spacer monomers are injected into elastomer molds to prepare uniform films of brush elastomers with well-defined [nsc, ng, nx] combinations (Section S1). This approach allows the incorporation of hydrophobic and hydrophilic side chains to control water uptake. (b) True stress–elongation curves of brushlike networks with different [nsc, ng, nx] triplets as indicated (Figure S7), where the Young’s modulus, E0, is defined as the stress–strain slope at λ → 1, and the strain-stiffening parameter β characterizes the increase of the modulus with deformation (eqs S2 and S4). The concurrent enhancement of softness and strain-stiffening of brushlike networks may result in an intersection of stress–strain curves, suggesting the existence of structurally dissimilar elastomers generating identical elastic stress, which controls the swelling-induced network deformation.

Table 1

Mechanical Properties of Dry Elastomers and Swollen Gels of Linear, Combs, and Bottlebrush PBA, PEG, and Their Copolymers

material	nscb	ngc	φ–1d	nxe	Gf (kPa)	βg	E0h (kPa)	Qeq,expi	Ggelj (kPa)
PBA	0	1	1	50	134	0.1	591	8.0	70.7
	0	1	1	100	67	0.04	344	10.6	15.7
	0	1	1	200	39	0.03	236	13.7	13.2
	11	1	12	50	10.3	0.21	43	7.8	7.6
	11	1	12	100	5.6	0.15	21	9.7	3.2
	11	1	12	200	2.4	0.1	8.3	14.2	1.4
	11	3	4.7	50	32.9	0.14	122	8.1	16.2
	11	3	4.7	100	18.9	0.1	66	11.4	6.9
	11	3	4.7	200	8.6	0.07	35	17.5	2.8
	11	5	3.2	100	7.6	0.06	37	18.9	2.5
	11	5	3.2	200	2.9	0.02	18	31.5	0.6
	11	10	2.1	50	47	0.11	166	10.2	20.4
	11	10	2.1	100	27.4	0.06	111	16.0	7.5
	11	10	2.1	200	15.8	0.03	72	20.1	4.1
	23	2	12.5	50	12.3	0.14	46	10.5	5.2
	23	2	12.5	100	5.5	0.12	20	14.4	2.0
	23	2	12.5	200	2.3	0.08	7.8	23.5	0.6
	23	4	6.8	50	19.4	0.2	80	9.7	16.0
	23	4	6.8	100	9.7	0.12	35	14.8	4.6
	23	4	6.8	200	3.8	0.07	12.6	25.0	0.8
	23	10	3.3	50	47.3	0.17	185	9.0	33.3
	23	10	3.3	100	26.7	0.11	94	13.2	11.2
	23	10	3.3	200	13.3	0.07	44	19.2	3.4
	41	2	21.5	50	4.2	0.23	18.4	13.7	2.0
	41	2	21.5	100	1.9	0.14	7	20.6	0.5
	41	2	21.5	200	0.7	0.07	2.1	38.0	NAk
	41	5	9.2	50	6.1	0.15	23	15.7	NA
	41	5	9.2	100	2.4	0.09	8.2	26.1	NA
	41	10	5.1	50	17.4	0.15	66	12.9	NA
	41	10	5.1	100	9.6	0.09	33	18.8	NA
PEG	9	1	10	150	15.2	0.093	52	11.3	20.5
	9	1	10	300	10.5	0.1	36	12.4	8.6
	9	1	10	600	8.1	0.085	27	13.9	7.3
	9	1	10	900	7.2	0.072	24	15.9	5.1
	9	1	10	1200	6.3	0.07	21	16.9	4.7
	19	1	20	150	10.9	0.204	45	11.6	12.0
	19	1	20	300	6.6	0.18	26	14.3	5.6
	19	1	20	600	3.6	0.16	13.8	16.0	5.3
PEG-co-PBAa, 90–10	9	1	10	100	18.3	0.104	63.9	NA	NA
PEG-co-PBAa, 80–20	9	1	10	200	9.5	0.077	31.8	NA	NA
PEG-co-PBAa, 60–40	9	1	10	400	3.8	0.052	12.3	4.1	31.3
PEG-co-PBAa, 80–20	9	1	10	400	3.7	0.071	12.3	7.1	11.4
PEG-co-PBAa, 90–10	9	1	10	400	7.2	0.054	23.3	11.2	8.0

Random graft copolymers with PEG and PBA side chains at different compositions, e.g., 90–10 corresponds to 90 wt % PEG.

Degree of polymerization of side chains from NMR.

DP of backbone spacer between neighboring side chains defined by n-BA molar fraction.

Brush parameter φ–1 = 1 + nsc/ng.

Targeted DP of brush backbone between cross-links defined by molar fraction of cross-linker, e.g., nx = 200 corresponds to 0.25 mol %.

Structural shear modulus from fitting stress–elongation curves with eq S1.

Firmness parameter from fitting stress–elongation curves with eq S1.

Young’s modulus as a stress–elongation slope at λ → 1 (eq S4).

Equilibrium swelling ratio calculated as a ratio of the swollen (12 h) to dry volumes, Qeq = V/V0.

Shear modulus of gels at Q = Qeq measured by tensile test (ε̇ = 0.005 s–1 and 20 °C) as Ggel = σtrue/(λ2 – λ–1).

Ggel is not reported for fragile gels due to large error.

Even though knowing the architectural parameters is vital for understanding molecular mechanisms of gel behavior, the equilibrium swelling ratio, Qeq, can be predicted from the mechanical properties of dry elastomers without specifying [nsc, ng, nx] values. For linear networks, Qeq is given by the Flory–Rehner model as Qeq ∼ G–3/8, where the elastomer shear modulus G ∼ nx–1 is proportional to its cross-link density.[10] In contrast, yet similar to biomaterials,[31] the strongly nonlinear deformation response of brushlike strands is instead described by two elasticity parameters (Figure b): G, structural shear modulus; and β, strain-stiffening parameter, both depending on [nsc, ng, nx] (eqs S3–S6).[32] The corresponding swelling ratios are deduced from the generalized Flory–Rehner equation by explicitly considering nonlinear network elasticity:[29]Here, K is a chemistry-specific constant, e.g., ΚPBA = 65.9 ± 1.1 kPa3/8 for PBA elastomers (Figure S14), and G(I1) is the instantaneous shear modulus of a dry elastomer, which increases with the extension of network strands aswhere I1 = λ2 + 2/λ = 3Q2/3 is the first deformation invariant, characterizing the elongation of network strands from the initial undeformed state, which universally describes both uniaxial deformation, λ = L/ L0, and isotropic expansion, Q = V/ V0 (eq S1).[33] By solving eqs and 2 for G at I1 = 3Qeq2/3, we obtain a functional dependence of Qeq on the mechanical characteristics of dry brush elastomers, G and β:

This equation is used to construct a 2D swelling map of PBA networks by drawing theoretical isochoric lines that correspond to β(G) dependencies for specific Qeq values, e.g., Qeq,theo = 3, 10, 20, 40, and 60 (Figure a). The obtained map empowers the ability to predict Qeq,theo for any given (G, β) pair in comparison to experimental observations (Table ). For instance, the Qeq,exp values of the [41, 2] series demonstrate proximity to the Qeq,theo = 10, 20, and 40 lines. The model is further verified by plotting the anticipated Qeq,theo and measured Qeq,exp values for different [nsc, ng, nx] combinations (Figure b).

Figure 3

Breaking the swelling-mechanics interdependence with brush architecture. (a) The strain-stiffening parameter, β, is plotted against the structural shear modulus, G, of PBA elastomers for selected values of the equilibrium swelling ratio Qeq,theo = 3, 10, 20, 40, and 60 (solid lines), using eq with ΚPBA = 65.9 ± 1.1 kPa3/8 (Figure S14c). The isochoric lines are overlaid with experimentally measured β(G) data for different brush architectures (Table ). Only selected [nsc, ng] series are shown to avoid crowding. The numbers next to the [41, 2] symbols correspond to Qeq,exp, while the diagonal dashed arrow corresponds to β ≅ Gφ–3/2/(kBT).[29] (b) The correlation between the experimentally measured (Table ) and theoretically predicted Qeq values of PBA elastomers suggests Qeq,exp ≅ Qeq,theo within 95% confidence. (c) For samples with different nx, Qeq increases with φ–1 = 1 + nsc /ng, the average degree of polymerization of side chain per repeat unit of the backbone, linearly proportional to mass grafting density. The data points depict the experimentally measured Qeq,exp(φ–1) from Table , where each vertical row of points corresponds to the samples with different [nsc, ng] (see the symbol legends in part a) at a given nx (Table ). The Qeq,theo(φ–1) relationships (dashed lines) are obtained from β, G(nsc, ng, nx) correlations established elsewhere (Figure S16).[32] (d) True stress versus uniaxial elongation (ε̇ = 0.005 s–1, 20 °C) of selected PBA gels swollen in toluene. At a similar equilibrium swelling ratio of Qeq = 14 ± 1, the gels display significant variation in shear modulus, Ggel, from 1.4 to 13.2 kPa corresponding to the moduli of dog lung and blood vessel, respectively.[34,35] (e) The measured Ggel versus their Qeq of PBA gels follow the Ggel ∼ Qeq–3 scaling (dashed lines). The gray bar corresponds to the gel samples in part d. The Ggel values are independently measured by uniaxial elongation and shear (Figure S11), including a separate study of the solvent evaporation effect (Figure S12) and isotropic swelling (Figure S13).

The [β, G] map shows that the swelling capacity progressively increases toward the bottom-left corner that corresponds to brush networks with increasing nx and nsc/ng (dashed arrows). A structural interpretation of the observed trend is exposed by plotting Qeq versus φ–1 = 1 + nsc/ng for different nx (Figure c) using the known correlations between G, β, and the architectural triplet [nsc, ng, nx].[30,32] The theoretical dashed lines indicate two main correlations: (i) Qeq increases with side chain length (∼nsc) and grafting density (∼ng–1) for a given nx, and (ii) likewise, Qeq increases with nx for a given brush structure φ–1. The experimental data demonstrate good agreement with the predicted trends (symbols in Figure c).

The isochoric β(G) lines in Figure a suggest that different brush architectures with different modulus values may have the same Qeq. This prediction is corroborated by measuring the modulus of swollen PBA networks with different architectures including linear chain networks by tensile and shear tests (Figure d and Figure S11). For example, different networks with nearly the same Qeq ≅ 14 demonstrate a Ggel variation from 1.4 to 13.2 kPa corresponding to the [11, 1, 200] bottlebrush and [0, 1, 200] linear PBA networks, respectively (Figure d). A complete set of Ggel(Qeq) data is presented in Figure e. For individual [nsc, ng] pairs, all networks follow the well-established Ggel ∼ Qeq–3 scaling law indicated by the dashed lines,[10] which is consistent with computer simulations of bottlebrush gels (Figure S18b). However, at the same Qeq, the gel modulus decreases with increasing φ–1 or nsc due to the swelling-induced stiffening of bottlebrush strands, indicating a breakdown of the Flory–Rehner theory (Section S4: Nonlinear Swelling of Polymer Networks).

This concept can be extended to other organogels such as brush networks with PIB or PDMS side chains (Figure S15 and Tables S2 and S3) and hydrogels with PEG side chains (Figure a). Like the organogels, PEG brush hydrogels closely follow the theoretically predicted Qeq (Figure b), and their modulus decreases as Ggel ∼ Qeq–3 (Figure c). However, the PEG hydrogels are significantly (ca. 10×) stiffer than the PBA organogels at the same Qeq as the less bulky side chains impose smaller backbone dilution at a given nsc. Synthesizing softer PEG hydrogels that match the tissue range 1–50 kPa would mandate a relatively high swelling ratio of Qeq ≅ 10–30, which significantly exceeds the tissuelike Qeq ≅ 3. Moreover, reaching the biological swelling ratio would require a high cross-link density yielding stiff gels with Ggel ∼ 1 MPa as evident from extrapolating the scaling lines to Qeq ≅ 3 (Figure c). To make soft hydrogels with controlled swellability, we synthesize hybrid bottlebrush networks with a mixture of hydrophilic and hydrophobic side chains (Figure a). The aqueous Qeq of the copolymer networks is controlled by the PEG fraction, while their stiffness is conventionally defined by the [nsc, ng, nx] triplet (Table ). Due to their similar solubility parameters δ ≈ 19 ± 1 MPa1/2,[35,36] the short PEG and PBA side chains with nsc ≤ 10 are miscible, yielding graft copolymers with a homogeneous morphology as confirmed by optical transparency (Figure d, inset), random copolymerization with NMR (Figure S4), and differential scanning calorimetry measurements (Figure S5). This strategy permits elastomer design with similar deformation responses and variable water uptakes. For example, we can gradually reduce the swelling ratio from Qeq ≅ 11.7 to 3.4 by decreasing the PEG side chains fraction from 100 to 60 wt %, while maintaining Gdry ≅ 4 kPa (block arrow in Figure d). Even though the decrease in swelling ratio results in modulus augmentation, the PEG-c-PBA hydrogels remain softer than the pure PEG gels (Figure c). As shown by the dashed arrow, the PEG-c-PBA hydrogels traverse the [Qeq, Ggel] landscape toward lower Qeq values, while maintaining the modulus <100 kPa.

Figure 4

Replicating tissue softness with controlled water concentration. (a) Bottlebrush networks with mixed hydrophobic PBA and hydrophilic PEG side chains prepared by random copolymerization of PEG and PBA macromonomers (Figure a). (b) Correlation between the experimentally measured (Table ) and theoretically predicted Qeq values of PEG bottlebrush elastomers (ng = 1) with two different side chain lengths (nsc = 9 and 19) as indicated. (c) Swelling dependence of gel modulus for selected PEG hydrogels [nsc = 9 and 19, ng = 1 (■)] and PBA organogels [nsc = 11 , ng = 1 (▲); and nsc = 41, ng = 2 (▼)]. Blending hydrophilic and hydrophobic side chains in PEG-c-PBA hydrogels (◑) allows broadly tuning the swellability without significantly altering the gel modulus. The horizontal light blue bar depicts the modulus range of soft tissue. The dashed lines indicate the Ggel ∼ Qeq–3 scaling law. (d) Modulus of dry PEG, PBA, and PEG-c-PBA elastomers against the measured equilibrium swelling ratio. The green bar indicates the swellability range of biological tissues around Qeq ≈ 3. The pink arrow highlights architectural tuning of the swelling ratio at a constant shear modulus. The bottom inset shows stress–strain of samples that differ in chemical composition but share the same stiffness. The top right inset demonstrates the optical transparency of a PEG-c-PBA 60:40 elastomer.

Conclusion

In conclusion, we have developed a design-by-architecture platform that enables the independent control of polymer gel softness from the equilibrium solvent fraction without altering chemical composition. Furthermore, brush elastomers achieve high swelling ratios up to Qeq = 38, which is unprecedented for nonionic gels. Due to its universal nature, the architectural methodology is applicable to a broad range of polymer/solvent combinations including tissue-mimetic hydrogels that have significant implications in the biomedical field. We seek to expand this platform to stimuli-responsive biomaterials by incorporating specific side chains and dynamic linkers that would predictably regulate both elastic and viscoelastic properties at constant volume.

Experimental Section

Materials

1,6-Hexanediol dimethacrylate (>90%), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO, 97%), ethyl α-boromoisobutyrate (EBiB, 98%), ethylene bis(2-bromoisobutyrate) (2-BiB, 97%), copper(II) bromide (CuBr2, 99.999%), tris[2-(dimethylamino)ethyl]amine (Me6TREN), methacrylic acid (99%), potassium tert-butoxide (potassium t-butoxide, 98%), and tetrabutylammonium bromide (TBAB, 98%) were used as received from Sigma-Aldrich. n-Butyl acrylate (n-BA, 99%) was obtained from Sigma-Aldrich and purified by passing through a column of basic alumina (Sigma-Aldrich, activated, basic, Brockman I) to remove inhibitor. Acetonitrile, anhydrous methanol, dichloromethane, acetone, anisole, and N,N-dimethylacetamide were used as received from Sigma-Aldrich. In addition, α,ω-methacryloxypropyl-terminated poly(dimethylsiloxane) (DMS-R18, average molar mass of 5000 g/mol, Đ = 1.15) was obtained from Gelest and purified using basic alumina columns to remove inhibitor.

Synthesis of Potassium Methacrylate

Approximately 25 g of potassium methoxide was dissolved in 50 mL of cold methanol in a sealed 250 mL round-bottom flask equipped with a stir bar. An equal molar portion of methacrylic acid was added dropwise over the course of 10 min with stirring in an ice bath generating a white potassium methacrylate precipitate. The reaction mixture was then shaken vigorously for 5 min and left to equilibrate to room temperature overnight. The precipitate was separated by passing the mixture through a porous glass filter and was washed immediately with additional 25 mL of anhydrous methanol. Solvent was removed under a vacuum, and the potassium methacrylate was stored for later use.

Synthesis of Poly(n-butyl acrylate) Macromonomers

Poly(n-butyl acrylate) samples with different degrees of polymerization were synthesized by supplemental activation reducing agent (SARA) atom transfer radical polymerization (ATRP) followed by a postpolymerization functionalization displacing the bromine end group with potassium methacrylate. In a typical synthesis, to a 500 mL air-free Schlenk flask, 120 g (0.94 mol) of butyl acrylate was combined with Me6TREN (10 μL, 37 μmol), CuBr2 (8 mg, 36 μmol), and EBiB, 15.2, 7.3, or 3.7 g (0.078, 0.037, or 0.019 mol) depending on the desired nsc, and the sample was diluted with an equal volume of acetonitrile. The reaction mixture was then cooled with an ice bath, and oxygen was removed by bubbling nitrogen gas for 1 h. The polymerization was initiated by adding a stir bar equipped with a clean Cu0 wire and transferring the sample to a 45 °C mineral oil bath. The reaction was monitored by 1H NMR and stopped near 80% conversion with the addition of chloroform (Figure S1). Excess catalyst was removed by washing in water ∼11 times, and excess solvent was removed by rotary evaporation at 45 °C under reduced pressure and redissolved in 7 parts N,N-dimethylacetamide. Potassium methacrylate was added in large excess (>3 mol equiv), and the reaction was left to stir for 3 days, turning a faint yellow color. Chloroform and 100 mL of water were then added separating the polymer into the organic phase. The organic phase was then washed in water 11 times until it became clear. Solvent was removed by rotary evaporation. See NMR spectra in Figure S2 and a GPC analysis in Figure S3.

Synthesis of Poly(n-butyl acrylate) Cross-Linker

Poly(n-butyl acrylate) macro-cross-linkers were synthesized using an equivalent procedure to the poly(n-butyl acrylate) macromonomers. The one exception is that a difunctional 2-BiB ATRP initiator was used to polymerize n-BA such that the corresponding macromonomer was functionalized at both ends of the polymer chain. The nsc = 80 cross-linker was synthesized by combining 24 g (0.19 mol) of butyl acrylate, Me6TREN (2 μL, 7.4 μmol), CuBr2 (1.6 mg, 7.2 μmol), and 2-BiB (0.67 g, 1.9 μmol) and diluting the mixture to 50% with acetonitrile. The reaction was then cooled in an ice bath and degassed for 1 h with bubbling nitrogen gas. The polymerization was initiated by the addition of a Cu0 wire and transferred to a 45 °C oil bath until the reaction reached ∼80% conversion. The reaction was then terminated by the addition of 50 mL of chloroform and washed 11 times in water. Solvent was removed by rotary evaporation at 45 °C under reduced pressure. The polymer was then functionalized by the addition of 7 parts N,N-dimethylacetamide and a large excess of potassium methacrylate and left stirring for 72 h. 50 mL of chloroform and 100 mL of water were then added separating the polymer into the organic phase. The organic phase was then washed in water 11 times until it became clear. Solvent was removed by rotary evaporation at 45 °C under reduced pressure.

Synthesis of Linear Poly(n-butyl acrylate) Elastomers

Linear poly(n-butyl acrylate) elastomers were synthesized by mixing 4 g of n-BA, 1,6-hexanediol dimethacrylate (1, 0.5, and 0.25 mol %), and BAPO (12 mg) followed by diluting the solution to 50% with anisole. The mixture was degassed with bubbling nitrogen for 1 h and then injected in 1.3 mm thick molds and left to polymerize under ambient light conditions in a nitrogen atmosphere. After 24 h, the samples were removed from their molds and washed 3 times in toluene. Gel fractions, as the mass of the dried postwashed sample divided by the mass of the dried prewashed sample, were measured by washing small sections of unwashed films in toluene 3 times over the course of 72 h.

Synthesis of Bottlebrush and Comb Poly(n-butyl acrylate) Elastomers

Bottlebrush poly(n-butyl acrylate) elastomers were synthesized by combining macromonomer (4 g), macro-cross-linker (1, 0.5, and 0.25 mol %, targeted DP of brush backbone between neighboring cross-links defined by molar fraction of cross-linker), and BAPO (1.5 wt %) which were diluted to 50% in anisole. Nitrogen gas was used to purge the oxygen for 1 h, and then, the mixture was injected into 1.3 mm thick elastomer molds and left to polymerize overnight in a nitrogen atmosphere. The corresponding film was separated from its mold, and a small portion was set aside to measure the samples’ corresponding gel fraction as described above. The larger bulk part of the film was washed 3 times in toluene and dried prior to measurement. For comb elastomers, n-BA as a spacer was used with predetermined molar ratios; e.g., for an [11, 10, 100] sample [nsc, ng, nx], 4 g of nsc = 11 macromonomer, 3 g of n-BA spacer (9 mol equiv), 0.0339 g (0.05 mol equiv) macro-cross-linker, and 5 mg of BAPO were used.

Synthesis of Poly(ethylene glycol) and PEG-c-PBA Brush Elastomers

Poly(ethylene glycol) (PEG) brush elastomers were prepared by one-step polymerization of poly(ethylene glycol) methyl ether methacrylate macromonomer (Mn ∼ 500 and 950 g/mol, Sigma-Aldrich) with different molar ratios of poly(n-butyl acrylate) macro-cross-linker. The initial reaction mixtures contained 50 wt % macromonomers, different molar ratios of macro-cross-linker, 1.5 wt % BAPO photoinitiator, and paraxylene as a solvent. First, the mixtures were degassed by nitrogen (N2) bubbling for 30 min. Subsequently, to prepare films, the mixtures were injected between two glass plates with a 2.3 mm spacer and polymerized at room temperature for 12 h under N2. Films were then washed with tetrahydrofuran (two times with enough to immerse and fully swell the films, each time for 8 h) in glass Petri dishes. The samples were finally dried at room temperature. The conversion of monomers to elastomers (gel fraction) was between 90 and 98 wt % in every case. A similar recipe was followed to synthesize poly(ethylene glycol)/poly(n-butyl acrylate) brush elastomers.

Uniaxial Tensile Tests

Dogbone-shaped sections of films were punched out to bridge dimensions of 12 mm × 2 mm × 1 mm and were loaded onto an RSA-G2 DMA instrument (TA Instruments). The samples were subjected to uniaxial extension at room temperature under a constant linear strain rate of 0.005 s–1. Samples were stretched until a break occurred at the bridge. All data has been reported as a function of the true stress, σtrue, against elongation ratio λ = L/L0, the length of the elongated sample normalized to the original length. The data was fitted using the relationships identified by eq S1.

Swelling Properties

PBA and PDMS elastomers were swollen in toluene for 12 h. PEG and PEG-co-PBA brush hydrogels were swollen in water for 12 h. The mass of the samples was measured before and after swelling (M0 and M, respectively). The equilibrium swelling ratio was calculated as a volume ratio of a swollen sample to the dry sample, Qeq = V/V0 = 1 + (x – 1) ρs/ρp, where x = M/M0; ρs = 0.867 or 0.997 g/mL, toluene or water mass density; ρp, mass density of PBA, PDMS, PEG: ρPBA = 1.09 g/mL, ρPDMS = 0.965 g/mL, and ρPEG = 1.125 g/mL, respectively. Note: samples were swollen, washed, and dried eliminating any unreacted reagents before swelling tests of the pure elastomers were performed.