Transformable Materials: Structurally Tailored and Engineered

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Transformable Materials: Structurally Tailored and Engineered Macromolecular (STEM) Gels by Controlled Radical Polymerization Julia Cuthbert,† Antoine Beziau,† Eric Gottlieb,† Liye Fu,† Rui Yuan,† Anna C. Balazs,*,‡ Tomasz Kowalewski,*,† and Krzysztof Matyjaszewski*,† †

Department of Chemistry, Center for Macromolecular Engineering, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Chemical Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: Structurally tailored and engineered macromolecular (STEM) gels constitute part of an emerging field of smart materials. STEM gels are polymer networks containing latent initiator sites available for postsynthesis modification. STEM gels synthesized by controlled radical polymerization (CRP) are presented. First, reversible addition−fragmentation chain transfer (RAFT) polymerization was used to copolymerize (meth)acrylate monomer, di(meth)acrylate cross-linker, and inimer for the subsequent atom transfer radical polymerization (ATRP) grafting-from process. The resulting STEM gels were infiltrated with a second monomer, which formed side chains grafted from the inimer sites by photoactivated ATRP. This approach permits significant spatial and temporal control over the structure of the resulting material. Here, the technique was used to transform primary STEM gels into single-piece amphiphilic and hard/soft materials.

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INTRODUCTION There is considerable interest in designing materials capable of postsynthesis modification,1,2 where an initially synthesized, primary “framework” can subsequently be altered to meet the requirements of multiple applications. Materials permitting such postsynthesis alteration are, however, challenging to fabricate. Polymer gels and networks present a potentially viable material for such tailorable frameworks because of their stimuli-responsive behavior1,3 and tunable mechanical properties.4−8 Moreover, due to their reversible swelling properties, polymer gels are “open systems,” which allow for the diffusion of nutrients or the introduction of new functionalities.9 Here, we show that structurally tailored and engineered macromolecular (STEM) gels prepared by controlled radical polymerization (CRP) permit facile postmodification and thus provide a route for new hierarchical, functional materials. In particular, with this level of postprocessing, the STEM gels are attractive candidates for tissue engineering,10,11 actuators,12,13 and 3D printing.14 STEM gels are polymer networks containing latent initiator sites (inimers) available for postsynthesis modification.15,16 The primary gel network was infiltrated with a secondary component (monomer) and subsequently functionalized by growing polymer side chains from the network to produce a modified STEM gel (Scheme 1). In other words, we refer to the initial network as “STEM-0” and the network with side chains as “STEM-1”. Like biological stem cells, the STEM-0 networks are undifferentiated templates (analogous to “mother” cells) that can be differentiated by introducing a variety of polymer © XXXX American Chemical Society

side chains, which impart the STEM-1 gels (“daughter” cells) with distinct properties. For example, in Scheme 1, the postsynthesis modifications involve grafting polymer side chains from bromoester inimers via atom transfer radical polymerization (ATRP).17−19 Previously, we reported STEM gels prepared by conventional free radical polymerization (FRP) and incorporation of a photoactive inimer based on 2-hydroxy-4′-(2-hydroxyethoxy)2-methylpropiophenone (Irgacure 2959).15 This paper expands on that system and other prior work based on grafting polymeric side chains from a network.16,20−22 The first STEM gel was a macroporous gel that was functionalized with hydrophobic, flourescent, conducting, or temperature responsive components.16 In addition, low-Tg side chains have been shown to act as a deluent, creating supersoft elastomers.22−25 Finally, relaxation dynamics were studied by comparing a primary hydrogel network and its modified networks containing linear polymer side chains grafted from ATRP initiator sites.21 The use of a CRP method to prepare the STEM gels permits control of the mesh size (i.e., network segments or distance between cross-link points) and composition of the network to produce a more homogeneous architecture than that obtained by FRP.26−30 In a more ordered structure, the properties are more uniform throughout the network, and the fraction of weak points caused by inhomogeneity is reduced. In FRP gels, high Received: February 28, 2018 Revised: April 28, 2018

A

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Scheme 1. (Top) Steps To Modify the STEM-0 Gels by Infiltrating Monomer and Catalyst and Then by Photo-ATRP with Possible Spatial Control To Produce STEM-1 Gels; (Bottom) Grafting Side Chains (Purple) from the Networks Using the Inimer (Red)

95%, Aldrich), methyl methacrylate (MMA, 99%, Aldrich), and poly(ethylene glycol) dimethacrylate (PEO750DMA, average molecular weight 750, Aldrich), were passed through basic alumina column to remove radical inhibitors prior to use. Ethyl α-bromoisobutyrate (EBiB, 98%, Aldrich), dichloromethane (DCM, ACS grade, Fisher Scientific), copper(II) bromide (CuBr2, 99%, Aldrich), 2,2′-azobis(4methoxy-2,4-dimethylvaleronitrile) (V70, Wako), 4-cyano-4[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CDTP, 97%, Aldrich), 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPADB, 97%, Aldrich), toluene (HPLC grade, Fisher Scientific), N,N-dimethylformamide (DMF, ACS grade, Fisher Scientific), water (H2O, HPLC grade, Fisher), deuterated chloroform (CDCl3, 99.8%, Cambridge Isotope Laboratories), deuterated dimethyl sulfoxide (DMSO-d6, 99.8%, Cambridge Isotope Laboratories), and tris[2(dimethylamino)ethyl]amine (Me6TREN, Aldrich) were used as received. Irga-MA was prepared as previously reported.15 HEMAiBBr was synthesized (see Supporting Information, Figure S1). Glass slides (Borosilicate, McMaster-Carr) were treated with Rain-X antiadhesive to prevent gel adhesion to the glass. Molds were made by sandwiching a silicone rubber seal (McMaster−Carr extreme temperature silicone rubber 1 mm thick) between two glass plates. General Procedure for STEM-0 Gel Synthesis (For Details, See Supporting Information). The monomer, PEO750DMA crosslinker, HEMA-iBBr inimer, and the RAFT agent were dissolved in toluene (Vmonomer = Vsolvent) in a Schlenk flask and degassed by nitrogen bubbling for 20 min. The radical initiator, V-70, was added air-free. This pregel solution was transferred to a degassed mold under nitrogen using a purged syringe and placed in water bath heated at 46 °C for 3 days. Conversion was determined by extraction of the unreacted monomer in 1H NMR solvent. The STEM gels were dialyzed in DCM. The obtained gels were first dried under air, then in an oven at 35 °C for 24 h, and finally placed in a desiccator under vacuum. General Procedure for Postsynthesis Modifications To Produce STEM-1 (For Details, See Supporting Information). The dried STEM gels were weighted and swollen overnight in an infiltration solution containing the secondary monomer, catalyst (CuBr2/Me6TREN), and solvent (DMF) (Vmonomer = Vsolvent) overnight (24 h). The infiltrated STEM gels were weighed and placed in a mold. The mold was covered and degassed under nitrogen for 20 min and then irradiated with a UV lamp (365 nm, 5 mW/cm2) for 4− 8 h. For the PBMA and the hydrophilic side chains conversion determination, a portion of a swollen gel was immersed in CDCl3 and vortexed to extract unreacted secondary monomer before and after UV irradiation. The monomer conversion was determined by 1H NMR. The modified STEM gels were dialyzed in DCM, dried under air, then in an oven at 35 °C for 24 h, and finally placed in a desiccator under vacuum. The gels were weighed again after 5 days in the desiccator.

molecular weight fractions form rapidly and nonhomogeneously, leading to dense network clusters, so that the final material contains various domains and mesh sizes.31,32 In CRP gels, the polymer chains grow concurrently and more uniformly. Extending this to the postsynthesis modifications, the CRP side chains are more uniform throughout the material, and their degree of polymerization (DP) can be controlled temporally in photoinduced ATRP (photo-ATRP).33−35 Photo-ATRP uses light as an external stimulus to photochemically reduce the radical deactivor complex (halide-Mtn+1/ ligand) to the activator complex (Mtn/ligand). Because of their potential spatial/temporal control, photoreactions are an effective method of tuning polymer networks.36,37 Recently, the degree of network cross-linking was controlled by photoinduced [4-4] cycloaddition of anthracene-terminated copolymers in order to tune the mechanical properties of elastomers.38 In addition, simulations have shown the potential of using photoinitiated CRP techniques for tailoring network architecture.39 For example, materials were modified by inserting trithiocarbonate moieties into the original network that allowed for postsynthesis network expansion.40 The entire process was referred to as “living additive manufacturing”. Similar chemistry was employed for self-healing or rearrangeable networks.41−44 Herein, the STEM-0 networks were prepared by reversible addition−fragmentation chain transfer (RAFT) polymerization and incorporated an ATRP initiator (inimer) available for orthogonal modifications. The advantage of combining both CRP techniques is that the ATRP inimer was directly polymerized into the network, and no postsynthesis deprotection step was needed. These highly tunable polymeric modifications were shown to transform hydrophobic STEM-0 into amphiphilic STEM-1 gels with the introduction of hydrophilic side chains. In addition, a temperature and pH response, as well as a raising or lowering of Tg, can be introduced. Building on our previous work on stackable gels,45−48 single-piece amphiphilic and flexible “flap” STEM gels were produced.

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EXPERIMENTAL SECTION

Materials. n-Butyl acrylate (BA, 99%, Aldrich), n-butyl methacrylate (BMA, 99%, Aldrich), 2-(dimethylamino)ethyl methacrylate (DMAEMA, 98%, Aldrich), 2-hydroxyethyl methacrylate (HEMA, 99%, Aldrich), 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA, B

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Scheme 2. Synthesis of the STEM-0 Gel Networks by Thermally Initiated RAFT Polymerization Using a Methacrylate Monomer, Cross-Linker, and Inimer (Red)

Swelling Measurements. To characterize the swelling capability of each gel (i.e., with and without side chains), each sample was dried under vacuum in a desiccator at room temperature for 7 days, with mass ranging from 15 to 40 mg, and then was immersed in excess solvent (water) at room temperature (22 °C). After 24 h, the samples were weighed. The swelling ratio was calculated as

swelling ratio =

WS WD

(GPC). The GPC system used a Waters 515 HPLC pump and a Waters 2414 refractive index detector using PSS columns (Styrogel 102, 103, and 105 Å) with THF as the eluent at a flow rate of 1 mL/ min at 35 °C using a linear poly(methyl methacrylate) (PMMA) standard. A MelodySusie UV lamp was used for the initial experiment for the postsynthesis modifications. Mechanical properties of the primary and modified STEM gels in the dry state were assessed using an Anton Paar MCR-302 rheometer fitted with a parallel plate tool. Disk-shaped samples of gels with a thickness of 1−2 mm and diameter D = 3.5−8 mm were subjected to periodic torsional shearing between two parallel plates under a constant normal load of 1 N. The analyses were performed in the linear viscoelastic response region of the studied samples. The frequency sweeps were carried out at room temperature (22 °C) at a constant applied shear strain of 0.1% (γ) over a frequency range 0.1− 100 rad/s. The temperature sweeps were carried at a constant ramp of 2 °C/min with a constant applied shear strain of 0.1% (γ) and a frequency (ω) of 6.28 rad/s. Mechanical properties of primary and modified gels were also assessed in compression mode. Disk-shaped samples of gels with a thickness of 1−2 mm and diameter D = 3.5−8 mm were placed between the parallel plates and subjected to a normal load increasing linearly in time from 0.25 to 20 N for softer samples or starting at 5 N and increasing to 30 N for more rigid samples. The loading rate was 0.1 N/s. The compression test data were recorded as load F vs the distance between the plates d. The initial linear portions of load vs distance curves were converted into the stress σ = F/(π × (D/2)2) and compression strain ε = (d0 − d)/d0, where the initial distance between the plates, d0 (corresponding to the initial sample thickness), was determined by the extrapolation to zero load. The Young’s modulus for compression, E, was calculated as a slope of the linear region of constructed stress−strain curves. Raman microscopy was performed using an XploRA ONE 532 confocal Raman microscope with a 532 nm laser and 50× objective. The STEM gel spectra were normalized to the integral of intensity between 1210 and 1260 cm−1 corresponding to the CS peak.

(1)

where WS and WD are the weights of the swollen and dry gels, respectively. This experiment was done in triplicate, and the range of the three values was reported as the uncertainty. The temperature responsive swelling was investigated as described at 5 °C increments from 25 to 60 °C in a water bath. The gels were placed in water (pH 8) or Tris/HCl buffer (pH 4) and allowed to equilibrate in 20 mL vials. The vials containing the gels in excess solvent were immersed in a water bath and heated in 5 °C increments from 25 to 60 °C. The gels were equilibrated at each temperature point for at least 3 h between measurements. Determination of the Side Chain DP. The theoretical molecular weight (MW) of the STEM-0 gel repeat unit was defined as the MW of a segment of the polymer chain without cross-linking. It was determined as follows:

MW repeat = (M)(M w ) + (X)(750 g/mol) + (inimer)(279.13 g/mol) + (RAFT) (2) where M = monomer, either BMA or MEO2MA, X = equiv crosslinker PEO750DMA, and RAFT = Mw of the RAFT agent, either CPADB or CDTP, which was 1 mol equiv in all cases (Table S1). The moles of initial STEM gel (just the primary network) used for each postsynthesis modification were determined as follows:

⎛ g ⎞ ⎟ mol STEM gel = mass STEM‐0 (g) ÷ MW repeat unit ⎜ ⎝ mol ⎠

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(3)

RESULTS AND DISCUSSION The STEM-0 gels were prepared by copolymerizing a (meth)acrylate monomer, di(meth)acrylate cross-linker, and (meth)acrylate ATRP initiator (inimer) by thermally initiated RAFT polymerization (Scheme 2 and Figure S2). The purpose of combining the two different CRP methods was to incorporate ATRP initiators (inimers) directly into the STEM-0 gel matrix and avoid the need for a subsequent deprotection step. Initially, STEM-0 gels were prepared by ATRP using the photoactive Irgacure moiety (Irga-MA) as the inimer (Scheme S1). However, generated acyl and isopropanol radicals reacted with Cu(II) and were therefore ineffective for postsynthesis ATRP side chain growth (Figures S3 and S4, Scheme S2). In photo-ATRP, the delivered photons excite the Cu(II)/ligand complex. Then, an electron is transferred, reducing the Cu(II) to Cu(I), and the ATRP begins. In this

The moles of inimer per STEM-0 gel were calculated as follows: mol inimer = mol STEM‐0 × equiv inimer per repeat unit

(4)

Assuming 100% inimer initiation efficiency, the DP of side chains in STEM-1 was calculated as secondary monomer per inimer. The mass of the secondary monomer incorporated was determined by gravimetry.

DP side chain = monomer per inimer =

2nd monomer (g) ÷ M w 2nd monomer mol inimer

( molg ) (5)

1

Material Characterization. H nuclear magnetic resonance (NMR) measurements were performed on a Bruker Avance 300 MHz spectrometer and used to determine the conversion of monomer in CDCl3. Molecular weight (Mn) and molecular weight distribution (Đ, Mw/Mn) were determined by gel permeation chromatography C

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Macromolecules way, radicals are generated from the reversible transfer of the Br atoms between the growing polymer chains and the Cu/ligand complex. A comparison of STEM-0 networks by FRP and RAFT polymerization was performed, which showed the quick formation of FRP microgels and the initial linear chains, followed later by high molecular weight fractions in RAFT (Figure S5). This demonstrated that RAFT polymerization produced a more controlled network. Several RAFT STEM-0 gel formulations were prepared by varying inimer density and the ratio of monomer to cross-linker, henceforth referred to as the target “mesh size”, which is the statistical distribution of monomer to cross-linker (Table 1). Initially, small mesh Table 1. Formulations of the STEM-0 Gels Prepared by RAFT Polymerization entry label 1 2

B40 B80

a

monomer

monomer/cross-linker/ inimer/RAFT agent (mol equiv)

monomer convb (%)

target mesh sizec

BMA BMA

122/3/10/1 250/3/10/1

68 83

40 80

a

B = BMA; subscript no. = mesh size. bDetermined by extraction of unreacted monomer in CDCl3 and 1H NMR. cStatistical monomer units between cross-links.

networks were synthesized. However, these were found to be unsuitable for postsynthesis modifications due to the low concentration of inimer sites and the high cross-link density (Table S2 and Figure S6). The STEM-0 gel networks were prepared with either hydrophobic n-butyl methacrylate (BMA) (Table 1) or the mildly hydrophilic 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) (Table S1). Postsynthesis Modifications. To graft the polymer side chains from the networks, the STEM-0 gels were infiltrated with a solution containing a second monomer, catalyst, and solvent (DMF) (Scheme 1). Then, the side chains were grown by photo-ATRP. An advantage afforded by photo-ATRP is spatial/temporal control over the modifications. It was possible to create single-piece materials containing different domains. The temporal control over the DP of the grafted side chains was achieved by infiltrating four B80 samples (Table 1) with a solution containing BMA monomer. The STEM-0 gel B80 samples were infiltrated with 62 equiv of BMA to 1 equiv of inimer (Table S3). The samples were exposed to UV light (λ = 365 nm) to perform photo-ATRP for 0.25−3 h, and the conversion of BMA was determined by extraction of unreacted monomer and subsequent 1H NMR (Figure 1A and Table S1). Initially, the polymerization rate was faster inside the infiltrated STEM-0 gel, but the conversion leveled off as the irradiation time progressed. In solution, however, the conversion of BMA was linear and consistent with ATRP polymerizations (Figure 1B, Table S4 and Figure S7). To investigate how the side chains transformed material properties, B40 and B80 were grafted with the n-butyl acrylate (BA) and hydrophilic monomers 2-(dimethylamino)ethyl methacrylate (DMAEMA) and hydroxyethyl methacrylate (HEMA) to produce modified STEM-1 gels (Table 2 and Table S5). The degree of polymerization (DP) of the side chains was determined by (1) extraction of the unreacted monomer and (2) gravimetric analysis. The DP was calculated assuming a 100% inimer initiation efficiency. In addition to the single grafted polymers (entries 1−5 and 7), PDMAEMA/ PHEMA copolymer side chains were grown (entry 6).

Figure 1. Conversion vs time (h) of PBMA in the network and in solution. (A) PBMA side chains grown from STEM-0 gel network B80. (B) Solution polymerization of PBMA chains.

The primary hydrophobic STEM-0 gels and modified hydrophilic STEM-1 gels were analyzed by Raman spectroscopy (Figure 2A). The STEM-1 gel spectra showed an increase in the aliphatic region (approximately 2850 to 3050 cm−1), corresponding to the grafted side chains and revealed the presence of the N−CH3 peaks in the PDMAEMA STEM-1 gels (2775 and 2830 cm−1). For a comparison, B40 were infiltrated with DMAEMA, catalyst, and solvent and analyzed prior to light exposure (Figure 2B). The characteristic DMAEMA N− CH3 stretches were observed in the infiltrated STEM-0 and STEM-1 gel, B40_D. Moreover, the vinyl peaks at 1640 and 3110 cm−1, present in the infiltrated sample, were not found in the STEM-1 gel. This confirmed that the mass increased determined by gravimetry was due to covalently bonded polymer chains, not unreacted monomer. Having confirmed the addition of polymer side chains, swelling measurements in water provided a fast, efficient way to assess the effect of PDMAEMA and PHEMA on the STEM gel physical properties. The PBMA STEM-0 gels were hydrophobic, swelling less than 10% in water (Figure S6). Adding hydrophilic side chains was hypothesized to transform the STEM-0 network into amphiphilic STEM-1 gels, which were swollen in HPLC grade water. In all cases, the side chains increased the hydrophilicity (Figure 3A,B). The PHEMAmodified gels swelled less than PDMAEMA ones, as did the denser network B40_D. In addition, PDMAEMA is both temperature- and pHsensitive.49 For the pure PDMAEMA grafts, a lower critical solution temperature (LCST) was observed around 45 °C (red D

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Table 2. Summary of the Postsynthesis Modifications by Grafting Hydrophilic Side Chains To Produce Modified STEM-1 Gels

a

entry

labela

grafted side chain

conv infiltrated monomerb (%)

DP side chain (1H NMR)

DP side chain (gravimetry)

wt %

1 2 3 4 5 6 7

B40_D B40_B B80_BM B80_D B80_H B80_DH B80_B

PDMAEMA PBA PBMA PDMAEMA PHEMA PDMAEMA-stat-PHEMA PBA

59 19 53 68 54 66 30

16 3 33 79 17 18-stat-18 19

17 ∼1 36 58 19 N/A ∼1

75 5.7 43 77 48 78 1

Refer to Table 1. bDetermined by extraction of unreacted monomer in CDCl3.

Figure 2. Raman scattering spectra of (A) the B80 STEM-0 gel and hydrophilic STEM-1 gels and (B) the B40 STEM-0 gel, infiltrated with a solution containing DMAEMA, and B40_D.

Figure 3. Swelling ratios of the hydrophilic STEM-1 gels in water. (A) The swelling from 25 to 60 °C. The range of the three values is the uncertainty. (B) Images of primary B40 and B40_D in the dry and swollen states. The pink color is due to the RAFT agent, CPADB.

open and closed circles Figure 3A), but the presence of HEMA affected the PDMAEMA LCST behavior. At lower pH, the protonated amine group raised the LCST. The same STEM-1 gels were swollen in a pH 4.0 buffer, and the swelling ratio was measured over a temperature range of 25−60 °C (Figure S8). The swelling of all the STEM-1 gels containing PDMAEMA increased. For example, B80_D increased from 3.5 to 11, and no LCST was observed between 25 and 60 °C. The swelling of the STEM-1 gels in water and buffer demonstrated a transformation of material properties. Mechanical Properties. The side chains were also expected to stiffen or soften the materials, depending on the choice of second monomer. To assess the impact of the grafted side chains on mechanical properties, the STEM gels were characterized in the dry state by dynamic mechanical analysis (DMA) using frequency and temperature sweeps (Figure S9 and Figure 4). Figure 4 shows the temperature dependence of the storage (G′) and loss (G″) modulus and the damping

factor, tan(δ) (tan(δ) = G″/G′). All the materials exhibited behavior typical of cross-linked networks. At lower temperatures, the STEM gels existed in a stiff, glassy state. As the temperature was increased, the moduli decreased as the gels passed through their glass transition temperature (Tg; observed in the tan (δ) local maximum), leading to a soft, rubbery plateau. The two STEM-0 gels, B40 and B80, showed similar behavior. The G′ and G′’ were observed to decrease over a temperature sweep from 20 to 80 °C. The Tgs were observed at ∼37 and ∼47 °C, respectively, in agreement with general expectations for a PBMA network, with the higher molecular weight network having a higher Tg. A self-similar STEM-1 gel was also prepared by grafting B80 with PBMA side chains (Figure S10). The side E

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Figure 4. Temperature dependence (6.28 rad/s) on the storage (G′) and loss (G″) moduli (A, B) and tan (δ) (C, D). STEM-0 gels: B40 and B80 (black). STEM-1 gels: B40_D and B80_D (red); B80_H (green); B80_DH (blue); and B40_B and B80_B (purple).

Tg for the PDMAEMA-stat-PHEMA STEM-1 was raised to 85 °C, which is approximately the average of the Tg of both polymers and consistent with a copolymerization. However, a significant shoulder was observed at 61 °C as result of the PBMA network’s Tg. As with the B80_H modified gel, this indicates some phase separation between the side chains and network. Therefore, by choosing the appropriate second monomer, the STEM-1 gel mechanical properties may be tailored and tuned to the desired specifications. Spatially Differentiated Materials. Photomediated polymerization offers the advantage of spatial control over the postsynthesis modifications. Having demonstrated that the STEM-0 could be transformed from hydrophobic to amphiphilic STEM-1 gels, we wanted to design a single-piece, spatially differentiated material that contains both the unmodified (STEM-0) and modified (STEM-1) domains. By infiltrating B80 with DMAEMA (as previously described) and covering half of the STEM-0 gel during photo-ATRP, a single piece, amphiphilic material was produced without the need for an interfacial linker or gluing agent (Figure S11). The same technique was used to create a flexible “flap” STEM gel, composed of adjacent hard and soft regions (Figure 5). To that end, a PMEO2MA STEM-0 network (M80), which is soft and flexible at room temperature (Tg = 0 °C), was infiltrated with a solution containing methyl methacrylate (MMA). As with the single piece amphiphilic gel, half of the surface was covered during photo-ATRP. This resulted in grafting PMMA side chains (approximately 16 wt %) that stiffened half of the STEM gel, creating a flexible flap, which could be bent 100 times without rupturing.

chains caused mild stiffening at room temperature, as seen in modulus values at low frequencies. Introducing PDMAEMA side chains also stiffened the material. It is noteworthy that the PDMAEMA side chains did not significantly affect the Tg from B80 to B80_D, and similar behavior was observed for the B40 and B40_D combination. Therefore, it was possible make a hydrophilic STEM-1 gel, while maintaining similar physical material properties to the original STEM-0. The same stiffening was observed for the copolymerization of DMAEMA and HEMA (B80_DH). Interestingly, the introduction of HEMA did not increase the modulus. Rather, for the pure PHEMA STEM-1, the initial G′ was 17 MPa and plateaued until the temperature approached 60 °C. The modulus decreased only by approximately 1 order of magnitude from room temperature to 120 °C. Finally, the networks could also be softened by the addition of a lower Tg polymer, PBA. Comparison of the tan(δ) traces elucidated how the side chains affected the network Tg. In both PBMA/DMAEMA combinations and PBMA/PBA combinations, a single tan(δ) maximum values was observed, indicating good miscibility between side chains and the network backbone. The introduction of HEMA, however, resulted in phase separation due to less favorable interactions between the PBMA backbone and PHEMA side chains. For the PHEMA grafted network, two local maxima were observed in the tan(δ): at 57 °C, attributed to the PBMA network, and 113 °C, attributed to the side chains. Moreover, the introduction of PHEMA and the side chains significantly shifted the material’s Tg up to above 100 °C and increased elasticity, as evidenced by the low tan(δ) values (tan(δ) < 1 over the entire temperature range). Similarly, the F

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Figure 5. Images of the “flap” STEM gel synthesized by grafting PMMA side chains from one-half of the material. The yellow color is due to the residual RAFT agent, CDTP. (A) Flexible M80 gel. (B) STEM gel with one-half containing PMMA side chains and the other half unmodified. (C) STEM gel used as a flap. (D) Temperature sweep on pristine B80 and covered half. (e) Temperature sweep on irradiated half with PMMA side chains. The measurements were performed at a constant frequency ω = 6.28 rad/s and at a constant strain = 0.1%.

Figure 6. Dual grafted STEM-1 gel with the top half containing PDMAEMA side chains and the bottom half containing PBMA side chains. The gel was swollen in a water solution containing the pink rhodamine B as a dye. Temperature sweeps revealed that each half has a different Tg. The measurements were performed at a constant frequency ω = 6.28 rad/s and at a constant strain = 0.1%.

Both halves of the flap STEM gel were thoroughly characterized by DMA. In comparison to the primary and covered M80, the PMMA side chains increased the shear

modulus by an order of magnitude (Figure S12). After compression testing, it was determined that the Young’s modulus was 1.4 and 1.3 MPa for the primary and covered G

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regions, respectively, and 13.7 MPa for the PMMA grafted half (Figure S13). In addition, the covered half showed the same temperature dependence as the primary material (Figure 5D) with the Tg = 0 °C and a rubbery plateau beginning at 20 °C. After adding PMMA side chains to the network, that material became stiff at room temperature. The Tg increased to 53 °C, and the rubbery plateau occurred above 100 °C. The modified half was also less lossy, as evidenced by the lower maximum tan(δ) value. Overall, this STEM-1 gel’s behavior was similar to a copolymer. Considering that linear PMMA undergoes the Tg around 100 °C, the modified half’s Tg is approximately an average of the network and side chains. Despite this, the two polymers were not completely miscible. A minor tan(δ) peak was observed at ∼−10 °C, which suggested some separation between the PMEO2MA and PMMA (inset Figure 5E). Further exploring STEM gel versatility and broadening the range of new mechanical and physical properties added to the materials, dual grafted STEM-1 gels were created. This variant involved growing different polymer side chains from two halves of the STEM-0 gel. To ensure that the monomers were infiltrated into separate parts of the gel, M80 was swollen in immiscible liquids, toluene containing BMA as the top organic layer and water containing DMAEMA as the bottom aqueous layer (Table S6 and Figure S14). To assess the change in hydrophilicity, the dual grafted STEM-1 gel was swollen in deionized water (DI) containing rhodamine B as a dye (Figure 6). On swelling in DI water at room temperature, the PDMAEMA region’s length increased from approximately 9 to 14 mm. The small increase in the swelling of the PBMA half from 5 mm in length to 6 mm was attributed to the contribution from the PMEO2MA backbone (Figure S15). As previously observed, attaching side chains to the primary networks led to a change in the modulus and a shift in the Tg. Both dual grafted domains were characterized by DMA temperature sweeps. The M80 and PDMAEMA side chains were miscible, resulting an increase in the Tg from 0 °C (primary, Figure S16) to 8 °C. In contrast, the M80 were found to be incompatible from the two Tg observed at 13 and 52 °C. In both instances, the introduction of side chains with higher Tgs than the original network increased the Tg of the material. However, the PMEO2MA and PBMA phase separated, whereas the PMEO2MA and PDMAEMA did not.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00442. 1 H NMR spectra, STEM gel synthesis details, swelling ratios, GPC traces, DMA frequency and temperature sweeps, and stress/strain curve (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.M.). *E-mail: [email protected] (T.K.). *E-mail: [email protected] (A.C.B.). ORCID

Eric Gottlieb: 0000-0002-7948-8031 Anna C. Balazs: 0000-0002-5555-2692 Tomasz Kowalewski: 0000-0002-3544-554X Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Department of Energy (Grant ER45998) is gratefully acknowledged. We acknowledge Chiaki Nishiura for his work on determining the interaction between Cu(II) and Irgacure generated radicals, Mateusz Olszewski for his assistance in designing the ToC, Michael Martinez for his assistance taking pictures, Sivaprakash Shanmugam for his discussion of results, and Travis Fu for assisting with the M80 swelling ratio.

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These highly tunable STEM gel networks were synthesized by RAFT polymerization and incorporation of ATRP initiator inimers. The primary STEM gels were transformed into modified materials with new properties by grafting polymer side chains from the networks by photo-ATRP. By carrying out spatial control over the modifications, single-piece amphiphilic and flap materials were produced without the need for a gluing agent. The presence of spatially differentiated components within the STEM gels is expected to allow these materials to exhibit a wide range of mechanical properties and active responses. For example, local modifications that create soft/ hard regions and gradients patterns could be used in soft robotics to mimic skin and muscle. We envision that these materials will have applications in actuators and, eventually, in 3-D printing and tissue engineering. H

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