“Grafting-Through”: Growing Polymer Brushes by Supplying

Article pubs.acs.org/Macromolecules

“Grafting-Through”: Growing Polymer Brushes by Supplying Monomers through the Surface Reihaneh Mohammadi Sejoubsari,† Andre P. Martinez,† Yasemin Kutes,‡ Zilu Wang,§ Andrey V. Dobrynin,*,§ and Douglas H. Adamson*,†,‡ Department of Chemistry and ‡Institute of Materials Science Polymer Program, University of Connecticut, Storrs, Connecticut 06269, United States § Department of Polymer Science, University of Akron, Akron, Ohio 44304, United States Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 17, 2018 at 10:21:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: We introduce a “grafting-through” brush polymerization mechanism where monomers are supplied through the surface on which the initiators are attached rather than from solution as in the “graftingfrom” technique. This is accomplished by attaching the initiator to the surface of a dialysis membrane and supplying monomers through the membrane to the growing brush. This avoids the growth of very long chains while promoting the growth of shorter chains by reversing the monomer concentration gradient found in the commonly used graftingfrom technique, where monomer concentration is lowest at the substrate and highest in the surrounding solution. Reversing this monomer concentration gradient results in shorter chains experiencing a higher local monomer concentration than longer chains, thus speeding up their growth relative to the longer ones. It is shown by AFM that brush layers made by this method are thicker and have lower roughness than brushes made by a grafting-from approach. Coarse-grained molecular dynamics simulations of brush polymerizations with monomers supplied through a permeable substrate provide insight into the mechanism of the grafting-through brush growth process. Simulations show that it is possible to obtain a brush layer with a chain dispersity index approaching unity for sufficiently long chains. FTIR, contact angle measurements, SEM, and kinetic studies are used to characterize and elucidate the growth mechanism of brushes synthesized by the new grafting-through strategy.

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INTRODUCTION Polymer brushes consist of polymers attached to a surface in high enough density that they are forced to extend from the substrate due to interactions with neighboring chains.1 This chain extension is responsible for the extraordinary properties that differentiate polymer brushes from surfaces simply coated by a polymer layer. Currently brushes are used as templates for bioelectrocatalytic systems, organic electronic devices,2 pHsensitive sensors for flow control across brush-coated channels,3 designer surfaces with a high degree of control over lubrication, adhesion, and wetting properties,4 low friction surfaces,5 and protective layers for nanoparticle stabilization. In addition, nature has used and relied on polymer brushes for demanding applications for millions of years: an example being the articular cartilage found on the sliding surface of joints, whose surfaces have been shown to owe their low friction properties to brush polymers.6,7 For synthetic approaches, there are two basic mechanisms for growing polymer brushes: “grafting to”8 and the more common “grafting from”.9−14 In the grafting-to approach, premade chains are attached to the substrate, providing brushes with well-characterized chains. However, attachment of the first few chains significantly hinders attachment of additional polymers © 2016 American Chemical Society

to the surface, resulting in low grafting density. In contrast, the grafting-from technique starts with initiators bound to a surface and grows the brush polymers from those initiators using monomer in the surrounding solution. This allows for a much higher grafting density but produces dispersity in chain lengths as a result of the monomer concentration gradient that develops in the brush layer.15−17 A schematic of the general approach can be found in Figure 1A. The concentration of monomer is highest in solution and lowest at the surface, meaning that the growing ends of the longer chains see a higher concentration of monomer than do the shorter chains. This results in the longer chains growing faster than the shorter chains, increasing chain length polydispersity.17,18 Thus, grafting-to gives lower brush dispersity and lower grafting density, while grafting-from gives high grafting density and higher dispersity. Nature does not use either method: the brushes found in nature are produced by the underlying cells of the cartilage,6,7 a “grafting-through” approach to brush growth. Received: January 26, 2016 Revised: March 4, 2016 Published: March 16, 2016 2477

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Scheme 1. Outline of the Surface Functionalization and Polymerization Reactions for the Grafting-Through Approach

the Supporting Information, indicate that the functionalization reaction occurs predominately on the outer surface of the membrane. The effect of supplying monomer through the surface is illustrated in Figure 2. The images shown are atomic force microscopy (AFM) studies of, from left to right, the surface of a blank dialysis bag membrane that has been subject to the same treatments as the other surfaces, except that no initiator is attached, a dialysis membrane after the growth of polymer brushes using a traditional grafting-from approach, and on the right a membrane after the growth of polymer brushes using the grafting-through mechanism. The only difference between Figures 2B and 2C is that in Figure 2C the monomer is placed in the dialysis bag and is thus supplied to the growing chains from the bottom. In Figure 2B, the monomer is supplied from the solution surrounding the bag rather than from inside the bag and thus is added from the top down. In addition to AFM, the morphologies of the surfaces are also observed by field emission scanning electron microscopy (FESEM, (JEOL 6335)) at 5 kV. Figures 3A and 3B show the surface of the dialysis membrane before and after polymerization. It is clear that the surface of the membrane changes after modification by polymer brushes. It can also be seen that the polymer brush uniformly covers the entire membrane surface. Changes in the surface properties of the dialysis membrane are apparent from variations of the contact angle of a water droplet on the surface as measured by a Ramé-Hart goniometer (Figure 4). The unfunctionalized dialysis membrane is found to be extremely hydrophilic, as expected, with θa = 28°. A larger contact angle, θa = 67°, is observed for the initiator-modified membrane due to the initiator making the membrane more hydrophobic. For the polymer-grafted membrane, θa = 36°, consistent with the hydrophilic nature of poly(N-hydroxyethyl acrylamide) grafted to the surface. The presence of the brush layer also modifies the adhesive properties of the substrate. To quantify this effect, we have measured force−distance curves for unmodified dialysis bags and polymer-grafted dialysis pages. In these experiments, an AFM tip modified by the attachment of a 5 μm silica sphere is detached from the surface and the retraction profile measured. This profile provides information on strength of adhesion between the polymer and the modified AFM tip. The result of this experiment shows different adhesion for nongrafted and poly(N-hydroxyethyl acrylamide)-grafted dialysis bags due to different interactions between the silica bead and these surfaces. For the polymer-grafted surface prepared by diffusing the monomer through the dialysis bag, the adhesion force is observed to be 0.33 ± 0.03 μN. For nongrafted dialysis bag we observe a stronger interaction 0.42 ± 0.01 μN, in agreement with the hydrophilic and dense structure of dialysis membrane.

Figure 1. Schematic representation of polymer brush growing approaches (monomers are shown by red circles, the red lines denote growing polymer chains, the green triangles represent surface bound initiator on the left, and active polymer chain ends on the right, and substrates are shown by blue rectangles). (A) Illustration of the “grafting-from” approach. (B) Illustration of “grafting-through” approach. (See text for details.)

The importance of chain density and polydispersity arises from the role chain extension plays in the properties of polymer brushes. Low grafting density and high polydispersity of the grafted chains weaken the interactions between chains and can result in an increase of the brush layer roughness and compressibility, adversely affecting the very properties responsible for the unique adhesive, lubricating and wetting properties of the brush layers. In this paper we develop a grafting-through approach that overcomes some of the inherent limitations in the traditional brush polymerization techniques. In our approach, the growth of chains tethered to the substrate is sustained by supplying monomers through the substrate, as illustrated in Figure 1B. This inverts the concentration gradient found in grafting-from approaches, where monomer concentration is higher further away from the surface, and solves the problem of longer chains growing faster than shorter ones by providing shorter chains with a higher monomer concentration than longer ones. This turns the tables and results in a much more uniform chain length distribution while maintaining a high number density of chains on the surface.

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RESULTS AND DISCUSSION A requirement for our grafting-through approach is that the monomers are uniformly supplied through the membrane surface. To satisfy this condition, we have utilized a dialysis membrane as a substrate for growing the brush layer. Monomers passing through the dialysis bag membrane emerge through the surface of the bag rather than through discrete pores; thus, monomers are supplied uniformly at the base of the growing polymer chains. Grafting-through polymerized brushes are prepared by first reacting 2-bromoisobutyryl bromide with the hydroxyl groups of a cellulose dialysis bag for 24 h at room temperature.19 The reaction scheme is shown in Scheme 1. The monomer, hydroxyethyl acrylamide (PHEA) in water, is then placed inside the bag, the bag sealed, placed in a water/methanol solution containing CuBr, CuBr2, and the ligand PMDTA, and allowed to react for 24 h. Prior to analysis, the dialysis bags are bath sonicated for 3 h with water, acetone, and THF in order to remove all surface-attached impurities. AFM studies, shown in 2478

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Figure 2. AFM images of (A) the surface of a pristine dialysis membrane treated exactly as those membranes with polymer, but with without initiator; (B) polymer brushes grown on the surface of a dialysis membrane from monomers supplied from the solution in a normal “growing from” approach; (C) polymer brushes grown on the surface of a dialysis membrane with monomer supplied from underneath, through the dialysis membrane surface. The vertical scale bar varies from −50 to 50 nm.

Figure 3. SEM images of (A) initiator functionalized surface of a dialysis bag prior to polymerization and (B) surface of dialysis bag after growth of polymer brush by monomer diffusing from inside the bag. Figure 5. FTIR spectra of substrate (blue), substrate after attachment of ATRP initiator (black), substrate after growth of polymer brushes from the membrane (red), and polymer grown separately (purple) as a control.

To monitor the reactions shown in Scheme 1, FTIR is employed to study chemical differences between the initiator functionalized and polymer-functionalized substrates, with details presented in the Supporting Information. Figure 5 shows the FTIR spectra of a blank membrane surface (blue), the membrane surface after attachment of the initiator (black), the surface after growth of the polymer brush (red), and PHEA polymer grown separately as a control (purple). Of particular note is a peak centered just above 1700 cm−1, attributed to the carbonyl absorption characteristic of an ester group containing a bromine on an adjacent carbon, found only for the initiator functionalized surface (black). All four spectra show absorption around 1650 cm−1. Additionally, the broad band around 3300− 3500 cm−1 implies that not all of hydroxyl groups on the surface of the membrane are reacted with initiator. After the membrane is surface-grafted with PHEA (red), the disappearance of the 1700 cm−1 peak indicates the reaction of the surface bound initiator and the presence of a polymer brush.

Although FTIR confirms the reaction steps of Scheme 1, it says nothing about the kinetics or how those chains grew. In order to show that the monomer in the bag does not simply diffuse into the surrounding solution and grow the brush by a conventional grafting-from mechanism, we carry out kinetic studies investigating the monomer concentration in the surrounding solution during the growth of polymer chains. As a control, we also monitor the concentration of monomer in an identical system, but without a growing brush. This is accomplished by placing 5 mL of monomer solution in dialysis bags submerged in 150 mL of a water/methanol mixture. At various intervals, 1 mL aliquots are collected from the surrounding solution, and 1H NMR, as shown in the Supporting Information, is used to determine the concentration

Figure 4. Contact angles measured using distilled water: (A) pristine dialysis bag, (B) initiator-grafted dialysis bag, and (C) polymer-grafted dialysis bag. 2479

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Macromolecules of monomer. Figure 6 shows the results. Monomer is seen to diffuse much more slowly through the growing brush, with the

Figure 7. Cross-section profiles indicative of the brush height relative to adjacent surface scratched with an AFM tip: (A) height of brush grown with the grafting-through approach and (B) brush grown with grafting-from approach with the same reaction time.

1 do suggest a brush density several times greater than that observed by conventional top-down monomer addition. For comparison, and shown in Figure 7B, we use the same approach to determine the grafting density of a grafting-from polymer synthesized from a dialysis membrane using the same conditions as the grafting-through brush, but with monomer supplied from the surrounding solution rather than through the membrane. The same possible sources of error noted for the grafting-through brush also apply. Finding a flat area of the brush, we determine the height to be approximately 20 nm. Using the same values used previously for the grafting-through brush, we find the grafting density to be 1.3 chains/nm2, still higher than expected, but much closer to typical literature values. To illustrate the concept and elucidate the mechanism of the grafting-through technique, we perform coarse-grained molecular dynamics simulations of brush formation with monomers supplied through a permeable membrane, as shown in Figure 8 (see Supporting Information for simulation details). In our simulations all species are modeled as beads with diameter σ. The permeable membrane with thickness 4.44σ is modeled by a rigid network having diamond structure, a lattice parameter a = 4.44σ, and a nearest interbead distance of 0.64σ (see Figure 8). Changing the distance between network vertices controls the permeability of the membrane. The monomers and solvent are represented by identical Lennard-Jones particles with diameter σ, interacting through pure repulsive WCA-potential with the interaction parameter εLJ = 1.0kBT (where kB is the Boltzmann constant and T is the absolute temperature) to ensure their miscibility. The monomers are initially placed below the membrane with solvent particles distributed above the membrane with equal densities (ρ = 0.729σ−3) as shown in Figure 8. To model monomer supply through the membrane, we have performed two sets of simulations. In the first set of simulations, the constant monomer flux through membrane is maintained by moving the permeable membrane down with a constant velocity, thus squeezing monomers out of the monomer reservoir located under the membrane (see Figure 8). Note that for these simulations we keep the position of a piston, made of a single layer of LJ beads and placed to separate monomers from solvent (see Figure 8), constant. In the second set of simulations, to facilitate monomer supply, we fix the location of the permeable membrane and move a piston up with a constant velocity. During simulation runs, either the membrane or piston is moving with a constant velocity v = 0.01σ/τ and 0.02σ/τ (where τ is the standard LJ time). The system is periodic in x, y, and z directions with dimensions 40σ × 40σ × 500σ. The periodicity along the z-axis ensures the

Figure 6. Comparison of monomer concentration versus time for dialysis bag functionalized with ATRP initiator and pristine dialysis bag. The vertical axis is the monomer concentration outside the bag divided by the expected concentration if all monomer inside the bag had diffused though. Both trials are supplied with monomer by diffusion from the interior of the bag.

concentration appearing to plateau in both cases around 20 h. Additionally, the final concentration of monomer in the outer solution remains lower for the system containing growing polymer chains. While it is not possible to rule out some contribution to the growing chains from monomers that have found their way to the outer solution before adding to a growing chain, it is clear the monomers interact with the growing brush as they diffuse from the inner to the outer solutions and do not simply diffuse out of the bag and then react from solution in a normal grafting-from mechanism. The brush grafting density can be estimated20 from the number-average molecular weight, Mn, of chains occupying a volume h/Σ and the density ρ of the polymer: Mn/NA = ρh/Σ (where NA is Avogadro’s number, h is the brush height, and Σ is the brush grafting density). The height of the brush layer is typically determined by elispometry of a brush grown on a hard, flat surface such as a silicon wafer.21 The dialysis membrane surface used in our work presents a challenge for the accurate measurement of height because of the roughness and softness of the dialysis bag, so we estimate the value by removing part of the brush from the surface by gentle scratching with an AFM tip in contact mode.4 This allows for a height comparison between the brush and the surface. We use a value of 10K for Mn, obtained by synthesizing a solution grown polymer using the same monomer concentration and reaction time as used for growing the brush. For a grafting-through grown brush, a value of 60 nm is estimated from the cross-sectional analysis between unscratched (brush surface) and scratched regions (bare substrate), as shown in Figure 7A. Using this value for the height, 1.1 g/cm3 as the density of PHEA, and 10 kg/mol as the Mn, the grafting density is found to be 3.9 chains/nm2. This value is very high, as typical values in the literature are less than 1 chain/nm2.20−22 Although we expect our grafting-through mechanism to result in very dense brushes on the surface of the substrate, the exact density measured may be affected by imprecise measurement of the brush height or our assumption of solution grown polymers being representative of the brush polymers not being valid. However, the AFM images in Figure 2480

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Figure 8. Snapshot of the simulation box during brush polymerization. Growing brush chains are shown in green; monomers and solvent beads are shown by yellow and blue points, respectively. The membrane and piston beads are colored in gray.

constant density distribution during a simulation run as movement of the piston up is accompanied by solvent particles filling the gap behind the piston, thus keeping the size of the simulation box filled with particles constant. Polymerization of the brush chains is initiated from the catalytic sites attached to the membrane. The catalytic sites are distributed over the surface of the membrane with density ρg equal to 0.0625σ−2, 0.16σ−2, and 0.25σ−2. A new monomer is added to a polymer chain with a probability 0.1 when a monomer is found within a capture radius rcup = 21/6σ. We check the proximity of the monomer to a catalytic site every 5.0τ. A new added monomer then becomes a new catalytic site. This monomer addition procedure is commonly used in simulations of brush polymerization.23 The connectivity of monomers into polymer chains is maintained by the FENE bonds with the spring constant K = 30kBT and the maximum bond length R0 = 1.5σ. The repulsive part of the bond potential is given by the pure repulsive WCA potential with εLJ = 1.0kBT and rcut = 21/6σ. Figure 9A shows the evolution of the number fraction distribution during the simulation run when monomers are supplied by moving a permeable membrane down with velocity 0.01σ/τ (our grafting-through approach). As one can see, the peak of the distribution gradually moves toward the larger degree of polymerization N with increasing time while the distribution remains narrow. For comparison, in Figure 9B we show the evolution of the number fraction distribution function obtained during simulations of brush polymerization using the conventional grafting-from approach. There is a significant

Figure 9. (A) Evolution of the number fraction distribution function of chains in a brush layer during polymerization by “grafting -through” technique for grafting density ρg = 0.16σ−2 and membrane moving with velocity 0.01σ/τ. (B) Evolution of the number fraction distribution function of chains in a brush layer during polymerization by “grafting-from” technique for grafting density ρg = 0.16σ−2. (C) Dependence of the dispersity index of the brush chains on the number-average degree of polymerization Nn. For three different brush grafting densities 0.0625σ−2 (black symbols), 0.16σ−2 (red symbols), and 0.25 σ−2 (green symbols) obtained during simulations with moving membrane (filled symbols) and moving piston (open symbols) and for two membrane and piston velocities v = 0.02σ/τ (triangles) and v = 0.01σ/τ (squares). The solid line is given by the equation y = 1 + (x + 1)−1 − 2(x + 1)−2. Inset shows dispersity index dependence for brush polymerization using the “grafting-from” technique for two brush grafting densities 0.16σ−2 (red symbols) and 0.25σ−2 (green symbols).

broadening of the distribution with time due to the screening of shorter chains by longer ones. Note that in grafting-from 2481

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bromide to attach the ATRP initiator as outlined in Scheme 1. Dialysis bags were immersed in a solution containing triethylamine (TEA) (2 mL) and a catalytic amount of 4-(dimethylamino)pyridine (DMAP) in tetrahydrofuran (THF), followed by the addition of 2-bromoisobuturyl bromide (6 g) added dropwise to the solution at 0 °C. The reaction was allowed to proceed overnight at room temperature while stirring. The dialysis bags were washed and ultrasonicated in acetone, THF, and water to remove residual reactants prior to their use in polymerization. Polymerization from Initiator-Modified Membrane. Dialysis tubing clips were used to seal one end of an initiator-modified dialysis membrane tube. N-Hydroxyethyl acrylamide (3 g) was then added to 3 mL of water and poured into the bag and sealed with a clip on the open end. The bag was placed in a water/methanol solution (6:1 ratio) containing CuBr, CuBr2, and pentamethyldiethylenetriamine (PMDTA) as the ligand and allowed to react for 24 h at room temperature. All grafting reactions were performed under an argon atmosphere. Adding CuBr2 prior to polymerization minimized chain termination by recombination. In some cases “sacrificial initiator” was added at the beginning of the reaction to produce free polymer in the solution. After polymerization, the grafted dialysis bags were thoroughly rinsed with THF, acetone, and water and dried under vacuum at 50 °C overnight. Atomic Force Microscopy (AFM). AFM was used to visualize the surface morphology of polymer brushes. Imaging of the surfaces was performed in tapping mode in air using a using an Asylum MFP-3D Multimode AFM using silicon tips (AC 160TS) with resonance frequencies of 200−400 kHz, according to the manufacturer. AFM Sample Preparation. A glass substrate was cleaned with acetone and then rinsed with purified water, ethanol, and chloroform. Double-sided tape was the attached to the center of the glass substrate, and 0.5 × 0.5 cm PHEA-grafted dialysis bags were placed on the tape followed by mounting into the AFM instrument. The laser signal was aligned, the deflection set point was set, and the cantilever and surface were engaged. Scan size was varied from 5 to 40 μm, and images were captured with slow scan. Kinetic Studies for the Diffusion of Monomer through Membranes. N-Hydroxyethyl acrylamide monomer was placed in a dialysis bag and then submerged in a flask that contained ligand, CuBr, CuBr2, and 150 mL of water/methanol mixture. The solution was stirred and maintained at room temperature. At designated intervals, 1 mL aliquots were collected. The monomer concentrations in the outer solution were determined using 1H NMR. As the concentration of PMEDTA ligand remains nearly constant, the monomer concentration can be determined relative to this standard compound for each designated time interval. The same condition was applied for all dialysis bags.

simulations chains are growing faster than in the case of the grafting-through simulations. This is due to a slower monomer supply through the permeable membrane. The quality of the brush is described by the brush dispersity index D defined as the ratio of the weight-average degree of polymerization Nw to the number-average degree of polymerization Nn, so D = Nw/ Nn. The dispersity index of our brush system first increases with increasing number-average degree of polymerization and then begins to decrease, approaching unity as brush chains increase in length (see Figure 9C). The low dispersity of the brush chains is a result of the monomer supply to the growing chains from the bottom up. Note that both sets of simulations of the grafting-through approach produce close brush distributions. It is interesting to point out that chain ends are located close to the diffuse boundary separating regions rich with monomers and rich with solvent, indicating that the brush growth is controlled by the diffusion of monomers (see Figure 8 and Supporting Information for details). This allows chains to maintain the concentration of monomers in the vicinity of chain ends. For comparison, in the inset we show the dependence of the dispersity index obtained in simulations of the grafting-from method. It follows from this figure that with increasing brush grafting density the screening effect is manifested at the earlier stages of the brush polymerization, resulting in significant increase of the brush dispersity index. Thus, our computer simulations show that the graftingthrough brush polymerization technique has the potential to grow a brush layer with a dispersity index approaching unity with continued chain growth. Simulations of grafting-from systems give the opposite result: polydispersity of the brush layer increases with chain growth.15,24 Additionally, the simulations indicate the mechanism responsible and clearly differentiate the expected brush morphologies resulting from our grafting-through and the traditional approach.

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CONCLUSION We have introduced a grafting-through approach for polymerization of brushes that involves the attachment of initiator molecules to a monomer permeable membrane. This allows for monomer to be supplied to the growing polymer chain ends from the bottom, rather than from the surrounding solution. The effect is to invert the normal grafting-from monomer concentration gradient so that the monomer concentration is higher near the surface than it is in the surrounding solution. This means shorter chains see a higher concentration of monomers than do longer chains, increasing their rate of polymerization and minimizing the polydispersity of the chains in the brush. This mechanism thus combines the advantage of low polydispersity from grafting-to with the high density advantage of the grafting-from approach. In addition, the concentration of monomer is known to play a role in the rate of termination in ATRP,25 so increasing the concentration of monomer near the surface decreases the rate of termination, again leading to a decrease in the polydispersity of the brush polymers. This decrease in polydispersity will lead to low friction and low adhesion surfaces with tunable properties and morphologies and allow for the synthesis of brushes made of chains with complex chemical architecture and varying composition such as block copolymer brushes.

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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.6b00183.

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AFM and SEM images of the inside of the dialysis bag, example 1H NMR spectra from kinetic study shown in Figure 6, and simulation details (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.V.D.). *E-mal [email protected] (D.H.A.).

EXPERIMENTAL SECTION

Notes

Attachment of Initiator to Membrane Surface. Hydroxyl groups on the cellulose surface were reacted with 2-bromoisobutyryl

The authors declare no competing financial interest. 2482

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(19) Feng, Q.; Hou, D.; Zhao, Y.; Xu, T.; Menkhaus, T. J.; Fong, H. Electrospun Regenerated Cellulose Nano Fi Brous Membranes Surface- Grafted with Polymer Chains/Brushes via the Atom Transfer Radical Polymerization Method for Catalase Immobilization. ACS Appl. Mater. Interfaces 2014, 6, 20958−20967. (20) Yonet-Tanyeri, N.; Evans, R. C.; Tu, H.; Braun, P. V. Molecular Transport Directed via Patterned Functionalized Surfaces. Adv. Mater. 2011, 23, 1739−1743. (21) Yang, C.-T.; Wang, Y.; Frank, C. W.; Chang, Y.-C. Chemoresponsive Surface-Tethered Polypeptide Brushes Based on Switchable Secondary Conformations. RSC Adv. 2015, 5, 86113−86119. (22) Bhat, R. R.; Tomlinson, M. R.; Wu, T.; Genzer, J. SurfaceGrafted Polymer Gradients: Formation, Characterization, and Applications. Adv. Polym. Sci. 2006, 198, 51−124. (23) Bain, E. D.; Turgman-Cohen, S.; Genzer, J. Progress in Computer Simulation of Bulk, Confined, and Surface-Initiated Polymerizations. Macromol. Theory Simul. 2013, 22, 8−30. (24) Martinez, A. P.; Carrillo, J.-M. Y.; Dobrynin, A. V.; Adamson, D. H. Distribution of Chains in Polymer Brushes Produced by a “Grafting From” Mechansim. Macromolecules 2016, 49, 547−553. (25) Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2004.

ACKNOWLEDGMENTS This work was supported by NSF Grants DMR-1409710 and CHE-1310453 and an AGEP GRS supplement to Grant DMR1004576.

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REFERENCES

(1) Milner, S. T. Polymer Brushes. Science 1991, 251, 905−914. (2) Barbey, R.; Lavanant, L.; Paripovic, D.; Schüwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Polymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications. Chem. Rev. 2009, 109, 5437−5527. (3) Ayres, N. Polymer Brushes: Applications in Biomaterials and Nanotechnology. Polym. Chem. 2010, 1, 769−777. (4) Sui, X.; Zapotoczny, S.; Benetti, E. M.; Schön, P.; Vancso, G. J. Characterization and Molecular Engineering of Surface-Grafted Polymer Brushes across the Length Scales by Atomic Force Microscopy. J. Mater. Chem. 2010, 20, 4981−4993. (5) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahla, D.; Fetters, L. J. Reduction of Frictional Forces between Solid Surfaces Bearing Polymer Brushes. Nature 1994, 370, 634−636. (6) Zappone, B.; Ruths, M.; Greene, G. W.; Jay, G. D.; Israelachvili, J. N. Adsorption, Lubrication, and Wear of Lubricin on Model Surfaces: Polymer Brush-like Behavior of a Glycoprotein. Biophys. J. 2007, 92, 1693−1708. (7) Seror, J.; Merkher, Y.; Kampf, N.; Collinson, L.; Day, A. J.; Maroudas, A.; Klein, J. Articular Cartilage Proteoglycans as Boundary Lubricants: Structure and Frictional Interaction of Surface-Attached Hyaluronan and Hyaluronan-Aggrecan Complexes. Biomacromolecules 2011, 12, 3432−3443. (8) Zdyrko, B.; Luzinov, I. Polymer Brushes by the “Grafting to” Method. Macromol. Rapid Commun. 2011, 32, 859−869. (9) Jordan, R. Ulman, a. Surface Initiated Living Cationic Polymerization of 2-Oxazolines. J. Am. Chem. Soc. 1998, 120, 243− 247. (10) Kim, J.; Bruening, M. L.; Baker, G. L. Surface-Initiated Atom Transfer Radical Polymerization on Gold at Ambient Temperature. J. Am. Chem. Soc. 2000, 122, 7616−7617. (11) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. Surface-Initiated Anionic Polymerization of Styrene by Means of Styrene by Means of Self-Assembled Monolayers. J. Am. Chem. Soc. 1999, 121, 1016−1022. (12) Zhao, B.; Brittain, W. J. Synthesis of Tethered PolystyreneBlock-Poly(methyl Methacrylate) Monolayer on a Silicate Substrate by Sequential Carbocationic Polymerization and Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 1999, 121, 3557−3558. (13) Buchmeiser, M. R.; Sinner, F.; Mupa, M.; Wurst, K. RingOpening Metathesis Polymerization for the Preparation of SurfaceGrafted Polymer Supports. Macromolecules 2000, 33, 32−39. (14) Yamamoto, S.; Ejaz, M.; Tsujii, Y.; Matsumoto, M.; Fukuda, T. Surface Interaction Forces of Well-Defined, High-Density Polymer Brushes Studied by Atomic Force Microscopy. 1. Effect of Chain Length. Macromolecules 2000, 33, 5602−5607. (15) Turgman-Cohen, S.; Genzer, J. Simultaneous Bulk- and SurfaceInitiated Controlled Radical Polymerization from Planar Substrates. J. Am. Chem. Soc. 2011, 133, 17567−17569. (16) Wittmer, J. P.; Cates, M. E.; Johner, A.; Turner, M. S. Diffusive Growth of a Polymer Layer by in Situ Polymerization. Europhys. Lett. 1996, 33, 397−402. (17) Milchev, A.; Wittmer, J. P.; Landau, D. P. Formation and Equilibrium Properties of Living Polymer Brushes. J. Chem. Phys. 2000, 112, 1606−1615. (18) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Polymers at Interfaces: Using Atom Transfer Radical Polymerization in the Controlled Growth of Homopolymers and Block Copolymers from Silicon Surfaces in the Absence of Untethered Sacrificial Initiator. Macromolecules 1999, 32, 8716−8724. 2483

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