Photocleavage of Covalently Immobilized Amphiphilic Block

Article pubs.acs.org/Macromolecules

Photocleavage of Covalently Immobilized Amphiphilic Block Copolymer: From Bilayer to Monolayer Eda Gungor and Andrea M. Armani* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States ABSTRACT: We developed and verified a method to create a photocleavable smart surface. Using the graf ting to approach, we covalently attached an intelligently designed tailor-made diblock copolymer to a silicon wafer. The photocleavable moiety, o-nitrobenzyl (ONB) ester, was integrated into the copolymer at the junction point between the hydrophilic poly(ethylene oxide) (PEO) and the hydrophobic polystyrene (PS) chains. The well-defined azide bearing amphiphilic block copolymer was synthesized via a general stepwise strategy that combines atom transfer radical polymerization (ATRP) and copper(I)-catalyzed azide−alkyne cycloaddition reaction (CuAAC), ending with azidation. The azide end-functionalized copolymer chains were covalently bound to the alkyneimmobilized silicon wafer by CuAAC. The smart surface was exposed to UV irradiation, resulting in photocleavage of the grafted ONB linker. As a result of the photocleavage and subsequent removal of the o-nitrosobenzaldehyde bearing PEO, the PS layer remained on the surface. To confirm the behavior, film thickness and wettability changes were investigated before and after UV irradiation using AFM and contact angle measurements. Integration of photocleavable polymers through covalent grafting to solid surfaces contributes responsiveness to such materials that can find a wide array of applications in advanced devices.

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INTRODUCTION Smart surfaces created by surface modification of the substrates with responsive materials have gained significant attention and found widespread application in diverse fields.1−5 Smart surfaces exhibit dramatic responses to even slight changes in their surrounding environment. One unique and straightforward method to produce smart surfaces is to integrate a responsive polymer film to a solid surface. Polymers containing reactive functionalities allow the physical or chemical properties of the surface to be tuned.6 These polymers can be intelligently designed to selectively respond to numerous stimuli, including light, temperature, pH, ionic factors, or solvents.7−10 Not surprisingly, light has been the most widely used stimuli since it is noninvasive, clean, and facile, and it can be remotely controlled in real time. Therefore, surfaces created from photoresponsive polymers are the most widely applied category of smart surfaces.11 One approach for forming a photoresponsive smart surface is to incorporate a responsive unit directly into the polymer chain. The o-nitrobenzyl (ONB) group is a well-known and widely studied photoresponsive moiety. Upon irradiation with UV light, it is able to undergo bond cleavage and produce onitrosobenzyl derivatives.12 Given its predictability, it was initially used as a protecting group to improve control and yield in organic reactions.13−15 However, more recently its photocleavage behavior has been leveraged as a key component in the development of optically responsive smart surfaces.2,16−19 © XXXX American Chemical Society

While the selectivity to the stimuli of interest is important, the stability of the smart surface plays an important role as well. In the past years, noncovalent attachment processes have been extensively utilized for the preparation of smart polymer films.20 However, these films can be easily detached from the substrate, thus limiting the operating conditions of smart surfaces.21 One strategy for improving the stability of the film−substrate interaction is to chemically anchor the polymer to the surface. For this reason, polymers that are bound with strong covalent bonds are in demand. Generally, the covalent tethering of the polymer to the surface is accomplished using either the graf ting f rom or the graf ting to method.22 In the graf ting f rom process, the polymer chains are grown directly from surface-bound initiators. Because the polymerization occurs directly on the substrate, the control over the polymerization reaction is limited, resulting in polymers with high polydispersities. Moreover, the synthesis strategies are limited, further restricting the diversity of polymers that can be utilized, and it is extremely challenging to characterize the final polymer structure.22−24 In contrast, the graf ting to process is a particularly attractive and flexible strategy for surface modification. It allows welldefined, tailor-made polymers with a wide variety of functional groups to be attached to substrates. In addition, the grafting to approach offers the opportunity to utilize virtually any reaction Received: July 25, 2016

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DOI: 10.1021/acs.macromol.6b01609 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Syntheses of (a) Azide End-Functionalized Photocleavable PS-b-PEO Copolymer and (b) Alkyne-Anchored Surface

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conditions including nearly all polymerization techniques, condensation reactions, and highly robust and efficient click reactions to prepare a desired polymeric surface.25−28 On the basis of a balanced analysis of the grafting methods as well as the deposition strategies, we designed and verified a smart surface via the graf ting to approach. To optimize the surface response and demonstrate the flexibility of the strategy, we used an amphiphilic diblock copolymer structure that consisted of a hydrophilic and a hydrophobic polymer building blocks linked by the ONB active group. The key design criterion was that the polymers would have different solubilities and hydrophilicities. Specifically, the photocleavable polystyrene-b-poly(ethylene oxide) (PS-b-PEO) amphiphilic copolymer with o-nitrobenzyl ester group was grafted to a silicon wafer through copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC). Upon UV irradiation, the ONB group was cleaved, and after the removal of the PEO sacrificial domain, the carboxylic acid-functionalized PS layer was revealed.

RESULTS AND DISCUSSION

The general polymer synthesis and attachment strategy has two key components: (1) the synthesis of the photocleavable block copolymer and (2) the fabrication of the alkyne-functionalized substrate. The PEO-b-PS containing ONB group was designed to attach to the SiO2/Si substrate through CuAAC, one of the high-yield coupling reactions.29 Synthesis of Photocleavable Block Copolymer. The photocleavable amphiphilic block copolymer PS-b-PEO with a photocleavable ONB junction was synthesized by combining ATRP of styrene and PEO attachment using the CuAAC (Scheme 1a). This pair of polymers was specifically chosen based on the difference in their hydrophobicities and solubilities. To introduce the ONB as a junction group and an alkyne group at the chain end of the polystyrene (PS), ATRP of styrene was carried out at 90 °C in toluene using 5-propargyl ether-2-nitrobenzyl bromoisobutyrate as an initiator in the presence of the Cu(I)Br/PMDETA catalyst system. The B


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Macromolecules number-average of molecular weight (Mn = 6100 g/mol) and polydispersity (Đ = 1.05) were calculated by GPC and show very good control over the ATRP. The other block of the copolymer, azide-terminated PEO, was synthesized in two steps. First, a tosylation was performed with tosyl chloride; next, an azidation was carried out with sodium azide. We chose to couple the PEO to PS using CuAAC instead of using an ONB ester unit containing PEO as a macroinitiator to improve reaction conditions. The formation of the well-defined photocleavable PS-b-PEO was confirmed by GPC (Figure 1). The spectrum verifies that

Figure 2. 1H NMR spectra of alkyne end-functionalized ONBcontaining PS and azide end-functionalized photocleavable PS-b-PEO copolymer.

at 7.9 ppm corresponding to the proton of the C−C double bond of the triazole ring appeared. Furthermore, a new signal at 3.9 ppm was observed. This peak was generated from the protons of the −CH2− group located between the triazole ring and PEO block. After the click reaction, the bromide end functionality of the PS-b-PEO copolymer was converted to an azide functionality. The characteristic signal of the −CH2− proton next to the bromide shifted from 4.4 to 3.9 ppm after the azidation of PS-b-PEO copolymer. Formation of Alkyne-Functionalized Substrate. The alkyne-modified SiO2/Si surface was prepared by a three-step method as depicted in Scheme 1b. First, hydroxylation took place through oxygen plasma method to increase the density and uniformity of hydroxyl groups on the oxide surface. This step also removes impurities. Next, amine functionalities were generated by vapor deposition of 3-aminopropyltrimethoxysilane (APTMS).31 Previous work has shown that silanization of silica results in multiple possible orientations of the APTES.32−34 Scheme 1b shows two representative possibilities. Because the silane groups are covalently bound to the silica surface, an APTMS self-assembly monolayer (SAM) is created.35 This monolayer provides anchored amine functionalities on the surface. In the last step, the alkyne functionalities were immobilized by an amidation reaction between the aminedecorated substrate and 4-pentynoic acid in the presence of 4(dimethylamino)pyridine (DMAP)/N,N′-dicyclohexylcarbodiimide (DCC) as a catalyst system. The X-ray photoelectron spectroscopy (XPS) measurements were taken for every modification step of the surface to ensure that the immobilization of the alkyne functionalities on the surface was successful (Figure 3). Si 2s and Si 2p peaks for the silicon appeared in all spectra at 153 and 103 eV, respectively (Figure 3a). Amination resulted in the appearance of a C 1s peak at 285.9 eV and a N 1s peak at 401.6 eV (Figure 3b,c), showing that the vapor deposition of the APTMS was successful. Further, deconvoluted C 1s signal revealed two components located at 285.7 and 286.8 eV which belong to the C−C/C−H and C−N groups, respectively. After the amidation step between the amine anchored surface and the 4-pentynoic acid, the peak intensity of C 1s and O 1s

Figure 1. GPC spectra of ONB containing alkyne-terminated PS, azide-terminated PEO, and photocleavable PS-b-PEO block copolymer.

the alkyne end-functionalized PS and azide-terminated PEO underwent a coupling reaction to form a block copolymer, resulting in a clear shift to the higher molecular weight. Based on the GPC result, all of the alkyne-decorated ONB-containing PS was consumed in the reaction. However, there was a small shoulder in the lower molecular weight region, corresponding to the unreacted PEO precursor, because an excess amount of PEO was used. Using dialysis with a cellulose membrane, the residual PEO block was removed. Very good control of the molecular weight (Mn = 17 200) and narrow polydispersity (Đ = 1.03) was achieved. Every step of the synthesis of the photocleavable copolymer was analyzed using 1H NMR. After the substitution reaction of the tosyl group of PEO with sodium azide, the 1H NMR peaks of the tosyl group disappeared, and a new peak attributable to the neighbor protons of the azide appeared at 3.4 ppm.30 The group fidelity was found to be well above 90%. The alkyne and ONB functionalized PS was also investigated. The resonances from the aromatic protons of the ONB group and the PS are clearly seen at 8.12 and 7.20−6.20 ppm, respectively (Figure 2). Based on the 1H NMR analysis in CDCl3, the degree of polymerization (DP) of the PS was calculated to be 54, which is in good agreement with Mn,GPC. The CuAAC and formation of the photocleavable PS-b-PEO copolymer were also confirmed using 1H NMR. The results in Figure 2 verified that the click reaction was successful and that there was no azide-functionalized PEO and alkyne-functionalized PS fragments remaining. After the click reaction, the characteristic peak of the alkyne disappeared, and the new peak C


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Scheme 2. Synthesis of Model CuAAC between AlkyneAnchored Surface and 1-Azido-4-bromobenzene

results. However, it is clear that the intensity of the aliphatic and aromatic C−C/C−H and CO/C−N signals increased in the deconvoluted spectrum, and the intensity of the C(O)N signal decreased because of the formation of triazole and the attachment of benzene ring. In addition, these carbons have slightly shifted binding energies due to nearest-neighbor effects from other hydrocarbon species. The formation of a triazole moiety was strongly supported by the analysis of the N 1s signal. The high-resolution N 1s data of the amine-grafted surface showed a peak centered at 401.6 eV (Figure 3c). After the CuAAC, the N 1s curve shifted slightly, changing the binding energies of N−H at 400.7 eV, and a new peak appeared at 402.3 eV, corresponding to the nitrogen in the triazole ring (N−N−N). Physically adsorbed unreacted azide species on the substrate would be expected at 405 eV.36 We did not observe such a peak in this region (Figure 3e). The XPS survey spectrum also indicated the presence of a new Br peak, and the intensity of the N 1s emission curve increased, which is in good agreement with the presence of an alkyne monolayer on the wafer (Figure 3g). Based on the XPS results, the ratios of the integrated areas of [Br]:[N]:[C] are [1]:[3.7]:[13.4]. These values are in good agreement with the corresponding theoretical ratios of [1]:[4]:[14]. Moreover, the XPS survey scan did not show any peaks around 933 eV due to Cu 2p3/2, indicating that no residual Cu(I) and Cu(II) species physically adsorbed on the monolayer surface (Figure 3a).37 Table 1 includes the atomic % calculated from the XPS scans for the three types of modified surfaces studied.

Figure 3. XPS spectra: (a) the survey scan overlay of amine, alkyne, and bromide-functionalized surfaces; high-resolution scans of (b) C 1s and its deconvolution results and (c) N 1s of the amine-functionalized surface; high-resolution scans of (d) C 1s of the alkyne-functionalized surface and its deconvolution results, (e) N 1s and (f) C 1s and their deconvolutions results, and (g) Br 3d of the bromide-functionalized surface.

increased. High-resolution scan C 1s signal supported the formation of alkyne end-functionalization. The signal was deconvoluted and fitted into three components: (a) a predominant peak located at 285.3 eV corresponding to C− C/C−H, (b) a signal at 286 eV assigned to C−N and CO, and (c) a binding energy signal centered at 287.6 eV ascribed to the electron-deficient C atom of carbonyl group in the amide unit (Figure 3d). As expected, N 1s data of the alkynefunctionalized surface showed exactly the same high-resolution scan of the amine tethered surface. XPS is a very important surface characterization method; however, it does not give bulk information because of the 10 nm limitation of penetration depth. To verify the reactivity of the surface, a model reaction was designed between the alkynefunctionalized surface and 1-azido-4-bromobenzene (Scheme 2). Unfortunately, even in this model system, the C 1s emission from the carbon−nitrogen bonds in the 1,2,3-triazole ring do not generate unique peaks because the emission from the oxygen- and nitrogen-bonded carbons overlap (Figure 3f). Therefore, they cannot be definitively identified from the XPS

Table 1. Elemental Compositions of Modified Surfaces Calculated by CasaXPS from XPS Analyses sample description

%C

%N

% Br

amine-functionalized surface alkyne-functionalized surface bromide-functionalized surface

11.08 12.5 15.7

2.94 1.33 3.63

0.96

Fabrication of Photocleavable Surface. As depicted Scheme 3, the PS-b-PEO photocleavable copolymer was grafted to the alkyne-functionalized silicon wafer using the CuAAC. Unbound polymers were removed by rinsing the surface with toluene, Et2O, DI water, IPA, and THF. The wafer with covalently bound PS-b-PEO photocleavable copolymer was then placed in the photoreactor with a commercially available UV lamps (λ = 350 nm). For AFM imaging, a portion of the D


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Scheme 3. Synthetic Route for Grafting of Photocleavable PS-b-PEO Copolymer via CuAAC and Photocleavage of ONB Junction

Figure 4. AFM analysis. (a) Schematic of smart surface before UV irradiation. (b) AFM image of the photocleavable diblock copolymer layer on the wafer. (c) Schematic of the surface after photocleavage. (d) AFM image of the wafer after photocleavage. The difference between the cleaved and uncleaved regions is clearly identifiable. (e) Statistical analysis of the height distribution across the polymer brushes from the AFM image in (b) and from the two regions of the AFM image in (d).

wafer was exposed to UV light for 6 h from the distance of 15 cm. We chose the UV irradiation time of 6 h to ensure complete photocleavage. After the cleavage of the PEO layer,

the surface was washed multiple times with toluene, DI water, methanol, IPA, and THF to remove the any residual cleaved PEO. UV exposure procedure was repeated to cleave the rest of E


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Macromolecules PEO and measure the contact angle value of remaining PS layer on the substrate (Scheme 3). In order to verify that the photocleavable polymer was covalently immobilized on the substrate, and not physically adsorbed on the surface, the chemical modifications of the surface before and after photocleavage were characterized by tapping mode atomic force microscopy (AFM) to determine the surface morphology and the thickness of the polymer brush. Samples were imaged before and after selective area UV exposure (Figure 4b,d). It should be noted that the images were taken in the dry state. As shown in Figure 4b, the surface tethered polymer brush was not highly dense because of steric hindrance during the graf ting to process. The differential dependence of height as a function of location on the sample was determined, and a histogram showing the frequency that a given height occurs was created (Figure 4e). The fits shown in Figure 4e are unweighted. As expected, there is a bimodal Gaussian distribution. The lowest peak (2 nm) corresponds to the intrinsic noise in the system and the free alkyneimmobilized surface. The second peak can be attributed to the thickness of polymer brushes before photocleavage and was found to have a mean value of 13 nm. Figure 4c is a schematic of the sample after the selective area photocleavage. In the uncovered region, the decrease in polymer brushes thickness due to the PEO removal upon photocleavage was clearly evident in both the AFM image and in the pair of histograms (Figure 4d,e). The mean thickness in the uncovered region reduced to 7 nm. Additionally, the variance around this value is similar to that measured for the pre-UV exposure film. The additional peak centered at 16 nm is probably uncleaved copolymer. However, while the cleaved PEO was easily removed from the immobilized PS surface, it adhered on the top of the uncleaved PEO in the covered region, despite vigorously cleaning the wafer surface.38 As a result, in the covered region, the polymer chains appeared to increase in height (Figure 4e). The conformation of grafted polymers on the substrate directly depends on molecular weight of polymer chains, grafting density (σ), and the surface property. The grafting density is not easy to determine through characterization methods. Usually, the equation based on the thickness of dry polymer and its molecular weight is used to calculate the grafting density of surface tethered polymers:39,40 σ=

Figure 5. Water contact angle measurements of (a) alkynefunctionalized surface (89° ± 1°), (b) the photocleavable PS-b-PEO copolymer grafted surface (65° ± 1°), and (c) the surface after photocleavage (80° ± 1°).

nature of the PEO. The amphiphilic nature of PS-b-PEO copolymer allowed the clear changes in contact angle to be observed and provided additional verification. The UV exposure resulted in the complete cleavage of the ONB group, followed by the release of the hydrophilic PEO from the substrate, leading to the formation of hydrophobic PS layer. Even though PS is a hydrophobic polymer, after photocleavage, the generated terminal carboxylic acid functionalities might slightly increase the hydrophilicity of the layer. Therefore, the contact angle measurements revealed that the grafting of PS-bPEO copolymer and the photocleavage process worked successfully.

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CONCLUSION In this work, we have demonstrated the synthesis of a covalently attached photocleavable smart polymeric surface. The photocleavable amphiphilic block copolymer featuring an o-nitrobenzyl ester moiety at the junction point was obtained via a route that sequentially combines atom transfer radical polymerization (ATRP) and copper(I)-catalyzed azide−alkyne cycloaddition reaction (CuAAC). The graf ting to approach was applied to anchor the azide-terminated photocleavable polymer brush to the alkyne-functionalized silicon wafer using CuAAC. The polymer film offers photoresponsiveness and stability due to light responsive moieties and the highly stable triazole rings, respectively. Upon exposure to UV light, the ONB linker underwent photolysis, and the sacrificial domain, PEO, was removed from the surface by rinsing with solvents. We believe that the strategy described here can be used in the fabrication of advanced multifunctional nanodevices for medical and energy applications.

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EXPERIMENTAL SECTION

Materials. Sodium azide (NaN3, 99.99%), 4-pentynoic acid (95%), copper(I) bromide (CuBr, 99.999%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 98%), N,N′-dicyclohexylcarbodiimide (DCC, 99%), 4-(dimethylamino)pyridine (DMAP, 99.99%), 3aminopropyltrimethoxysilane (APTMS, 97%), and 1-azido-4-bromobenzene solution (0.5 M in tert-butyl methyl ether, 95%) were purchased from Sigma-Aldrich and were used as received. Tosyl chloride (TsCl, 99%) was purchased from Fluka and was used as received. Poly(ethylene oxide) monomethyl ether (PEO113-OCH3, Aldrich, Mn = 5000 g/mol) was dried by azeotropic cycles with toluene. Styrene (St, Aldrich, 99%) was purified by passing through activated basic alumina (Aldrich) columns to remove inhibitors prior to use. All other solvents and reagents were commercially purchased at extra-pure grade and used without further purification. Silicon wafer with 2 μm thermal oxide (P-doped) was purchased from WRS Materials. Synthesis of 5-Propargyl Ether-2-Nitrobenzyl Bromoisobutyrate. The ATRP initiator, 5-propargyl ether-2-nitrobenzyl bromoisobutyrate containing the photocleavable ONB ester junction and bromide functionality, was synthesized following previously published protocol.43 Briefly, an atom transfer radical polymerization (ATRP) initiator with o-nitrobenzyl (ONB) and a terminal alkyne functionality

ρ × h × NA Mn

where ρ is the polymer density (1.067 for PS-b-PEO), h is the brush thickness found by AFM, Mn is the molecular weight of copolymer, and NA is Avogadro’s number. The grafting density of the covalently attached photocleavable copolymer surface was found 0.67 chain/nm2. According to the literature, a grafting density higher than 1 chain/nm2 is considered as a dense surface.41,42 As previously mentioned, highly dense polymeric surface is challenging via the graf ting to approach. The contact angle measurements provide the evolution of the wettability of the surface. We started the series of contact angle measurements from the alkyne-decorated substrate (Figure 5). The hydrophobic character of the alkyne selfassembled monolayer resulted in a contact angle value of 89° ± 1°. After completing the CuAAC reaction between the azideterminated PS-b-PEO and the alkyne monolayer, the contact angle value decreased to 65° ± 1° because of the hydrophilic F


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Macromolecules

triazole), 7.20−6.20 (b, ArH of PS), 5.23 (s, 2H, CH2−O−CO), 4.60 (s, 1H, triazole−CH2−O−), 4.40 (t, 1H, CH−Br), 3.90 (t, 2H, CH2−triazole), 3.62 (b, 113H, aliphatic H of PEO), 3.39 (s, 3H, CH3O−PEO). Azidation of Photocleavable PS-b-PEO Copolymer. PS-b-PEO copolymer (1 g, 0.1 mmol) was dissolved in DMF (15 mL), and NaN3 (63 mg, 1 mmol) was added to this solution. After stirring the reaction mixture at room temperature overnight, DMF was removed under reduced pressure. The residue was diluted in DCM and washed with water. The organic phase was dried over Na2SO4 and filtered, and the solvent was removed under reduced pressure. The residue was precipitated in hexane, filtered, and dried overnight in vacuo at 30 °C, resulting in a white powder. Yield: 0.86 g (86%). 1H NMR (600 MHz, CDCl3) δ: 8.12 (m, 1H, ArH in ortho position of NO2, ONB), 7.86 (s, CHC in triazole), 7.20−6.20 (b, ArH of PS), 5.23 (s, 2H, CH2−O− CO), 4.60 (s, 1H, triazole−CH2−O−), 3.90 (b, 3H, CH2−triazole and, CH−N3), 3.62 (b, 113H, aliphatic H of PEO), 3.39 (s, 3H, CH3O−PEO). Fabrication of Alkyne-Anchored on Si Wafer Surface. The surface of a freshly cleaved and cleaned SiO2/Si wafer (1 cm2) was hydroxylated by an oxygen plasma (120 W, 30 sccm, 5 min), using an Anatech SP100. After activation, the wafer was removed from the chamber and immediately transferred to a desiccator containing APTMS. The hydroxylated surface of the wafer was exposed to the amine-terminated silane coupling agent (APTMS) using vapor deposition grafting technique at 25 °C for 30 min. The wafer was removed from the desiccator, thoroughly washed in THF, methanol, acetone, and IPA, and then dried under vacuum. The surface was then alkynylated with an amidation reaction at the amine end groups using 4-pentynoic acid in the presence of DCC and DMAP. The wafer was left to stand in toluene, THF, hexane, ethyl acetate, acetone, and IPA for 24 h for each solvent by slowly shaking to remove all residual chemicals and then dried at 40 °C. Grafting of PS-b-PEO Copolymer Containing ONB Junction to the Alkyne-Anchored Si Wafer through CuAAC. Copper(I) catalyzed alkyne−azide cycloaddition was carried out between the alkyne-functionalized silica surface (1 cm2) and the azide endfunctionalized PS-b-PEO containing ONB in a 10 mL Schlenk flask with a shelf. The shelf allowed the wafer to be separated from the magnetic stir bar inside the Schlenk flask. The shelf has holes to circulate solvent efficiently. After the solution of PS-b-PEO (0.7 g, 0.06 mmol) in DMF (7 mL) and PMDETA (1 mol % relative to azide) were added to the Schlenk flask, the reaction mixture was degassed by purging with argon for 15 min, and then Cu(I)Br (1 mol % relative to azide) was added. The reactants in the Schlenk flask were degassed by argon once again and then stirred at a low rate at room temperature for 24 h. The wafer with a polymer grafted surface was cleaned with toluene, Et2O, DI water, IPA, and THF separately sonicated for 2 h and then dried at 40 °C. Photocleavage of Grafted PS-b-PEO Copolymer on the Si Wafer. The photocleavable PS-b-PEO on silicon wafer was placed in the photoreactor. Half of the wafer was covered up with a mask and exposed to UV light for 6 h. The experiment was performed at room temperature, and eight top lamps were used. By the end of 6 h, PEO was cleaved off and washed off with toluene, DI water, methanol, IPA, and THF separately sonicated for 3 h and then dried at 40 °C. Material Characterization and Equipment. Photoreactor. UV light exposure was conducted in a Luzchem ICH-2 photoreactor with temperature control and rotating sample stage. The substrate was irradiated with eight Hitachi FL8BL-B lamps on the top position at a distance of 15 cm. The typical intensities were measured using a Smart Sensor digital lux meter and ranged from 4 to 6 mW/cm2 with center lamp wavelength at 350 nm. Nuclear Magnetic Resonance (NMR) Spectroscopy. 1H NMR spectra were recorded on Varian Mercury 400 and 600 spectrometers. Samples were analyzed in chloroform-d, and all chemical shifts are stated as part per million (ppm) from external tetramethylsilane (TMS) (δ = 0 ppm). X-ray Photoelectron Spectroscopy (XPS). XPS data were acquired using an AXIS Ultra DLD spectrometer equipped with a

was prepared in three steps starting from commercially available 5hydroxy-2-nitrobenzyl alcohol. 5-Hydroxy-2-nitrobenzyl alcohol was reacted with propargyl bromide to give 5-propargyl ether-2-nitrobenzyl alcohol in 80% yield. This step was followed by an esterification reaction between 5-propargyl ether-2-nitrobenzyl alcohol and 2bromoisobutyryl bromide to prepare 5-propargyl ether-2-nitrobenzyl bromoisobutyrate with 87% yield. Preparation of Alkyne End-Functionalized ONB Containing Polystyrene with ATRP. A dry 25 mL Schlenk flask equipped with a magnetic stir bar was filled with St (5 mL, 44 mmol), 5-propargyl ether-2-nitrobenzyl bromoisobutyrate initiator (77 mg, 0.22 mmol), PMDETA (0.045 mL, 0.22 mmol), and toluene (5 mL). The mixture was degassed by three freeze−pump−thaw cycles to deoxygenate the solution and backfilled with nitrogen. CuBr (0.031 g, 0.22 mmol) was added to the reaction mixture under a N2 atmosphere and followed by immersing the flask into an oil bath at 90 °C. The laboratory and the fumehood’s lights are shielded against UV light to protect the ONB group. The reaction lasted for 5 h by cooling and exposing the reaction mixture to air. The mixture was diluted with THF and then passed through a neutral alumina column to remove the copper complex; the solution was precipitated into methanol. The final polymer was isolated by filtration and dried in vacuo at 35 °C for 24 h. Mn,GPC = 6100, Đ = 1.05, Mn,NMR = 6000. 1H NMR (400 MHz, CDCl3) δ: 8.15 (d, 1H, ArH in ortho position of NO2, ONB), 7.52 (s, 1H, ArH in metha position of NO2, ONB), 7.20−6.20 (br, ArH in PS), 5.25 (s, 2H, ONB−H2C−OCO), 4.60 (s, 2H, H2C−O−ONB), 4.40 (t, 1H, CH−Br), 2.50 (t,1H, CHC−CH2). Synthesis of α-Methoxy-ω-tosyl-poly(ethylene oxide). CH3O−PEO−OH (Mn = 5000 g/mol, DP = 113) (3 g, 0.6 mmol), DMAP (0.37 g, 0.3 mmol), and TEA (0.42 mL, 3 mmol) were dissolved in DCM. To this solution, toluene-4-sulfonyl chloride (tosyl chloride) (0.57 g, 3 mmol) in DCM was added dropwise at 0 °C. The reaction mixture was stirred overnight at room temperature. After removal of the insoluble salts by filtration, the mixture was treated with 4 M HCl solution, washed with brine and distilled water, and dried over Na2SO4. The residue was purified twice by dissolution/ precipitation with DCM/cold Et2O, filtered, and dried overnight in vacuo at 30 °C.The product obtained was a white powder. Yield: 2.7 g (90%). 1H NMR (400 MHz, CDCl3) δ: 7.80 and 7.34 (d, 4H, −OSO2C6H4CH3), 4.16 (t, 2H, −CH2CH2−OSO2C6H4CH3), 3.63 (450H, OCH2CH2 repeating unit of PEO), 3.46 (t, 2H, −CH2CH2− OSO 2 C 6 H 4 CH 3 ), 3.37 (s, 3H, −OCH 3 ), 2.44 (s, 3H, −OSO2C6H4CH3). Synthesis of Azide End-Functionalized Poly(ethylene oxide) (PEO-N3). Tosylated-PEO (2.7 g, 0.57 mmol) was dissolved in DMF (15 mL), and NaN3 (0.37 g, 5.7 mmol) was added to this solution. After stirring the reaction mixture at room temperature overnight, DMF was removed under reduced pressure. The residue was diluted in DCM and washed with water. The organic phase was dried over Na2SO4 and filtered, and the solvent was removed under reduced pressure. The residue was precipitated into cold Et2O, filtered, and dried overnight in vacuo at 30 °C, generating a white powder Yield: 2.5 g (93%). Mn,GPC = 8200, Đ = 1.02. 1H NMR (400 MHz, CDCl3) δ: 3.63 (m, 450H, OCH2CH2 repeating unit of PEO) and 3.33−3.39 (m, 5H, CH3O−PEO and CH2CH2O−N3). Synthesis of Photocleavable PS-b-PEO Block Copolymer via CuAAC Reaction. Alkyne end-functionalized ONB containing PS (1.2 g, 0.2 mmol, 6000 g/mol based on Mn,NMR) and PEO-N3 (1.2 g, 0.24 mmol) were dissolved in nitrogen-purged DMF (5 mL) in a Schlenk tube equipped with magnetic stirring bar. CuBr (4.3 mg, 0.3 mmol) and PMDETA (63 μL, 0.3 mmol) were added, and the reaction mixture was degassed by three freeze−pump−thaw cycles, left in argon, and stirred at room temperature for 5 h. The reaction mixture was passed through an alumina column to remove the copper salt, precipitated into hexane, and dried in vacuo at 25 °C. The THF solution of block copolymer was dialyzed against pure water for a week in a cellulose tube (Spectra/Por 1 Dialysis corresponding to a cutoff molecular weight of 6000−8000) to remove residual PEO-N3. Mn,GPC = 17 200, Đ = 1.03, Mn,NMR = 12 400. 1H NMR (600 MHz, CDCl3) δ: 8.12 (m, 1H, ArH in ortho position of NO2, ONB), 7.86 (s, CHC in G


Macromolecules

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monochromatic Al Kα X-ray source (1486.6 eV), hemispherical analyzer, and an eight channeltron multidetector. Survey scans were carried out over the 1000−0 eV range and an analyzer pass energy of 160 eV. The Br 3d (68−75 eV), C 1s (280−292 eV), N 1s (395−408 eV), and Cu 2p3/2 (926−938 eV) regions were investigated in detail. High-resolution scans were run with 20 eV of the analyzer pass energy. The sample penetration depth for the XPS measurements was determined to be 10 nm. All samples were freshly prepared, dried, and kept under vacuum. Atomic Force Microscopy (AFM). The AFM experiments were performed with a JSPM-5200 microscope (JEOL-USA, Peabody, MA) in alternative current amplitude and phase modes to avoid damaging the sample and to increase the image spatial resolution. Standard silicon probes (Bruker) with a 125 μm long cantilever, nominal force constant of 40 N/m, and resonance frequency of 300 kHz for tapping mode surface topography were used as received. All experiments were carried out in ambient conditions. All images were processed using the Gwyddion 2.9 SPM data analysis framework. Contact Angle Goniometer. Water contact angle values were measured with a goniometer (Ramé−Hart Model 290-F1). A 5 μL water droplet placed on the surface by a microsyringe. Three spots were measured for each sample and averaged. Gel Permeation Chromatography (GPC). The average molecular weights (Mn) and polydispersity indices of polymers were estimated by a Waters GPC system equipped with a Waters 2707 autosampler, a Waters 1515 isocratic HPLC pump, a Waters 2414 refractive index detector, and four Waters Styragel columns (Styragel HR1, HR4, HR4E, and HR5E). The analyses were performed at 35 °C, using tetrahydrofuran (THF) with a flow rate of 1.0 mL/min as eluent. The GPC was calibrated against polystyrene standards of known Mw.

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Article

REFERENCES

(1) Senaratne, W.; Andruzzi, L.; Ober, C. K. Self-Assembled Monolayers and Polymer Brushes in Biotechnology: Current Applications and Future Perspectives. Biomacromolecules 2005, 6, 2427−2448. (2) Brown, P. K.; Qureshi, A. T.; Moll, A. N.; Hayes, D. J.; Monroe, W. T. Silver Nanoscale Antisense Drug Delivery System for Photoactivated Gene Silencing. ACS Nano 2013, 7, 2948−2959. (3) Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533−1554. (4) Choi, I. S.; Chi, Y. S. Surface Reactions On Demand: Electrochemical Control of SAM-Based Reactions. Angew. Chem., Int. Ed. 2006, 45, 4894−4897. (5) Liang, Y. Q.; Cui, Z. D.; Zhu, S. L.; Li, Z. Y.; Yang, X. J.; Chen, Y. J.; Ma, J. M. Design of a Highly Sensitive Ethanol Sensor using a Nano-coaxial p-Co3O4/n-TiO2 Heterojunction Synthesized at Low Temperature. Nanoscale 2013, 5, 10916−10926. (6) Anastasiadis, S. H.; Retsos, H.; Pispas, S.; Hadjichristidis, N.; Neophytides, S. Smart Polymer Surfaces. Macromolecules 2003, 36, 1994−1999. (7) Matsumoto, N. M.; Prabhakaran, P.; Rome, L. H.; Maynard, H. D. Smart Vaults: Thermally-Responsive Protein Nanocapsules. ACS Nano 2013, 7, 867−874. (8) Dai, S.; Ravi, P.; Tam, K. C. pH-Responsive Polymers: Synthesis, Properties and Applications. Soft Matter 2008, 4, 435−449. (9) Ercole, F.; Davis, T. P.; Evans, R. A. Photo-responsive Systems and Biomaterials: Photochromic Polymers, Light-triggered Selfassembly, Surface Modification, Fluorescence Modulation and Beyond. Polym. Chem. 2010, 1, 37−54. (10) Fuhrmann, K.; Połomska, A.; Aeberli, C.; Castagner, B.; Gauthier, M. A.; Leroux, J.-C. Modular Design of Redox-Responsive Stabilizers for Nanocrystals. ACS Nano 2013, 7, 8243−8250. (11) Rodríguez-Pulido, A.; Kondrachuk, A. I.; Prusty, D. K.; Gao, J.; Loi, M. A.; Herrmann, A. Light-Triggered Sequence-Specific Cargo Release from DNA Block Copolymer−Lipid Vesicles. Angew. Chem., Int. Ed. 2013, 52, 1008−1012. (12) Aujard, I.; Benbrahim, C.; Gouget, M.; Ruel, O.; Baudin, J.-B.; Neveu, P.; Jullien, L. o-Nitrobenzyl Photolabile Protecting Groups with Red-Shifted Absorption: Syntheses and Uncaging Cross-Sections for One- and Two-Photon Excitation. Chem. - Eur. J. 2006, 12, 6865− 6879. (13) Han, G.; Mokari, T.; Ajo-Franklin, C.; Cohen, B. E. Caged quantum dots. J. Am. Chem. Soc. 2008, 130, 15811−15813. (14) Critchley, K.; Zhang, L.; Fukushima, H.; Ishida, M.; Shimoda, T.; Bushby, R. J.; Evans, S. D. Soft-UV Photolithography using SelfAssembled Monolayers. J. Phys. Chem. B 2006, 110, 17167−17174. (15) Chien, Y.-H.; Chou, Y.-L.; Wang, S.-W.; Hung, S.-T.; Liau, M.C.; Chao, Y.-J.; Su, C.-H.; Yeh, C.-S. Near-Infrared Light Photocontrolled Targeting, Bioimaging, and Chemotherapy with Caged Upconversion Nanoparticles in Vitro and in Vivo. ACS Nano 2013, 7, 8516−8528. (16) Kang, M.; Moon, B. Synthesis of Photocleavable Poly(styreneblock-ethylene oxide) and Its Self-Assembly into Nanoporous Thin Films. Macromolecules 2009, 42, 455−458. (17) Zhao, H.; Gu, W.; Sterner, E.; Russell, T. P.; Coughlin, E. B.; Theato, P. Highly Ordered Nanoporous Thin Films from Photocleavable Block Copolymers. Macromolecules 2011, 44, 6433−6440. (18) Zhao, H.; Gu, W.; Thielke, M. W.; Sterner, E.; Tsai, T.; Russell, T. P.; Coughlin, E. B.; Theato, P. Functionalized Nanoporous Thin Films and Fibers from Photocleavable Block Copolymers Featuring Activated Esters. Macromolecules 2013, 46, 5195−5201. (19) Kumar, S.; Dory, Y. L.; Lepage, M.; Zhao, Y. Surface-Grafted Stimuli-Responsive Block Copolymer Brushes for the Thermo-, Photo- and pH-Sensitive Release of Dye Molecules. Macromolecules 2011, 44, 7385−7393. (20) Lee, M. E.; Gungor, E.; Armani, A. M. Photocleavage of Poly(methyl acrylate) with Centrally Located o-Nitrobenzyl Moiety: Influence of Environment on Kinetics. Macromolecules 2015, 48, 8746−8751.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.M.A.). Funding

This work was supported by the Office of Naval Research [N00014-11-1-0910]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Center for Electron Microscopy and Microanalysis (CEMMA) for the use of the XPS, Prof. Moh El-Naggar for the use of the AFM, Dr. Shuxing Li for his help with AFM imaging, and Prof. Malancha Gupta for the use of the goniometer.

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ABBREVIATIONS AFM, atomic force microscopy; APTMS, 3-aminopropyltrimethoxysilane; ATRP, atom transfer radical polymerization; CuAAC, Cu(I)-catalyzed azide−alkyne cycloaddition; CuBr, copper(I) bromide; DCC, N,N′-dicyclohexylcarbodiimide; DMAP, 4-(dimethylamino)pyridine; DCM, dichloromethane; DMF, dimethylformamide; Et2O, diethyl ether; GPC, gel permeation chromatography; HCl, hydrochloride; IPA, isopropyl alcohol; Mn, the number-average molecular weight; NaN3, sodium azide; Na2SO4, sodium sulfate; NMR, nuclear magnetic resonance; ONB, o-nitrobenzyl; PEO, poly(ethylene oxide); PEO-N3, azide-terminated poly(ethylene oxide); PS, polystyrene; PMDETA, N,N,N′,N″,N″-pentamethyldiethylenetriamine; TEA, triethylamine; THF, tetrahydrofuran; TMS, tetramethylsilane; TsCl, tosyl chloride; UV, ultraviolet; XPS, X-ray photoelectron spectroscopy. H


Article

Macromolecules (21) Rühe, J. In Polymer Brushes; Wiley-VCH Weinheim: Germany, 2004; pp 1−31. (22) 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. (23) Husseman, M.; Malmström, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Controlled Synthesis of Polymer Brushes by “Living” Free Radical Polymerization Techniques. Macromolecules 1999, 32, 1424−1431. (24) Turgman-Cohen, S.; Genzer, J. Simultaneous Bulk- and SurfaceInitiated Controlled Radical Polymerization from Planar Substrates. J. Am. Chem. Soc. 2011, 133, 17567−17569. (25) Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. Y-Shaped Amphiphilic Brushes with Switchable Micellar Surface Structures. J. Am. Chem. Soc. 2003, 125, 15912−15921. (26) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Controlling Polymer-Surface Interactions with Random Copolymer Brushes. Science 1997, 275, 1458−1460. (27) Zhao, H.; Sterner, E. S.; Coughlin, E. B.; Theato, P. oNitrobenzyl Alcohol Derivatives: Opportunities in Polymer and Materials Science. Macromolecules 2012, 45, 1723−1736. (28) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse Chemical Function from a Few Good Reactions. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (29) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective “Ligation” of Azides and Terminal Alkynes. Angew. Chem., Int. Ed. 2002, 41, 2596−2599. (30) Gungor, E.; Cote, G.; Erdogan, T.; Durmaz, H.; Demirel, A. L.; Hizal, G.; Tunca, U. Heteroarm H-shaped Terpolymers Through Click Reaction. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1055−1065. (31) Hunt, H. K.; Soteropulos, C.; Armani, A. M. Bioconjugation Strategies for Microtoroidal Optical Resonators. Sensors 2010, 10, 9317. (32) Acres, R. G.; Ellis, A. V.; Alvino, J.; Lenahan, C. E.; Khodakov, D. A.; Metha, G. F.; Andersson, G. G. Molecular Structure of 3Aminopropyltriethoxysilane Layers Formed on Silanol-Terminated Silicon Surfaces. J. Phys. Chem. C 2012, 116, 6289−6297. (33) Zhu, M.; Lerum, M. Z.; Chen, W. How To Prepare Reproducible, Homogeneous, and Hydrolytically Stable AminosilaneDerived Layers on Silica. Langmuir 2012, 28, 416−423. (34) Hicks, J. C.; Jones, C. W. Controlling the Density of Amine Sites on Silica Surfaces Using Benzyl Spacers. Langmuir 2006, 22, 2676−2681. (35) Jönsson, U.; Olofsson, G.; Malmqvist, M.; Rönnberg, I. Chemical Vapour Deposition of Silanes. Thin Solid Films 1985, 124, 117−123. (36) Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D. Mixed Azide-Terminated Monolayers: A Platform for Modifying Electrode Surfaces. Langmuir 2006, 22, 2457−2464. (37) Meda, L.; Cerofolini, G. F. A Decomposition Procedure for the Determination of Copper Oxidation States in Cu-zeolites by XPS. Surf. Interface Anal. 2004, 36, 756−759. (38) Altinpinar, S.; Zhao, H.; Ali, W.; Kappes, R. S.; Schuchardt, P.; Salehi, S.; Santoro, G.; Theato, P.; Roth, S. V.; Gutmann, J. S. Distortion of Ultrathin Photocleavable Block Copolymer Films during Photocleavage and Nanopore Formation. Langmuir 2015, 31, 8947− 8952. (39) Lego, B.; François, M.; Skene, W. G.; Giasson, S. Polymer Brush Covalently Attached to OH-Functionalized Mica Surface via SurfaceInitiated ATRP: Control of Grafting Density and Polymer Chain Length. Langmuir 2009, 25, 5313−5321. (40) Patil, R. R.; Turgman-Cohen, S.; Šrogl, J.; Kiserow, D.; Genzer, J. On-Demand Degrafting and the Study of Molecular Weight and Grafting Density of Poly(methyl methacrylate) Brushes on Flat Silica Substrates. Langmuir 2015, 31, 2372−2381.

(41) Liu, Y.; Klep, V.; Zdyrko, B.; Luzinov, I. Polymer Grafting via ATRP Initiated from Macroinitiator Synthesized on Surface. Langmuir 2004, 20, 6710−6718. (42) Yoshikawa, C.; Goto, A.; Tsujii, Y.; Fukuda, T.; Yamamoto, K.; Kishida, A. Fabrication of High-Density Polymer Brush on Polymer Substrate by Surface-Initiated Living Radical Polymerization. Macromolecules 2005, 38, 4604−4610. (43) Schumers, J.-M.; Gohy, J.-F.; Fustin, C.-A. A Versatile Strategy for the Synthesis of Block Copolymers Bearing a Photocleavable Junction. Polym. Chem. 2010, 1, 161−163.

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Photocleavage of Covalently Immobilized Amphiphilic Block

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