Phototriggered Growth and Detachment of Polymer Brushes with

Feb 5, 2018 - In addition, obtained polymers contain a few short chains. These short chains might form at the end of the polymerization process where ...
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Letter Cite This: ACS Macro Lett. 2018, 7, 239−243

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Phototriggered Growth and Detachment of Polymer Brushes with Wavelength Selectivity Xinhong Xiong, Lulu Xue, and Jiaxi Cui* INM - Leibniz Institute for New Materials, Campus D2 2, Saarbrücken 66123, Germany S Supporting Information *

ABSTRACT: Both phototriggered growth and removal of polymer chains from surfaces are efficient ways to finely tune interface properties. Combining these two capabilities in one system with independent control can significantly increase the feasibility of photoregulation on surface modification but has not been reported yet. Herein we describe a novel approach to control both the growth and the detachment of polymer brushes independently by light with different wavelengths. The approach is based on a nitrodopamine-based initiator (NO2− BDAM) which contains a catechol structure for surface modification, alkyl bromide group for radical polymerization, and o-nitrophenyl ethyl moiety for photolysis. When dimanganese decacarbonyl (Mn2(CO)10) was applied together with NO2−BDAM as an initiating system, visible light (460 nm) can be used to trigger the site-specific growth of polymer brushes. Resulting polymer brushes can be selectively removed by UV light (360 nm). This method is suitable for different monomers on various substrates, suggesting a facile and robust method to regulate surface properties.

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Our strategy is based on a nitrodopamine-based initiator, NO2−BDAM (Figure 1a). It contains three functional moieties: (1) the catechol structure which allowed for its modification on nearly all types of substrates with strong binding,30−32 (2) an alkyl bromide group for forming radical-initiating species for grafting polymerization,33,34 and (3) a photocleavable onitrophenyl ethyl moiety.35 Irradiation with UV light (typically, 360 nm) would lead to a molecular cleavage on the β-position of the ethylene group, and thus a detachment of the anchoring polymer chain on the amino group would be expected. NO2− BDAM can be prepared efficiently by an amidation reaction between 6-nitrodopamine and 2-bromoisobutyryl bromide (Figures S1−2). For independent control over the growth and the detachment of polymer chains, dimanganese decacarbonyl (Mn2(CO)10) was selected as a visible-lightactivated initiator for surface-initiated polymerization. Under irradiation, it generates a highly active radical •Mn(CO)5 to abstract halides from organohalogen compounds to form radical species for initiating polymerization.36−39 We first evaluated the wavelength selectivity of NO2−BDAM and Mn2(CO)10 by visible light of 460 nm. The absorption of Mn2(CO)10 rapidly decreased under irradiation, suggesting a distinct degradation of Mn2(CO)10 (Figure S3). Under the same irradiation conditions, no detectable decomposition was observed in NO2−BDAM (Figure S4) even when Mn2(CO)10

he modification of a surface by polymer brushes is an effective strategy to tailor interface physical and chemical properties for a wide range of applications, such as antifouling coatings, chemical sensing, and stimuli-responsive materials.1−7 For getting high grafting density, surface-initiated radical polymerizations are often applied.8−10 These polymerizations can be triggered by different stimuli including temperature, chemical additive, electrochemistry, light, etc.1,4 Among the methods currently available, light-induced surface polymerization takes particular advantages of spatiotemporal control, remote modulation, and room-temperature operation.11−15 In this approach, special chromophores are employed to trigger in situ activation of initiating/propagating radicals for the sitespecific growth of polymer chains from the surface and thus allow for patterned substrates,14 gradient surfaces, and even complex 3D structures.16 In addition to the growth of polymer brushes, light is also able to postmediate the properties of grown polymer brushes by phototriggered reactions to initiate catalyst,17 activate functional groups,18−20 postmodify,21−23 remove polymer chains, etc.24−26 When more than two chromophores that respond to the light with different wavelengths are applied together, the activities of these molecular species on surfaces can be regulated independently.27,28 This concept of wavelength selectivity has been utilized on the caged surface for preparing an orthogonal surface29 but not been attempted in the field of polymer brushes yet. Herein we describe a robust and facile method for regulating the growth and the removal of polymer brushes independently by switching light wavelength. © XXXX American Chemical Society

Received: December 21, 2017 Accepted: February 1, 2018

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DOI: 10.1021/acsmacrolett.7b00989 ACS Macro Lett. 2018, 7, 239−243

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of thin NO2−BDAM-based coatings on these substrates was confirmed by both surface wettability (Figure 2a) and X-ray

Figure 2. (a) Thickness and water contact angle of organic layers on silicon (Si) and titanium wafers under different conditions. S−Br and S-PNIPAAm: initiator- and PNIPAAm-modified substrates, respectively. S-photolysis: substrates after phototriggered detachment of PNIPAAm brushes. Data are presented as the mean ±SD (n = 3). (b) XPS spectra of Si−Br, Si−PNIPAAm, and Si−photolysis.

photoelectron spectroscopy (XPS, Figure 2b). On a coated Si wafer, the water contact angles (WCAs) increase from 24° to 32.1°, and a signal due to bromine appears in the XPS spectrum. This is accompanied by a decrease in the signal due to silicon. NO2−BDAM-modified Si substrate (Si−Br) and monomer N-isopropylacrylamide (NIPAAm) were selected as a model system to study the phototriggered growth and detachment of polymer brushes. When the substrate was immersed in the NIPAAm solution in the presence of Mn2(CO)10, a polymer layer indeed formed under visible-light irradiation (460 nm, Figure 2a). The growth of the PNIPAAm brush alters the composition on the top (Figure 2b) and thus the WCA (Figure 2a). It was expected that the polymerization process with Mn2(CO)10 and alkyl bromide as the initiating system followed a free radical polymerization mechanism because of the irreversible abstract reaction between •Mn(CO)5 and alkyl bromide.37,38 We confirmed this by monitoring the molecular weight (Mw) of the polymers obtained from different irradiation time in the solution state (Figure S6). The polymers obtained show similar Mw regardless of the irradiation time, implying a free radical polymerization mechanism. Based on this polymerization mechanism, we thus could control the Mw of the polymer chains on substrates by the monomer concentration (Figure 3a). A nearly linear relationship between the final thickness of polymer brushes and the monomer concentration was observed. Although the abstract reaction is irreversible, the photodecomposition of Mn2 (CO)10 is suggested to be reversible.43 Obtained •Mn(CO)5 would reform Mn2(CO)10 in a diffusion-limited rate under a nonirradiated state. In other words, radical species only formed under an irradiation state. It implied a simple light-switchable way to regulate the growth of the polymer chains. We probe this by monitoring the thickness of the polymer layers obtained from different irradiation time (then irradiation energy). As expected, the thickness of the polymer layers increases with the irradiation energy until it reaches a stable state (Figure 3b). The increase in thickness was attributed to the increase in grafting density, rather than the length of polymer chains. When the substrate was fully covered, the grafting reaction was restricted. The quenching of the •Mn(CO)5 was very fast, which even allows for a light-switched “on−off” on the grafting polymerization reaction (Figure 3c).

Figure 1. (a) Schematic of the photoregulated procedure on a surface. NO2−BDAM contains three functional moieties. (b) Photolysis conversion (%) of Mn2(CO)10 and NO2−BDAM in solution under different irradiation dose at 460 nm. Data were calculated from the absorbance values at λmax = 349 nm by assuming 100% conversion at full photolysis (Figures S3 and S4). (c) Photocleavage conversion (%) of NO2−BDAM under different irradiation dose at 360 nm. Data were calculated from the absorbance values at λmax = 351 nm by assuming 100% conversion at full photolysis (Figure S5).

was fully converted (Figure 1b). It indicated a high selectivity to grow polymer chains on a surface by using a light of 460 nm without inducing any photodegradation of NO2−BDAM. When UV light of 360 nm was applied, NO2−BDAM decomposed drastically (Figure 1c), implying the feasibility to detach the polymer chains connected to its amine moiety. In other words, light with wavelengths of 460 and 360 nm could be used to control the growth and the detachment of polymer brushes, respectively, in the designed system. The catechol moiety of NO2−BDAM in principle allows it to be modified on a wide range of substrates. We tested this universality by four kinds of substrates, i.e., titanium (Ti), silicon (Si), Au, and glass wafers. The typical protocol was applied to modify these wafers. Briefly, the substrates were immersed in an agitated alkaline solution (pH = 8.5) containing NO2−BDAM. Because of the electron-withdrawing nitro substitute, the NO2−BDAM solution is stable in air at room temperature, which allows us to control the deposition modes on different substrates. For example, a two-step codeposition method in which NaIO4 was added as an oxidant to induce the polymerization of NO2−BDAM was applied to prepare the initiating layer on silicon, Au, and quartz surfaces (see the details in Supporting Information). The dopamine moiety of NO2−BDAM was oxidized and created a polymerized network through the formation of quinoidal structures to realize the immobilization of the NO2−BDAM.40 On the Ti substrate, the oxidant is not necessary since the strong chelating interaction of the catechol group with the Ti atom can lead to a selfassembled monolayer (SAM) of NO2−BDAM.41,42 The photolabile o-nitrophenyl ethyl structure could be fully maintained in the SAM on a Ti substrate, compared to the complicated polymeric structures formed through oxidationinduced polymerization on the other substrates. The formation 240

DOI: 10.1021/acsmacrolett.7b00989 ACS Macro Lett. 2018, 7, 239−243

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Figure 3. Photoinduced growth of PNIPAAm on silicon substrates. (a) Thicknesses of the PNIPAAm layers obtained from different NIPAAm concentrations. (b) Thickness of the PNIPAAm layer on Si substrate versus the irradiation dose. (c) Thickness change of the polymer layer during an on−off irradiation. The time off slot is 10 min. In (a), (b), and (c), the concentration of Mn2(CO)10 was 0.25 mM, and the polymerizations were conducted at room temperature under the irradiation of 460 nm LED light. Data are presented as the mean ± SD (n = 3).

BDAM)-based bottom layer. The removal of polymer chains was further confirmed by the changes in thickness, surface wettability, and chemical composition of the organic layer (Figure 2a and b). After full photolysis, the thickness of residual organic coatings is very close to that of the poly(NO2−BDAM) coatings. Without PNIPAAm brushes, the surface becomes hydrophilic again. The decrease in the thickness of the polymer layer depends on the irradiation dose (Figure 4b). We further compared the photolysis of poly(NO2−BDAM)-based and NO2−BDAM-based polymer brush layers. The poly(NO2− BDAM)-based layer on the Si substrate shows slightly lower photolysis efficiency than the NO2−BDAM-based one. It was attributed to the integrated structure of o-nitrophenyl ethyl structure in the NO2−BDAM SAM on the Ti substrate.44 The photoinduced detachment of polymer chains from the initiating layer allowed us to collect the tethered polymer chains for structure study. The polymer brush prepared on a Ti substrate (thickness: 169 ± 3 nm) was selected because of the efficient photolysis. The polymers obtained can completely dissolve in dimethylformamide, implying a non-cross-linked polymer structure (the most probable structure should be linear polymers). Gel permeation chromatography (GPC) measurement of the polymers suggests an average molecular number (Mn , see the details in Supporting Information) of 2.41 × 105 g/mol and a PDI of 2.5 (Figure S9). These polymers show smaller molecular weight and higher PDI than that obtained from free radical polymerization in solution. It can be explained

The same polymer system was used to study the detachment of tethered chains under UV light irradiation (360 nm). We first studied the photolysis of the poly(NO2−BDAM) initiator layer by UV spectroscopy. The absorption of the poly(NO2− BDAM) decreases upon irradiation, indicating that the polymer coatings maintain the photocleavable properties of NO2− BDAM (Figure S7).35 A similar decrease is also observed in the case of PNIPAAm brushes under irradiation (Figure 4a). Since

Figure 4. Photodegradation process of PNIPAAm brushes grown from the NO2−BDAM-based initiating layer. (a) UV spectra of PNIPAAmcoated quartz slides under different irradiation dose. (b) Thickness changes of the PNIPAAm layers on a silicon wafer and a titanium wafer under different UV irradiation dose. 360 nm LED light was used.

PNIPAAm does not show visible absorption in the region of >250 nm, the irradiation condition does not induce any damage to the PNIPAAm chains (Figure S8). The decrease was thus attributed to the photolysis that occurred on the poly(NO2−

Figure 5. Photolithography demonstration of PNIPAAm brushes on silicon wafers (scale bars are 400 μm). (a−d) One-step patterned brushes obtained by visible-light irradiation (460 nm) with different photomasks. (e, g) Schematic diagram of two-step patterning strategy by using different photomask combinations. (f) Polymer brush patterns made from the substrate shown in (b) by using a striped photomask under UV irradiation of 360 nm. (h) Two-step brush patterning based on the patterns formed in (d) by using a square photomask under UV irradiation of 360 nm. 241

DOI: 10.1021/acsmacrolett.7b00989 ACS Macro Lett. 2018, 7, 239−243

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applications on functional surface coatings, reactive interface engineering, photodegradable materials, and photoinduced releasing systems.

by the fact that the surface prevents monomers from delivering to the chain ends from all directions and thus reducing the effective concentration of monomers near the active chain ends.45 With the Mn , we then estimated the initial grafting density γ by using the equation γ = hdryρNA/Mn where hdry is the dry height of the polymer brushes; NA is Avogadro’s number; and ρ is the bulk volume density of polymers (1.1 g/ cm3 for PNIPAAm).46 It has been suggested that the Mn of PNIPAAm obtained from GPC with polystyrene as the standard is very close to the real Mn.47 As a result, a grafting density of 0.46 chain/nm2 is obtained, which is comparable with the grafting density of dense polymer brushes. In addition, obtained polymers contain a few short chains. These short chains might form at the end of the polymerization process where the grafted polymer chains restricted the growth of propagating chains. We probed the diversity of the current strategy by using different monomers and substrates. In addition to NIPAAm, a hydrophilic amide, we also used two other monomer types including acrylic acid (acidic) and butyl acrylate (hydrophobic). All these monomers can be grown from poly(NO2−BDAM)coated silicon wafers via visible-light-induced surface polymerization and also can be removed by UV light (Figure S10). This wavelength-selective growth and detachment could also be applied to different materials surfaces, such as Au and quartz surfaces (Figures S10−11). It was expected that the current strategy could be used for spatiotemporal control and postregulation of polymer brushes because of the light-regulated growth and detachment. To demonstrate this feasibility, we prepared patterned PNIPAAm brushes on Si substrates. Striped and square masks were used, which led to distinctly polymer brush patterns (one-step patterning strategy, Figure 5a−d). Although Mn2(CO)10 has been dramatically used for light-induced (surface-initiated) polymerization, it was the first time that it was used for preparing polymer brush patterns. This capability was contributed from its reversible decomposition. The protocol (visible-light irradiation under a photomask) was simple and efficient. The grown polymer brush patterns could be further changed by UV light (two-step patterning strategy, Figure 5e and 5g). Based on the one-step PNIPAAm patterns on surfaces, subsequent UV irradiation of 360 nm generated new patterns by removing the polymer chains on exposal regions, implying incredible feasibility in making polymer brush patterns. In conclusion, we have developed a novel wavelengthselective approach to modify surfaces by finely controlling the growth and the detachment of polymer brushes, based on the combination of Mn2(CO)10 and NO2−BDAM. When visible light (460 nm) was used to trigger the formation of polymer brushes, these grown polymer chains could be finely removed by UV light (360 nm). Although the photoinduced polymerization followed a free radical polymerization manner, the structure of polymer brushes can be controlled by monomer concentration (length of polymer chains) and irradiation energy (grafting density). The growth of polymer brushes could be switched by light because of the reversible decomposition of the Mn2(CO)10. This method can be applied with different monomers on various substrates. Moreover, onestep and two-step patterns could be prepared during this lightinduced growth and detachment of polymer brush processes. Therefore, our research provides a facile pathway of lighttriggered attachment and detachment of polymer chains on material surfaces and opens new opportunities for the

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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/acsmacrolett.7b00989. Experimental detail and supported figures are supplied (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiaxi Cui: 0000-0002-2550-979X Author Contributions

J.C. supervised the research. X.X. and L.X. performed the experiments. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J. C. acknowledges the support from BMBF under the project of the Leibniz Research Cluster “Organic/synthetic multifunctional microproduction units - New ways to the development of the active ingredient” with an award number 031A360D. NO2− BDAM was designed and synthesized in Prof. del Campo’s group.

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DOI: 10.1021/acsmacrolett.7b00989 ACS Macro Lett. 2018, 7, 239−243