Letter Cite This: ACS Macro Lett. 2018, 7, 100−104
pubs.acs.org/macroletters
Substrate-Independent Approach to Dense Cleavable Polymer Brushes by Nitroxide-Mediated Polymerization Wei Wei,†,§ A. Balamurugan,†,§ Jonathan H. Dwyer,† and Padma Gopalan*,†,‡ †
Department of Materials Science and Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States
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S Supporting Information *
ABSTRACT: High grafting density polymer brushes are grown on an inimer coating bearing nitroxide-mediated polymerization (NMP) inimers and glycidyl methacrylate (GMA). The inimer coating is cross-linked on the substrate to provide an initiator layer with needed stability during long exposure to organic solvents at moderate to high temperatures. Surface-initiated nitroxide-mediated polymerization (SI-NMP) is conducted to grow polystyrene (PS) brushes on the coating with a sacrificial layer designed to cleave the brushes. The cleaved brushes have larger molecular weights than the corresponding free polymers. The grafting density of the brushes is as high as 1.12 chains/nm2 throughout the brush growth, which is among the densest PS brushes reported so far. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) depth profiling are used to reveal the surface morphology and kinetics of the growth.
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containing cross-linkable polymer coating which forms a stable and uniform initiator layer regardless of the substrate.15,16 Use of the cross-linked inimer coating avoids any issues with blend miscibility as might be present for a multicomponent curable mixture, while resulting in high chain density on a range of substrates.17 Although SI-ATRP has proven to be the most efficient and versatile SI-CRP technique, SI-NMP does offer some advantages. This includes a mechanistically distinct pathway for orthogonal brush growth that does not use transition metal catalysts and hence is valuable for creating interfaces for biological and electronic applications.6 So far, SAMs have been the most commonly used means of anchoring NMP initiators to the surface and grow, for example, polystyrene (PS) brushes with a grafting density of ∼0.5 chains/nm2.18 However, when compared to SI-ATRP it is difficult to achieve high grafting density for SI-NMP brushes starting with SAMs due to the bulky nitroxide groups on the initiator.19,20 To resolve this issue, electrografting of the alkoxyamine-derived acrylate monomer was used to functionalize conducting surfaces.21,22 Though limited to conducting substrates this method provides abundant NMP initiators resulting in PS brushes with a grafting density of roughly 0.7−0.8 chains/nm2 on a steel plate and 1.7 chains/nm2 on carbon fibers. Similarly, the Langmuir− Blodgett deposition method has also been used to increase the NMP initiator density and to achieve a grafting density of >1 chains/nm2.23−25 Other methods such as plasma surface
urface modification with polymer brushes is an attractive method for tuning of physical and chemical properties of interfaces such as wetting, adhesion, electronic, catalytic, or biological activity.1−8 Surface-anchored polymer chains commonly known as “polymer brushes” are a broad class of materials where tethering of one chain end provides mechanical stability to withstand a variety of postprocessing steps.9 Of the two methods used for growth of polymer brushes, the “grafting to” approach offers the advantage of complete characterization of the brushes, but the grafting density is usually low.10 To achieve high grafting density, the “grafting from” method is preferred where initiators anchored to the substrate are used to grow polymers using anionic, cationic, and ring-opening metathesis or free radical polymerizations.4 Of these methods surface-initiated controlled radical polymerization (SI-CRP) is widely used due to the ease of polymerization, accessibility to initiators, and applicability to a wide range of monomers.5 These SI-CRP methods include surface-initiated atom transfer radical polymerization (SIATRP), surface-initiated reversible addition−fragmentation chain-transfer polymerization (SI-RAFT), and surface-initiated nitroxide-mediated polymerization (SI-NMP).5 These initiators are typically anchored onto a variety of substrates by the formation of a self-assembled monolayer (SAM).5 While this strategy has worked well, SAMs have limited stability to various reagents and are not substrate-independent, as they require a new initiator for every substrate type.5 Furthermore, when dissimilar mixed molecules are used to create a SAM, a truly well mixed layer is unlikely and difficult to assess.11−14 We recently introduced a unique way to anchor an ATRP initiator to the surface via a single-component ATRP inimer © XXXX American Chemical Society
Received: December 17, 2017 Accepted: December 27, 2017
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DOI: 10.1021/acsmacrolett.7b00979 ACS Macro Lett. 2018, 7, 100−104
Letter
ACS Macro Letters
mol, respectively, both with narrow dispersity around 1.3 (Figure S6). Scheme 2 shows an overview of the SI-NMP strategy. P1 was thermally cross-linked to form an inimer coating which
activation have also been used for growing PS brushes via SINMP that claim high surface coverage, but the molecular weight of these brushes is unknown.26,27 In spite of these advances a robust strategy for implementing SI-NMP which is substrate independent, well-controlled, synthetically less complex, and yields high grafting density is still a challenge. We present an approach to grow dense polymer brushes via SI-NMP from an ultrathin polymer coating which is compatible with a wide range of substrates. We designed a new NMP inimer of ethylbenzene-TEMPO derivative to grow polymer brushes via SI-NMP. Previous reports have shown that ethylbenzene-TEMPO derivatives when used as unimolecular initiators can result in the successful NMP/SI-NMP of styrene to afford polymers/ polymer brushes with controlled molecular weights and low dispersity.21,28,29 However, their inimer version has not been reported so far. The synthesis of NMP inimer M1 comprises three consecutive synthetic steps as shown in Scheme 1.
Scheme 2. Schematic Illustration for the SI-NMP of PS, from Inimer Mat Formation to Brush Cleavage
Scheme 1. Schematic Illustration of the NMP Inimer (M1), Copolymer (P1), Homopolymer (P2), and the Free Initiator (3) provided high density NMP initiators on the substrate. From this inimer coating, a two-step sequential NMP process was carried out to create bilayer polymer brushes. The first layer was designed to be a sacrificial layer to release the second brush layer, hence allowing direct and more realistic measurement of the molecular weight and the grafting density. The sacrificial layer circumvents the need to design a synthetically complex cleavable inimers. The stability of the cross-linked inimer coating is essential for the polymer brush growth. An optimized cross-linking condition should provide near quantitative cross-linking to provide a stable coating as well as minimizing the loss of any nitroxide initiators due to their thermal lability. Hence, we used the nitrogen to carbon atomic percentage ratio (N/C) as an indicator of the relative initiator density. Spin-coating 0.3 wt % P1 from toluene solution on the Si wafer resulted in a thin film (∼8.0 nm). X-ray photoluminescent spectroscopy (XPS) analysis showed the N/C in the film to be 0.046. This as-spun coated film was used as the reference to normalize thicknesses and N/C ratios after cross-linking by thermal annealing. Thermal analysis by Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA) showed that P1 was thermally activated above 110 °C and no significant decomposition was observed below 200 °C (Figure S7), hence the cross-linking temperature was chosen within this range. Note that all the films were subjected to toluene wash after the cross-linking to remove uncross-linked chains. In Figure 1a, the annealing time was fixed at 30 min with annealing temperature varying from 110 to 160 °C. Our data shows that annealing at 140 °C gives reasonably high thickness without a substantial loss in the initiator density. At an annealing temperature at 140 °C, 60 min annealing time provided the best results (Figure 1b). In all further studies 140 °C/60 min was used to cross-link the inimer coating. Note that GMA is necessary for cross-linking since P2 did not provide a comparable thickness to P1 under the same condition (Table S1). For the growth of polymer brushes from this coating 1phenyl-1-(2,2,6,6-tetramethyl-1-piperidinyloxy)ethane (3) was used as the free initiator to control the polymerization. The first short sacrificial layer (∼5 nm) was designed to be a
First, bromo-α-methylbenzyl alcohol was converted into its chloro derivative 1 (Figure S1) by using SOCl2 at 0 °C, followed by coupling of 1 with TEMPO by atom transfer reversible addition reaction to get alkoxyamine product 2 (Figure S2). The final inimer of ethylstyrene-TEMPO (M1) (Figure S3) was obtained by reacting compound 2 and tributylvinyl tin using Pd(PPh3)4 as the catalyst via Stille coupling reaction. The purified inimer M1 was readily copolymerized with glycidyl methacrylate (GMA) by RAFT to give a statistical copolymer P1 (Figure S4). P1 carries both the NMP initiating sites and the cross-linkable GMA units in a single chain. Similarly, the homopolymer of inimer M1 was also synthesized by RAFT to obtain P2 (Figure S5). 1H NMR spectra confirmed the successful formation of both the homopolymer and the copolymer. The characteristic peaks of inimer M1 and GMA were at 4.5 and 3.5 ppm, respectively (Figure S4). From NMR analysis, the actual incorporation ratio of inimer M1 and GMA was estimated as 85 (M1):15(GMA). GPC analysis revealed the number-average molecular weights of P1 and P2 to be 7700 g/mol and 5900 g/ 101
DOI: 10.1021/acsmacrolett.7b00979 ACS Macro Lett. 2018, 7, 100−104
Letter
ACS Macro Letters
Figure 2. SI-NMP of the second layer of PS brushes in the presence of the free initiator on the Si wafer (see Figure S11 and Figure S12 for details on other substrates). (a) Molecular weights of free PS in the solution and cleaved PS from the surface vs the monomer conversion. (b) First-order kinetics of the polymerization with presence of the free initiator. (c) Thickness of PS brushes vs molecular weight of the cleaved PS brushes. (d) Grafting density of the PS brushes at different conversions.
Figure 1. Cross-linking studies on the inimer coating (a) at different temperatures with annealing time fixed at 30 min and (b) at 140 °C with different annealing time. Thicknesses and N/C ratios are both normalized to the as-spun coated inimer coating.
independent of the substrate on which the inimer coating is applied (Figure S12). Although theoretical studies have argued that polymer brushes on planar surfaces have lower molecular weights than free polymers, which has been validated in many experimental studies,5,35 our counter results are not the only exception to this rule.23,24,36 Unlike bulk polymerization, the major problem for brush growth on planar surfaces is the insufficient mixing of free radicals and monomers, due to the slower diffusion time of monomers to the surface compared to the kinetics of the reaction.37 This mixing issue primarily causes the molecular weight of brushes to be smaller than that of the free polymers. However, if the diffusion of monomer to the surface can be fast enough, the molecular weight of brushes can indeed be equal to or larger than the free polymers. Upon comparison of the studies which show higher molecular weight in brushes grown on planar surfaces than the corresponding free polymers,23,24,36 there are two interesting commonalities: (1) the polymerization temperatures are high (≥100 °C) and (2) the brush grafting densities are high (≥0.8 chains/nm2). The high temperature promotes the monomer diffusion to the surface, hence improving the mixing of the free radicals on the surface and monomers in the solution, and the high initiator density on the surface increases the rate of initiation and hence the rate of polymerization.38,39 In our system, in fact when the inimer coating thickness is closer to a monolayer, the molecular weight of the PS brush grown from the coating is less than that of the free polymer following the theoretical predictions; however, with a thicker coating the trend is reversed (Table S3). The first-order kinetic plot showed that the SI-NMP of PS is a living polymerization (Figure 2b). The dry brush thickness is also proportional to the molecular weight of cleaved brushes (Figure 2c). From the dry brush thickness (h) and the molecular weight of the cleaved PS brushes (Mn), the chain grafting density (σ) was calculated using the following equation15
statistical brush of styrene and MMA, namely, poly(styrenestatistical-methyl methacrylate) (PS-r-PMMA), where the PMMA segments can be readily degraded under ultraviolet (UV) light. Given that the reactivity ratio of styrene and methyl methacrylate is close to 0.5, we expect only short segments of PS and PMMA alternating in PS-r-PMMA.30 Other experiments have estimated the length of both segments to be 1−2 units long when the monomer feed ratio is close to 0.5, which will allow for complete degradation of the sacrificial layer.31 After growing an ∼5 nm thick sacrificial layer, a second layer of PS brush was grown. Exposure to 254 nm UV light was used to degrade PMMA32 in the sacrificial layer and to release the PS brushes. Note that PS has a large absorption coefficient around this wavelength (∼50 cm−1),33 which might interfere with delivering enough energy to the sacrificial layer if the thickness of PS brushes is too large. However, an estimation using Beer’s law shows more than 99% transmittance of ∼254 nm UV through a 1 μm thick PS layer, which is sufficient to pass through the PS brush layer in our studies. Another concern is UV-induced cross-linking or decomposition of PS. Our previous study showed that 0.6 J/cm2 of 254 nm UV causes little cross-linking of PS thin film on a SiO2 substrate,34 and the PS powder sample exposed to the same UV did not show significant change in molecular weight and dispersity (
