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Stimuli-Responsive Binary Mixed Polymer Brushes and Free-Standing Films by LbL-SIP Nicel C. Estillore and Rigoberto C. Advincula* Department of Chemistry and Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-5003, United States
bS Supporting Information ABSTRACT: We report a facile approach to preparing binary mixed polymer brushes and free-standing films by combining the layer-by-layer and surface-initiated polymerization (LbLSIP) techniques. Specifically, the grafting of mixed polymer brushes of poly(n-isopropylacrylamide) and polystyrene (pNIPAM-pSt) onto LbL-macroinitiator-modified planar substrates is described. Atom transfer radical polymerization (ATRP) and free radical polymerization (FRP) techniques were employed for the syntheses of pNIPAM and pSt, respectively, yielding pNIPAM-pSt mixed polymer brushes. The composition of the two polymers was controlled by varying the number of macroinitiator layers deposited on the substrate (i.e., LbL layers = 4, 8, 12, 16, and 20); consequently, mixed brushes of different thicknesses and composition ratios were obtained. Moreover, the switching behavior of the LbLmixed brush films as a function of solvent and temperature was demonstrated and evaluated by water contact angle and atomic force microscopy (AFM) experiments. It was found that both the solvent and temperature stimuli responses were a function of the mixed brush composition and thickness ratio where the dominant component played a larger role in the response behavior. Furthermore, the ability to obtain free-standing films was exploited. The LbL technique provided the macroinitiator density variation necessary for the preparation of stable free-standing mixed brush films. Specifically, the free-standing films exhibited the rigidity to withstand changes in the solvent and temperature environment and at the same time were flexible enough to respond accordingly to external stimuli.
’ INTRODUCTION Stimuli-responsive polymer films are a class of intelligent or “smart” materials with immense implications for fundamental research and industrial and biomedical applications.1 Highdensity surface-tethered polymer chains as thin coatings allow for the manipulation of surface properties that are useful in a host of applications including coatings, ion selectivity, drug delivery, and biomedical applications.26 These high-density surfacetethered polymers, commonly referred to as polymer brushes, have been widely investigated for their pronounced response to perturbations in the environment.79 Of the various brush compositions that include homopolymer, block copolymer, and random copolymer, binary mixed polymer brushes have received increasing attention because of their reversible responsive properties.1020 Binary mixed polymer brushes can be composed of two incompatible polymers (i.e., hydrophilic and hydrophobic polymers) that are statistically distributed and grafted irreversibly on the same substrate.10 However, a challenge plaguing the synthesis of mixed brushes is the inherent incompatibility and interference of reaction conditions affecting the polymerization of each polymer component. Therefore, a key enabling factor for the preparation of mixed brushes is the selective activation of two different polymerization techniques. r 2011 American Chemical Society
The photo and thermal activation of a derivatized azobisisobutyrylnitrile (AIBN) initiator has been demonstrated for the synthesis of various mixed brushes.2123 Mixed brushes have been grown from a single initiator through incomplete photodecomposition and thermal activation of an AIBN initiator, enabling the successful grafting of a second brush component. Although there has been limited success using this route, recent advances in living free radical polymerization such as atom transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMP) have led to more well-controlled brush growth.24,25 Both the “grafting to”2631 and “grafting from”3237 methods have been applied to the synthesis of binary mixed polymer brushes. For instance, polystyrene (PS) and poly(2-vinylpyridine) (P2VP) mixed brushes and polyelectrolyte mixed brushes have been prepared by employing the grafting to approach.2631 In this case, the grafting conditions (i.e., time, temperature, and concentration) during the polymerization of the first brush were regulated to ensure that unreacted functional groups remain for the second brush to be grafted. Because Received: January 8, 2011 Revised: March 23, 2011 Published: April 22, 2011 5997
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Scheme 1. Ultrathin Film Fabrication via the Layer-by-Layer (LbL) and the Subsequent Surface-Initiated Polymerizations (SIP) of NIPAM and Styrene on Modified Planar Substrates
preformed polymers are used in a grafting to method, the accumulation of these polymers on the surface hinders the diffusion of more polymers from reacting at the interface, resulting in a low grafting density and film thickness.38a Because monomers can diffuse more readily toward functionalized surface-bound initiators, the grafting from or surface-initiated polymerization (SIP) approach has been more widely explored for the synthesis of dense polymer brushes.38b PMMA-PS mixed polymer brushes have been reported from mixed monolayers of ATRP and NMP initiators as well as from asymmetric difunctional initiators (Y-SAM) possessing both ATRP and NMP moieties.3236 In this case, the mixed brushes are synthesized simultaneously through different activation mechanisms. Thus, the grafting from approach affords highly stretched polymer brushes that can undergo reversible conformational changes as a result of various external stimuli such as solvent, pH, temperature, and light, making them particularly attractive for the preparation of a variety of “smart” surface coatings.79 External stimuli including solvent and pH have been utilized in regulating the switching properties of binary mixed polymer brushes.2136 Because of the immiscibility of mixed polymer brushes, induced self-reorganizations have been observed. Various groups have demonstrated the solvent switching behavior of PSPMMA mixed brushes.21,3236 The subtle interplay between the quality of the solvent and the chain length (or molecular weight) affects the self-reorganization of these films. In a nonselective solvent, a “rippled” morphology can be observed, resulting from the lateral phase segregation of two incompatible brushes, whereas a selective solvent reveals a perpendicular phase segregation creating a “dimpled” or layered morphology.3236 Theoretical calculations have also been extensively used for the verification of the phase segregation in binary mixed polymer brushes.3945 Although the SIP technique affords highly stretched polymer chains, control of the initiator density remains crucial in
establishing a proper brush conformation. Genzer and co-workers have studied the crossover from the mushroom to brush regime on a gradient-functionalized initiator substrate.46 The initiator concentration gradient varied along the substrate in which the thickness of the grafted brush layer correlated directly to the grafting density. Consequently, at a low initiator concentration, the mushroom regime was obtained but a high initiator concentration produced crowded polymer chains in the brush regime.46 The brush morphology is more desirable because by end grafting at one end it is possible to observe a reversible swelling or collapsing as a result of a change in the environment.38 Therefore, scrupulous control of the initiator density requires accounting for the number of available initiating sites on a surface as well as the length of time the substrate is in contact with the initiator solution.47,48 Recently, the use of water-soluble macroinitiators offered an advantage over the typical synthesis of silane-based initiators.49 More specifically, these macroinitiators allow for deposition onto various substrates using water as the solvent medium compared to using organic solvents.49 The combined layer-by-layer (LbL) and SIP techniques (LbL-SIP) have emerged as a promising method for the synthesis of very dense, “smart” nanomaterials. The Advincula group was one of the first to report this concept of employing the combined LbL and SIP techniques for controlling the underlying layers and subsequent growth of polymer brushes, respectively.50,51 This offers a unique advantage in that the LbL underlayers can be prepared to respond to various external stimuli while at the same time the outer layer is coated with polymer brushes. More interestingly, the sole deposition of macroinitiators has proven effective in the control of initiator density because each layer can act as a grafting site for which the brushes are grown (i.e., varying brush thicknesses are obtainable).52 One emerging application in the area of SIP is the fabrication of free-standing or self-sustaining polymer films.53 The ability to obtain films without substrates is expected to broaden the field of 5998
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Figure 1. Multilayer buildup of the LbL ATRP/ACVA macroinitiator films as monitored by ellipsometry. The thickness of the film was directly related to the number of deposited macroinitiators on the surface.
sensors, separation membranes, micromechanical devices, and wound dressing.5457 Although the LbL technique has been extensively utilized for the preparation of free-standing films,58 the incorporation of stimuli-responsive brushes has not been fully explored. The work presented in this article strives to investigate more closely the various aspects of the combined LbL-SIP approach for the synthesis of binary mixed polymer brushes and free-standing films. Specifically, binary mixed polymer brushes composed of poly(n-isopropylacrylamide) and polystyrene (pNIPAM-pSt) were synthesized via ATRP and free radical polymerization (FRP) techniques, respectively (Scheme 1). An increase in the initiator density was confirmed by ellipsometry and X-ray photoelectron spectroscopy (XPS) data. The successful grafting of the binary mixed brushes was verified by ellipsometry, water contact angle, FT-IR (transmission and imaging modes), XPS, and atomic force microscopy (AFM) measurements. The fine tuning of surface properties was demonstrated by exploiting various external stimuli (i.e., solvent and temperature) to regulate the switching behavior of the LbLmixed brush films. Because pNIPAM and pSt brushes are incompatible, phase segregation was observed upon exposure to selective solvents. Moreover, the preparation of stimuli-responsive free-standing mixed brush films was realized with the current LbL-SIP system through the use of a sacrificial layer, poly(vinyl alcohol) (PVOH), for the detachment of the polymer film from the substrate. It was concluded that the composition of the two polymers is crucial in dictating the surface properties as well as the stability, durability, and integrity of the free-standing films.
’ RESULTS AND DISCUSSION Multilayer Growth of the LbL Macroinitiator Films. A relatively high grafting density is crucial in establishing the polymer brush conformation.38 Because control of the initiator layers can yield more feasible manipulation of the resulting polymer brush, the LbL technique was used for the fabrication of five different films (LbL layer = 4, 8, 12, 16, and 20) with varying amounts of the macroinitiator layers deposited on the surface. Two ATRP macroinitiators and a low-molecular-weight 4,40 -azobis(4-cyanovaleric acid) (ACVA) FRP initiator were
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utilized for the LbL assembly. On the basis of the ellipsometry data shown in Figure 1, the LbL film thickness was proportional to the number of macroinitiator layers deposited on the substrate with an average thickness of 8 Å per bilayer deposited.51,52 The LbL macroinitiator film thickness ranged from 13 to 26 nm, which also corresponded to an increase in the initiator density on the surface.52 Essentially, each of the layers within the multilayer acts as an initiating site capable of participating in the polymerizations of NIPAM and styrene. It is expected, therefore, that the LbL 20 and 4 films will yield the thickest and thinnest mixed brushes, respectively, and that the other LbL films (8, 12, and 16) will have intermediate brush thicknesses. Water contact angle measurements taken for the LbL macroinitiator films suggested relatively hydrophilic surfaces (∼70°) as a result of the water-soluble macroinitiators used. Furthermore, XPS was employed to probe the chemical identification of the LbL macroinitiator films. Figure 2b (bottom black plot) displays a representative survey scan taken from the LbL 20 macroinitiator film (26 nm thickness). Of particular interest is the bromine (Br 3d at 73 eV) signal that is the elemental marker for the ATRP macroinitiators.24 Thus, an increase in the Br 3d signal translates to more active sites on the surface; therefore, thick polymer brushes should be obtainable.52 As the LbL macroinitiator film increased, so did the Br 3d signal, which is a further evidence of the increased initiator density on the surface with the LbL 20 macroinitiator film yielding the highest bromine concentration. SIP Growth of the LbL-PNIPAM-PSt Mixed Brushes. The sequence of the polymerizations is crucial to the SIP growth of the two brushes (Scheme S1). The growth of the first brush ultimately determines the availability and accessibility of the underlying initiating layers. Thus, an undesirable scenario would be present when the first brush is so thick such that the initiating layers underneath are no longer available for the polymerization of the second polymer brush. To address this possible problem, ATRP was performed first, followed by the FRP of the second monomer. This polymerization sequence effectively controls the molecular weight of the initial brush and simultaneously avoids problems with uncontrolled diffusion-limited termination in the subsequent step.24,59 Moreover, the ATRP technique allows certain monomers to be polymerized under ambient conditions (i.e., RT in aqueous solution),24 which is desirable in this study because the ACVA initiator is activated via heat or light of a specific wavelength.59 Thus, the optimum conditions were to conduct the ATRP of NIPAM followed by the FRP of styrene, yielding the pNIPAMpSt mixed polymer brushes. Figure 3 summarizes the thicknesses after the SIP polymerizations of the pNIPAM homopolymer and pNIPAM-pSt mixed brushes. As expected, a direct relationship was observed with the pNIPAM brush thickness and the LbL macroinitiator films. PNIPAM brush thicknesses of 28, 56, 71, 138, and 159 nm were produced for the LbL 4, 8, 12, 16, and 20 films, respectively. Likewise, after the growth of the second brush pSt, the obtained pNIPAM-pSt mixed brush thickness also increased proportionally to the LbL macroinitiator films. These results indicate that with more macroinitiator layers deposited on the surface, the initiator density increases accordingly, thereby producing thicker polymer brushes. However, associated with the increasing pNIPAM brush thickness was a decrease in the ability of the underlying ACVA initiators to polymerize styrene. More interestingly, an inverse relationship was observed with the pSt brush thickness 5999
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Figure 2. Surface characterizations of the LbL macroinitiator and brush films taken from the LbL 20 film. (a) Representative FT-IR (transmission mode) spectra of the LbL macroinitiator (bottom), LbL-pNIPAM (middle), and LbL-pNIPAM-pSt mixed (top) brush films. (b) Representative XPS survey spectra of the LbL macroinitiator (bottom), LbL-pNIPAM (middle), and LbL-pNIPAM-pSt mixed (top) brush films.
Table 1. Thickness of the LbL-PSt Brushes and Water Contact Angle Measurements for the LbL-PNIPAM and PNIPAM-PSt Mixed Brushes static water contact angle (θ)
Figure 3. Thickness measurements for the pNIPAM homopolymer (9) and pNIPAM-pSt mixed (4) brush films. The pNIPAM and overall film thicknesses increased accordingly with increasing LbL macroinitiator films.
and the LbL macroinitiator films. By taking the difference (Δ) between the pNIPAM-pSt and pNIPAM thicknesses, the relative thickness of the pSt brush was determined. The Δ values are summarized in Table 1. These results imply that the availability of the ACVA initiator to polymerize styrene decreased because of the increase in the density of the initial pNIPAM brushes. In other words, although the LbL 20 macroinitiator film had more macroinitiator layers deposited as compared to the LbL 4 film, the thick pNIPAM brushes prevented the diffusion of the styrene monomer toward the ACVA initiators. Furthermore, the importance of the pNIPAM brush as a temperature-sensitive polymer also played a crucial role during the FRP of styrene. The pNIPAM brush has been reported to exhibit a lower critical solution temperature (LCST) of 32 °C whereas above and below this temperature the pNIPAM chains alter their properties as well as conformational states.3,7,51 Because the FRP of styrene was conducted at an elevated temperature of 60 °C, the pNIPAM brushes were in a collapsed and dehydrated conformation. This provides further evidence of
LbL macroinitiator
pSt brush thickness
pNIPAM
pNIPAM-pSt
films
(nm)
(deg)
(deg)
4
31
65 ( 1.4
88 ( 1.3
8
26
62 ( 4.3
89 ( 2.9
12 16
24 6
66 ( 0.2 64 ( 0.7
88 ( 0.5 88 ( 6.7
20
2
65 ( 1.7
91 ( 1.9
the steric hindrance imposed by the dense, collapsed pNIPAM brushes preventing the diffusion of the styrene monomer to the underlying ACVA initiators. The successful FRP of styrene is evident by the water contact angle measurements as well as the free pSt polymer produced in solution. Table 1 provides the contact angle measurements for the pNIPAM homopolymer and pNIPAM-pSt mixed brushes. The LbL macroinitiator films became hydrophilic after the growth of the pNIPAM brushes (∼64°) and hydrophobic following the growth of pSt brushes (∼89°). These values are consistent with previously studied pNIPAM and pSt brushes.51,60 FT-IR and XPS Analysis of the LbLBrush Films. Surface characterizations by FT-IR and XPS were performed to evaluate the LbLbrush films further. Figure 2a displays representative FT-IR (transmission mode) spectra of the LbL macroinitiator film (bottom), pNIPAM homopolymer brushes (middle), and pNIPAM-pSt mixed brushes (top). Peak assignments for the pNIPAM homopolymer brush are as follows: 33003400 cm1, NH stretching; 28003000 cm1, CH(CH3)2 isopropyl group of pNIPAM; 1650 and 1510 cm1, amide I (CdO) and amide II (NH), respectively. All assignments correlated with previously studied pNIPAM brushes.51 For the pNIPAMpSt mixed brushes, the peaks affiliated with pNIPAM brushes decreased accordingly but the peaks attributed to pSt brushes, mainly the aromatic CH stretching (28003000 cm1), should have increased. Although these peaks were present as 6000
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Figure 4. Overlaid FT-IR spectra of the LbL 4 (pink), 8 (olive), 12 (blue), 16 (black), and 20 (red) films. The left and right plots display the pNIPAM homopolymer and pNIPAM-pSt mixed brush spectra, respectively.
shown in Figure 4, it was not proportionate for each LbL layer as observed with ellipsometry. It is possible that the aromatic CH functional group is weaker relative to the carbonyl and amine groups. When all of the LbLbrush films were overlaid, the intensities of the carbonyl and amide peaks were proportional to the composition of LbL films, LbL 20 with the highest and LbL 4 with the lowest intensities. These results also implied that the LbL 20 film was the thickest overall film, thereby giving the highest absorbance intensities. Figure 4 illustrates the overlaid spectra of the pNIPAM homopolymer and pNIPAM-pSt mixed brush films. A closer examination of the CH stretching region after the growth of pSt brushes revealed a decrease in the peaks affiliated with the pNIPAM brush for all of the LbL films. In addition, it was expected that there will be an increase in the overall CH stretching region because of the addition of pSt and a decrease mainly in the NH (3300 cm1) and CdO (1600 cm1) regions. The thickness data (Table 1) suggests that the LbL 4 film should display the greatest aromatic CH stretching. Because pSt grew 31 nm as compared to only 2 nm for the LbL 20 film, this was not the case. Additional analysis of the films using IR imaging and XPS was performed to confirm the successful grafting of the pNIPAM-pSt mixed brushes. FT-IR imaging was also performed to probe the functional group distribution and density on the LbLbrush films.53a,61 In particular, the surface chemical mapping of the relatively thick LbL 20brush film (pNIPAM, 159 nm; pNIPAM-pSt, 161 nm) was examined over an area of 160 160 μm2. Figure 5 displays the FT-IR 3-D images of the pNIPAM homopolymer and pNIPAM-pSt mixed brush films with a false color assignment and spectral range of 1500 to 3500 cm1 per image. The maximum and minimum absorbances were taken from the CdO, CH, and NH regions centered at 1600, 2900, and 3300 cm1, respectively. The color assignments are as follows: green (CdO), yellow (CH), and blue (NH). The LbL 20 film was saturated with pNIPAM brushes (159 nm), and the thinner pSt brushes only grew 2 nm. Consequently, the film was mostly enriched in the pNIPAM brushes. In addition, the statistical distribution of the pNIPAM-pSt mixed brushes meant that the surface functionalities were randomly configured on the surface. As illustrated in Figure 5, the color intensities provided insight into the functional group distribution and film homogeneity. Although a particular functional group was focused at the
corresponding wavenumber, traces of the other colors were also present, highlighting the mixed brush composition or simply functional group distribution. For instance, the carbonyl group was centered at 1600 cm1 where the green color saturated the surface, but there were traces of the aliphatic CH groups as evident from the yellow color. Likewise, in the aliphatic CH region centered at 2900 cm1, the yellow color became more intense and its distribution proliferated in this region. The blue color was dominant in the amine region at 3300 cm1, but again, there were traces of the green as well as the yellow color, confirming the presence of the carbonyl and CH groups, respectively. Figure S4 provides the actual spectra taken from the three major functional groups of the LbLbrush films from the focal plane array (FPA) detector. The peaks centered at 1600, 2900, and 3300 cm1 were all attributed to the pNIPAM brushes. It is evident, however, that the maximum absorbances in the three different regions for the pNIPAM-pSt mixed brush all decreased when compared to that of the pNIPAM homopolymer brushes (Figure 6). These results confirm the data acquired from the transmission-mode FT-IR in which the succeeding growth of the pSt brushes led to a decrease in the major peaks affiliated with the pNIPAM brushes. To confirm the successful grafting of the binary mixed brushes, XPS was also employed to elucidate the elemental composition of the LbLbrush films (Figure 2b). Unlike the FT-IR data, the overall carbon signal increased accordingly with the LbL films after the grafting of pSt brushes (Figure S5). In addition, the comparison of the N 1s signal between the pNIPAM homopolymer and pNIPAM-pSt mixed brushes indicated the greatest and smallest decreases were acquired for the LbL 4 and 20 films, respectively. Table 2 summarizes the change (or amount of decrease) in the N 1s signal that was determined by calculating the area under the nitrogen peak. These results corroborated the thickness change for the pSt brushes (refer to Table 1). Although the LbL 4 film possessed the least amount of deposited macroinitiators, it yielded a 90% decrease in nitrogen relative to the carbon signal after the growth of the pSt brushes, indicating that a thick pSt brush layer was grafted; thus this film produced a better balance (or symmetric composition) between the pNIPAM (28 nm) and pSt (31 nm) brushes. However, the LbL 20 film still had a significant amount of detectable nitrogen even after pSt brush polymerization with only a 40% decrease in the nitrogen 6001
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Figure 5. Surface chemical mapping probed by FT-IR imaging of the LbL 20brush films. The maximum and minimum absorbances were taken at the three major functional groups of pNIPAM: (a) CdO, 1600 cm1; (b) CH, 2900 cm1; and (c) NH, 3300 cm1 groups.
signal relative to that of carbon, indicating that a thin pSt brush layer was grafted; thus it produced an imbalance (or asymmetric composition) between the pNIPAM (159 nm) and pSt (2 nm) brushes. Stimuli Response of the Binary LbLMixed Polymer Brushes. The solvent and temperature responses of the LbLmixed brush films were then investigated. The stimuli response of binary mixed brushes comprises a complex interplay between the two brushes, causing them to undergo phase segregation.10 The composition (and therefore the thickness) of each brush will determine its behavior upon perturbations in the environment. Solvent Response. The induced surface reorganization depends on the quality of the solvent chosen. In this study, toluene and a mixed solvent of H2O/MeOH were employed as the solvents either to swell or collapse the pNIPAM-pSt mixed
brushes.3,7,27,51 Briefly, the LbLmixed brush films were exposed to each solvent for 1 h at RT and immediately dried with compressed air prior to the contact angle and AFM measurements. A complete solvent cycle was performed to examine the reversible switching of the surface properties. Although solvent response studies often involve heating the solvent,27,33 we opted not to apply heat because it causes the pNIPAM brushes to attain the collapsed, dehydrated state.3,7,51 The change in surface wettability of the LbLbrush films upon exposure to the solvents was measured using the water contact angle. Figure 7 provides the contact angle measurements for each film after exposure to the solvents. Toluene is a selective solvent for the pSt brushes, causing it to be in a swelled and extended state, but simultaneously acts as a poor solvent for the pNIPAM brushes. All of the films were hydrophobic after soaking in toluene (∼88°), which indicated that the pSt brushes 6002
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Table 2. Atomic Concentrations of the N 1s Signal for the LbL-PNIPAM and PNIPAM-PSt Mixed Brush Films atomic concentration (N 1s) LbL macroinitiator films
Figure 6. Corresponding bar graphs maximum absorbance for the different The green bars show the maximum brushes, and the gray bars show the pNIPAM-pSt mixed brushes.
depicting the decrease in the functional groups of pNIPAM. absorbance for the pNIPAM maximum absorbance for the
were at the airbrush interface, dictating the wetting properties. The films were then soaked in a mixed solvent of H2O/MeOH, which is now a better solvent for the pNIPAM brushes but at the same time acts as an unfavorable solvent for the hydrophobic pSt brushes. It was quite evident that only the LbL 4 and 8 films remained relatively hydrophobic (∼82°). These values imply that both polymers were present at the airbrush interface even though the polar solvent was more favorable for the pNIPAM brushes. Moreover, with increasing incompatibility in the binary mixed brush composition, the surface property was mainly dictated by one brush. For instance, the LbL 12, 16, and 20 films had dramatically altered wetting properties after soaking in the polar solvent. A clear transition from a hydrophobic (∼90°) to hydrophilic (∼60°) surface was attributed to the enrichment of the surface by the pNIPAM brushes. Finally, the LbL 12, 16, and 20 films never attained their initial hydrophobic property, and even after an overnight soaking in the toluene solvent, contact angle values of 80, 78, and 64° were obtained, respectively. In contrast, the LbL 4 and 8 films did regain their hydrophobic properties, implying that these more symmetric films exhibited a better reversible switching of surface properties. To confirm the solvent-induced surface reorganization, AFM analyses of the LbL 8 and 12 brush films were examined (Figure 8). It is evident from the AFM images that the thicker or dominant brush saturated the surface. Even though the LbL 8 and 12 films produced almost identical pSt brush thicknesses, 26 and 24 nm, respectively, their surface morphologies were distinctively different under the toluene solvent. LbL 8 displayed more globular domain features characteristic of the behavior of pSt brushes.60 However, with increasing incompatibility or asymmetrical composition, the LbL 12 film showed a unique morphology. The large incompatibility for which the surfaces were mainly composed of pNIPAM brushes caused these brushes to dominate the response behavior over the pSt brushes even when the films were soaked in toluene. The wormlike morphology of the LbL 12 film suggested that very little phase segregation existed because of the dominance of pNIPAM over the pSt brushes. When the solvent quality decreased for the pSt brushes, a significant morphological change was observed for the LbL 8
pNIPAM
pNIPAM-pSt
%Δ
4
9.40
1.64
90.2
8
10.34
4.08
80.1
12 16
10.65 11.09
4.15 5.20
77.5 61.7
20
11.35
7.15
40.2
film. It is noticeable that this film underwent a surface rearrangement (after soaking in H2O/MeOH) where the solvophobic pSt brushes formed the core and the solvophilic pNIPAM brushes shielded the pSt. This observation is similar to published reports where exposure to a selective solvent led to a “dimpled” morphology.3236 This “dimpled” morphology arises from clusters formed when one component is exposed to a selective solvent, and the tendency of the brush is to minimize contact with the solvent. The random distribution of the binary mixed brushes on multiple layers of the macroinitiator on the surface suppressed the aggregates from homogeneously and uniformly covering the surface. As shown in Figure 8, the LbL 8 film displayed isolated clusters on the surface. In contrast, there was no obvious change in the wormlike morphology for the LbL 12 film, but the polar solvent did cause some swelling of the pNIPAM brushes with an increase in the surface roughness. The reversible switching behavior of the surface properties was confirmed as both of the LbL films reverted back to their original morphology as displayed in Figure 8. Both the AFM and contact angle data provide evidence of a successful surface reorganization induced by a change in the solvent. It is concluded that the saturation of one brush over the other largely dictated the surface properties and that the interplay between solvent selectivity and brush thickness affected the reorganization of the binary mixed brushes. Free-Standing Mixed Polymer Brush Films (Scheme 2). The integrity, stability, and durability of the free-standing films are determined largely by their mechanical strength.58 These films must exhibit rigidity to be able to withstand mechanical forces but also the flexibility to respond to perturbations in the environment. For example, the freely floating film must be able to undergo contraction and/or swelling as a result of an external stimulus while retaining its original structure (or dimensions). If the free-standing film is too stiff, then it may or may not undergo the proper conformational changes, but if it is too flexible, then there is a high probability that the film will disintegrate. In this study, the LbL 16 mixed brush film was employed for the preparation of the free-standing films. In particular, the overall thickness obtained for this film was 144 nm with a composition of 138 nm of pNIPAM and 6 nm of pSt. The composition of each brush component will have a prominent effect on the behavior of the free-standing films, and as stated earlier, whichever brush is the more dominant of the two will dictate the behavior of the freely floating films. The solvent and temperature response experiments of the free-standing mixed brush films were conducted under conditions similar to those for films still attached to the substrate. A 10 wt % aqueous solution of poly(vinyl alcohol) (PVOH) was drop-casted on the LbLmixed brush film and dried 6003
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Figure 7. Static water contact angle measurements for the solvent response of the LbLbrush films. A transition from a hydrophobic to a hydrophilic surface was obtained for the LbL 12, 16, and 20 mixed brush films.
Figure 8. Solvent response of the LbLmixed brush films studied by AFM. The topographic AFM images (5 μm 5 μm) of the LbLmixed brush films depict the evolution of the surface morphology after exposure to the solvents. A complete solvent cycle was conducted to test the reversibility of the films.
overnight. (The approximate thickness of the PVOH layer was about 130 nm.) After drying overnight, the film was pliable and easily detached using tweezers. The successful detachment of the free-standing film from the substrate was confirmed using ATRIR (Figure S6). As expected, the pure PVOH film displayed a broad hydroxyl group (OH) between 3100 and 3500 cm1. The peeled-off PVOH mixed brush film also displayed the broad hydroxyl group, but this region could also include the amine group (NH) originating from pNIPAM. The 28003000 cm1 CH region was also present. The distinguishing peak from the pure PVOH and PVOH mixed brush film lies in the carbonyl region at 1650 cm1, which was absent in the pure PVOH film.
Furthermore, after the dissolution of the PVOH layer in water, the mixed brush film was released and transferred to a new Si wafer substrate. The front side of the film displayed ridgelike features (Figure S7) that were due to a surface reorganization upon dissolution of the PVOH layer and the subsequent transfer and drying of the freely floating film on the new substrate.53a The stability of the free-standing film was also examined, and for this purpose, the LbL 8 brush film was also prepared as a comparison to the LbL 16 film. After the dissolution of the PVOH layer, the released mixed brush films remained in the water for an additional 1 h in order to observe their behavior. It is 6004
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Scheme 2. Preparation of the Free-Standing Mixed Brush Films from PVOH as the Sacrificial Layer
film, which had a thicker pNIPAM brush than the LbL 8 film. In this case, the thicker pNIPAM brushes were rearranging to minimize their interaction with the nonpolar solvent. The LbL 8 film showed some insolubility, but the thicker pSt brushes had more affinity for this (toluene) solvent. The reversibility was tested by placing the freely floating films back into the H2O/ MeOH mixed solvent. Both films became less opaque and more transparent. It is apparent that during the solvent treatment experiments both of the free-standing films were prone to disintegrate. This was especially true during the swelling and contraction under the solvent conditions. This implies that the free-standing films were so flexible that when undergoing conformational changes they broke into several pieces. Several groups have reported the incorporation of inorganic components, including clay and nanoparticles, to give the free-standing films structural rigidity.58 As for the temperature response, only the LbL 16 film was examined (Figure 11). As the temperature was elevated to above the LCST of pNIPAM, the film started to become more opaque but remained swelled in the water. The opaqueness was attributed to the transition from coil to globule of the pNIPAM brushes as well as the contributing insolubility of the pSt brushes.53a
Figure 9. Stability of the obtained free-standing mixed brush films examined by leaving the films suspended in water for an additional 1 h after the dissolution of the PVOH layer.
clearly evident that the LbL 8 film that possessed the thicker pSt brush (26 nm) started to fold because of the polar solvent, whereas the LbL 16 that had a thinner pSt brush (6 nm) remained swelled in water (Figure 9). The response behavior of the free-standing mixed brush films to solvent and temperature changes were also demonstrated. Figure 10 provides macroscopic images while the films were suspended in each solvent. It is apparent that in the H2O/MeOH mixed solvent the LbL 8 and 16 films were in a swelled conformation; however, as the solvent went from a polar to a nonpolar solvent, the freely floating films became increasingly insoluble. This insolubility was more evident with the LbL 16
’ CONCLUSIONS The preparation of binary mixed polymer brushes and freestanding films was demonstrated on the basis of the combined LbL and SIP techniques. Specifically, sequential ATRP and FRP techniques were employed for the synthesis of the pNIPAM-pSt mixed brushes. The thickness and XPS measurements confirmed an increase in the initiator density with increasing LbL macroinitiator film. The composition of the two brushes was controlled by varying the number of macroinitiator layers deposited on the surface during the multilayer buildup of the films. Consequently, symmetric and asymmetric binary pNIPAM-pSt mixed polymer brushes were obtained. Because the LbL 20 film yielded the thickest pNIPAM brush, the subsequent grafting of pSt produced a large compositional imbalance, leaving the saturation of the pNIPAM on the surface. The reversible switching of the surface properties was illustrated by alterations in the quality of solvent and tracked by water contact angle and AFM measurements more prominently with symmetric composition LbL 4 and 8 films. The phase segregation of the mixed brushes revealed unique morphologies obtained for the LbL 12 and 16mixed brush films for which there was a large compositional imbalance. More interestingly, the transition from a hydrophobic to a hydrophilic surface was evident in these films. Furthermore, the preparation of free-standing mixed brush films was exploited for the current LbL-SIP system. The freely floating films were 6005
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Figure 10. Macroscopic observations of the freely floating films during the solvent exposure experiment. Digital images of the LbL 8 (top) and LbL 16 (bottom) free-standing films are shown.
Figure 11. Temperature response for the LbL 16 free-standing mixed brush film. Photographs were taken after 1 h soaking in water at the specified temperature.
subjected to perturbations in the surroundings, and their response was macroscopically observed. Although dense polymer brushes are desirable for induced solvent and temperature changes, the response of the brushes is compensated for by the crowding and limited space for rearrangement to occur. Hence, the LbL-SIP technique is a valuable tool for controlling the initiator density with varying brush thickness.
’ EXPERIMENTAL SECTION Materials. Styrene (St; Reagent Plus, 99%, Aldrich) was passed through an activated basic alumina column. N-Isopropylacrylamide (NIPAM, >98% TCI America) was recrystallized in n-hexanes to remove the inhibitor. 3-Aminopropyltrimethoxysilane (APS, 95% Acros Organics), Cu(I)Br (98%, Alfa Aesar), and N,N,N0 ,N0 ,N00 -pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich), 4,40 -azobis(4-cyanovaleric acid) (ACVA, >98%, Fluka), and poly(vinyl alcohol) (PVOH, Mw = 13 00023 000 g/mol, 98% hydrolyzed, Aldrich) were used as received without further purification. All other reagents were purchased from Aldrich and utilized as received. Milli-Q water with a resistivity of 18 MΩ was used for the multilayer preparation, ATRP polymerization of NIPAM, and solvent response experiments. Toluene was freshly distilled and collected immediately prior to use. The ATRP macroinitiators employed for this study were previously synthesized by our group and similar to the ones that Armes’ group synthesized.51,52 Instrumentation. Fourier transform infrared (FT-IR) absorption spectra of the LbL brush films on silicon wafer substrates were recorded with a Digilab FTS7000 series spectrometer (Varian). Spectra (4000700 cm1) were collected at 4 cm1 resolution using a mercurycadmiumtelluride (MCT) detector with 16 scans being averaged per spectrum. FT-IR imaging was conducted on a Digilab Stingray imaging system consisting of a Digilab FTS 7000 spectrometer, a UMA 600 microscope, and a 32 32 mercurycadmiumtelluride IR imaging focal plane array (MCT-FPA) image detector with an average spatial area of 176 μm 176 μm in transmission mode. An 8 cm1
nominal spectral resolution and an undersampling ratio (UDR) of 4 for the imaging setup and spectral data were collected with 16 scans. Attenuated total reflectance (ATR)-IR surface analysis of the freestanding mixed brush films and modified membranes was recorded with a Digilab FTS7000 series spectrometer (Varian) and a germanium (Ge) crystal. Spectra (4000700 cm1) were collected at 4 cm1 resolution with 16 scans being averaged per spectrum. All image and spectral processing was carried out using the Win-IR Pro 3.4 software package. Static water contact angle goniometry was conducted using a KSV CAM 200 instrument (KSV Ltd.) using the bubble drop method with Milli-Q water. 1H NMR spectra were recorded on a General Electric QE 300 spectrometer (300 MHz). X-ray photoelectron spectroscopy (XPS) was carried out on a Physical Electronics 5700 instrument with photoelectrons generated by nonmonochromatic Al Kr irradiation (1486.6 eV). Photoelectrons were collected at a takeoff angle of 45° using a hemispherical analyzer operated in fixed retardance ratio mode with an energy resolution setting of 11.75 eV. Null ellipsometry was used to determine the thickness of the LbL macroinitiator and brush films. All measurements were conducted using a null ellipsometer operating in polarizercompensatorsample analyzer (Multiskop, Optrel Berlin) mode. As a light source, a HeNe laser (λ = 632.8 nm) was applied, and the angle of incidence was set to 60°. A multilayer flat film model was used to calculate the thicknesses of the LbL macroinitiator and brush films from the experimentally measured ellipsometric angles, Δ and ψ, assuming refractive indices of n = 1.55 (for the LbL macroinitiator films), n = 1.50 (for pNIPAM), and n = 1.5447 (for pNIPAM-pSt). All atomic force microscopy (AFM) images were recorded in air under ambient conditions on a PicoScan system (Agilent Technologies, formerly Molecular Imaging Corp.) equipped with an 8 8 μm2 scanner. Magnetic ac (MAC) mode (noncontact mode) was used for all imaging. A MAC lever, which is a silicon nitride-based cantilever coated with a magnetic film, was used as an AFM tip. 6006
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Langmuir A digital camera (Panasonic DMC-ZS7, Lumix) was utilized for the macroscopic analysis and observation of the obtained free-standing mixed brush films. LbL Macroinitiator Film Preparation. Prior to the LbL multilayer growth of the macroinitiator films, the substrates were first functionalized with 0.5 wt % APS and subsequently stored in 0.1 M HCl. The positively charged substrates were alternately dipped (manually) in the polyelectrolyte ATRP macroinitiator solutions (1 mg/mL; no pH adjustment was necessary) and ACVA initiator solution (10 mg/mL; the pH was adjusted to 11.44 with 0.1 M NaOH) for 20 min, followed by a 5 min water rinse. The LbL process was repeated until the desired number of layers was achieved. The LbL macroinitiator films were dried prior to dipping into the oppositely charged solution. SI-ATRP of NIPAM. The polymerization of NIPAM was conducted under ambient conditions with a 1:1 by volume consisting of H2O/ MeOH (methanol) as solvents. Briefly, NIPAM (1.0 g, 8.84 mmol) and PMDETA (55.5 μL, 0.266 mmol), mixed in 17.5 mL of H2O/MeOH, were added to a 25 mL Schlenk flask and degassed to remove any dissolved gases and impurities for 1 h. This homogeneous solution was then transferred via a cannula to another Schlenk flask equipped with a small stir bar containing 12.7 mg (0.088 mmol) of Cu(I)Br. The degassed, green solution was finally transferred to a 25 mL Schlenk flask containing the LbL-macroinitiator-modified silicon wafer substrate. The polymerization was conducted at RT for 1 h and was terminated by repeated exposure to air. The substrate was rinsed and soaked in a H2O/ MeOH mixed solvent (at RT) to remove residual catalyst and any physisorbed material for 12 h. The LbL-pNIPAM film was dried under vacuum prior to surface analysis and styrene polymerization.
Surface-Initiated Free Radical Polymerization (SI-FRP) of Styrene. In a 25 mL Schlenk flask, styrene (3.0 mL, 26.18 mmol) was mixed with 1.5 mL of dry toluene and degassed to remove any dissolved gases and impurities for 1 h. This homogeneous solution was transferred via a cannula to another Schlenk flask containing the LbL-pNIPAM brush film. The solution was placed in an oil bath set at 60 °C for 24 h. The polymerization was terminated by repeated exposure to air. The substrate was rinsed and soaked in toluene (at RT) to remove any physisorbed material for 12 h. The resulting LbL-pNIPAM-pStmixed brush film was dried under vacuum prior to surface analysis. Solvent Treatment of the LbLMixed Brush Films. Toluene and a mixture of H2O/MeOH were employed as the selective solvents for the pSt and pNIPAM brushes, respectively. A complete solvent cycle exposure was conducted in order to examine the reversibility of the films. Briefly, the LbLmixed brush films were first immersed in toluene at RT for 1 h and immediately dried with compressed air prior to water contact angle and AFM measurements. Subsequently, the same film was immersed in a H2O/MeOH mixed solvent under the same conditions, and finally, the film was placed back in the toluene solvent.
Temperature Response of the LbLMixed Brush Films. The LbLbrush films were exposed to Milli-Q water at varying temperatures above and below the LCST (32 °C) for 1 h. After each immersion in water at the specified temperature, the films were dried with compressed air prior to the water contact angle measurements. Preparation of Free-Standing Mixed Brush Films. To detach the polymer film from the substrate, a 10 wt % solution of PVOH was drop-casted onto the LbLmixed brush films and dried slowly under an air atmosphere. The edges of the dried LbLmixed brush films were scratched to facilitate the detachment process prior to immersing the films in RT water. The complete dissolution of the PVOH sacrificial layer was achieved after 1 h of soaking in water; during the 1 h period, fresh water was replaced at least three times to get rid of the dissolved PVOH particulate. The obtained free-standing films were transferred either to a clean Si wafer substrate or a TEM grid and dried under air for further analysis and characterization. For the solvent and temperature
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response experiments, conditions similar to those of the films still attached to the substrate were used.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional thickness and contact angle measurements, AFM images, FT-IR imaging spectra, and 1H NMR spectrum of precipitated pSt. AFM and digital images of the free-standing films and membranes. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge partial funding from NSF DMR-10-06776, CBET-0854979, CHE-1041300, and Texas NHARP 01846. Technical support from Agilent Technologies, Malvern Instruments, Biolin Scientific (KSV Instruments), and Optrel is also acknowledged. ’ REFERENCES (1) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; M€uller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010, 9, 101–113. (2) Zhou, F.; Huck, W. T. S. Phys. Chem. Chem. Phys. 2006, 8, 3815– 3823. (3) Kaholek, M.; Lee, W.-K.; LaMattina, B.; Caster, K. C.; Zauscher, S. Nano Lett. 2004, 4, 373–376. (4) B€unsow, J.; Kelby, T. S.; Huck, W. T. S. Acc. Chem. Res. 2010, 43, 466–474. (5) Ducker, R.; Garcia, A.; Zhang, J.; Chen, T.; Zauscher, S. Soft Matter 2008, 4, 1774–1786. (6) Ayres, N. Polym. Chem. 2010, 1, 769–777. (7) Chen, T.; Ferris, R.; Zhang, J.; Ducker, R.; Zauscher, S. Prog. Polym. Sci. 2010, 35, 94–112. (8) Jennings, G. K.; Brantley, E. L. Adv. Mater. 2004, 16, 1983–1994. (9) Luzinov, I.; Minko, S.; Tsukruk, V. V. Soft Matter 2008, 4, 714– 725. (10) Uhlmann, P.; Merlitz, H.; Sommer, J.-U.; Stamm, M. Macromol. Rapid Commun. 2009, 30, 732–740. (11) Sheparovych, R.; Motornov, M.; Minko, S. Langmuir 2008, 24, 13828–13832. (12) Motornov, M.; Sheparovych, R.; Tokarev, I.; Roiter, Y.; Minko, S. Langmuir 2007, 23, 13–19. (13) LeMieux, M. C.; Lin, Y.-H.; Cuong, P. D.; Ahn, H.-S.; Zubarev, E. R.; Tsukruk, V. V. Adv. Funct. Mater. 2005, 15, 1529–1540. (14) Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Shulha, H.; Tsukruk, V. V. Nano Lett. 2005, 5, 491–495. (15) Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125, 15912–15921. (16) Ionov, L.; Minko, S.; Stamm, M.; Gohy, J.-F.; Jerome, R.; Scholl, A. J. Am. Chem. Soc. 2003, 125, 8302–8306. (17) Wang, X.; Bohn, P. W. Adv. Mater. 2007, 19, 515–520. (18) Li, D.; Sheng, X.; Zhao, B. J. Am. Chem. Soc. 2005, 127, 6248– 6256. (19) Zhao, B.; Zhu, L. J. Am. Chem. Soc. 2006, 126, 4574–4575. (20) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Hoy, O.; Luzinov, I.; Minko, S. Adv. Funct. Mater. 2007, 17, 2307–2314. (21) Feng, J.; Haasch, R. T.; Dyer, D. J. Macromolecules 2004, 37, 9525–9537. (22) Sidorenko, A.; Minko, S.; Schenk-Meuser, K.; Duschner, H.; Stamm, M. Langmuir 1999, 15, 8349–8355. 6007
dx.doi.org/10.1021/la200089x |Langmuir 2011, 27, 5997–6008
Langmuir (23) Minko, S.; Usov, D.; Goreshnik, E.; Stamm, M. Macromol. Rapid Commun. 2001, 22, 206–211. (24) (a) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921–2990. (b) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelback, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716–8724. (c) Ramakrishnan, A.; Dhamodharan, R.; R€uhe, J. Macromol. Rapid Commun. 2002, 23, 612–616. (d) Kaholek, M.; Lee, W.-K.; Ahn, S.-J.; Ma, H.; Caster, K. C.; LaMattina, B.; Zauscher, S. Chem. Mater. 2004, 16, 3688–3696. (25) Hawker, C. J.; Basman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661–3688. (26) Ionov, L.; Sidorenko, A.; Stamm, M. Macromolecules 2004, 37, 7421–7423. (27) Minko, S.; Luzinov, I.; Luchnikov, V.; Muller, M.; Patil, S.; Stamm, M. Macromolecules 2003, 36, 7268–7279. (28) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896–3900. (29) Houbenov, N.; Minko, S.; Stamm, M. Macromolecules 2003, 36, 5897–5901. (30) Motornov, M.; Sheparovych, R.; Katz, E.; Minko, S. ACS Nano 2008, 2, 41–52. (31) Hinrichs, K.; Aulich, D.; Ionov, L.; Esser, N.; Eichhorn, K.-J.; Motornov, M.; Stamm, M.; Minko, S. Langmuir 2009, 25, 10987– 10991. (32) Zhao, B. Polymer 2003, 44, 4079–4083. (33) Zhao, B.; He, T. Macromolecules 2003, 36, 8599–8602. (34) Zhao, B.; Haasch, R. T.; MacLaren, S. Polymer 2004, 45, 7979–7988. (35) Zhao, B.; Haasch, R. T.; MacLaren, S. J. Am. Chem. Soc. 2004, 126, 6124–6134. (36) Zhao, B. Langmuir 2004, 20, 11748–11755. (37) Wang, Y.; Brittain, W. J. Macromol. Rapid Commun. 2007, 28, 811–815. (38) (a) Polymer Brushes: Synthesis, Characterization, Application; Advincula, R. C., Brittain, W. J., Caster, K. C., R€uhe, J., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (b) Barbey, R.; Lavanant, L.; Paripovic, D.; Sch€uwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Chem. Rev. 2009, 109, 5437–5527. (39) Lai, P.-Y. J. Chem. Phys. 1994, 100, 3351–3357. (40) Soga, K. G.; Zukermann, M. J.; Guo, H. Macromolecules 1996, 29, 1998–2005. (41) Minko, S.; Muller, M.; Usov, D.; Scholl, A.; Froeck, C.; Stamm, M. Phys. Rev. Lett. 2002, 88, 035502–1. (42) Muller, M. Phys. Rev. E 2002, 65, 030802–1. (43) Santer, S.; Kopyshev, A.; Donges, J.; Yang, H.-K.; Ruhe, J. Langmuir 2006, 22, 4660–4667. (44) Santer, S.; Kopyshev, A.; Donges, J.; Ruhe, J.; Jiang, X.; Zhao, B.; Muller, M. Langmuir 2007, 23, 279–285. (45) Zhu, L.; Zhao, B. J. Phys. Chem. B 2008, 112, 11529–11536. (46) Wu, T.; Efimenko, K.; Genzer, J. J. Am. Chem. Soc. 2002, 124, 9394–9395. (47) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265–1269. (48) Bao, Z.; Bruening, M. L.; Baker, G. L. Macromolecules 2006, 39, 5251–5258. (49) Edmondson, S.; Armes, S. P. Polym. Int. 2009, 58, 307–316. (50) Fulghum, T. M.; Patton, D. L.; Advincula, R. C. Langmuir 2006, 22, 8397–8402. (51) Fulghum, T. M.; Estillore, N. C.; Vo, C.-D.; Armes, S. P.; Advincula, R. C. Macromolecules 2008, 41, 429–435. (52) Edmondson, S.; Vo, C.-D.; Armes, S. P.; Unali, G.-F.; Weir, M. P. Langmuir 2008, 24, 7208–7215. (53) (a) Fujie, T.; Park, J. Y.; Murata, A.; Estillore, N. C.; Tria, M. C. R.; Takeoka, S.; Advincula, R. C. ACS Appl. Mater. Interfaces 2009, 1, 1404–1413. (b) Amin, I.; Steenackers, M.; Zhang, N.; Beyer, A.; Zhang, X.; Pirzer, T.; Hugel, T.; Jordan, R.; Golzhauser, A. Small 2010, 6, 1623–1630.
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(54) (a) Tang, Z.; Kotov, N.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413–418. (b) Jiang, C.; Markutsya, S.; Pikus, Y.; Tsukruk, V. V. Nat. Mater. 2004, 3, 721–728. (55) Mamedov, A. A.; Kotov, N.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190–194. (56) Mallwitz, F.; Laschewsky, A. Adv. Mater. 2005, 17, 1296–1299. (57) Fujie, T.; Okamura, Y.; Takeoda, S. Adv. Mater. 2007, 19, 3549– 3553. (58) (a) Nolte, M.; Fery., A. IEE Proc., Nanobiotechnol. 2006, 153, 112–120. (b) Jiang, C.; Tsukruk, V. V. Adv. Mater. 2006, 18, 829–840. (c) Wang, Y.; Angelatos, A. S.; Carus, F. Chem. Mater. 2008, 20, 848–858. (59) Prucker, O.; R€uhe, J. Langmuir 1998, 14, 6893–6898. (60) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. Macromolecules 2000, 33, 8821–8827. (61) Miller-Chou, B. A.; Koenig, J. L. Macromolecules 2003, 36, 4851–4861.
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