Swelling of Hydrophilic Polymer Brushes by Water and Alcohol Vapors

May 26, 2016 - The sample was subsequently analyzed by contact angle measurement using deionized water (DIW) as a probing liquid and then dried with a...
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Swelling of Hydrophilic Polymer Brushes by Water and Alcohol Vapors Casey J. Galvin† and Jan Genzer* Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States S Supporting Information *

ABSTRACT: We examine the effect of end-tethering, grafting density (σ), chemistry of polymer side chain, and solvent type on the vapor swelling of hydrophilic polymer brushes. Using a library of samples derived by postpolymerization modification of a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brush, we determine the extent of vapor swelling and solvent uptake at different vapor pressures of water, methanol, and ethanol using spectroscopic ellipsometry. We compare the results from neat PDMAEMA and PDMAEMA quaternized by methyl iodide with chemically analogous samples prepared by spincasting bulk PDMAEMA. We find that brush samples swell to greater extents than spuncast samples, indicating a role for end-tethering in the vapor uptake process. Furthermore, vapor swelling of polymer brushes depends strongly on both polymer and solvent chemistry. We demonstrate that the extent to which σ affects vapor sorption inside the brush depends on polymer side chain chemistry, indicating an interdependence of the observed parameters on each other. The implications of these findings for the use of polymer brushes in technologies such as vapor sensing applications are discussed.

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sensitivity of nanocantilever vapor detection systems.13 The swelling of polymer brushes in solvent vapors has also been explored as a technique to determine the areal grafting density (σ) of polymer chains on the surface in a nondestructive way.14 This last possibility is especially important, since σ is a critical parameter in the behavior of polymer brush systems, affecting processes from biofouling15 to brush stability16 to polyzwitterionic complex formation in humid environments.17 Using poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) and its chemical derivatives exposed to water and organic alcohol vapors, we have conducted a series of experiments aiming to understand how vapor swelling of polymer brushes is affected by key features and parameters of polymer brushes, including the effect of covalent grafting, thickness, σ, side chain chemistry, and polymer−solvent interactions. We also employed spuncast polymer films as a benchmark for our studies involving polymer brushes. Figure 1 depicts schematically the various systems investigated in this work. In exploring the effect of side chain chemistry, we have employed a postpolymerization modification (PPM) scheme18,19 to ensure consistency of brush parameters between different sample chemistries. While we have attempted to consider each parameter in isolation, our results suggest that parameters may be highly coupled, and that the influence of each parameter depends strongly on the side chain chemistry of

INTRODUCTION Polymer brushes, a class of dense assemblies of macromolecules typically covalently grafted to solid surfaces, have been the subject of wide ranging studies for over the past three decades.1,2 The scientific interest in polymer brushes stems from both the potential application of these grafted films in technologies applied to fields such as biotechnology3 and antifouling surfaces,4,5 and the unusual phenomena, which arise upon confining polymer chains to nanoscale environments.6 While some of these phenomena also occur in non-grafted polymer thin films (e.g., those produced by spincasting polymer solutions on solid substrate), such as variation in glass transition temperature as a function of location within the polymer thin films,7,8 other examples are unique to polymer brushes as a result of their covalent attachment to the solid substrate. These include stretching of the grafted chains away from the substrate,9 and regulation of charges within polyelectrolyte brushes resulting in deviations from bulk pKa values.6,10,11 Understanding how confinement to the surface affects the physical behavior of the polymer chains is tantamount to the successful application of polymer brushes in technological applications. The majority of work pertaining to polymer brushes has concentrated on examining the behavior of these systems in liquid environments. Yet, polymer brushes also exhibit unique behavior in gaseous and vapor-enriched environments that are of both fundamental and physical importance. For instance, polymer brushes have been employed as active layers in vapor phase membrane technology12 and served to enhance the © XXXX American Chemical Society

Received: January 16, 2016 Revised: May 13, 2016

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DOI: 10.1021/acs.macromol.6b00111 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Schematic of spuncast polymer films (left) and polymer brushes (right) swelled by solvent vapors (blue circles). While both spuncast films and polymer brushes can be prepared such that they attain the same thickness, the structure of the two films is inherently different. Whereas a spuncast film features polymers that are packed relatively densely, the polymer brush contains interstitial sites. The interaction between chemistries in the polymer chain repeat units and in the solvent, quantified through the Flory−Huggins χ parameter, influence strongly the extent of swelling. In this study, alkyl functional groups in the polymer side chain and solvent are varied systematically to examine the effect on solvent uptake. Synthesis of PDEAEMA Brushes. An identical procedure was employed for PDEAEMA brush synthesis, except without the OTS deposition step in order to produce a polymer brush with homogeneous grafting density (σ), i.e., number of polymer grafts per unit area of the substrate. The polymerization proceeded with molar ratios of [DEAEMA]:[CuCl]:[Bpy] of [1]:[0.080]:[0.036]. Specifically, the PDEAEMA polymerization solutions contained 45 mL of DMSO (0.634 mol), 45 mL of DEAEMA (0.224) purified by passing through a column containing inhibitor remover), 2.813 g of 2,2′-bipyridyl (0.018 mol), and 0.8394 g of CuCl (0.008 mol). The polymerization lasted for 4 h, after which point the sample was removed, rinsed extensively with ethanol, sonicated in ethanol for 20 min, and finally dried with a stream of N2 gas. Chemical Modification of PDMAEMA Brushes. The postpolymerization modification (PPM) reactions on PDMAEMA were carried out using 0.1 M solutions of MeI, PrI, or PS in acetonitrile at 40 °C for 48 h in an orbital shaker.21,22 Modified samples were rinsed extensively with acetonitrile and THF and then dried under a stream of N2 gas. Bulk Synthesis of PDMAEMA. A 10 mL aliquot of DMAEMA monomer (0.059 mol) was purified by passing through a column with inhibitor remover. 9.2 mg of AIBN (5.6 × 10−5 mol) was dissolved into the bulk monomer while degassing solution by bubbling with N2 for 1 h in a Schlenk flask. The molar ratio of [DMAEMA]:[AIBN] = 1050. The flask was sealed and immersed in an oil bath at 90 °C. The reaction was allowed to proceed for 4 h, after which the flask was opened and the contents dissolved in THF and precipitated in hexanes. Repeated precipitation from THF into hexanes recovered a white solid. Preparation of Spuncast PDMAEMA Films. Silicon wafer segments measuring 1.5 cm × 1.5 cm were sonicated in ethanol, dried by a stream of N2 and treated in a UV−ozone chamber for 20 min. Bulk PDMAEMA was dissolved in THF to create either 1 or 2 wt % solutions. PDMAEMA solution was pipetted onto clean wafers and spuncast at 2000 rpm for 60 s with a 500 rev/sec ramp. For OTSbased samples, clean wafers were first inverted over pure OTS in an enclosed plastic vessel for 10 min. These OTS-modified substrates were sonicated in ethanol for 20 min, then dried under a stream of N2 gas, and characterized by ellipsometry. PDMAEMA solution was pipetted onto the OTS-modified substrate and spuncast at 2000 rpm for 60 s with a 250 rev/s ramp.

the polymer brush. We hope that this study will serve as a useful guidepost in designing polymer brush coatings for use in vapor environments, and point the way toward future experiments.

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EXPERIMENTAL SECTION

General methods. Acetonitrile, 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-(diethylamino)ethyl methacrylate (DEAEMA), methyl iodide (MeI), propyl iodide (PrI), 1,3-propane sultone (PS), dimethyl sulfoxide (DMSO), methanol, ethanol, tetramethylene ethylenediamine (TMED), 2,2′-bipyridyl, CuCl, and inhibitor remover packing were purchased from Sigma-Aldrich and used as received. nOctyltrichlorosilane (OTS) was purchased from Gelest and used as received. The ATRP initiator, [11-(2-bromo-2-methyl)propionyloxy] undecyltrichlorosilane (BMPUS), was synthesized following a previously published procedure.20 Silicon wafers (0.5 mm thick, 100 mm diameter, p-doped, orientation [100]) were purchased from Silicon Valley Microelectronics. Synthesis of PDMAEMA Brushes with Gradient in Grafting Density. A silicon wafer measuring 4.5 cm × 5 cm was sonicated in methanol, dried with a stream of N2 gas, and treated in a UV−ozone chamber for 20 min. This wafer was then placed horizontally next to a reservoir containing a 4:1 mixture of mineral oil:OTS for 7 min in an enclosed plastic Petri dish to generate a gradient in OTS along the length of the wafer. After OTS vapor deposition, the wafer was placed into a solution of 30 μL of 5 vol % BMPUS in anhydrous toluene and 30 mL of anhydrous toluene and incubated at −20 °C overnight. The wafer was then removed from solution, rinsed with ethanol, dried with a stream of N2 gas, sonicated in ethanol for 20 min and dried with a stream of N2 gas. The sample was subsequently analyzed by contact angle measurement using deionized water (DIW) as a probing liquid and then dried with a stream of N2 gas before immersion into a custom-built glass reactor containing the polymerization solution. The polymerization proceeded with molar ratios of [DMAEMA]:[CuCl]: [Bpy] of [1]:[0.067]:[0.030]. Specifically, the polymerization solution comprised 50 mL of DMAEMA (0.297 mol; purified by passing through a column containing inhibitor remover), 50 mL of DMSO (0.582 mol), 3.1251 g of 2,2′-bipyridyl (0.020 mol), and 0.9339 g of CuCl (0.009 mol). The reaction was allowed to proceed for 3 h. Following polymerization, the sample was removed, rinsed extensively with ethanol and sonicated in ethanol for 20 min before being dried under a stream of N2 gas. B

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Figure 2. Comparison of brush and spuncast films with different thicknesses and substrate hydrophobicity. Samples includes a 90 nm PDMAEMA brush (red filled squares), 180 nm PDMAEMA brush (red stars) and PDMAEMA−MeI brush (red open squares) derived from a 90 nm PDMAEMA brush. Analogous spuncast samples includes a 90 nm PDMAEMA spuncast film (blue filled squares), 180 nm PDMAEMA spuncast film (blue stars) and PDMAEMA−MeI spuncast film (blue open squares) derived from a 90 nm PDMAEMA spuncast film. A 90 nm PDMAEMA spuncast film on an OTS-modified Si substrate (green squares) is also included. (a) Swelling factor calculated from thickness values derived from fitting SE data for brushes and spuncast films of PDMAEMA and PDMAEMA−MeI. (b) Solvent fraction inside the PDMAEMA and PDMAEMA− MeI brushes and spuncast films determined by fitting SE data. (c−e) Values of the Flory−Huggins interaction parameter (χ) calculated from data in part b. software (J. A. Woollam) using a liquid cell with windows fixed at an incidence angle of 70° (relative to the sample normal). Contained within the cell were two inverted vial caps (1 cm in diameter) holding either pure methanol or pure ethanol. The vessel was sealed with a steel lid fitted with an O-ring to prevent vapor escape. Spectroscopic ellipsometry measurements were performed every 5 min using the “dynamic scan” option in the WVASE32 software at an incidence angle of 70° from 400 to 1000 nm. The duration of each measurement was 3 min. Fitting Spectroscopic Ellipsometry Data. For humidity measurements, the data collected by spectroscopic ellipsometry measurements at ≈10% RH were fit to a model comprising a Si substrate, SiOx layer (thickness 1.5 nm), and a Cauchy layer. The Si and SiOx layers used material files supplied with the WVASE32 software. The Cauchy layer was fit using thicknesses, and the Cauchy equation parameters An and Bn. All other data at higher RH levels were fit to a similar layer, except that the refractive index of the polymer brush layer was modeled by employing an effective medium approximation (EMA) between the Cauchy film and H2O, which used a material file supplied with the WVASE32 software. The An and Bn values obtained at 10% RH were used for the Cauchy film and were held constant. A similar model was used for measurements on samples exposed to alcohols, including identical An and Bn values for the Cauchy film, with material files constructed from literature values for methanol23 and ethanol.24 The thickness and volume fraction of H2O, methanol or ethanol of the EMA were fit and used to construct plots below. Infrared Variable Angle Spectroscopic Ellipsometry (IRVASE). Measurements were performed on an IR-VASE (J. A. Woollam) controlled by WVASE-IR software (J. A. Woollam) at an

Chemical Modification of Spuncast PDMAEMA Film. A silicon wafer supporting a PDMAEMA spuncast film was incubated in an enclosed glass vial containing 100 μL of MeI for 48 h. The sample was then removed and exposed to a stream of N2 gas prior to characterization by infrared variable angle spectroscopic ellipsometry. Spectroscopic Ellipsometry Measurements under Controlled Humidity Conditions. Measurements were performed on a variable angle spectroscopic ellipsometer (J. A. Woollam) controlled by WVASE32 software (J. A. Woollam) using a liquid cell with windows fixed at an incidence angle of 70° (relative to the normal). Contained within the cell were two inverted vial caps (1 cm in diameter) holding either pure KOH or a saturated aqueous solution of K2SO4. SE measurements were performed every 5 min using the “dynamic scan” option in the WVASE32 software at an incidence angle of 70° from 400 to 1000 nm. The duration of each measurement was 3 min. A custom poly(methyl methacrylate) lid with a single opening was used to allow access for the RH-temperature probe (Omega Engineering). The probe was connected via USB to a computer and recorded temperature and RH level every 5 min at the start of each measurement. The final RH level inside the cell during a measurement was calculated as RHt=0 + (3/5) × (RHt=1 − RHt=0), using the assumption that the increase in RH was linear over the measurement period. The approximation was found to be accurate within 1% RH. Note that above ≈85% RH, the change in RH during a measurement is 1, indicating a hydrophobic environment. The decreasing trend may be explained by the same hypotheses put forth for Figure 2. As in Figure 3, the high and low density data sets in Figure 4a exhibit similar χ values, while the intermediate data sets exhibit similar χ values that are larger in magnitude than the high density and low density data sets. The data in Figure 4b for the PDMAEMA−PS sample exhibit markedly different behavior, however. The PDMAEMA−PS measurement points at high and low density exhibit similar behavior; similarly, the points at intermediate grafting densities also exhibit similar behavior to each other. We have explained previously the trends of the high and low density data points in Figure 4b as originating from the ability of the betaine functionalities to form inter- and intramolecular zwitterionic complexes, respectively.17 The molecular structure of a zwitterionic complex is shown in the inset of Figure 4b. In contrast, the intermediate grafting densities are not able to form these complexes, leading to the observed χ value trends. The mechanism underlying the ability or inability of betaine H

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Macromolecules in a more hydrophobic environment and increase in χ, as illustrated in the lower right drawings in Figure 4. One implication of the suggested model is that the environment felt by the water molecules in the polymer brush is similar at both the lowest and highest Σ levels measured, due to either intra- or intermolecular interactions among side groups in the brush, respectively. This notion is supported by the similar χ values for σ levels for the low and high density regions in both of the brushes, since similar densities in either functional group may result in similar numbers of intra- or intermolecular interactions, whether that density is achieved by chain collapse at low σ or chain crowding at high σ. Furthermore, if the betaine moieties form a complex and charge compensation occurs, the resulting chemical functional groups (i.e., quaternary ammonium, propyl chain and sulfonate “counterion”) resemble those of a previously analyzed propyl-iodide modified PDMAEMA brush (PDMAEMA−PrI) brush.17 The PDMAEMA−PrI brush also incorporates a propyl chain onto the quaternized ammonium group, as well as a condensed iodide counterion. Given the similar chemical environment produced by complexation in the PDMAEMA−PS brush and the native state of the PDMAEMA−PrI brush, one might expect similar χ values for these two samples. Indeed, comparison of χ values for dense brushes of PDMAEMA−PS and PDMAEMA−PrI reveals comparable χ values at the highest RH levels (cf. Figure 8). The speculated mechanism advanced in the preceding paragraph is limited to a molecular scale. It is also worth considering the data in Figure 4 on the scale of the entire brush. As postulated below in the discussion of Figure 6, the presence of moisture in the PDMAEMA brush promotes the uptake of additional moisture. As reported previously17 for a densely grafted PDMAEMA brush, the hydration of the brush starts at the polymer/air interface, and penetrates deeper into the brush with increasing RH level. At a RH level of ≈10% RH, the polymer air/interface shows hydration, suggesting that the brush retains moisture that will influence the uptake of additional water molecules with increasing RH. Since different Σ values were not evaluated for the NR measurements in ref 17, we can only speculate how changing Σ would affect this hydration at low RH. One possibility is that as Σ decreases, hydration at low RH penetrates deeper into the brush due to a reduction in steric hindrance from a lower areal concentration of chains, and a higher concentration of ambient moisture relative to polymer on the surface. Furthermore, it may be that at higher Σ, it is increasingly difficult to remove completely water molecules, resulting in retained moisture that affects the uptake of additional water molecules. When coupled together, these two possibilities could also lead to the dependence of the hydration behavior of the PDMAEMA brush on Σ in Figure 4. In terms of the impact of σ on technological applications of the polymer brushes studied here, the weak dependence of PDMAEMA swelling on σ suggests these brushes would be robust against small deviations in σ between samples. Our results show that in the case of interacting side chain chemistry (i.e., zwitterionic chemistry), the behavior of the brush depends on σ. In this case, σ becomes a much more important factor and would require higher tolerance in the manufacturing process. Given the strong effect of side chain chemistry on vapor swelling seen in these two samples, we further explore this parameter in the next section, with an interest in tuning the sensitivity of the PDMAEMA brush to other organic solvents.

Role of Amine Substituent Functional Group in Vapor Swelling. The functional groups present in the side chain of a polymer affect strongly the interaction of that polymer with a solvent. The tertiary amine of PDMAEMA presents an attractive site to explore how side chain functionality affects interactions between polymer and solvent vapor. By taking advantage of a PPM reaction scheme,18,19 the side chain chemistry of PDMAEMA can be modified through a quaternization reaction, while preserving the degree of polymerization and σ of the original PDMAEMA brush. Quaternization by an alkyl halide results in the generation of a quaternary ammonium group and introduction of an alkyl group into the PDMAEMA side chain. Furthermore, a variety of PDMAEMA analogues with different alkyl moieties substituted on the amino group are available commercially. In this section, we examine the influence of amine-substituted functional groups on swelling behavior in water vapor using poly(2-diethylamino)ethyl methacrylate (PDEAEMA),43 which bears ethyl moieties compared to the methyl moieties of PDMAEMA, and PDMAEMA brushes modified with methyl iodide (PDMAEMA−MeI) and propyl iodide (PDMAEMA− PrI). Figure 5 plots data for PDMAEMA (orange) and PDEAEMA (dark cyan) brushes exposed to different RH levels. The PDMAEMA data are the same as the 90 nm thick brush in

Figure 5. Influence of amine substituent chemistry on humidity uptake for PDEAEMA (dark cyan) and PDMAEMA (orange; same as Figure 2, PDMAEMA 90 nm data). (a) Swelling factor calculated from thickness data obtained from SE measurements plotted as a function of RH %. (b) Solvent volume fraction obtained from SE measurements plotted as a function of RH %. (c) Flory−Huggins interaction parameter (χ) as a function of RH %. Both species show similar swelling behavior and χ parameters at low RH levels. Above ≈50% RH, PDMAEMA uptakes more H2O and swells more extensively than PDEAEMA. Both samples exhibit a decreasing χ parameter with increasing RH level. I

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Macromolecules Figure 2. The PDEAEMA data were collected using a brush with a thickness of 62 nm synthesized using an identical scheme as the PDMAEMA brush. Parts a−c of Figure 5 plot respectively the SF, φ, and χ values of each brush as a function of RH level. The abscissa scales of Figure 5, parts a and b, are chosen to emphasize the behavior of PDEAEMA, such that the PDMAEMA data at higher RH levels fall outside of the plotted range. Figure 2 plots complete ranges of the SF and φ values for PDMAEMA. Examination of the data in Figure 5, parts a and b, reveals that PDEAEMA exhibits a similar trend of increasing SF and φ with increasing RH level. This behavior is consistent with the mechanism of swelling described above, wherein the brush layer absorbs more water vapor with increasing RH level (i.e., φ increases), and this increased vapor uptake leads to swelling with increasing RH level. Up to ≈60% RH, the PDMAEMA and PDEAEMA samples exhibit similar SF and φ values. Above ≈60% RH, PDMAEMA increases rapidly for both of these parameters, while PDEAEMA exhibits less swelling and solvent uptake. At 95% RH, PDEAEMA achieves an SF value of ≈1.11 and φ value of ≈0.08, compared to PDMAEMA values of ≈1.50 and ≈0.31, respectively. This low level of swelling in PDEAEMA is consistent with previous reports examining its response to humid vapors.43 Figure 5c plots the χ value calculated from φ data for PDMAEMA and PDEAEMA as a function of RH level. PDEAEMA exhibits a similar trend as the PDMAEMA brush and spuncast films in Figure 2, wherein higher values of χ are seen at lower to intermediate RH levels, and above a critical RH level, χ shows a decreasing trend with increasing RH level. One interesting feature apparent in the PDEAEMA φ data in Figure 5b is a plateau region between ≈20−75% RH. Above 75% RH, φ appears to increase with an increasing rate as RH level increases. The inflection point at ≈75% RH corresponds to an increase in the rate of swelling in Figure 5a, and marks the onset of decreasing χ with increasing χ in Figure 5c. This plateau is not apparent in the PDMAEMA data or other analogous polymer species examined,17 but that may be due to the relatively large values and rate of increases of φ that mask this shape of the data. Quaternizing the tertiary amine of PDMAEMA also provides a route to increasing the alkyl content of the side chain, while simultaneously generating a hydrophilic quaternary ammonium group bearing a condensed counteranion. In this regard, quaternization leads to an amphylotic character of the polymer side chain. Figure 6a plots the mole fraction of solvent absorbed into PDMAEMA (orange), PDMAEMA−MeI (purple), PDMAEMA−PrI (green) and PDEAEMA (dark cyan) as a function of RH level. The calculation of mole fraction from φ is detailed in the Supporting Information. The PDMAEMA−PrI data used in the calculation is taken from ref 17, while the PDMAEMA, PDMAEMA−MeI, and PDEAEMA are taken from Figures 2b and 5b. The error bars were determined by propagating the error bars associated with φ data. The lines are fits to the data using polynomial equations and intended to guide the eye. The data in Figure 6a show a similar stratification as seen for φ data, in which mole fraction at a given RH level decreases as PDMAEMA−MeI > PDMAEMA−PrI > PDMAEMA > PDEAEMA. This ordering follows the expected decrease in hydrophilicity of the functional groups present in the side chain of the polymer brush, with the quaternized moieties exhibiting the highest mole fractions, followed by PDMAEMA and finally PDEAEMA. The data for the quaternized samples also appear

Figure 6. (a) Mole fraction of H2O in tertiary amine-based polymer brushes bearing different chemical functional groups in the polymer side chains. Samples include PDMAEMA (orange), PDMAEMA−MeI (purple), PDMAEMA−PrI (green) and PDEAEMA (dark cyan). Solid lines are polynomial fits to guide the eye. (b) Results of water cluster number (NC) analysis following Brown analysis plotted as a function of the number of water molecules per polymer repeat unit as calculated from solvent mole fraction in panel a. PDMAEMA, PDMAEMA−MeI, and PDMAEMA−PrI plot together two separate humidity cycles. Only one cycle was performed for PDEAEMA.

to possess a different shape than the unmodified samples. PDMAEMA−MeI and PDMAEMA−PrI both follow what appears to be a concave path, while PDMAEMA and PDEAEMA (above ≈75% RH) follow a convex path. The concave shape of the PDMAEMA and PDEAEMA data suggests that water is absorbed into the brush at an increasing rate with increasing RH level, while the convex shape of PDMAEMA−MeI and PDMAEMA−PrI indicates a decreasing rate of moisture absorption with increasing RH level. The latter trend is due to already high water content within the brush. To gain insight into the mechanism underlying these opposing trends, we calculated the water cluster number (NC) using Brown’s analysis,44−46 and plotted it as a function of the number of water molecules per polymer repeat unit in Figure 6b. A comparison of results from Brown’s method and the Zimm-Lundberg method of calculating NC is shown in Figure S5 in the Supporting Information, and demonstrates nearly identical values for the two calculation methods. For convenience and clarity, we opt to use just the results from Brown’s analysis. A description of Brown’s analysis is provided in the Supporting Information, along with plots used in the calculations in Figure S6. The abscissa is calculated as x/(1 − x), where x is the solvent mole fraction. NC provides a measure of whether water molecules in the polymeric film tend to associate with the polymer repeat units or other water molecules. An NC value of unity implies that water molecules are isolated from other water molecules and interact only with the polymer repeat units. Higher values of NC imply that water molecules are interacting with each other to form a cluster within the polymer film, and the value of NC provides an average number of water molecules in each cluster. The RH range covered by the data in Figure 6b is from 60% RH and higher. Below 60% RH, all samples exhibit values of NC close to unity. J

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Macromolecules As the number of water molecules in the brush increases, all of the samples exhibit an increasing trend in NC with increasing mole fraction, suggesting that an increased number of water molecules in the brush leads to increased interactions between water molecules in the brush. Furthermore, PDMAEMA, PDMAEMA−MeI, and PDMAEMA−PrI exhibit a concave trend, appearing to increase toward a plateau value of NC. PDEAEMA does not exhibit such a shape in the range examined, but may not reach sufficiently high numbers of water molecules to demonstrate this shape. Despite these similarities, the chemical functional groups in the polymer side chain affect strongly the increase in NC. The most hydrophilic polymer, PDMAEMA−MeI, appears to plateau toward a value of NC of ≈2.5, even with an average of ≈10 water molecules per polymer repeat unit. In contrast, PDEAEMA reaches an NC value of ≈9 despite supporting an average of only one water molecule per repeat unit. PDMAEMA−PrI reaches an NC value of ≈3 with ≈6 water molecules per unit. PDMAEMA achieves an NC value of ≈5.5 with ≈4.5 water molecules per repeat unit. These data can be considered in another context by calculating the average number of water clusters per polymer repeat unit (i.e., the ratio of the value on the ordinate to that on the abscissa in Figure 6b). The results of this calculation are plotted in Figure S7 in the Supporting Information, and suggest that water prefers to form many smaller clusters in the PDMAEMA−MeI brush, compared to fewer, larger clusters in the PDEAEMA brush, with PDMAEMA and PDMAEMA−PrI falling in between. The value of NC does not provide any indication of the distribution of water molecules or clusters within the polymer film. Prior studies have reported on a nonhomogeneous distribution of vapor molecules in both polymer brush and spuncast films.14,17,33 It is thus important to emphasize that NC in this situation is best used as a proxy for whether water vapor prefers to associate with itself or with the polymer functional groups. In this regard, the trends at the extremes are clear when the data in Figure 6, parts a and b, are considered together. While water molecules may be willing and able to partition into the PDEAEMA brush to a limited extent, these water molecules tend to only associate with each other and avoid the ethyl moieties of the polymer brush. In contrast, water vapor appears to partition readily into the PDMAEMA−MeI brush, and the absorbed water molecules interact with the quaternary ammonium moieties. PDMAEMA−PrI offers a similarly favorable environment for water vapor, but the propyl moieties lead to increased preference for associations between water molecules. Finally, PDMAEMA provides an environment with the lowest alkyl content, but lacks the quaternary ammonium group of the quaternized species. As a result, it falls between PDEAEMA and PDMAEMA−PrI. To put it another way, for PDMAEMA−PrI and PDMAEMA−MeI, water enters the brush because it prefers the brush environment relative to the air. For PDMAEMA and PDEAEMA, an increasing amount of water enters the brush because there is an increased amount of water in the brush. Influence of Solvent Functionality on Polymer Brush Swelling. The previous section examined the influence that functional groups in the polymer side chain can have on the uptake of water vapor, and demonstrated that increasing hydrophobicity of the polymer inhibits uptake of water vapor. This final portion of this work considers how increasing the hydrophobicity of the solvent affects the swelling of the same series of polymer brushes by using methanol and ethanol as solvents.

Figure 7. Swelling factor (a and b) and solvent volume fraction (c and d) for brushes incubated in methanol vapors (a and c) and ethanol vapors (b and d) derived from SE data collected at various incubation times for PDMAEMA (orange), PDMAEMA−MeI (purple), PDMAEMA−PrI (green), and PDEAEMA (dark cyan).

Figure 7 displays swelling factor and φ data derived from spectroscopic ellipsometric measurements for PDMAEMA (orange), PDMAEMA−MeI (purple), PDMAEMA−PrI (green) and PDEAEMA (dark cyan) brushes exposed to methanol vapors (panels a and c) and ethanol vapors (panels b and d) plotted as a function of exposure time. The data are presented as a time series, where t = 0 represents the moment when the ellipsometry sample chamber has been closed and simultaneously the first measurement has started. As incubation time increases, the partial pressure of solvent vapor increases due to evaporation of the solvent from a liquid reservoir located adjacent to the sample. While the use of a digital RH probe during water vapor based measurements allowed correlation of RH level and ellipsometric data, no such probe for methanol and ethanol vapor was immediately available. Nonetheless, we reason that the increase in swelling factor and φ with increasing time is due to the increase in solvent vapor partial pressure, since the polymer film will maintain equilibrium with its environment. After a certain period of time, the environment in the sample chamber will become saturated with solvent vapor and the partial pressure will not change. At the point of saturation, in the absence of condensation of vapor within the brush, the brush will exhibit no change in φ (and therefore, in SF), resulting in the plateau shape of the data. The exception to this behavior is the PDMAEMA incubated in methanol vapor, which increases in SF and φ seemingly without bound. Upon opening the sample chamber at 60 min, visual inspection revealed liquid condensed on the brush sample. This liquid evaporated quickly, consistent with high volatility of methanol. This liquid condensate was not observed on other samples, which instead showed a smooth and rapid progression of colors during the deswelling process. We conclude, then that the anomalous behavior of the PDMAEMA/methanol data is likely due to condensation of methanol vapor in the polymer brush. It is interesting to note that data up to 30 min for PDMAEMA/methanol are K

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Macromolecules consistent with the plateauing seen in the other samples, suggesting the onset of condensation after this point. Therefore, the brush appears to swell by a factor of ≈2.5 due to vapor, the highest SF seen in these experiments. Examination of the data in Figure 7 leads to the same conclusion reached for water vapor experiments; namely that the increase in SF originates from an increase in φ due to an increase in vapor partial pressure. Since these experiments occurred at the same ambient temperature, the saturation vapor pressure inside the sample chamber should be the same for each sample. Therefore, the different SF and φ values observed for each sample are due to variations in the interactions between the solvent vapor and polymer brush functional groups. Starting with the data for methanol and setting aside PDMAEMA for the reasons detailed above, the data in Figure 7a indicate no appreciable difference in SF for PDMAEMA− MeI and PDMAEMA−PrI, and only a slight reduction in SF for PDEAEMA, with all obtaining SF values of ≈1.6. However, Figure 7c shows that φ values decrease as PDMAEMA−PrI > PDMAEMA−MeI > PDEAEMA, though these values all fall in the range of 0.34 ± 0.04. Ethanol vapor produces greater variation in the SF and φ values. Figure 7b indicates that SF values decrease as PDMAEMA (≈2.0) > PDEAEMA (≈1.6) > PDMAEMA−PrI (≈1.24) > PDMAEMA−MeI (≈1.08). The φ data in Figure 7d follows the same ordering, with PDMAEMA (≈0.55), PDEAMA (≈0.32), PDMAEMA−PrI (≈0.18) and PDMAEMA−MeI (≈0.03). It is important to note that there was humidity in the sample chamber during these measurements; no attempt was made to reduce it from the ambient condition of ≈35% RH. It is likely that a cosolvent will affect the uptake of vapor into the brush, especially one that is miscible with both the polymer and the alcohol solvents. However, 35% RH is relatively low, with PDMAEMA showing little change in SF and φ at this level. The quaternized brushes will be more sensitive to these RH levels. Given the exceedingly small uptake of ethanol in PDMAEMA− MeI, we conclude that our results are not distorted strongly by this level of RH. Thus, we reach the conclusion that modifying the side chain chemistry of the polymer brushes alters the sensitivity of the brush to different solvents. This tunability opens the possibility to produce sensors for a variety of vapors from the same polymer species. This approach thus represents an efficient strategy for manufacturing, since the thickness and σ of the underlying polymer brush only needs to be controlled for one species. By assuming that the relative partial pressure of solvent vapor in the chamber is equal to unity at the last data point collected for each sample (30 min for PDMAEMA and 50 min for PDMAEMA−MeI in methanol), we can calculate the value of χ for each sample in methanol (orange) and ethanol (green). These values are plotted in Figure 8, along with the χ values for each sample at 95% RH (blue). The spuncast PDMAEMA 90 nm film from Figure 2 is included also, with data for water and ethanol swelling. For water at 95% RH, PDMAEMA−MeI exhibits the lowest χ parameter at ≈0.70, followed by PDMAEMA at ≈0.9, PDMAEMA−PrI at ≈1.15 and PDEAEMA at ≈1.8. In contrast, ethanol follows nearly the complete opposite order, with PDMAEMA at ≈0.75, PDEAEMA at ≈1.0, PDMAEMA−PrI at ≈1.4, and PDMAEMA−MeI at ≈2.6. Setting aside PDMAEMA, methanol falls between water and ethanol, with PDMAEMA−MeI, PDMAEMA−PrI, and PDEAEMA all exhibiting values of ≈1.0. Furthermore, methanol and PDMAEMA were the only

Figure 8. Flory−Huggins χ parameters for samples in methanol vapors (orange circles) and ethanol vapors (green triangles) calculated from data in panels b and d at saturation conditions (60 min, except for PDMAEMA in methanol, which was calculated at 30 min).

combination to exhibit any evidence of condensation, suggesting that χ ≈ 0.65 as an upper limit of magnitude. Finally, the spuncast PDMAEMA film exhibits a χ parameter for water of ≈1.3 and ≈0.7 for ethanol. We note that the somewhat higher value of χ for water and lower value for ethanol are consistent with the conclusion reached from Figure 2 that the spuncast samples present a more hydrophobic environment compared to the brush samples. Overall, the results reveal a strong dependence of χ on the type of functional group present in the polymer side chain and solvent vapor. For water, modification of the side chain chemistry to introduce a quaternary ammonium group reduces significantly the χ parameter value, while increasing alkyl chain length increases the χ parameter. For ethanol, the opposite trend occurs, with the highest χ parameters observed for the quaternized species, and lower χ parameters observed for lower alkyl chain lengths. PDMAEMA and methanol offer a compromise on the extremes in hydrophilicity and hydrophobicity presented by the other samples and solvents. Both possess hydrophobic methyl groups (and the polymer backbone for PDMAEMA), which are not as hydrophobic as the ethyl moieties in PDEAEMA and ethanol and propyl moieties in PDMAEMA−PrI. Both possess a hydrophilic moiety, although not as hydrophilic as quaternary ammonium or water. It appears to be this moderation of the extremes within PDMAEMA and methanol that enables these species to be compatible with the other samples, and particularly so with each other. While our data do not provide specific insight into the mechanism, by which these associations occur, we speculate that spatial orientation of the hydrophobic and hydrophilic regions of the solvent molecules around these same regions of the polymer side chain are important. Thus, methanol may have an easier time to position its hydroxyl functional group near the amine or quaternary ammonium, and align its methyl group with the other pendant alkyl groups. L

DOI: 10.1021/acs.macromol.6b00111 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Present Address

CONCLUSION This article aims to examine some of the key parameters affecting the swelling of polymer brushes in solvent enriched vapor environments. These parameters under investigation include end-grafting, molecular weight (Mn), grafting density (σ), side chain functionality, and the functionality of the solvent vapor. While the experimental results presented above have attempted to isolate the individual role of each parameters, our findings indicate that these parameters are connected mutually and influence each other. End-grafting polymer chains to the substrate leads to greater swelling of the polymer film compared to films that are not chemically grafted. In an application such as a vapor sensor using a polymer brush as the sensing layer, the greater swelling would increase the sensitivity of the sensor. The extent to which σ affects the vapor uptake of the polymer brush depends on the functional groups in the side chain of the polymer. While we find a small effect of σ on the swelling PDMAEMA brush, it does not change substantially the swelling associated with a given RH level. Thus, a technology using a PDMAEMA brush may exhibit robustness in terms of required tolerance in σ during manufacturing. In contrast, the zwitterionic brush may swell the most, suggesting the highest sensitivity in a sensing application, yet shows a strong dependence on σ and would require high tolerance during manufacturing. Finally, using a postpolymerization modification (PPM) strategy, we show that from a single polymer species we can adjust the sensitivity to additional vapor molecules by incorporating increasingly hydrophobic side chain chemistry. In this regard, our findings guide the design of coating parameters that must be taken into account when creating polymer brush coatings for technological applications. While we have attempted to be thorough in our study, this work also points toward unresolved questions and additional experiments for related systems. One aspect of the quaternized PDMAEMA brushes left unexplored in this work is the influence of counterion species, including protonation of the amine, which is known to affect strongly the swelling43 and hydrophobicity47 of polyelectrolyte brushes. Finally, the homopolymer brushes studied here are just one branch of the surface-grafted macromolecule family. Cross-linked,48,49 diblock,50,51 and biomolecular37,52 systems, among many others, may exhibit unique characteristics that may produce surprising phenomena in certain vapor environments.53

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†

Okinawa Institute of Science Technology Graduate University, Onna-son, Okinawa 904−0497 Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Science Foundation, Grant No. DMR-0906572. Partial support from Grant No. DMR-1404639 is also appreciated.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00111. IR-VASE data, swelling factor versus solvent volume fraction, calculating reduced grafting density, water cluster analysis, mole fraction calculation, water cluster number (NC) calculation, plots used in calculation of derivative in calculation of NC by Brown’s analysis, and average number of water clusters per polymer repeat unit (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(J.G.) E-mail: [email protected]. M

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