Humidity Driven Swelling of the Surface-Attached Poly(N

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Humidity Driven Swelling of the Surface-Attached Poly(N‑alkylacrylamide) Hydrogels C. K. Pandiyarajan, Oswald Prucker, and Jürgen Rühe* Laboratory for Chemistry and Physics of Interfaces, Department of Microsystems Engineering (IMTEK), University of Freiburg, 79110 Freiburg, Germany S Supporting Information *

ABSTRACT: We describe a systematic investigation on the humidity driven swelling behavior of a homologous series of surface-attached N-alkylacrylamide hydrogels. To this, first a series of copolymers is synthesized, each comprising an N-alkylacrylamide with an appropriate chain length of the alkyl substituent and a UV-active cross-linker, (methacryloyloxy)benzophenone (MABP). Thin films of the copolymers are deposited onto silicon substrates precoated with a self-assembled monolayer of benzophenone (BP) silane or directly onto polymer films deposited on solid substrates. Upon irradiation with UV light (λ = 254 nm), the BP moieties present in the copolymer (and when applicable at the surface of the substrate) are activated, which leads to the simultaneous cross-linking of the polymer chains and surface attachment of the forming gel. The cross-link density of the gels is tuned by varying the mole fraction of MABP in the copolymer ranging from 1% to 10.0% MABP. The swelling of the gels is studied using optical waveguide spectroscopy (OWS) while the (relative) humidity of air in contact with the samples is carefully controlled. To obtain information on the energy of hydration, microcalorimetric experiments are carried out. Swelling in humidity or water and the energy of hydration are compared, and the implications of these parameters on nonspecific protein adsorption and cell adhesion are discussed. platform, i.e., from stiff metals to soft elastomers.14,16,20 Using this approach, reactive groups such as benzophenone or sulfonyl azide are activated by heat or light, forming reactive intermediates which react with neighboring chains in the glassy state under (formal) C,H-insertion (C,H-insertion cross-linking, CHic mechanism). The obtained films are very stable under swelling and even rather strong shear-stress conditions due to the formation of covalent bonds to the surface and within the layer.21 Swelling of the hydrogels films in aqueous systems has been extensively studied under different conditions as a function of pH,22,23 ionic strength, and salt concentration, employing surface analytical tools such as spectroscopic ellipsometry (SE),23 surface plasmon resonance (SPR) spectroscopy,24 optical waveguide spectroscopy (OWS),25 and neutron or Xray reflectometry.26 Particularly when it comes to the vapor (water) phase swelling, many efforts were made on the polyelectrolyte (PE) system,27,28 where the presence of charges facilitates the selective extraction of water molecules from contacting moist air.29 Early research attempts were mainly focused on a polyelectrolyte system composed of poly(Nmethyl-4-vinylpyridine) iodide brush (positively charged)30 and poly(styrenesulfonate)31 (negatively charged).27 It has been demonstrated that the extent of swelling depends on the relative

1. INTRODUCTION Over the years, surface coatings consisting of polymeric thin films have gained increasing attention due to their wide range of applications in the field of bioelectronics,1 tissue engineering,2 blood contacting devices,3 and microfluidics,4 to mention a few. They either provide directly the desired functionalities or are used as layers which enhance the wettability5 of the surfaces (e.g., metals, glass, or silicon) or improve the adhesion of additional coatings such as paints or antifouling materials.6 Especially, hydrophilic polymeric surfaces7 (i.e. in most cases hydrogels) are interesting candidates due to their tendency to exhibit strong swelling when exposed to an aqueous medium, which is considered to be one of the primary reasons that these materials resist a nonspecific adsorption of proteins.8,9 Recently, films generated from hydrogels10 such as poly(2-hydroxyethyl methacrylate),11 poly(ethylene glycol), or poly(ethylene oxide),12 poly(acrylamide), and polyzwitterions e.g., poly(3sulfopropyl-N,N-dimethylammomium ethyl methacrylate),13 have been demonstrated to resist nonspecific adsorption of protein and consequently minimize the adhesion of bacterial or mammalian cells.10,12 Thin layers of these polymers can be obtained by a variety of techniques, including photo14,15 or thermal16 cross-linking of the polymer films, surface-initiated polymerization (leading to the formation of polymer brushes),17 plasma polymerization,11 and polymer analogous reactions.18,19 Coatings produced through either photo or thermal crosslinking mechanisms are an interesting new approach to thin film deposition which is highly versatile and can be applied to any © XXXX American Chemical Society

Received: June 28, 2016 Revised: September 27, 2016

A

DOI: 10.1021/acs.macromol.6b01379 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Synthesis of UV-Active Cross-Linker (Methacryloyloxy)benzophenone (MABP) and Poly(x% N-alkylacrylamide-co-y % MABP)a

a

Z represents the number of carbon atoms of the alkyl chains of the N-substitution. In the case of poly(DMAAm-co-y% MABP), the mole fraction of MABP is varied between 1%, 2.5%, 5%, and 10%.

Figure 1. Schematic depiction of the OWS setup employed in the humidity swelling measurements of the surface-attached networks.

applications either directly or indirectly come in contact with water vapor-enriched environments. Here, we report a systematic investigation on the humidity driven swelling behavior of neutral, surface-attached poly(Nalkylacrylamide) networks comprising chemically similar structures. A homologous series of copolymers incorporating N-alkylacrylamide (methyl, ethyl, dimethyl, propyl, and butyl) and a UV-active cross-linker (methacryloloxy)benzopheonone (MABP) have been synthesized through free-radical polymerization. The systems are photochemically cross-linked through C,H-insertion based cross-linking (CHic),35 and the cross-link density of the coatings is tuned by varying the molar fraction of MABP in the copolymer, i.e., poly(N-alkylacrylamide-co-y% MABP), where y ≈ 1.0%, 2.5%, 5%, and 10% MABP (cf. Scheme 1). Such a system enables us to study the effect of cross-link density and the influence of molecular structure on the swelling capacity of the gels under humid conditions in a systematic fashion. Thin films of these copolymers were deposited onto gold-coated glass substrates. Swelling studies were performed by employing optical waveguide spectroscopy (OWS) as depicted

humidity of the environments and mostly swell at high humidity (i.e., RH ≥ 60−70%) to the factor of 1.3−1.4 (i.e., at high humidity the films had water content of ≈30−40%).32 As most thin film measurements are carried out under ambient conditions, this water content cannot be neglected for quantitative evaluations whenever the thickness of a “dry” film in air is discussed.33 Otherwise, this will lead to a significant measurement error. The latest report includes the humidity driven swelling of the quaternized poly(N,N-dimethylethyl methacrylate) (PDMAEMA) brushes, where the combination of spectroscopic ellipsometry and neutron reflectivity techniques was employed to probe the water uptake of the electrolyte layers.26,34 One of the key findings of the work is that the side-chain chemistry (alkyl chain length of the quaternized DMAEMA) had a significant role in controlling the humidity swelling. However, the swelling behavior of the neutral hydrogels in humid environments is still an underexplored concept. It is central to perform such investigation because most of the biological B


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Samples were then thoroughly extracted using the corresponding solvents followed again by annealing (∼60−70 °C) and dried in a stream of dry nitrogen. 2.6. Humidity Swelling of the Networks. For optical waveguide spectroscopy (OWS) the sample was mounted to a flow cell as depicted in Figure 1. A thermostat was used to keep the cell at a constant temperature of ∼25.0 ± 0.1 °C. The relative humidity inside of the flow cell was controlled by passing through a humidified air stream from the wash bottle, which was loaded with a saturated salt solution. The salt present in the solution reduces the vapor the pressure of the water and thereby the relative humidity of the contacting air (cf. Table 1). A

in Figure 1. The extent of swelling was measured as a function of relative humidity by determining the change in film thickness when exposed to moisture (swollen thickness, dRH) and nitrogen (dry film thickness, ddry). The results are discussed based on the selective absorption of water molecules from the moist air.30

2. MATERIALS AND METHODS 2.1. Materials. All solvents and reagents were purchased from Sigma-Aldrich, USA, and used as received (p.a. grade or higher). Monomers such as N-methylacrylamide (MAAm), N-ethyl acrylamide (EAAm), N-propyl acrylamide (PAAm), N-butylacryl amide (BAAm), and diethylacrylamide (DEAAm) were obtained from ABCR GmbH & Co KG, Germany, and used as received (p.a. grade or higher). Goldcoated LaSFN9 substrates for optical waveguide spectroscopy (OWS) measurements were obtained from Res-Tec GmbH, Germany. 2.2. Instrumentation. The molecular weight of the copolymer was determined using GPC, Agilent 1100 (polymer standard service), Santa Clara, CA. Polystyrene and poly(methyl methacrylate) (PMMA) were employed as a standard with the concentration of 1−3 mg/mL in DMF or THF column at the flow of 1 mL/min. 1H NMR spectra were acquired using an Avance 250 MHz spectrometer from Bruker. The samples were dip coated using a Z 2.5 tension testing machine from Zwick GmbH. OWS measurements were performed in the Kretchmann configuration with an instrument constructed from RES-TEC adopted with a He−Ne laser as a light source with a wavelength of 633 nm (λ = 632.8 nm). The optical components of the films were obtained through the software winspall 3.02 developed by the Max Planck Institute for Polymer Research, Mainz, Germany. 2.3. Synthesis of UV-Active Cross-Linker (Methacryloyloxy)benzophenone (MABP). MABP was synthesized according to the procedure described by Prucker14 and Pandiyarajan et al.21 Briefly, ∼5.25 g of methacryloyl chloride in 20 mL of dichloromethane was added to the solution of 5.06 g of 4-hydroxybenzophenone in 100 mL of dichloromethane at 0 °C. The mixture was allowed to reach room temperature and stirred for ∼16 h. After completion of the reaction monitored via thin layer chromatography (TLC), the reaction mixture was filtered and the organic phase was washed with the solution of 5% HCl, 5% NaHCO3, and water. The solution was dried over anhydrous MgSO4, added to ∼200 mL of cold n-hexane, and allowed to complete precipitation at −20 °C overnight. The precipitated monomer was filtered and subsequently recrystallized using a ethyl acetate−hexane mixture. 1H NMR (300 MHz, CDCl3): δ = 2.1 ppm (s, 3H, *CH3), δ = 5.7−6.3 ppm (2s, 2H, *CH2=), δ = 7.1−7.9 ppm (various m, 9H, *C− Haro). FT-IR: 3049 (=C−H), 2980−2927 (−C−H), 1732 (−O−C O), 1653 (−CO), 1596 (−CC−), and 1446 cm−1 (−C−H). 2.4. Synthesis of Poly(N-alkylacrylamide-co-MABP). A dry Schlenk tube (∼100 mL) was charged with the calculated amount of the corresponding acrylamide), MABP, and AIBN dissolved in ∼20−25 mL of methanol for MAAm, DMAAm, EAAm, and PAAm. In the case of BAAm and DEAAm, THF was used as a solvent. The Schlenk tube was closed carefully and degassed under nitrogen through three freeze and thaw cycles: polymerization was carried out at 60 °C overnight in a thermostatic water bath (cf. Scheme 1). After the polymerization reaction was terminated, the polymer was precipitated in diethyl ether or n-hexanes and repeated three times. The molecular weight (Mw) and spectroscopic details of the copolymers are presented in the Supporting Information (cf. S1). 2.5. Preparation of Surface-Attached Networks on Au Substrates. A freshly cleaned LaSNF9 glass substrate covered with a ∼50 nm gold layer was deposited with a thin layer (5−10 nm) of medical grade polyurethane (BASF, Ludwigshafen) (∼5−10 mg/mL in THF) or polystyrene. The solution of the copolymers with a concentration of ∼30−50 mg/mL (in ethanol for PMAAm, PEAAm, PPAAm, and PDMAAm and in THF for PBAAm and PDEAAm) was spun cast onto the substrate and annealed at slightly elevated temperature (∼60−70 °C) for ∼5−10 min to evaporate the solvent completely from the coatings. The layers were then irradiated with UVA light (λ = 254 nm) with a total UV dose of 4 J/cm2 for 4 min.

Table 1. Relative Humidity of Air in Contact with Saturated Salt Solutions at 25 °C sat. salt solution

humidity (%)

sat. salt solution

humidity (%)

LiCl K2CO3 NaBr

25 46 60

NaCl KCl KNO3

77 84 92

peristaltic pump (Ismatec, type 1SM596D, Switzerland) was employed to supply the moist air from a wash bottle to the flow cell, while the relative humidity of the air was continuously probed through a humidity sensor (Testo-635, Germany). For swelling measurements, angular scans were recorded in the presences of nitrogen and humid air as stated above. Layer thickness and refractive index of the coatings were obtained by fitting the angular scans according to the Fresnel formalism.36 The swelling factor (α) can be determined from the ratio of the swollen thickness (dRH) and dry thickness (ddry), i.e., α = dRH/ ddry. It is worth mentioning here that the anchoring layer of PU is relatively hydrophobic and insoluble in water. Hence, the polymer does not swell either in the liquid water (αliq = 1.0) or water vapor (αvap = 1.0, data not shown). In addition, the thickness of PU layer was kept very low (i.e., 5−10 nm) compared to the poly(alkylacrylamide) layers (300−600 nm). Therefore, the effect of PU layer on the humidity swelling of these poly(alkylacrylamide) networks can be considered as negligible. 2.7. Calorimetry Measurements of the Precursor Polymers. The heat released during the process of dissolution of precursor polymers that are used in the study is investigated using a solution calorimeter (Thermometric precision solution calorimeter, LKB 8700, Sweden). In a typical run, 10−20 mg of polymer powder is placed in a sealed glass ampule (2222-150 for solid samples), and a dissolution process is initiated by breaking the ampule in the beaker containing 100 mL of deionized water and the heat released is recorded. The system was calibrated electrically (i.e., by adding a known amount of energy (≈10 J) before and after the reaction. The heat exchange with the surroundings and the heat carried into the system by stirring were subtracted mathematically using respective obtained baseline temperatures before and after the reaction, and finally the heat of dissolution was calculated.

3. RESULTS AND DISCUSSION 3.1. Generation of Surface-Attached Hydrogel Networks. Surface-attached poly(N-alkylacrylamide-co-y% MABP) networks with varying chain length of the alkyl substituent and varying cross-link density are synthesized as described previously and the spectroscopic details of the copolymers are summarized in the Supporting Information (cf. S1). Briefly, a photoactive precursor copolymer containing benzophenone units was deposited on top of a gold substrate that carried a thin layer of polyurethane (5−10 nm). After evaporation of the solvent (by annealing at ∼60−70 °C for 5−10 min), the sample was irradiated with UV light at 254 nm for 4 min, i.e., applying a total UV dose = 2−4 J/cm2 (cf. Figure 2). The photoactive benzophenone present in the copolymer absorbs UV light and instantaneously forms triplet biradicals at the carbonyl group C


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Figure 2. (a) Schematic diagram of the simultaneous surface-attachment and cross-linking process. (b) Cross-linking mechanism of the UV active benzophenone.

(−C·−O·). These radicals then abstract a hydrogen atom from any CH2 group (backbone or side chain) in the vicinity which leads to the creation of two carbon-based radicals (−C··C−). Recombination of these two radicals yields a covalent (−C− C−) bond connecting two polymer chains. If this process occurs at several groups along the polymer chain, this leads ultimately to the cross-linking of the whole polymer film as well as an attachment of the forming polymer network to the polyurethane surface and thus produces surface-bound networks. Additional information regarding the MABP cross-linking reactions can be found in previous publications.16,21,35 3.2. Swelling Measurements. Optical waveguide spectroscopy (OWS) has been employed to investigate the swelling characteristics of the surface-attached films. OWS is an efficient surface analytical tool that measures thickness and refractive of the coatings independently from each other.36 Generally, a higher order waveguide mode appears at the lower angle of incidence which is connected to the layer thickness. In contrast to this, the waveguide mode at a higher angle of incidence is more sensitive to the change in refractive of the films. Therefore, the layer thickness of the copolymers was adjusted to ∼350− 650 nm in order to achieve at least two or more waveguides in the dry state. The temperature of all measurements was maintained at 25 °C, and the relative humidity was recorded using a humidity sensor close to the location of the measurement. One of the key questions for a quantitative analysis of the swelling process (liquid or vapor) is whether under the chosen conditions the system has attained the equilibrium degree of swelling within the experimental time frame. To probe this, kinetic measurements were performed by selecting an angle (θkin ≈ 40°−50°) roughly 5° below the reflectivity minimum when the sample is kept in the dry state, i.e., under dry nitrogen. At this angle, the reflectivity was measured as a function of time (cf. Figure 3). After recording the baseline for a few minutes (>5 min), the dry nitrogen was replaced by moist air, and the

Figure 3. OWS kinetic measurement of a surface-attached poly(DMAAm-co-1% MABP) film.

measurement was continued for several hours. It was observed that any changes in the reflectivity leveled off after a few (5−10) minutes so that no change in the reflectivity was observed. Thus it can be safely assumed that an equilibrium state was obtained. Regardless of this, the kinetic measurement was continued for at least an hour or two until the exact relative humidity of the saturated salt solution is recorded in the humidity sensor (for example, RH of the saturated NaCl solution ∼77 ± 2%). In this way, we make sure that there are no changes in the reflectivity as well as the relative humidity inside the cell (cf. Figure 1). Upon completion of the kinetic measurements, OWS scan were recorded to determine the swollen thickness and refractive index of the layers. The swelling ratio of the surface-attached networks at different relative humidity (αvap) was determined from the ratio of swollen (dswell) and dry (ddry) layer thicknesses as depicted in eq 1. α vap ≈ D

swollen thickness (dswell) dry thickness (ddry )

(1) DOI: 10.1021/acs.macromol.6b01379 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Reflectivity scans of surface-attached poly(DMAAm-co-2.5% MABP) layers with a thickness of (a) 49 nm and (b) 716 nm at different relative humidity. (c) Swelling ratio of thin (49 nm) and thick (716 nm) films are plotted against the relative humidity. Symbols and solid lines represent the measured reflectivity and the model calculations based on Fresnel formalism, respectively, and dini denotes the initial (dry) film thickness measured at N2 atmosphere.

Figure 5. OWS reflectivity scans obtained from a surface-attached poly(DMAAm-co-y% MABP) network at different relative humidity and y = (a) 1.0%, (b) 2.5%, (c) 5.0%, and (d) 10.0%. Symbols represent the measured reflectivity values, and solid lines are the results of model calculations based on Fresnel formalism. E


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Figure 6. (a) Swelling ratio (αvap) of the surface-attached poly(N-alkylacrylamide-co-3% MABP) networks as a function of the relative humidity, where the alkyl substituent is (Z) = methyl (1), ethyl (2a), dimethyl (2b), propyl (3), butyl (4a), and diethyl (4b). (b) Swelling ratio (αvap) of the surfaceattached poly(DMAAm-co-y% MABP) networks as a function of relative humidity, where y = 1.0%, 2.5%, 5.0%, and 10%. (c) Solvent fraction (φvap s ) is plotted against the swelling ratio at the highest relative humidity (RH) ∼ 92%, and dini denotes the initial (dry) film thickness measured at N2 atmosphere.

incidence (Δθ ≈ 5°−12°) at high RH. Higher relative humidities than 92% were not employed. The reason for this is that very close to the dew point of the system water condensation due to small temperature fluctuations occurred, and the thickness values obtained close to 100% RH showed a strong scattering. The change in the thickness and the refractive index of the surface-attached PDMAAm layers were determined by modeling the data using the Fresnel formalism (cf. Supporting Information S3). Figure 6a shows the swelling ratio of various surface-attached poly(DMAAm-co-y% MABP) gels as a function of relative humidity. In general, the film thickness increased as the relative humidity of the system was raised due to swelling. At the same time, a decrease of the refractive index has been observed due to the water uptake because the refractive index of water (nwater) = 1.33 is smaller than that of the PDMAAm networks (nPDMAAm = 1.50). Interestingly, the water sorption behavior of the PDMAAm gels was significantly affected by the amount of MABP in the copolymers. For example, at a given RH (≥90%), poly(DMAAm-co-1% MABP) and poly(DMAAm-co-10% MABP) exhibit swelling ratios of 1.36 and 1.18, respectively. A higher cross-linker content in the copolymer renders the polymer less polar because of the more hydrophobic MABP units. Additionally, with higher MABP contents the polymer becomes more strongly cross-linked and exhibits a lower degree of swelling. In addition, the solvent fraction, i.e., amount of solvent incorporated into the swollen network, was calculated from the thickness measurement as shown in eq 3, where the vap φvap are the polymer and solvent fraction of the gel, p and φs respectively.

3.2.1. Influence of Film Thickness on Humidity Swelling. To examine the influence of the layer thickness on vapor swelling, surface-attached poly(DMAAm-co-2.5% MABP) was studied as a model system. The thickness of the coatings was adjusted to 49 nm (thinner film) and 716 nm (thicker film) via adjusting the spin-coating conditions. The measured reflectivity scans of the films at various relative humidity and the corresponding Fresnel calculations are presented in Figures 4a and 4b. Both thinner (49 nm) and thicker film graphs (716 nm) showed a significant change in the angle θ as the relative humidity of the surrounding is raised from 5% to 92%. Upon fitting these data, the layer thickness was found to increase in both cases with raising the RH of the surroundings; i.e., the thinner film thickness increased from 49 to 65 nm, and thicker film changed from 716 to 974 nm. However, when the film thicknesses are normalized (cf. Figure 4c), the measured curves strongly instigate that the gels respond very similarly to the moisture exposure independent of the film thickness. The observed differences in the swelling of these two films were αvap ≈ 0.03−0.04, which is small compared to the overall swelling of the film, and it is within the experimental error of the SPR and OWS technique (cf. Supporting Information S2). Such slight variations of the swelling of different layers may be due to small differences in the film processing and subsequent small changes in the cross-link density. Therefore, any influence of layer thickness on the swelling behavior can be safely neglected in the following discussions. 3.2.2. Influence of Cross-Linker Contents on Humidity Swelling. To evaluate the effect of cross-link density on humidity swelling, hydrophilic poly(DMAAm-co-y% MABP) was selected as a model system, where the molar content of the photo-cross-linker MABP was gradually varied between 1%, 2.5%, 5%, and 10%. The OWS reflectivity scans of the PDMAAm networks are presented in Figure 5. A significant change in the waveguide spectra occurs as the relative humidity of the moisture is varied from 0% to 92%, i.e., the resonance angle of the waveguides typically shifted to higher angles of

α vap =

1 1 vap = φp 1 − φsvap

(2)

Upon rearranging φsvap = F

1 1 − α vap

(3) DOI: 10.1021/acs.macromol.6b01379 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. OWS reflectivity scans of poly(DMAAm-co-y% MABP) in nitrogen air and water. Symbols and solid lines represent the measured reflectivity and the model calculations based on Fresnel formalism, respectively.

Figure 6c plots the solvent fraction (φvap s ) as a function of the swelling ratio (αvap) obtained at a RH of 92%. The amount of solvent incorporated into the network increased (linearly) as the swelling ratio increased. As mentioned earlier, the degree of cross-linking plays a major role in controlling the extent of swelling; i.e., the higher the cross link density between the polymeric chains of the matrix (10% MABP), the smaller the degree of swelling. This is simply because the chains cannot extend as freely as in the case of weakly bound networks (1% MABP) upon exposure to the water vapor. 3.2.3. Influence of the Alkyl Substituent on the Humidity Swelling Behavior. To probe the influence of the length of the alkyl substituent on the amide groups on the humidity driven swelling behavior, poly(alkylacrylamide) networks made from polymers with a constant amount of built in cross-linker (3%) were chosen. The hydrophobicity of structurally otherwise identical hydrogels was tuned by varying the alkyl chain length (Z) at N-substitution in a systematic fashion, i.e., Z = 1 (methyl), 2a (ethyl), 2b (dimethyl), 3 (propyl), 4a (butyl), and 4b (diethyl) acrylamides. The swelling ratio of the gels was again derived from the OWS reflectivity scans recorded in an N2 and humid air medium (Supporting Information S4). Upon fitting the reflectivity scans using the Fresnel formalism, the dry and swollen layer thicknesses were derived to determine the swelling ratio using eq 1. Figure 6b depicts the swelling ratio of the surface-attached poly(alkylacrylamide) networks as a function of relative humidity. The copolymers with shorter alkyl substituent (i.e., Z < 2) such as MAAm, EAAm, and DMAAm display a significant degree of swelling in the presence of moisture (RH > 50−60%). In contrast, copolymers containing larger alkyl substituent (i.e., Z > 2) such as PAAm, BAAm, and DEAAm show weak or poor swelling under such conditions.

Furthermore, among these homologous series of gels, the hydrophilic MAAm was found to extract the maximum amount of water into the network (ca. ∼30%), whereas the hydrophobic BAAm and DEAAm were found to retain less than 10% of water in the matrix. Such difference in the swelling ratio is a direct evidence of the change in the alkyl substituent length (Z). When Z > 2, the system behaves rather like a hydrophobic than hydrophilic gel. The alkyl chains reduce the interaction between the polar amide groups and water molecules, and accordingly the swelling decreases with increasing alkyl substituent length (Z). 3.3. Water Swelling. In addition to the water vapor swelling, we measured the swelling of these gels in the presence of liquid (deionized) water (αliq) as a function of MABP concentration which is proportional to the cross-link density. The measured OWS reflectivity scans of the water-swollen systems and their corresponding Fresnel calculations are presented in the Supporting Information. Some examples for OWS curves of polymer networks with a varying cross-linker density are depicted in Figure 7. A significant difference between the measurement in the presence of nitrogen atmosphere and in water was noticed. For example, in the case of poly(DMAAm-co10% MABP) it showed one waveguide under nitrogen air; in contrast, it displayed multiple and narrow waveguides in water. The calculated film thickness (d) and the refractive index (n) of the polymer films are shown in Table 2. The corresponding measurements of poly(N-alkylacrylamide) networks with varying chain length of the alkyl substituent have been reported elsewhere.21 The swelling behavior of poly(DMAAm-co-y% MABP) gels in liquid water displayed a similar trend as in the case of water vapor. The degree of swelling was significantly affected by the extent of cross-linking (cross-link density or amount of crossG


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change in elastic energy due to chain stretching is estimated to be around ΔSel/ν ≈ 0.27−0.33 kT per polymer subchain of the surface-attached network (cf. Supporting Information S5).37 This contribution to the Gibbs free energy of the hydration process is very small compared to the contribution from the enthalpy of hydration. The difference between the entropy of vaporization at 298 K (entropy loss) and the mixing entropy of water in the polymer (entropy gain) is expected to be small compared to the enthalpic components and more or less independent of the structural details of the polymer. Accordingly, they will be neglected in the following discussions, so that the Gibbs free energy for vapor swelling can be approximated to ΔGhydr = ΔHhydr − TΔS ≈ ΔHhydr. The swelling equilibrium constant Kswell which describes the partion of water in the air [H2O (air)] to water in the film [H2O (film)] can be expressed asb

Table 2. Calculated Optical Components of the SurfaceAttached PDMAAm Networks (Figure 7)a N2 (atm) polymer poly(DMAAm-co-1% MABP) poly(DMAAm-co-2.5% MABP) poly(DMAAm-co-5% MABP) poly(DMAAm-co-10% MABP)

deionized water

ddry (nm)

ε′

dswell (nm)

ε′

αliq

175

2.2299

700

1.9008

4.0

334

2.318

1135

1.994

3.4

215

2.2412

537

1.9721

2.5

273

2.2991

453

2.1408

1.66

ε′ is the permeability constant (= n2), where n is the refractive index of the film.

a

linker (MABP) in the precursor polymer), i.e., weakly crosslinked or the one with lower amount cross-linker (1% MABP) swell by a factor of S ≈ 4.0 compared to the densely cross-linked gels (10% MABP) (S ≈ 1.6). The swelling of poly(N-alkylacrylamide) networks also changed very significantly with the length of the alkyl substituent (Z). This can be attributed to changes the fact that with longer alkyl substituent the network becomes more hydrophobic, especially noticeable when Z ≥ 3. Consequently, the interaction between the amide groups and water becomes less favorable, and the degree of swelling is significantly deminished as seen in the case of the propyl, butyl, and diethylacrylamide gels.21 3.4. Thermodynamic Considerations. In order to extract water from the humid air to swell the hydrogel, the free energy of vaporization of the water needs to be overcome. The driving force for this extraction process is the strong interaction between polar amide groups of the polymer with the water molecules. If the Gibbs free energy of solvationa/hydration ΔGhyd is higher than the energy ΔGvapor required to condense water from the gaseous phase into the gel, the system becomes hygroscopic and begins to take up water from the contacting air due to the strong interaction between water and hydrogel: ΔGswell = ΔGvapor + ΔG hydr

K swell = [H 2O(air)]/[H 2O(film)] = exp( −ΔGswell /RT ) ∼ exp( −ΔHhydr /RT )

(5)

The concentration of water in the air is equivalent related to the concentration at saturation by the relative humidity (RH): [H 2O(air)] =

RH csat 100

(6)

The concentration of water in the film is directly connected to the degree of swelling: [H 2O(film)] = =

dswell − ddry dswell

=

ddryα vap − ddry ddryα vap

1 α vap − 1 = 1 − vap vap α α

(7)

Here csat is the water concentration in the air at the dew point (which is a constant at a given temperature) and the relative humidity (expressed in %). If we combine eqs 5, 6, and 7, we see that the swelling equilibrium is directly connected to the relative humidity and the swelling coefficient α of the film:

(4)

K swell =

The entropy changes occurring during swelling of the gel and the corresponding stretching of the polymer subchains of the gel are rather small. Even when we consider hydrophilic gels with Z ≤ 2, the gels swell to the factor of the 1.36−1.42 at the highest humidity studied (RH ∼ 92%). Under such circumstances, the

(RH/100)csat 1−

1 ∝ vap

⎛ ΔHhydr ⎞ exp⎜ − ⎟ RT ⎠ ⎝

(8)

Upon rearranging eq 8 and combining all constants (taking into account that ΔHhydr is also constant for a given polymer network), we obtain

Figure 8. (a) 1/αvap of the surface-attached poly(N-alkylacrylamide-co-3% MABP) networks is plotted against the relative humidity (RH/100), where the alkyl (Z) = methyl (1), ethyl (2a), dimethyl (2b), propyl (3), butyl (4a), and diethyl (4b). (b) 1/α of the surface-attached poly(DMAAm-co-y% MABP) networks as a function of relative humidity (RH/100), where y = 1.0%, 2.5%, 5.0%, and 10%. The solid red line denotes the linear fit of the data, and dini denotes the initial (dry) film thickness measured at N2 atmosphere. H


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1 ∼ RH (9) ∝ vap This partition coefficient between water in the air to water in the film suggests that the water fraction increases linearly with the relative humidity in good agreement with the data shown in Figure 8. When we compare different polymers, however, the energies of hydration at the same relative humidity (RH = constant) vary, and we obtain

Figure 10). For example, weakly cross-linked poly(DMAAm-co1% MABP) swelled to the factor of 1.36 in moist air (RH ≈ 92%) and 4.0 in water (liquid). The densely cross-linked gel synthesized from poly(DMAAm-co-10% MABP) swelled to 1.18 in the humid air and 1.66 in water. Similar results were obtained when we compare the hydrophilic poly(MAAm-co-3% MABP), i.e., αvap ≈ 1.42 and αliq ≈ 5.37, and the hydrophobic poly(BAAm-co-3% MABP) coatings (αvap ≈ 1.08, αliq ≈ 1.18). The swelling in water depends also on the enthalpy of hydration. However, in contrast to water swelling for a system swollen in bulk water, the heat of vaporization plays no role. In the case of swelling in bulk water, the enthalpy of hydration is balanced against entropy change due to stretching of the chains, which now becomes a critical parameter. In this case, the system swells until the entropy loss of the surface-attached chains compensates the enthalpy of hydration: ΔHhydr = TΔSelastic. Note that for surface-attached gels as discussed here the swelling is quite different from that of the swelling of free gel,37 and the entropy loss can be calculated according to

1−

1−

⎛ ΔHhydr ⎞ 1 ⎟ vap ∼ exp⎜ − RT ⎠ ∝ ⎝

⎛ 1 ⎞ ln⎜1 − vap ⎟ ∼ ΔHhydr ⎝ ∝ ⎠

(10)

(11)

Equation 9 indicates that the amount of water (1 − 1/αvap) in the film depends exponentially on the energy of hydration. In order to determine the energy of hydration ΔHhyd, calorimetric experiments were carried out on structurally identical, but free (i.e., not surface-attached), hydrogels as free polymer and network have essentially the same compostion and the photochemical cross-linking process induces important structural, but only minor compositional changes to the system. To this, an ampule containing small amounts of the respective dry polymer powder was placed in an calorimeter, and after temperature equilibration, the ampule was broken and the heat released was recorded. The enthalpies thus determined were then compared to the swelling of the gels in water and humid air. Figure 9 shows the enthalpy of hydration (ΔHhyd) against the MABP fraction in the poly(DMAAm-co-y% MABP). The

ΔHhydr ⎛d⎞ ΔSelastic = −υ⎜ ⎟[(α liq)2 − 1 − ln α liq ] = ⎝ ⎠ R 2 RT (12)

Here, d is dimension in which the layer can swell, which is one (d = 1) in the case of the surface-attached hydrogels studied here, and υ is the number of cross-links in the network.37 If we again combine all constants, we obtain (α liq)2 − 1 − ln α liq ∼ Hhydr

(13)

Although the values of swelling in humid air and the swelling in water are numerically quite different, they follow the same trend when the driving force for water uptake, the energy of hydration, is varied by changing the polarity of the polymer (cf. Figure 10 and Figure S6).38 ⎛ 1 ⎞ [(α liq)2 − 1 − ln α liq ] ∼ ΔHhydr ∼ ln⎜1 − vap ⎟ ⎝ ∝ ⎠

(14)

4. CONCLUSIONS The swelling of photochemically cross-linked, surface-attached poly(alkylacrylamide) networks caused by the humidity of a contacting medium can be followed using optical waveguide spectroscopy. The driving force behind the observed hygroscopicity is the strong interaction of the polar amide groups present in the polymers that extract water from the air and bind it rather strongly. The obtained results suggest that the most hydrophilic polymers from this series swell in humid air quite strongly. At the highest humidity studied (92% RH at 25 °C) they can swell by a factor of 1.36−1.42; in other words, they can take up so much water from the ambient air that the whole film contains about 25−30% of water. These values underline the importance of the determination of humidity swelling for all quantitative evaluations of such hydrophilic surface-attached layers. If this water uptake is not taken into account, the amount of polymer in the layer will be overestimated quite significantly, i.e., in the case of the polymers studied here by 30−40%, which is a considerable error. The cross-linker contents and the length of the alkyl groups in the side chains influence cross-link density and the hydrophobicity of the networks, which are the two dominating factors that affect the swelling behavior in water and humid air.

Figure 9. Enthalpy of hydration (ΔHhydr) is plotted against the MABP mole fraction of the poly(DMAAm-y% MABP), where y = 1.0%, 2.5%, 5%, 7.5%, and 10%.

dissolution process is highly exothermic in the case of polymers carrying low amounts of the photo-cross-linker MABP and becomes smaller as the MABP content is increased. For example, ΔHhyd ≈ −9.7 kJ/mol for poly(DMAAm-co-1% MABP), which shows excellent solubility in water, and for poly(DMAAm-co-10% MABP), ΔH ≈ −1.3 kJ/mol (almost no solubility in water). This must be compared to the enthalpy of vaporization of water, which is ΔHvap water ≈ 40 kJ/mol and which can be used to calculate the Gibbs free energy close to the dew point. Finally, we compared the swelling characteristics of the identical gels in the presence of liquid water and vapor (moist air). Gels that showed a significant swelling in humid air exhibited strong swelling in liquid water and vice versa (cf. I


Article

Macromolecules

Figure 10. Swelling ratio of the surface-attached poly(DMAAm-co-y% MABP), where y = 1.0%. 2.5%, 5.0%, and 10% measured in (a) humid air (αvap) (RH = 92%, T = 23 °C) and (b) water (αliq) (T = 23 °C) is plotted against the enthalpy of hydration (ΔHhydr). (c) Correlation between the swelling ratio measured in humid air (αvap) and water (αliq), where the ln (1 − 1/αvap) is plotted against the (αliq)2 − 1 − ln(αliq) according to eq 14. The solid lines added are guides to the eye.

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Microcalorimetry measurements demonstrate that with increasing content of hydrophobic units in the polymer the energy of hydration is strongly reduced. As the swelling of the gels in humid air and swelling in water depend both on the energy of hydration, these two phenomena show a strong correlation, i.e., gels that showed strong swelling water tend to uptake more water from moist air, and vice versa. It should be noted, as shown in other publications,21 that the swelling of the surface-attached gels in water is strongly correlated with the protein adsorption behavior and consequently also with the adhesion of biological cells to the surfaces of such systems. This correlation is valid for systems where strong enthalpic interactions between the proteins and the surface-attached gels can be excluded, which means in other words that Coulombic or hydrophobic interactions between hydrogel and protein do not dominate the system as it is the case for neutral, surface attached hydrogels carrying no hydrophobic groups. An interesting consequence of this correlation might be that a simple measurement of the film thickness of such surface-attached neutral hydrogels at high humidity and a measurement of the dry film thickness is sufficient to predict the swelling in water and accordingly the tendency of the surfaces for (unspecific) protein adsorption. Subsequently, as cell adhesion is generally strongly correlated with the extent of protein adsorption, such simple measurements might also predict the strength/extent of cell adhesion to the employed surfaces. We are currently continuing further research along these lines.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01379. Spectroscopic details and supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.R.). Present Address

C.K.P.: Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Fan Wu for his help on some of the vapor swelling measurements, Natalia Schatz for her help to synthesize copolymers, Martin Schoenstein for his help on calorimetric measurements, and the Deutsche Forschungs Gemeinschaft (DFG) for financial support of the work.

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ADDITIONAL NOTES Solvation is the more general term. However, as we study only hydrogels in aqueous environments, we employ exclusively the term hydration in the following. b From a principle point activities should be taken rather than concentrations. However, for the systems studied (neutral gels a

J


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in water) the difference will be very small and well within the experimental error of the system. However, for charged systems this difference could become important.

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Humidity Driven Swelling of the Surface-Attached Poly(N

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