Reorganization of an Amphiphilic Glassy Polymer Surface in Contact

Jul 3, 2018 - We address the question of how a surface of a glassy polymer reorganizes after coming in contact with water. Because contact angle hyste...
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Reorganization of an Amphiphilic Glassy Polymer Surface in Contact with Water Probed by Contact Angle and Sum Frequency Generation Spectroscopy Nishad Dhopatkar, Emmanuel Anim-Danso, Chao Peng, Saranshu Singla, Xinhao Liu, Abraham Joy, and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States Downloaded via UNIV OF SUSSEX on July 3, 2018 at 18:54:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: We address the question of how a surface of a glassy polymer reorganizes after coming in contact with water. Because contact angle hysteresis measurements are also affected by surface roughness and chemical heterogeneity, we have used surface-sensitive sum frequency generation spectroscopy (SFG) in conjunction with water contact angles to answer this question. To increase the magnitude of the surface reorganization, we have designed an amphiphilic polymer, poly(α-hydroxymethyl-n-butyl acrylate) (PHNB), to study the changes in the structure of polar hydroxy groups and nonpolar (methyl and methylene) groups at the interface. The SFG and the water contact angles show that reorganization does occur for PHNB below Tg. However, complete reorganization requires heating the sample above the bulk Tg. These heating experiments were conducted by first heating the sample in the presence of water and then followed by cooling the sample to room temperature in the presence of water to lock the changes in the surface structure (we refer to this treatment as water annealing). The polar contribution to the total surface energy of PHNB, determined by Owens−Wendt−Rabel−Kaelble (OWRK) method at room temperature, increases after water annealing above Tg. This is consistent with our SFG results that show an increase in concentration of polar hydroxy groups at room temperature after water annealing the PHNB film above Tg. For PHNB, the contact angle hysteresis is higher for samples that are water annealed above Tg. This is consistent with the surface energy and SFG results. For a low-Tg polymer, poly(n-butyl acrylate), which has the same nonpolar side group but lacks the hydroxyl group, surface reorganization takes place immediately after contact with water, and these changes are reversible.

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INTRODUCTION

reorganization is possible for glassy polymers, materials that are commonly used in biomedical implants. Koberstein and co-workers10 demonstrated that surface reorganization does occur for end-functionalized glassy polymers over prolonged periods (hundreds of hours) after exposure to saturated water vapor environment at different temperatures below Tg. However, these reorganization dynamics are limited by the diffusion rates of end-functional polymers, and they concluded based on the activation energy that these dynamics were associated with sub-Tg β-relaxation process. Other multicomponent amphiphilic systems such as end-grafted or block copolymers also show surface reorganization.11 In the case of phase-separated systems, the low glass transition temperature of one of the blocks plays an important role in surface segregation. To achieve surfaces that can reorganize quickly, Genzer and co-workers12 have used the strategy of lowTg amphiphilic homopolymers containing hydrophobic and

Polymers are commonly used in food packaging and biomedical devices such as drug-eluting stents and scaffolds.1−4 However, it is not clear what is the surface of a polymer film that is in contact with water. This knowledge becomes highly relevant if the polymer has been designed to place certain functional groups for interacting with biological molecules.5−7 The characterization of these surfaces in a vacuum is not sufficient because the interfacial energy can be lowered by reorganization of pendant groups and/ or migration of more polar groups to the interface upon contact with water. The wetting properties are controlled by very local changes in surface concentrations.8 It was surprising when Holly and Refejo9 first demonstrated that even hydrogels, which contain a significant fraction of water, show high advancing water contact angles (θadv), indicating that the surfaces of these hydrogels are covered with hydrophobic pendants or backbone nonpolar groups. These materials also show a high contact angle hysteresis, indicating that the surfaces that are hydrophobic in contact with air reorganize upon exposure to water. The time scales of these arrangements are extremely fast for hydrogels. However, this raises the question of whether such surface © XXXX American Chemical Society

Received: March 26, 2018 Revised: May 28, 2018

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DOI: 10.1021/acs.macromol.8b00640 Macromolecules XXXX, XXX, XXX−XXX

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measurements, a 3.5 wt % solution in tetrahydrofuran (THF) (Sigma-Aldrich, HPLC grade, 99.9%) was used to spin-coat ∼280 nm thick films (measured using ellipsometer) on a sapphire prism surface at 2000 rpm for 1 min. Because of rapid evaporation of THF during spin-coating, the films were rougher. For water contact angles such microroughness could be detrimental; hence, to form smoother films a 2 wt % solution was used to solvent cast films over Si wafers. Contact angle and atomic force microscopy (AFM) measurements were performed on casted films. The polymer solution was deposited over a clean Si wafer using a pipet. To prevent fast evaporation of THF which could lead to roughness of the film, the samples were surrounded by THF bath in an enclosed box so that the samples slowly dry out in an almost saturated THF vapor environment for at least 8 h, similar to the procedure used by Lam et al.24 Then the films were annealed at 110 °C in a vacuum for at least 8 h to remove the excess solvent and to equilibrate the polymer surface structure. For poly(n-butyl acrylate) (PnBA) (MW ∼ 200 000 g/mol), the solutions were made in toluene (Sigma-Aldrich, HPLC grade, 99.9%), and samples were prepared using the same procedure we used for PHNB for SFG and contact angle measurements. Before coating, both sapphire and Si-wafer substrates were cleaned by piranha treatment. Caution should be used when using piranha because it is extremely corrosive. The substrates were rinsed with copious amounts of DI water, dried by N2 jet, and plasma treated (PDC-32G by Harrick Scientific) for 5 min followed by spin-coating or solution-casting. The advancing and receding contact angles were measured by monitoring the shape of 5 μL droplets of Millipore ultrapure water (∼18.2 MΩ) after increasing and decreasing the droplet volume by 0.5−2 μL, respectively, using a Ramé-Hart goniometer. The droplet volume was not increased or decreased further once the contact line moved. ImageJ was used to calculate the advancing and receding contact angles. Contact angles were measured for pristine PHNB films as well as after subjecting them to water annealing protocol. In this protocol, films coated on the Si wafers were exposed to water bath at different temperatures for 30 min. The goal was to facilitate the kinetics of reorganization under water. Then the films along with the water bath were cooled down to room temperature so as to retain the new surface structure achieved under water at corresponding temperatures. After drying the films using a N2 stream, the contact angles were measured again. We are referring to this treatment as water annealing. The SFG experiments were performed in a total internal reflection geometry using a sapphire prism as a substrate with incident angles of 42° and 16° for polymer−air and polymer−water interfaces, respectively, with reference to the surface normal of an equilateral sapphire prism. In total internal reflection geometry, there could be some contribution from the buried sapphire/polymer interface to the polymer−air and polymer−water interfaces. Using wavelength-dependent refractive indices for sapphire25 and water (D2O),26,27 we calculated the contribution from the buried interface to the free polymer interfaces for a range of polymer thickness (260−300 nm), as described in the Supporting Information (Figures S5−S8). Assuming that the signal strength (Aq) is comparable at both buried and free interfaces, the maximum contribution from the polymer−sapphire interface is ≈2.5% to polymer−air interface and ≈5−15% to polymer−D2O interface in the polymer −OH region (3100−3250 cm−1) for a 280 nm thick PHNB film. The details of the SFG instrumentation, the in situ heating and cooling stage, and the vacuum chamber to minimize the convective heat exchanges have been described elsewhere.28

hydrophilic pendant groups that show a rapid decrease of contact angle (a change of 2°/s). It appears from these few studies monitoring reorganization of polymeric surfaces using water contact angles that the glassy polymers can reorganize below bulk Tg. However, direct measurements of surface reorganization have reached opposite conclusions. Wang et al. have reported infrared−visible sum frequency generation spectroscopy (SFG) data for poly(methyl methacrylate) (PMMA), poly(n-butyl methacrylate) (PBMA), and poly(n-octyl methacrylate) (POMA), in contact with air and water.13,14 The SFG signals are generated only at noncentrosymmetric interfaces, and SFG intensity is a function of concentration and orientation of interfacial molecules.15−19 The authors observed no surface reorganization for PMMA in water. However, for PBMA and POMA with sub-roomtemperature Tg, the surface reorganization was manifested by loss of SFG signal associated with the n-alkyl pendant groups. Even for the same polymer, the conclusions drawn about the surface reorganization are not consistent. Horinouchi et al.20 reported that surfaces of glassy PMMA show changes in water contact angles below Tg due to reorganization of carbonyl groups upon exposure to water. Although the changes in water contact angles were small, their conclusion was that the surfaces do arrange upon contact with water below Tg. Here, we revisit this outstanding question on whether there is enough mobility below Tg for polymer surfaces to reorganize after contact with water. To attain higher contact angle hysteresis and to observe clearer changes in the SFG spectra after bringing polymer films in contact with water, we have studied the surface reorganization using an amphiphilic polymer, poly(α-hydroxymethyl-n-butyl acrylate) (PHNB). PHNB has a hydrophobic ester n-butyl and hydrophilic hydroxymethyl pendant groups in each repeating unit as shown in Figure 1A.21,22 Because water contact angles are also affected by surface

Figure 1. Chemical structures of (A) poly(α-hydroxymethyl-n-butyl acrylate) (PHNB) and (B) poly(n-butyl-acrylate) (PnBA) synthesized by the scheme published elsewhere.21,22

roughness, we have used the combination of SFG and contact angle hysteresis to derive conclusions on surface reorganization. In addition, we have designed annealing experiments where the samples are heated above Tg to speed up the reorganization and then cooled down in contact with water. This allowed us to compare the extent of reorganization below the bulk Tg of PHNB. We have also compared the surface reorganization of the glassy PHNB with a low-Tg poly(n-butyl acrylate) (PnBA) (Figure 1B), which contains the same n-butyl hydrophobic pendant groups but lacks the OH polar pendant groups. We expect that the surface dynamics will be much faster for PnBA because of its low Tg (−54 °C).23

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RESULTS AND DISCUSSION The advancing and receding water contact angles for PHNB and PnBA are shown in Figure 2C. The differences between advancing and receding water contact angles have been used in the past to reflect surface reorganization of polymer thin films.29,30 PHNB surface (static contact angle ∼81 ± 1°) shows an advancing contact angle of 81 ± 1° and the receding contact angle of 55 ± 1°, resulting in the hysteresis of 26°. Poly(n-butyl acrylate) (PnBA) (static contact angle ∼91 ± 1°), which

MATERIALS AND METHODS

Poly(α-hydroxymethyl-n-butyl acrylate) (PHNB) with a MW of 57 800 g/mol with a Tg of ∼81 ± 2 °C (Supporting Information Figure S1) was synthesized and purified as previously described.21,22 For SFG B

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air, we have used SFG to probe the polymer−air and polymer− water interfaces. Figures 3A and 3B show SFG spectra at the PHNB−air interface in PPP and SSP polarizations, respectively. The peaks observed in PPP correspond to CH2,sym (2850 cm−1, d+), CH3,sym (2880 cm−1, r+), CH2,asym (2915 cm−1, d−), CH3,Fermi (2945 cm−1), and CH3,asym (2970 cm−1, r−) assigned to the nbutyl pendant groups,32 whereas in SSP polarization the peaks correspond to r+ (2885 cm−1), d− (2915 cm−1), and CH3,Fermi (2945 cm−1). Similar peaks positions are observed for SFG spectra collected at the PnBA−air interface in PPP and SSP polarizations (Figures 3C and 3D). The dominance of r+ vibrational bands in SFG spectra for both polymers suggests a highly ordered hydrophobic n-butyl group at the air interface.13,32 Additionally, the PHNB−air SFG spectrum shows a broad peak around 3200 cm−1 which we assign to the −OH pendant group. This assignment also overlaps with the water bands, and there exists a possibility of condensed water on the PHNB surface. In previous studies on the hydrophilic mica surface, at 40% relative humidity, there were bands observed for both ice-like and water-like structures at 2400 and 2500 cm−1, respectively, when D2O vapors were used.33 For much more hydrophobic PHNB surface we expect much less condensation of water at 40% relative humidity. Therefore, the presence of −OH band is most likely due to −OH groups from PHNB. The presence of this polar hydroxymethyl group at air interface explains why the static and the advancing angles are lower for PHNB compared with PnBA. To understand surface structure at the receding contact line, we need to probe the PHNB−water interface. To avoid any spectral overlap between the vibrational band of PHNB −OH groups and those from water, we have used D2O instead of H2O in these experiments. The SFG spectra for the PHNB−D2O interface are strikingly different from those collected for

Figure 2. Representative images of probe droplets for (A) advancing and (B) receding contact angles on PHNB measured with an initial drop volume of 5 μL. The droplet volume is increased or decreased by 0.5−2 μL for the contact line to advance or recede, respectively. (C) Advancing and receding angles on PnBA and PHNB surfaces. Error bars were plotted for standard error calculated using two droplets on each substrate, and four to six independent substrates were used for each polymer.

contains the same n-butyl hydrophobic but no polar pendant groups, shows advancing contact angle, receding contact angle, and hysteresis of 91 ± 1°, 71 ± 1°, and 20 ± 1°, respectively. Since both the film surfaces have similar RMS height variation of 0.3 ± 0.1 nm, the difference in their contact angle hysteresis is likely due to the difference in their interfacial energies (γSV and γSL). Moreover, since both PHNB and PnBA contain the same nbutyl hydrophobic pendant group which is expected to order at the air interface,31 the difference in their static or advancing angles could be related to the surface concentration of polar hydroxy groups and the difference in the surface kinetics due to difference in their Tg. It is also possible that the contact angle hysteresis is affected by surface roughness and surface inhomogeneity. So to verify whether PHNB surface restructures on going from air to water interface and to identify the differences between PHNB and PnBA surfaces in contact with

Figure 3. SFG spectra in PPP polarization (A) for PHNB−air (red circle) and PHNB−D2O (blue triangle); in SSP polarization (B) for PHNB−air (red circle) and PHNB−D2O (blue triangle); in PPP polarization (C) for PnBA−air (red circle) and PnBA−D2O (blue triangle); and in SSP polarization (D) for PnBA−air (red circle) and PnBA−D2O (blue triangle) with a common left intensity axis. All the spectra at air interface are obtained at 42° and at D2O interface at 18° incident angles. Black lines represent fits obtained using Lorentzian function. All the left axes are for SFG intensities at polymer−air and all the right axes are for SFG intensities at polymer−D2O interfaces, except for panel D, where the polymer−D2O spectrum has negligible counts. C

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water annealing below Tg are similar. The advancing and static contact angles reduce to ∼79 ± 0.4° and 66 ± 0.8°, respectively, only after water annealing the samples at 90 °C, above the polymer Tg. The most striking observation was a much larger drop in receding angles from 55 ± 0.4° to 40 ± 0.6° upon water annealing above Tg. Thus, the contact angle hysteresis increases from 26° to 39° upon water annealing the PHNB films above its Tg. It should be noted that the RMS roughness of PHNB film also increases from 0.3 ± 0.1 to 2.2 ± 0.7 nm. So the larger roughness may have resulted in increasing the contact angle hysteresis instead of additional reorganization. To confirm that the water annealing process is not chemically degrading the sample, we collected ATR-IR spectra for PHNB films (Figure S3) before and after water annealing. The similarity in the IR spectra did not detect any changes due to water annealing, and we can hence rule out the possibility of chemical degradation during water annealing. The ellipsometer thickness after water annealing at 90 °C and N2 drying was 284 ± 4 nm, similar to that of the pristine films, suggesting lack of film swelling. Also, the surface structure recovers after the films were annealed under vacuum above Tg (Figure S4). To determine the changes in critical surface free energy, we measured the contact angle on PHNB films using different probe liquids based on the Owens−Wendt−Rabel−Kaelble method. This approach was used to calculate the dispersion and polar components of the surface free energy using the following equation:34,35

PHNB−air interface (Figure 3A,B). For the PHNB−D2O interface, the hydrocarbon features observed in the PHNB−air spectra are still observed. However, instead of a predominant r+ mode observed for the air interface, we observe a dominant r− mode for the aqueous−polymer interface. Decrease in the amplitude ratio r+:d+ from about ∼15 to ∼0.3 suggests loss of ordering, and increase in the amplitude ratio r−:r+ from ∼0.4 to ∼4 implies change in orientation of the n-butyl pendant groups going from PHNB−air to PHNB−D2O. For PnBA, we did not observe any hydrocarbon signals in SSP polarization, indicating disordering of the surface pendant groups. These results for PnBA are consistent with previous reports.13 To ensure that the PnBA films were still intact and not dewetted, the microscopy images before and after exposure to water are shown in Figure S2. The broad peak centered around 3150 cm−1 is again assigned to the hydroxymethyl group of PHNB. The relative increase in the amplitude strength of the −OH peak with respect to the peaks in hydrocarbon region after the sample is exposed to water indicates increased number density and/or orientational order of the polar hydroxymethyl groups and could be the reason behind the lower receding contact angles for PHNB in comparison to those measured for PnBA, which does not have polar hydroxymethyl functional groups. Thus, the surface reorganization is evident for PHNB based on disordering of nbutyl groups and relative increase in SFG amplitude for polar hydroxymethyl groups under water. Even though we do observe surface reorganization for glassy PHNB, it is also possible that kinetics barriers limit the extent of reorganization necessary to reach a thermodynamic equilibrium conformation at the aqueous interface. To understand the role of Tg in controlling the reorganization of PHNB, the films are annealed under water at different temperatures before cooling them down to room temperature in the presence of water. In Figure 4, we have plotted the static, advancing, receding contact angles, and the contact angle hysteresis on water annealed PHNB. We observed that the advancing angles before and after

(1 + cos θLP)γL/2 γLD =

γSD +

γLP /γLD γSP

(1)

Here, θLP is Young’s static contact angle, γL is the liquid surface tension, and γDL and γPL are the dispersion and polar components of liquid surface tensions, respectively. The terms γDS and γPS are the dispersion and polar components of polymer surface energy, respectively. Thus, based on contact angle measurements using different liquids, the dispersion and polar components of polymer surface energy are obtained from the slope and intercept of a linear fit to the experimental data using eq 1. The liquids that are used and their surface tensions are listed in Table 1. Table 1. Surface Tension Components of Liquids Used for Contact Angle Measurements surface energy (mJ/m2)

H2O

formamide

ethylene glycol

propylene glycol

γDL γPL γL

21.8 51.0 72.8

39.5 18.7 58.2

32.8 16.0 48.8

26.4 9.0 35.4

Figure 5 shows a plot and fitting curves for pristine PHNB and after water annealing at 90 °C. For pristine PHNB, the dispersion component (γDS ) is higher than the polar (γPS ) surface energy component, consistent with the observation of a hydrophobic surface and higher static/advancing water contact angles. For water annealed sample, the γDS decreases, whereas γPS increases upon annealing and almost doubles compared to γPS for the pristine sample. Even though there are assumptions in using static contact angles in eq 1, the results do indicate that the surface reorganization has occurred after water annealing above Tg. The higher contact angle hysteresis of water annealed films raises two questions. First, why do the samples annealed at temperatures below Tg not show any changes in advancing

Figure 4. Static, advancing, and receding contact angles (left axis) on PHNB surfaces, with varying under water annealing temperatures, measured with an initial drop volume of 5 μL. The droplet volume is increased or decreased by 0.5−2 μL for the contact line to advance or recede, respectively. The angles are measured after annealing the polymer films under water at respective temperatures for 30 min, followed by cooling under water to room temperature and drying by N2 jet. The corresponding contact angle hysteresis is plotted on the right axis. Standard errors are plotted as error bars obtained by averaging over at least six sample surfaces with each sample surface probed using at least two water droplets. Dashed lines are only to guide the eye. D

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strengths of −OH:r+ (Figure 6b) shows an insignificant change from 3.8 ± 0.5 for pristine PHNB surface to 4.7 ± 0.5 for samples water annealed at 70 °C. However, for the samples water annealed at 90 °C the amplitude ratio −OH:r+ increases to 8.7 ± 0.1. We expect these changes in spectral features to reflect the advancing contact angles. The small changes in the −OH:r+ amplitude ratio and ordered methyl groups, for samples water annealed below Tg, explain the similar advancing contact angles observed for films water annealed below Tg. Only upon water annealing above Tg, we observe that the reorganization does not recover after drying at room temperature using nitrogen gas as indicated by the disordered methyl groups and a significantly higher −OH:r+ ratio that result in lower static and advancing contact angles for PHNB. To understand the receding contact angle for the water annealed films, we have measured the SFG spectra of annealed films at the PHNB−D2O interface (Figure 7a). All the spectra were collected at 25 °C after water annealing. The SFG peaks assigned to hydrocarbon stretches are less sharper upon water annealing below Tg, and the ratio of the amplitude strengths of −OH:r− increases slightly upon water annealing below Tg (Figure 7b). These small changes are consistent with small changes in receding contact angles. The largest changes were observed for samples water annealed above Tg. Both the hydrocarbon and the ratio of −OH:r− are significantly affected, consistent with the changes in receding water contact angles. Finally, we discuss the changes in the water structure at room temperature next to PHNB interface as a function of water annealing temperature (Figure 7c). The D2O spectra show bimodal spectral shape with peaks around 2500 and 2600 cm−1, corresponding to the dihedral bonded (liquid-like) and loosely clustered water molecules,36, respectively. With increasing water annealing temperature, the liquid-like component increases. For films water annealed at 70 °C, we also observe increasing contribution of ice-like OD peaks. Upon water annealing at 90 °C, we observe a dominant ice-like peak around 2400 cm−1. These observations are consistent with the red-shift observed for the PHNB −OH peak, thereby suggesting an increased interactions between D2O and polar PHNB groups. It is also possible that there is a slight broadening of the polymer−water interface (interfacial swelling), and this could lower the surface Tg and increase mobility. This lowering of surface Tg in contact with water could also be a function of the hydrophilicity of the repeat unit of the glassy polymer. In future, neutron or X-ray reflectivity experiments are necessary to measure the changes in interfacial width after exposure to water above Tg.

Figure 5. Analysis of dispersion and polar surface energy components for PHNB at 25 °C (pristine, red) and upon water annealing at 90 °C (blue). The error bars are obtained by measurements over at least four sample surfaces with at least two probe droplets used on each sample surface.

angles, even though there are clear changes in the receding angles? Additionally, can the changes in contact angle hysteresis be a result of only the increase in surface roughness after water annealing the samples above Tg? We have again used SFG to highlight the changes taking place after water annealing. Figure 6a shows spectra at the PHNB−air interface for dried films after the sample was annealed under water at different

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SUMMARY In this work, we addressed the question of whether a glassy polymer surface will restructure after exposure to water below its bulk Tg. By choosing a polymer with an amphiphilic repeat unit, we show using contact angle hysteresis and surface-sensitive SFG spectroscopy that the hydrophobic pendant groups disorder upon exposure to water and the amplitude strength of −OH:r+ increases after contact with water. The extent of this reorganization increases slightly with temperature, for samples annealed under water at temperatures below Tg (water annealed samples). The samples that were water annealed above Tg show further increase in contact angle hysteresis, decrease in ordering of methyl groups, and increase in the ratio of amplitude strength of −OH:r+ (and −OH:r−). This indicates that there is further increase in surface concentration and/or ordering of −OH groups in water annealed PHNB films above Tg. This implies

Figure 6. (a) SFG spectra in PPP polarization for PHNB at air interface at room temperature as annealed (red circle) and after heating for 30 min under D2O at 70 °C (blue square) and 90 °C (pink triangle) followed by drying with N2 jet. All the spectra are fitted by Lorentzian function. (b) Ratio of SFG peak amplitudes of −OH (3200 cm−1) to CH3,sym (2880 cm−1, r+) from the fitting results for the spectra shown in (a). The error bars are obtained by fitting and averaging over at least three spectra.

temperatures and cooled to room temperature under water (water annealed samples). For the sample water annealed at 70 °C, the spectral features are very similar to that of the pristine PHNB film. Dramatic decrease in d+ and CH3,Fermi peaks are observed only after water annealing the sample at 90 °C. The decrease in the amplitude strength of r+:d− modes also indicates disordering of the methyl groups. The ratio of amplitude E

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Figure 7. (a) SFG spectra in PPP polarization for PHNB−D2O interface at room temperature as annealed (orange circle), after heating for 30 min under D2O and cooling down to 25 °C from 40 °C (green square), 70 °C (blue triangle), and 90 °C (pink diamond). All the spectra are fitted by a Lorentzian function. (b) Ratio of SFG peak amplitudes of −OH (3100 cm−1) to CH3,asym (2970 cm−1, r−) from the fitting results for the spectra shown in (a). (c) SFG spectra in PPP polarization for PHNB−D2O interface in the D2O region. The spectra were not fitted due to two overlapping broad peaks.

Present Addresses

that a complete extent of reorganization can only take place when the glassy polymer films are annealed under water above the polymer Tg. Therefore, surfaces of glassy polymers partially reorganize below Tg and larger scale chain motions above Tg are necessary to lower the interfacial energy in contact with water even further. These results shed important light in designing antimicrobial surfaces, surfaces that promote cell growth by attaching growth factors, and antifog or self-cleaning surfaces. In all these cases surface reorganization could either increase or decrease the populations of functional groups on the surface. These results also shed light on the kinetics of the surface reorganization upon adding or removing water and the relationship between contact angle hysteresis and surface reorganization.

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N.D.: Avery Dennison Polymers, Adhesives, and Coatings Center of Excellence, Mill Hall, PA 17751. E.A.-D.: Solvay Specialty Polymers, 4500 McGinnis Ferry Road, Alpharetta, GA 30005. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Alex Nyarko for helping with the AFM and ATR-IR measurements. We also thank Sukhmanjot Kaur and Yang Zhou for their remarks in preparing this manuscript. The funding was provided by NSF-DMR 1610483.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Agrawal, C.; Ray, R. B. Biodegradable polymeric scaffolds for musculoskeletal tissue engineering. J. Biomed. Mater. Res. 2001, 55, 141−150. (2) Soppimath, K. S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinski, W. E. Biodegradable polymeric nanoparticles as drug delivery devices. J. Controlled Release 2001, 70, 1−20. (3) Sauer-Budge, A. F.; Mirer, P.; Chatterjee, A.; Klapperich, C. M.; Chargin, D.; Sharon, A. Low cost and manufacturable complete microTAS for detecting bacteria. Lab Chip 2009, 9, 2803−2810. (4) Ishihara, K.; Takai, M. Bioinspired interface for nanobiodevices based on phospholipid polymer chemistry. J. R. Soc., Interface 2009, 6, S279−S291. (5) Israelachvili, J.; Wennerström, H. Role of hydration and water structure in biological and colloidal interactions. Nature 1996, 379, 219−225. (6) Zdrahala, R. J.; Zdrahala, I. J. Biomedical applications of polyurethanes: a review of past promises, present realities, and a vibrant future. J. Biomater. Appl. 1999, 14, 67−90.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00640. DSC thermogram, RMS roughness of PHNB films measured by AFM, optical microscopy images of PnBA films, ATR-IR spectra of PHNB, and SFG spectral fitting parameters (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.D.). ORCID

Chao Peng: 0000-0002-5672-6324 Abraham Joy: 0000-0001-7781-3817 Ali Dhinojwala: 0000-0002-3935-7467 F

DOI: 10.1021/acs.macromol.8b00640 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (7) Zhang, C.; Myers, J. N.; Chen, Z. Elucidation of molecular structures at buried polymer interfaces and biological interfaces using sum frequency generation vibrational spectroscopy. Soft Matter 2013, 9, 4738−4761. (8) Whitesides, G. M.; Laibinis, P. E. Wet chemical approaches to the characterization of organic surfaces: self-assembled monolayers, wetting, and the physical-organic chemistry of the solid-liquid interface. Langmuir 1990, 6, 87−96. (9) Holly, F. J.; Refojo, M. F. Wettability of hydrogels I. Poly (2hydroxyethyl methacrylate). J. Biomed. Mater. Res. 1975, 9, 315−326. (10) Wong, D.; Jalbert, C. A.; O’Rourke-Muisener, P. A. V.; Koberstein, J. T. Surface Dynamics of Polymer Glasses: Sub-Tg Surface Reorganization in End-Functional Polymers. Macromolecules 2012, 45, 7973−7984. (11) Inutsuka, M.; Yamada, N. L.; Ito, K.; Yokoyama, H. High Density Polymer Brush Spontaneously Formed by the Segregation of Amphiphilic Diblock Copolymers to the Polymer/Water Interface. ACS Macro Lett. 2013, 2, 265−268. (12) Crowe, J. A.; Genzer, J. Creating responsive surfaces with tailored wettability switching kinetics and reconstruction reversibility. J. Am. Chem. Soc. 2005, 127, 17610−17611. (13) Wang, J.; Woodcock, S. E.; Buck, S. M.; Chen, C.; Chen, Z. Different surface-restructuring behaviors of poly (methacrylate) s detected by SFG in water. J. Am. Chem. Soc. 2001, 123, 9470−9471. (14) Hankett, J. M.; Liu, Y.; Zhang, X.; Zhang, C.; Chen, Z. Molecular level studies of polymer behaviors at the water interface using sum frequency generation vibrational spectroscopy. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 311−328. (15) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (16) Eisenthal, K. B. Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy. Chem. Rev. 1996, 96, 1343−1360. (17) Boyd, R. W. Nonlinear Optics, 3rd ed.; Academic Press: 2008. (18) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Molecular Structure of Polystyrene at Air/ Polymer and Solid/Polymer Interfaces. Phys. Rev. Lett. 2000, 85, 3854− 3857. (19) Nanjundiah, K.; Hsu, P. Y.; Dhinojwala, A. Understanding rubber friction in the presence of water using sum-frequency generation spectroscopy. J. Chem. Phys. 2009, 130, 024702. (20) Horinouchi, A.; Atarashi, H.; Fujii, Y.; Tanaka, K. Dynamics of Water-Induced Surface Reorganization in Poly (methyl methacrylate) Films. Macromolecules 2012, 45, 4638−4642. (21) Peng, C.; Joy, A. Baylis-Hillman Reaction as a Versatile Platform for the Synthesis of Diverse Functionalized Polymers by Chain and Step Polymerization. Macromolecules 2014, 47, 1258−1268. (22) Peng, C.; Joy, A. Self-emulsion polymerization of baylis-hillmanderived α-hydroxymethyl-substituted acrylates. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1743−1747. (23) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R. Polymer Handbook; Wiley: New York, 1989; Vol. 7. (24) Lam, C.; Wu, R.; Li, D.; Hair, M.; Neumann, A. Study of the advancing and receding contact angles: liquid sorption as a cause of contact angle hysteresis. Adv. Colloid Interface Sci. 2002, 96, 169−191. (25) Malitson, I. H. Refraction and dispersion of synthetic sapphire. J. Opt. Soc. Am. 1962, 52, 1377−1379. (26) Bertie, J. E.; Ahmed, M. K.; Eysel, H. H. Infrared intensities of liquids. 5. Optical and dielectric constants, integrated intensities, and dipole moment derivatives of water and water-d2 at 22. degree. C. J. Phys. Chem. 1989, 93, 2210−2218. (27) Odhner, H.; Jacobs, D. Refractive index of liquid D2O for visible wavelengths. J. Chem. Eng. Data 2012, 57, 166−168. (28) Anim-Danso, E.; Zhang, Y.; Alizadeh, A.; Dhinojwala, A. Freezing of water next to solid surfaces probed by infrared-visible sum frequency generation spectroscopy. J. Am. Chem. Soc. 2013, 135, 2734−2740. (29) Joanny, J.; De Gennes, P.-G. A model for contact angle hysteresis. J. Chem. Phys. 1984, 81, 552−562. (30) Gao, L.; McCarthy, T. J. Contact angle hysteresis explained. Langmuir 2006, 22, 6234−6237.

(31) Crowe-Willoughby, J. A.; Stevens, D. R.; Genzer, J.; Clarke, L. I. Investigating the molecular origins of responsiveness in functional silicone elastomer networks. Macromolecules 2010, 43, 5043−5051. (32) Kweskin, S.; Komvopoulos, K.; Somorjai, G. Molecular restructuring at poly (n-butyl methacrylate) and poly (methyl methacrylate) surfaces due to compression by a sapphire prism studied by infrared-visible sum frequency generation vibrational spectroscopy. Langmuir 2005, 21, 3647−3652. (33) Miranda, P.; Xu, L.; Shen, Y.; Salmeron, M. Icelike water monolayer adsorbed on mica at room temperature. Phys. Rev. Lett. 1998, 81, 5876. (34) Owens, D. K.; Wendt, R. Estimation of the surface free energy of polymers. J. Appl. Polym. Sci. 1969, 13, 1741−1747. (35) Carré, A. Polar interactions at liquid/polymer interfaces. J. Adhes. Sci. Technol. 2007, 21, 961−981. (36) Ji, N.; Ostroverkhov, V.; Tian, C.; Shen, Y. Characterization of vibrational resonances of water-vapor interfaces by phase-sensitive sum-frequency spectroscopy. Phys. Rev. Lett. 2008, 100, 096102.

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DOI: 10.1021/acs.macromol.8b00640 Macromolecules XXXX, XXX, XXX−XXX