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Vibrational Sum Frequency Spectroscopy on Polyelectrolyte Multilayers: Effect of Molecular Surface Structure on Macroscopic Wetting Properties Emil Gustafsson,†,‡ Jonas Hedberg,§ Per A. Larsson,‡ Lars Wågberg,†,‡ and C. Magnus Johnson*,§ †

KTH Royal Institute of Technology, School of Chemical Science and Engineering, Wallenberg Wood Science Center, Teknikringen 42, SE-100 44 Stockholm, Sweden ‡ KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden § KTH Royal Institute of Technology, School of Chemical Science and Engineering, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: Adsorption of a single layer of molecules on a surface, or even a reorientation of already present molecules, can significantly affect the surface properties of a material. In this study, vibrational sum frequency spectroscopy (VSFS) has been used to study the change in molecular structure at the solid−air interface following thermal curing of polyelectrolyte multilayers of poly(allylamine hydrochloride) and poly(acrylic acid). Significant changes in the VSF spectra were observed after curing. These changes were accompanied by a distinct increase in the static water contact angle, showing how the properties of the layer-by-layer molecular structure are controlled not just by the polyelectrolyte in the outermost layer but ultimately by the orientation of the chemical constituents in the outermost layers.

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fibers modified with poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) turned hydrophobic when subjected to heat curing.7,8 Because similar results were obtained on PAH/PAA films deposited on SiO2 surfaces the mechanism was proposed to be a migration and a reconformation of hydrophobic groups within the multilayer film; however, so far, no direct measurements to test the proposed mechanism have been given due to technical limitations. It is well known that the contact angle at an interface is influenced by the outermost few nanometers of a material and thus conventional spectroscopic equipment lack the required surface sensitivity.9 In the present work, the interface-sensitive vibrational sum frequency spectroscopy (VSFS) technique, which commonly only probes the top few monolayers,10,11 was used to investigate the proposed molecular rearrangements at the solid−air interface following heat treatment of PAH/PAA. Because VSFS has the advantage of solely probing the surface of centrosymmetric media, in contrast with, for example, infrared and Raman spectroscopy, it allows studies of heat-induced molecular changes that can be directly correlated to changes in the contact angle. In a few previous studies, VSFS has been applied to study other types of

INTRODUCTION The layer-by-layer (LbL) technique1 to consecutively treat a solid substrate with oppositely charged polyelectrolytes or nanoparticles has over the last 20 years rapidly emerged as a new and effective way of engineering surfaces to achieve desired properties. One application is to influence the physical properties of fibrous networks,2 where the early focus was to improve these by increasing the adhesive strength between the individual fibers.3 It has been observed by use of dynamic contact-angle analysis that adsorption of LbLs onto cellulose fibers alters the wetting properties of the fibers and that the contact angle is strongly dependent on the type of polyelectrolytes used.4 Furthermore, the polyelectrolyte adsorbed in the outermost layer clearly influenced the wetting, an effect that has also been observed for LbL films on flat substrates.5,6 Observations have also been made that the surface properties of LbLs depend on the surrounding media; for example, an LbL film is less hydrophilic in air than in water. This has been explained to be a result of mobility of the components in the film where hydrophobic groups of the polyelectrolytes within the film can migrate to the surface in air while in water the surface is more hydrophilic.6 The proposed migration has, however, been difficult to detect other than by indirect methods due to the demands for measurement techniques with exceptional lateral resolution along the surface normal. In a recent study it was found that paper made from © 2015 American Chemical Society

Received: March 17, 2014 Revised: March 13, 2015 Published: April 10, 2015 4435

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Langmuir polyelectrolyte multilayers;12,13 however, no attempts have been made to correlate a macroscopic property such as the contact angle with a molecular reorientation in the surface region.

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(2) χ (2) = χNR +

(2)

n

where the sum is taken over the n different vibrations contributing to the spectrum. The different vibrations can interfere constructively or destructively with each other or with the nonresonant background, implying that the spectral features can appear more complicated than in linear spectroscopies such as IR and Raman. The second-order susceptibility is related to the orientational average of the molecular hyperpolarizability () according to eq 3.

EXPERIMENTAL SECTION

Materials. PAH (Mw = 15 000 g mol−1) and PAA (Mw = 8000 g mol−1) (Figure 1) were purchased from Sigma-Aldrich (Munich,

χR(2) =

N (2) ⟨β ⟩ ε0 R

(3)

where N is the number of contributing oscillators and ε0 is the dielectric permittivity. Thus, in centrosymmetric bulk media where the orientational average is zero, the sum frequency signal is zero. The hyperpolarizability depends on the IR (μc) and Raman (αab) transition moments (eq 4). ααβμγ β (2) ∝ ωn − ωIR − i Γn (4)

Figure 1. Repeating unit for (a) poly(allylamine hydrochloride) (PAH) and (b) poly(acrylic acid) (PAA). Germany) and used without further purification. Polished silicon wafers (P-doped with boron), with an oxide layer of 1.4 to 1.7 nm, were purchased from MEMC Electronic Materials (Novara, Italy). Microscopy slides (Kindler, Freiburg, Germany) and CaF2 windows (CeNing Optics, China) were used for IR and Raman spectroscopy. Sample Preparation. SiO2 substrates were cleaned with water and ethanol followed by plasma treatment (PDC-002 plasma cleaner, Harrick Scientific, Pleasantville, NY) for 2 min at 30 W. Glass slides and CaF2 windows were cleaned with Deconex and further cleaned in the same way as the SiO2 substrates. LbLs were formed through dipping substrates in 100 mg L−1 solutions of PAH and PAA with 10 mM of added NaCl and the pH adjusted to 7.5 and 3.5, respectively. The immersion time was 10 min, and each adsorption step was followed by 3 × 2 min rinsing in Milli-Q-purified water. The samples were gently blown dry with nitrogen gas after the final adsorption cycle. LbL films consisting of 2.5, 7.5, and 65.5 bilayers, that is, ending with PAH as outer layer, were prepared, and some samples were heattreated at 160 °C for 90 min. The samples were stored and heattreated in an argon atmosphere to minimize the risk of contamination. Films of the individual polyelectrolytes were prepared by spin-coating of 1 wt % solutions of PAH and PAA onto cleaned glass for 60 s at 2000 rpm using a KW-4A spin-coater (Chemat Technology, Northridge, CA). Vibrational Sum Frequency Spectroscopy. The VSF spectrometer used in this work consisted of an Ekspla (Vilnius, Lithuania) Nd:YAG laser (27 ps, 1064 nm, 20 Hz) and an OPG/OPA from Laservision (Bellevue, WA) and has been described in detail elsewhere.14 The energy of the infrared beam in the CH stretching region was ∼300 μJ. Two different polarization combinations of the beams were used, SSP and PPP, corresponding to the polarizations of the sum frequency, visible, and infrared beams, respectively. Here S stands for a polarization perpendicular to the plane of incidence, and P is when the beam is polarized parallel to this plane. The VSF spectra were collected in a N2 atmosphere in a Teflon chamber, which is described elsewhere.15 Theory VSFS. Vibrational sum frequency spectroscopy is a secondorder laser spectroscopy technique with an extreme surface specificity.10,11,16 If two centrosymmetric media (e.g., a solid and air) are in contact with each other, the sum frequency signal will only originate from their interface under the electric dipole approximation. The technique is applicable for all interfaces accessible by the laser beams, for example, the air−solid, liquid−solid, and liquid−liquid interfaces. The sum-frequency intensity, IVSF, is proportional to the incident electric fields, IIR and Ivis, as well as the effective second-order susceptibility, χ(2) eff , which depends on the Fresnel factors and the second-order susceptibility, χ(2), that contains information about the molecules residing at the surface, as shown in eq 1. (2) 2 IVSF ∝ |χeff | I visIIR

∑ χR,(2)n

where ωn and Γn denote resonance frequency and damping constant of the nth vibrational mode, respectively. Equation 4 further shows that when the laser frequency ωIR is in resonance with a vibrational frequency of the surface molecules, the VSF intensity is enhanced. The spectra have been fitted with five resonances according to eq 5, where Lorentzian line shapes of the resonances are assumed (2) IVSF(ωIR ) ∝ |χNR,eff +

(2) 2 | ∝ ∑ χR,eff n

(2) χNR,eff +

∑ n

An ωn − ωIR + i Γn

2

(5) where An is the amplitude of each vibration. IR and Raman Spectroscopy. FT-IR measurements were performed using a Nicolet iS10 FT-IR spectrometer in transmission mode. For the Raman spectroscopy, a Horiba HR800 instrument was used. The laser wavelength was 514 nm and an Olympus 50× objective was employed. Raman spectra were collected from three different spots on the surface and then averaged. CaF2 windows were used as substrates, and the same samples were analyzed using both techniques. Contact-Angle Measurements. Static water contact angles were measured in a room with a controlled climate of 23 °C and 50% RH using Milli-Q water in a CAM200 contact angle system (KSV Instruments, Helsinki, Finland). Drop sizes of 5 μL were used in all measurements. To allow for measurements at other RHs than 50%, the contact-angle meter was temporarily enclosed by a custom-built plastic cover and the RH was adjusted with an air flow that was either dry (15% RH) or moist (85% RH). The RH was monitored by a sensor placed near the drop dispensing needle. Four measurements were conducted for each surface, and the drop was allowed to equilibrate for 30 s before the angle was recorded. Atomic Force Microscopy. The surface morphology of the LbL films was imaged using a Nanoscope IIIa AFM (Veeco Instruments, now Bruker AXS, Santa Barbara, CA) with a type E piezoelectric scanner. Images were acquired in tapping mode in air using RTESP Si cantilevers (Bruker Probes, Camarillo, CA).

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RESULTS AND DISCUSSION Origin of Increased Hydrophobicity of PAH/PAA Multilayers upon Heating. For the VSFS measurements, 7.5 bilayers of PAH/PAA were assembled on glass. The use of glass, in contrast with silicon with a native oxide layer of 1 to 2 nm, hereafter referred to as SiO2, which is more commonly used for LbL studies, was motivated by the fact that VSFS is preferably carried out on transparent substrates to reduce the influence of the nonresonant background, which makes the interpretation of the spectra less straightforward.16 To correlate

(1)

The second-order susceptibility involves two terms, a nonresonant part (χ(2) NR) mainly originating from the substrate and a resonant part (χ(2) R ) originating from the surface molecules (eq 2) 4436

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For all individual samples, the intensity of the strongest peak in the region 2910−2940 cm−1 in SSP has been arbitrarily normalized to one. The intensity of the corresponding PPP spectrum is then normalized with a similar constant. Thus, the intensities in SSP and PPP for the same kind of sample can be compared directly, but the absolute intensities between different samples cannot be compared, only the relative SSP and PPP intensities. All spectra have been fitted according to eq 1 with five resonances. The VSF spectra for nonheated samples exhibit peaks at approximately 2855, 2875, 2915, 2935, and 2965 cm−1 under the SSP polarization and one peak at 2965 cm−1 in PPP (although there also is a broad band extending over the whole spectrum below ∼2930 cm−1 in PPP). Upon heat treatment the peak at ∼2915 cm−1 in SSP, only present as a shoulder for the nonheated sample, increased in intensity. Otherwise, the VSF spectra were quite similar. In the spectra of the individual nonheated polyelectrolytes, PAH and PAA exhibit peaks at approximately the same frequencies in the SSP polarization combination, centered at around the same frequencies as for the PAH/PAA multilayer; however, especially the region above 2900 cm−1 is difficult to fit because it contains several overlapping bands but only one clear peak. In the PPP polarization combination, the signal was weak and broad features were observed. PAH exhibits a peak at ∼2965 cm−1 and a broad band centered at ∼2890 cm−1, whereas PAA displays intensity in a large part of the spectrum with a maximum at ∼2965 cm−1. In particular, in the region 2910− 2970 cm−1 that contains overlapping bands, several fits with essentially similar errors can be obtained. Hence, the fitting parameters (i.e., peak center, bandwidth, and amplitude) should not be considered definitive. It is, however, apparent that the peaks in the CH-stretching region for the two individual polyelectrolytes appear at approximately the same frequencies, and thus these spectra cannot aid significantly in the spectral assignments. It is, however, to be noted that the bands in the region 2850−2880 cm−1 are relatively more intense than the region ∼2940 cm−1 for PAH than for PAA. The fact that the SSP spectrum of the PAH/PAA film shows an intensity ratio between the (2850−2880)/2940 cm−1 regions, that is, between the ratio in the spectra of the individual polyelectrolyte films, thus qualitatively indicates that both PAH and PAA are present in the surface region. The IR and Raman spectra (Figure 4), which are non-surface-sensitive, are identical for the heattreated and non-heat-treated samples, and thus the VSF spectra of the heat-treated samples do not reveal any molecular changes in the bulk for the CH and CH2 groups.

the change in wetting behavior to the composition of molecules at the air−solid interface, we performed contact-angle measurements on the same substrates and in connection to the VSFS measurement, which was performed the day after the LbL deposition. The samples were stored in argon between measurements to protect them from contamination. The contact-angle measurements (Table 1) show that a heat treatment for 90 min at 160 °C promoted a decrease in Table 1. Contact Angles Measured on (PAH/PAA)7.5 Adsorbed onto SiO2 and Glass Substratesa PAH/PAA7.5 on SiO2 PAH/PAA7.5 on glass a

no heat treatment

heat treated

47.9 ± 0.6 39.4 ± 0.7

69.8 ± 0.4 64.6 ± 0.6

Values are given with 95% confidence limits.

hydrophilicity for (PAH/PAA)7.5. This is in accordance with previous results for (PAH/PAA)2.5 assembled on SiO2 as well as on cellulose model surfaces and cellulose fibers.7,8 The contact angles measured on SiO2 and glass substrates were similar and were also in good agreement with those measured for (PAH/PAA)2.5 films, indicating a limited influence of film thickness on the contact angle. VSF spectra were collected to probe the very top surface of the multilayer of nonheated and heat-treated films of (PAH/ PAA)7.5 (Figure 2). Spectra were also collected for nonheated

Figure 2. VSF spectra of nonheated and heated (PAH/PAA)7.5 under SSP and PPP polarizations.

spin-coated films of homogeneous PAH and PAA to assist the spectral assignment (Figure 3). FTIR and Raman measurements were performed as a complement to probe possible bulk changes in the multilayer upon heat treatment (Figure 4).

Figure 3. VSF spectra of homogeneous spin-coated films of PAH and PAA in (a) SSP polarization and (b) PPP polarization. 4437

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Figure 4. (a) IR and (b) Raman spectra showing the absence of bulk effect from heat treatment of (PAH/PAA)7.5.

2940 cm−1 originate from CH groups significantly tilted from the surface normal or from the symmetric stretch of CH2 groups, whereas the peak at 2965 cm−1, which is more pronounced in the PPP spectrum, originates from an antisymmetric CH2 stretching vibration.11 Multiple CH2 peaks have previously been observed when the methylene group is situated close to polar groups.22 This can be the reason why PAA, which only has one type of CH2 group, has more vibrational bands than expected for one sole methylene group. Peaks at 2915−2940 cm−1 SSP are most likely attributed to CH2 symmetric stretches and their Fermi resonances,11,23 as CH bands have not been seen in this frequency range. However, the polarization rules also comply with the peaks at 2875, 2935, and 2965 cm−1 originating from chain-end methyl groups,11 which have previously been reported in VSF spectra of polymer films.24 Nevertheless, the fact that all peaks (especially those at 2875, 2935, and 2965 cm−1) in the VSF spectra within a few wavenumbers have corresponding counterparts in the IR and Raman spectra (Figure 4) indicates that the features in the VSF spectra indeed originate from CH and CH2 groups because the number density of chain end methyl groups is too low to result in an IR or Raman signal. The fact that VSF spectra of substantially thicker (PAH/ PAA)65.5 films (Figure S1 in the Supporting Information) were similar to those from (PAH/PAA)7.5 support the fact that the polymer−air interface contributes strongly to the VSF signal. Thus, considering the method-specific demands for a mode to be VSFS active, it is evident that the detected CH and CH2 groups form an ordered structure and reside in an environment where the centrosymmetry is broken for both the nonheated and heated sample.16 Taking the correlation between the increased contact angles (Table 1) and the change in the interface-sensitive VSF spectra upon heat treatment, it is suggested that the heat treatment induces reorientation of the polymer chains to expose more hydrophobic groups at the polymer−air interface, as manifested by the increase in intensity of the band at 2915 cm−1 in SSP. The intensity increase is suggested to originate from a surface enrichment of the functional group (likely CH2), giving rise to the peak at 2915 cm−1 and possibly an enhanced alignment along the surface normal because dipole transitions perpendicular to the surface are probed in SSP. A similar mechanism of more hydrophobic groups migrating to the surface has also been suggested by Feng et al.25 for nanofibers of poly(vinyl alcohol), which are highly hydrophobic as opposed to a smooth film of the same material. In that study, angle-resolved XPS was used for analysis of the surfaces, and an enrichment of hydrophobic CH2 groups was observed at the solid−air interface as compared with the

Figure 5 displays SSP spectra acquired in the OH stretching region (3000−3800 cm−1) to study the presence of water in the

Figure 5. VSF spectra showing the OH region for nonheated spin coated films of PAH and PAA as well as nonheated and heat-treated (PAH/PAA)7.5..

surface region. No internal reference exists between the different samples, and hence the absolute intensities should not be compared. All spectra show indications of water on the surface, but the low intensity (and thus the low S/N ratio) indicates that the amount of water is low or that the water molecules are fairly disordered or oriented in such a way that the transition moments mostly are not aligned along the surface normal. The presence of some water is not surprising because both polyelectrolytes contain hydrophilic groups that are able to form hydrogen bonds with water molecules. The broad water bands are centered at 3300−3400 cm−1, predominately indicating a medium strength of the hydrogen bonds of the water molecules in the surface region. To the best of our knowledge there are no clear assignments for the bands in the CH/CH2 stretching region for either PAH or PAA. Although several assignments are found in the literature, they are not concordant, assumingly partly due to different preparation methods; however, a general agreement is that the bands between ∼2850 and ∼2990 cm−1 originate from CH and CH2 vibrations in PAH and PAA. Therefore, only tentative assignments are made, based on the spectra in Figures 2 and 3, and previous studies.17−21 The so-called VSFS polarization rules can be applied to aid in the spectral assignments.11 These rules give indications on whether a vibration has a symmetric or antisymmetric character based on a comparison of the intensity of a certain peak in different polarization combinations. By utilizing this for the intensities of the peaks in the SSP and PPP spectra, the conclusion can be drawn that the peaks in the range 2850− 4438

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Langmuir bulk material. VSFS studies on polymer films have shown that the hydrophobic parts of a polymer prefer to reside at the polymer−air surface,24,26,27 and the phenomenon of hydrophobic parts of polymers migrating to the polymer−air interface has been observed for various polymer films.19,24,26,28−31 The present work is, however, to the best of our knowledge, the first observation of these phenomena for LbL films. Another possible explanation for the increased contact angle resulting after heat curing is a change in the surface topography. To explore this possibility, we used high-resolution AFM to capture images of (PAH/PAA)7.5 films on SiO2. The images in Figure 6 display cluster-like features both before and after heat

change in chemical composition at the interface, demonstrated by VSFS, combined with these inherent surface structures on both fiber and paper level, is enough to create a hydrophobic surface. Effect of Time, Humidity, and Rewetting on the Wetting of PAH/PAA Multilayers. The reference contact angles for nonheated samples in Table 1 were measured 24 h after the LbL assembly to make the correlation to the VSFS measurements as accurate as possible; however, in previous work for cellulose fibers,8 it was noted that aging could result in hydrophobization similar to the that achieved by heat treatment. Contact angles were therefore measured on freshly made (PAH/PAA)7.5 as well as samples aged for 24 h and 60 days. Figure 7 shows a clear increase from 10 to 40° in contact

Figure 6. Tapping mode AFM images (5 × 5 μm) of (PAH/PAA)7.5 on SiO2: (left) nonheated and (right) subjected to 160 °C for 90 min.

treatment of the film, and no significant topographical changes can be identified as a result of the heat treatment. This observation is also supported by the root-mean-square roughness of the films, the values being 16 and 12 nm, respectively, for the nonheated and heated film. The VSFS results presented herein provide a plausible mechanism for the heat-induced increase in the contact angle of PAH/PAA adsorbed on flat SiO2. In previous work we have shown that heat treatment of paper sheets made from PAH/ PAA-modified cellulose fibers resulted in a drastic shift in the wetting properties, from rapidly absorbing to highly hydrophobic,8 and the discrepancy between the wetting of paper and a flat model surface is yet to be explained. Zhai et al.32 have reported an increase in advancing contact angle from 60 to 115° as a result of heat treatment of a microstructured LbL film of PAH/PAA. Prior to the heat treatment the smooth film was treated with acid to create a porous and rough surface (surface roughness >400 nm with an average pore size of 10 μm). This suggests that heat-induced reconformation32 in combination with the rough surface, with surface features and roughness bearing large similarities to that of a fiber network in a paper sheet, resulted in a hydrophobic film made from the same polyelectrolytes that are used in the present work. These observations are contradictory to the general view that hydrophobicity is only improved by microstructures if the contact angle of the flat substrate is >90°; however, a growing number of publications suggests that by introducing surface features of certain geometries highly hydrophobic surfaces can be obtained for previously slightly hydrophilic substrate.25,33−35 These claims are further supported by theoretical work by Liu et al.,36 who conclude that certain surface geometries, including mushroom-like microstructures and hierarchical micro- and nanostructures, may enable the creation of highly hydrophobic surfaces from rather hydrophilic substrates. If these theories can be applied on cellulose fiber networks, which indeed contain structures similar to the previously mentioned structures, the

Figure 7. Contact angle as a function of dwell time between LbL assembly and contact-angle measurement for nonheated (PAH/ PAA)7.5 on glass substrate. Samples were stored in at 23 °C and 50% RH. The contact angle for a heat-treated (HT) sample is added for comparison.

angle during the first 24 h, suggesting a rather rapid change of surface properties when the LbL film is removed from aqueous environment and incubated in the comparably hydrophobic air. Further aging for 60 days increases the contact angle to 50°. The decreased hydrophilicity that is seen after heat treatment can, however, not be reached in room temperature, at least not within the studied time frame. In addition to the herein proposed reorientation of polymer chains due to increased mobility during the heat treatment, evaporation of water from the LbL film is another possible explanation for the decreased hydrophilicity. The influence of the surrounding relative humidity, which will affect the water content in the LbL film37,38 was evaluated by measuring the contact angle under dry conditions (15% RH) and humid conditions (85% RH). Samples were allowed to equilibrate for at least 10 min prior to measurement. Figure 8 shows contact angles for nonheated, same-day (PAH/PAA)7.5 and heattreated (PAH/PAA)7.5, both on glass substrates. For the nonheated sample there is a slight increase in contact angle when the RH is lowered to 15%. This could be interpreted as an effect of evaporation of water from the newly made film, which, even though it has been dried with N2 gas, still might contain more water than a film that has been aged longer. Evaporation of bound water together with a reorientation of CH or CH2 groups, as suggested from the VSFS measurements, would then explain the large increase in contact angle during the first 24 h, where the contact angle increased from 10 to 40°. For heat-treated samples, the static contact angle is unaffected 4439

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does not provide enough energy and mobility to overcome the energy barrier to again reorientate the polymer chains to create a more hydrophilic interface. VSF spectra in the SSP polarization of a heat-treated (PAH/PAA)7.5 film after 24 h rewetting followed by drying with N2 (Figure 10) still have the

Figure 8. Contact angle shortly after LBL assembly as a function of surrounding RH for nonheated (PAH/PAA)7.5 and heat-treated (HT) (PAH/PAA)7.5.

by the surrounding RH in the range 15−85% RH, suggesting that the increased contact angle after heat treatment is not solely an effect of exclusion of water from the LbL film because exposure to 85% RH does not reduce the contact angle. Additionally, VSF spectra in the OH stretching region show very small signals for surface water molecules, as shown in Figure 5, for neither nonheated nor heated samples, which further strengthens the conclusion that bound water has little influence on the measured contact angle, at least not after 24 h. Heat-treated (PAH/PAA) 7.5 on glass substrate were immersed into Milli-Q water at room temperature to investigate the reversibility of the decrease in hydrophilicity and thereby further elucidate the mechanism. Samples were submerged for 15 min, 24 h, and 72 h and were subsequently dried with a stream of N2, after which static contact angle was measured. Figure 9 shows that immersion of heat-treated samples in water resulted in a lowering of the contact angle and thus a

Figure 10. VSF spectra under SSP polarization showing the effect of 24 h rewetting of heat treated (PAH/PAA)7.5.

peak around 2915 cm−1, characteristic for the heat-treated film and attributed to more hydrophobic CH2, but it is broadened and partly also has the characteristics of the spectra for the nonheated film. This is in accordance with the partly decreasing contact angle after rewetting and suggests that the reorientation of polymer chains at the interface is partly nonreversible at least on the time scales and temperatures studied in this work. It should, however, be noted that the rewetting process appeared to be very sensitive to the conditions and the spectral features of the bands in the region 2910−2940 cm−1 as well as the relative intensity between the band below and above 2900 cm−1 could change between different samples. They did, however, always look like a mixture between the heated and nonheated SSP spectra in this region. There was no major change in the PPP polarization due to the rewetting (Figure S2 in the SI), which was an expected result because no change was noted in PPP in Figure 2 due to heat treatment. Also, the OH stretching region was largely unaffected by the rewetting (Figure S3 in the SI), indicating that the nitrogen-dried film did not contain more bound water in the surface region after the rewetting. It is known that a heat treatment of PAH/PAA films can create amide cross-links between the two polymers.39−42 Even a low degree of cross-linking within the outermost layers of the reoriented, more hydrophobic PAH/PAA film would significantly affect the chain mobility and thus create a relatively stable interface. VSF spectra were collected in the 1200−1800 cm−1 range, and no peaks attributed to amide bonds (∼1540 and 1670 cm−1) were seen (Figure S4 in the SI); however, the absence of amide bands does not exclude their presence, as the amide groups have to have a net orientation not in the surface plane and reside in a noncentrosymmetric environment for them to be able to give a sum frequency signal. The only band observed in this spectral region is the symmetric COO− stretching vibration at 1415 cm−1.21

Figure 9. Contact angle as a function of post heat-treatment immersion time in H2O for heat-treated (PAH/PAA)7.5 on glass substrate. The samples were dried with N2 gas prior to the contactangle measurement.

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more hydrophilic surface. The effect was, however, limited, and soaking in water for 72 h at room temperature decreased the contact angle by a mere 15 to ∼55°, which is significantly above the 40° contact angle of a nonheated surface stored for 24 h. This result implies that heat treatment at 160 °C induces a conformational change at the polymer−air interface that is largely nonreversible; that is, rewetting at room temperature

CONCLUSIONS VSFS was used to probe heat-treatment-induced changes at the polymer−air interface of (PAH/PAA)7.5 films on glass substrates. The results showed an increased CH2 signal after heat treatment, indicating reorientation and migration of polymer chains to minimize the surface energy. This was also 4440

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Langmuir

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supported by contact-angle measurements that showed an increase in contact angle due to heat treatment and thus a less hydrophilic surface. The effect was stable and could only be partially reversed by reimmersion of heat-treated samples in water. The study highlights the applicability of VSFS for fundamental work in surface science and demonstrates a powerful tool for the understanding of molecular mechanisms involved in the changes of the interfacial properties of LbL thin films. These changes may also be of great importance for the adhesive properties of LbL-modified surfaces. The study furthermore demonstrates how important the outermost surface layer is for the macroscopic wetting properties of a material.

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ASSOCIATED CONTENT

S Supporting Information *

Additional VSF spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46-(0)8-7909911. E-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.G. and L.W. acknowledge the financial support from the Wallenberg Wood Science Center (wwsc.se). P.A.L. thanks the BiMaC Innovation Excellence Centre and the Swedish Governmental Agency for Innovation Systems (VINNOVA) for funding. C.M.J. acknowledges the Swedish Center for Biomimetic Fiber Engineering and the Swedish Foundation for Strategic Research (SSF) for financial support through an Ingvar Carlsson grant.

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REFERENCES

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DOI: 10.1021/la5046207 Langmuir 2015, 31, 4435−4442