Aggregation States of Polystyrene at Nonsolvent Interfaces - American

May 15, 2014 - Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan. ABSTRACT: The aggregation states of polystyrene (PS) thin films at interfaces...
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Aggregation States of Polystyrene at Nonsolvent Interfaces Ayanobu Horinouchi,† Norifumi L. Yamada,§ and Keiji Tanaka*,†,‡ †

Department of Applied Chemistry and ‡International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Neutron Science Division, Institute for Materials Structure Science, High Energy Acceleration Research Organization, 203-1 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan ABSTRACT: The aggregation states of polystyrene (PS) thin films at interfaces with nonsolvents such as water, methanol, and hexane were examined by specular neutron reflectivity and sum-frequency generation vibrational spectroscopy. The density profiles of the PS thin films along the direction normal to the interface with water and methanol were comparable to that in air. However, this was not the case for the film in hexane exhibiting a diffuse interfacial layer due to swelling. Also, the local conformation of PS in the outermost region of the films was quite sensitive to the surrounding environment and consequently responded to a change in its environment. This was the case for typical nonsolvents such as water and methanol. The extent of the conformational change might be explained in terms of the interfacial energy.

1. INTRODUCTION Many polymers have found a wide range of applications as functional materials in the presence of a liquid such as some used as solid electrolytes and separator films for cells,1,2 liquid filtration membranes,3,4 biochips for tailor-made diagnosis,5,6 contact lenses,7,8 and so forth. In these cases, the liquid serves as a nonsolvent for the polymer for the obvious reason of not dissolving the polymer. For a better understanding of how the design and construction of these highly functionalized materials could be improved, the aggregation states and physical properties of such polymers at the nonsolvent interfaces should be systematically studied. We have been conducting research aimed at understanding the local conformation and the hierarchical dynamics of polymer chains at solid/liquid interfaces on the molecular level. As a result, the water-induced conformational change of poly(methyl methacrylate) (PMMA) at the outermost surface was revealed by sum-frequency generation (SFG) vibrational spectroscopy.9−11 PMMA is a material of interest for us because of its good compatibility to human tissue, leading to a diverse bioapplications such as in lenses, dentures, bone cement, and so forth.12−14 Such conformational changes have also been confirmed in other poly(n-alkyl methacrylate)s as well.15,16 This is simply because hydrophilic carbonyl groups in the side chains are segregated or oriented toward the water phase to form hydrogen bonds with water molecules. The impact of hydrophilic carbonyl groups on the polymer interface with water could be clearly seen in comparison with poly(methyl 2propenyl ether) (PMPE), which is structurally similar to PMMA except for lacking a carbonyl group.17 Polystyrene (PS) is a typical hydrophobic polymer as it simply does not contain any polar functional groups in its structure. Because of this, the chemical interaction between PS and a polar liquid such as water is sure to be much weaker © 2014 American Chemical Society

compared to that involving a polar liquid and any poly(n-alkyl methacrylate)s. Therefore, investigating how PS chains will behave upon contact with a nonsolvent can significantly contribute to the concepts that we have established so far in understanding polymer/nonsolvent interfaces. In this study, the density profiles of a PS film in water, methanol, and hexane are examined by neutron reflectivity (NR) to confirm whether PS in the outermost region in the film is changed upon contact with the liquids. Then, the local conformation of PS at the liquid interfaces is determined by SFG vibrational spectroscopy. Our results indicate that the PS surface can be reorganized at a molecular level upon contact with a liquid even if the liquid does not form strong favorable interactions with PS.

2. EXPERIMENTAL SECTION 2.1. Neutron Reflectivity (NR) Measurement. As a sample, perdeuterated PS (dPS) with a number-average molecular weight (Mn) of 317K and a molecular weight distribution (Mw/Mn), where Mw is a weight-average molecular weight, of 1.05 was purchased from Polymer Source Inc. dPS was here used instead of the conventional (protonated) PS to confer a contrast at the interfaces with liquids. The bulk glass-transition temperature (Tg) of the dPS found by differential scanning calorimetry (DSC6220, SII Nanotechnology Inc.) is 376 K. A dPS film with a thickness of approximately 60 nm in a dried state was prepared from a toluene solution on a quartz block with a size of 60 × 60 × 8 mm3 by a spin-coating method. The film was annealed under vacuum at 393 K for 24 h. Water, methanol, and hexane, which are typical nonsolvents for PS, were used. Prior to the measurements, the dPS film was aged in these respective liquids for 2 h, which was apparently enough to cause a structural change in the Received: March 2, 2014 Revised: May 14, 2014 Published: May 15, 2014 6565

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film. The density profiles of the dPS film in contact with the liquids were examined by NR measurements using horizontal-type neutron reflectometer SOFIA at a materials and life science experimental facility, J-PARC.18,19 All measurements were made using a single dPS film. Once the measurement in a liquid was completed, the film was taken away from the setup. Then, it was well-dried and again annealed under vacuum for 12 h at 393 K. The liquids were used in the following order: water, methanol, and hexane. The reflectivity was also calculated on the basis of the scattering length density (b/V) profile along the depth direction using Parratt32 software, a freeware program from the Hahn-Meitner Institute.20 The (b/V) values of SiO2, dPS, water, methanol, and hexane used for the calculations were 3.48 × 10−4, 6.22 × 10−4, −5.61 × 10−5, −3.74 × 10−5, and −5.75 × 10−5 nm−2, respectively. 2.2. Sum-Frequency Generation (SFG) Vibrational Spectroscopy. As a sample, PS with Mn = 62k and Mw/Mn = 1.08 was used. Since the initiator fragments and end groups of PS were observed in SFG spectra,21 PS was synthesized by anionic polymerization using potassium naphthalenide and deuterated methanol as an initiator and a terminator, respectively. The Tg of the PS by DSC is 374 K. PS films were spin-coated onto quartz prisms from a toluene solution and were annealed under vacuum at 393 K for 24 h. The thickness of the films determined by ellipsometry was approximately 200 nm. As nonsolvents, deuterium oxide (D2O), deuterated methanol (d4-methanol), and deuterated hexane (d14-hexane) were used instead of water, methanol, and hexane to avoid peak overlapping of the vibrational mode of PS with those of the liquids. The SFG spectra were collected at a visible wavelength of 532 nm with tunable infrared beams traveling through the prism and polymer film and overlapping at the air and liquid interfaces on the polymer films as described in a previous report.9,21 The visible beam with a wavelength of 532 nm was generated by frequency doubling the fundamental output pulses from a picosecond Nd:YAG laser. The IR beam, which is tunable over the wavenumber range from 1000 to 4300 cm−1, was generated from an EKSPLA optical parametric generation/amplification and difference frequency generation (OPG/OPA/DFG) system based on LBO and AgGaS2 crystals. The measurements were carried out with an ssp (SF output, visible input, and infrared input) polarization combination.

such as t and σ representing the total thickness of the film and Gaussian roughness to express the interfacial width, respectively, are shown in Table 1. When the calculated NR curves are Table 1. Parameter Used to Fit the Experimental Reflectivities Shown in Figure 1a environment

t/nm

t/tair

(b/V) × 104/ nm−2

air water methanol hexane

58.8 58.8 59.2 70.6

1.00 1.00 1.01 1.20

6.22 6.22 6.22 5.26

σ/nm 0.2 0.4 0.4 1.4

liquid content (bulk)/% 0 0 14.2

χ2 7.9 5.8 8.3 8.8

× × × ×

10−3 10−3 10−3 10−3

in good accordance with the experimental one, it can be claimed that the (b/V) profiles shown in panel (b) well reflect the density profiles of the dPS film along the direction normal to the interface. The (b/V) profiles of the dPS film in water looked similar to that of the dPS film in methanol, suggesting that the aggregation states of the PS film are comparable in both liquids. The interface of the film with water was also in good accordance with the data by Seo and Satija.22 On the other hand, in hexane, the (b/V) value in the internal region of the dPS film was much lower than those in the two previous cases. This result makes it clear that hexane molecules penetrated deeply into the film. This is the reason that the film became thicker. That is, the dPS film was significantly swollen by hexane. The interfacial width between dPS and hexane was much broader than the dPS interfaces with the other two liquids. This interfacial broadening induced by nonsolvents was also observed in the system of PMMA/alcohols.23 We then discuss the local conformation of PS at the liquid interfaces using SFG spectroscopy, which has the best depth resolution among available techniques.24,25 Figure 2(a) shows

3. RESULTS AND DISCUSSION Panel (a) of Figure 1 shows the scattering vector, q [= (4π/λ) sin θ], dependence of NR for a dPS film in contact with air,

Figure 1. (a) Neutron reflectivity for a dPS film in air, water, methanol, and hexane. Open symbols depict experimental data, and solid lines represent the reflectivity calculated on the basis of the scattering length density (b/V) profiles shown in (b). For clarity, the values in panel (a) are offset from one another (on the y axis, reflectivity) by a factor of 10.

Figure 2. (a) SFG spectra for PS at the air, D2O, d4-methanol, and d14hexane interfaces with the ssp polarization combination. (b) SFG intensity ratio of ν20b and ν2 stretching modes as a function of ϕ calculated on the basis of eq 1

the SFG spectra for PS at air, D2O, d4-methanol, and d14-hexane interfaces with the ssp polarization combination. On the basis of our previous study using deuterated PS (d8-PS) with a sec-butyl end group and d8-PS with an n-butyl end group,21 the SFG peaks around 2847 and 2921 cm−1 are assignable to symmetric and antisymmetric C−H stretching vibrations of methylene (CH2 s and CH2 as) groups. The peaks appearing at 2902 cm−1, which has not been assigned, seem to be from C−H stretching vibrations of methyne (CH) groups.26 In addition,

water, methanol, and hexane. For clarity, each data set for the dPS film in liquids was intentionally offset by a decade. The open symbols and solid curves denote experimental data and calculated reflectivities based on the model (b/V) profiles shown in panel (b) of Figure 1, respectively. Only in the case of the measurement in air were neutron beams guided into the sample from the air side. The parameters used for the fitting 6566

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Figure 3 shows the SFG spectrum for water on the PS film with the ssp polarization combination. For comparison, the

typical peaks originating from phenyl groups were observed in the wavenumber region of 3000 to 3100 cm−1.27,28 Recently, Patterson’s group has revisited the assignment of SFG peaks originating from phenyl rings and reported that the vibrational modes except for ν20b, ν2, and ν7a would not appear in the SFG spectra.29,30 In our spectra, the peaks around 3020 and 3060 cm−1, which are assignable to the ν20b and ν2 modes, were clearly observed. Although the backbone of PS is composed only of hydrophobic groups, the SFG spectrum changed its shape upon contact with D2O. This spectrum change was also observed in the case of d4-methanol. This implies that the local conformation of PS at the outermost region in the film changed to a more stable conformation in contact with other phases (such a liquid) as long as the chain mobility permits. The tilt angle of phenyl rings relative to the interface normal (ϕ) can be discussed on the basis of the intensity ratio of the ν20b to ν2 modes, the most dominant peaks in our spectra.29−32 ⎛β ⎞ ⎞ Α ν20b 2(⟨cos 3ϕ⟩ − ⟨cos ϕ⟩) caa, ν 20b ⎛ ⎟⎜ = ⎜⎜ ⎟ ⎟ Αν2 ⎝ βaac, ν2 ⎠⎝ (7 + 2r )⟨cos ϕ⟩ + (1 − 2r )⟨cos 3ϕ⟩ ⎠

Figure 3. SFG spectra for water on PS and quartz in the O−H vibrational region with the ssp polarization combination. (1)

result for water on the quartz substrate, which was pretreated with a piranha solution, is also presented in Figure 3. In the spectrum acquired for the PS film, a clear peak and a shoulder were observed at around 3150 and 3450 cm−1, respectively. The former can be assigned to the O−H vibrational mode of water molecules, which interact with one another via strong hydrogen bonding. Although the name of this peak is controversial,36 here we call this icelike water as commonly used.37−39 The latter is assignable to the O−H vibration of ordered water molecules perhaps induced by an attractive interaction with π electrons of the phenyl rings of PS. Such a peak at around 3400−3600 cm−1 was also observed on PMMA,12 poly(2-hydroxyethyl acrylate),40 and poly(vinyl alcohol).41 These polymers possess functional groups which can attractively interact with water. In the case of water on the hydrophilic quartz, this can be clearly seen. Intense peaks corresponding to the icelike and ordered water were observed around 3150 and 3450 cm−1. In particular, the latter is probably due to the O−H vibration of water hydrogen bonded to the quartz surface. Taking into account the above discussion, it is plausible that the π electrons of the PS phenyl rings interact with water molecules at the interface. Panel (a) of Figure 4 shows SFG spectra for methanol on PS and quartz in the O−H vibrational region with the ssp polarization combination. In the case of methanol on the PS film, no other siginificant peak except for the peaks due to the phenyl groups (ν20b and ν2) were observed. This indicates that hydroxyl groups of methanol could be hardly aligned at the

Here, Aν2 and Aν20b represent the amplitudes of the peaks for the ν2 and ν20b modes, respectively. βaac,ν2 and βcaa,ν20b are their respective hyperpolarizabilities. The r value, 1.13, is given by the βccc/βaac ratio for the ν2 mode.28 Unfortunately, there is no information regarding the βaac,ν2 and βcaa,ν20b values. However, the tilt angle of phenyl rings can be somehow compared among the different interfaces by observing the trend in the peak amplitudes. Panel (b) of Figure 2 shows the SFG intensity ratio of the ν2 to ν20b modes as a function of ϕ calculated on the basis of eq 1, provided that the ratio of βcaa,ν20b to βaac,ν2 is assumed to be 1.0. Figure 2(b) makes it clear that the intensity ratio of ν20b to ν2 increases with increasing ϕ. The Aν20b/Aν2 values from the SFG spectra at the air, D2O, d4-methanol, and d14-hexane interfaces are 0.92, 0.69, 0.82, and 0.43, respectively. On the basis of these values, it seems most likely that the phenyl rings are the most tilted at the air interface and that the extent follows the order of d4-methanol, D2O, and d14-hexane interfaces. Invoking the fact that the change in ϕ is mainly enthalpy-driven, the interaction between the phenyl rings and the liquid should be crucial. Taking into account that the surface free energies of air, water, methanol, and hexane are 0, 72.8, 22.3, and 18.4 mJ·m−2,33 respectively, it is plausible that the tilt angle of phenyl rings is correlated to the interfacial energy except in the case of the hexane interface. To conclude, the PS interface with other nonsolvents should be made. For the moment, the reason that the hexane case is ruled out of the correlation has yet to be established. However, since the interface was diffuse and the inside of the film was swollen, as shown in panel (b) of Figure 1, the situation may not be simply compared to others. The surface free energy of a phenyl ring is larger than those of methylene and methyne groups of the main chain.34 Hence, it seems reasonable that the orientation of phenyl rings toward a liquid with a higher surface free energy is effective at minimizing the interfacial free energy. Also, the interfacial orientation of the phenyl rings would be better for the interaction of the π electrons with hydroxyl groups of water and methanol via weak hydrogen bonding.35 To discuss and establish the relationship of hydrogen bonding to our observations, SFG measurements for water and methanol on PS films were carried out in the O−H vibrational region.

Figure 4. SFG spectra for PS and quartz at the methanol interface in the (a) O−H and (b) C−H vibrational regions with the ssp polarization combination. 6567

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Figure 5. Time dependence of the intensity of the SFG signal at 3060 cm−1 before and after contact with (a) D2O, (b) d4-methanol, and (c) d14hexane. For the purpose of direct comparison, each data point was normalized by the average value of the signal intensity at the air interface in the time range from 0 to 60 s. The dashed line is ideal value estimated by eqs 2−5.

before and after contact with (a) D2O, (b) d4-methanol, and (c) d14-hexane. For comparison, each data set was normalized by the average value of the signal intensities at the air interface. In all cases, the signal intensities drastically decreased right after introducing a liquid onto the film surface. Here, a decrease in signal intensity could signify a decrease in the number of oriented phenyl rings; however, full consideration must be given to the optical nature of the interface, which requires us to discuss the Fresnel factors.9,41 Even if the number of phenyl rings oriented remains constant, the signal intensity changes at each interface. Thus, it is necessary to predict to what extent the signal intensity changes at each liquid interface. The signal intensity with the ssp polarization combination is proportional to the square of the absolute value of the effective sum frequency susceptibility tensor of the surface (χ(2) eff,ssp), which is given as follows

interface. Given that the hydroxyl groups of methanol interact with the π electrons of PS phenyl rings, it seems that the interaction is not sufficiently strong enough to orient the hydroxyl groups at the PS interface. On the other hand, for methanol on the quartz, in which the strong interaction existed, a clear peak appeared at around 3150 cm−1. This means that the hydroxyl groups of methanol face the quartz surface with some extent of orientation. To discuss the orientation of methanol at the PS interface, SFG measurements were also performed in the C−H vibrational region. Panel (b) of Figure 4 shows the SFG spectrum acquired at the methanol/PS interface in the C−H vibrational region with the ssp polarization combination. For comparison, the data at the methanol/quartz interface is also presented. Four peaks were observed for the methanol/PS interface. Two peaks observed were assigned to the ν20b and ν2 modes of PS phenyl rings, as already discussed. Since the peaks around 2840 and 2951 cm−1 were not observed using d4-methanol, it is clear that they originated from methanol at the PS interface. The former and latter are due to the C−H symmetric stretching vibration of methyl groups42 and the Fermi resonance of the vibration with the overtone of the methyl bending mode.42 SFG with the ssp polarization combination enables us to gain access to the dipole, or the functional group, along the perpendicular direction. Hence, the peak at around 2840 cm−1 reveals that the methyl groups of methanol are oriented along the direction normal to the interface. On the contrary, for the methanol/quartz interface, there were no clear peaks derived from the methyl groups of methanol. Although the baseline of the spectrum gradually increased with increasing wavenumber, it corresponds to the left shoulder of the peak arising from hydroxyl groups. This indicates that the methyl groups of methanol can hardly align on the quartz surface. Thus, it can be claimed that the chemical nature of a solid surface changes the aggregation states of a liquid at the interface. That is, the kind of liquid in contact with a polymer alters the aggregation states of the polymer at the interface, and at the same time, the kind of solid in contact with a liquid alters the aggregation states of the liquid at the interface. To what extent the phenyl rings of PS oriented at each liquid interface is discussed here. Our experimental design to realizing this is to pursue the signal intensity as a function of time after introducing a liquid onto the PS surface. Figure 5 shows the time dependence of the SFG signal intensities at 3060 cm−1

(2) χeff,ssp = Lyy(ωSF) Lyy(ωvis) Lzz(ωIR ) sin θIR χyyz

(2)

where Lii(i = x, y, z) and θIR are Fresnel coefficients at frequency ω (SF, SFG; vis, visible beam; IR, infrared beam) and the incident angle of the IR beam. Also, χyyz is the secondorder nonlinear susceptibility tensor in the laboratory coordinate system, which depends on the vibrational mode of the functional group.9,43 Since the SFG signal is detected here at a given wavenumber, the χyyz value can be assumed to be constant. Thus, the calculation of only Lii is needed as follows Lxx(ω) =

2n1(ω) cos θ1 n2(ω) cos θ1 + n1(ω) cos θ2 ×

Lyy(ω) =

(3)

2n1(ω) cos θ1 n1(ω) cos θ1 + n2(ω) cos θ2 ×

6568

cos θ3 2n2(ω) cos θ2 × n3(ω) cos θ2 + n2(ω) cos θ3 cos θ1

2n2(ω) cos θ2 n2(ω) cos θ2 + n3(ω) cos θ3

(4)

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on Innovative Area “New Polymeric Materials Based on Element-Blocks” (no. 25102535) program and a Grant-in-Aid for Scientific Research (B) (no. 24350061). The neutron reflectivity experiment was approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (proposal no. 2009S08).

2n1(ω) cos θ1 n2(ω) cos θ1 + n1(ω) cos θ2 ×

2n2(ω) cos θ2 n × 1 n3(ω) cos θ2 + n2(ω) cos θ3 n3

⎛ n (ω) ⎞2 ×⎜ 3 ⎟ ⎝ n′(ω) ⎠

■

(5)

where ni and θi (i = 1, 2, 3) are the refractive index of medium i and the incident angle in medium i with respect to the normal. Subscripts 1, 2, and 3 correspond to layers 1 (quartz), 2 (PS), and 3 (a liquid), respectively. The refractive index of the PS/ liquid interfacial layer is taken as n′. The n values of quartz, PS, air, D2O, d4-methanol, and d14-hexane used in the calculation were 1.46, 1.59, 1, 1.33, 1.32, and 1.37, respectively.9,44−46 Also, the values of n′, which are unknown, were simply given by averaging the refractive indices between PS and a liquid. The ordinate of Figure 5 was normalized by the signal intensity for the ν2 mode at the air interface, and thus, it was unity before introducing a liquid onto the film surface. On the other hand, the normalized signal intensities at D2O, d4methanol, and d14-hexane interfaces were calculated to be 3.8, 3.7, and 2.3, respectively, as drawn by the dashed line in Figure 5. The intensity after the liquid introduction was much lower than the corresponding value calculated on the assumption that the number of phenyl rings oriented at each liquid interface is the same as that at the air interface. This makes it clear that the orientation of phenyl rings became more random upon contact with the liquid. Also, the decrement depended strongly on which kind of liquid was used. After D2O contact, the intensity became smaller than the calculated value by 92%. This decrement is larger than those for the cases of d4-methanol (80%) and d14-hexane (78%). Thus, it is clear that the aggregation states of PS in the outermost region in the film were most altered by contact with water among the three liquids employed. This may be again explained in terms of the interfacial energy. Either way, the interaction between the phenyl ring and liquid molecule should be studied more using other nonsolvents with the aid of first-principles calculations. This will be reported in the near future.

4. CONCLUSIONS The aggregation states of PS thin films interfacing with typical nonsolvents such as water, methanol, and hexane were studied. Our results revealed that water and methanol molecules could hardly penetrate the PS thin films, unlike hexane which deeply penetrated the film, resulting in a diffuse interfacial layer. In terms of the local conformation of PS in the outermost region of the films, all three nonsolvents caused a conformational change to varying degrees. These could happen even with other nonsolvents, which may result in different degrees of change.

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Corresponding Author

*Tel: +81-92-802-2878. Fax: +81-92-802-2880. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the JST SENTANKEISOKU (13A0004) and JSPS KAKENHI Scientific Research 6569

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dx.doi.org/10.1021/la500829p | Langmuir 2014, 30, 6565−6570