Swelling Structure of Thin Poly(methyl methacrylate) Films in Various

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Swelling Structure of Thin Poly(methyl methacrylate) Films in Various Alkyl Length Alcohols Hironori Atarashi,† Hiroshi Morita,‡ Dai Yamazaki,§ Masahiro Hino,z Toshihiko Nagamura,† and Keiji Tanaka*,† †

Department of Applied Chemistry, Kyushu University, Fukuoka 819-0395, Japan, ‡Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8568, Japan, §Quantum Beam Science Directorate, Japan Atomic Energy Agency, Ibaraki 319-1184, Japan, and zResearch Reactor Institute, Kyoto University, Osaka 590-0494, Japan

ABSTRACT Thin films of a typical glassy polymer in alcohol nonsolvents were structurally characterized by specular neutron reflectivity (NR) and were found to be discernibly swollen. The extent of penetration by the nonsolvent was determined by the chain length of the alcohol. Treating this situation as one of a macroscopic phase separation, the interaction χ parameters for the polymer and nonsolvents combinations were extracted. This observation leads us to investigate the factors that control the nonsolvent/polymer interface, being an unusual example of a liquid/liquid interface. The fractional amount of nonsolvents at the substrate interface was higher than that in the internal region of the film. This segregation of a component in a phase-separated domain was explained in terms of an entropic factor. SECTION Macromolecules, Soft Matter

I

n the last two decades, it has been accepted that aggregation structures and physical properties of polymer materials at the air-facing surface are different from those in the corresponding bulk.1-3 However, the surface will contact with a different phase if the polymer is used in applications such as biomaterials,4,5 nanocomposites,6,7 and so forth. Thus, the interfaces contacted with different phases should be studied structurally and dynamically as a next step. Previously, we have examined the density profile of a perdeuterated poly(methyl methacrylate) (dPMMA) thin film spin-coated on a substrate in water, being a typical nonsolvent, along the direction normal to the interface.8 The dPMMA interface with water was diffuse in comparison with the pristine surface with air. In addition, a swollen layer of water molecules was formed beneath the diffused interface. The amount of absorbed water molecules in the film was strongly related to how the film was prepared. This means that the water absorption into the film was probably not an equilibrium process. Thus, an understanding of what happens when a polymer thin film contacts with a nonsolvent requires experiments that reach the equilibrium state or, at least, a quasi-equilibrium state. We have also previously studied the structure of a dPMMA film spin-coated on a substrate in methanol.8,9 In this case, methanol penetrates deeply into the dPMMA thin film and segregates at the substrate interface. This swelling can be equilibrated within a few hours, depending on the thickness of the film. Thus, pairing PMMA with short alkyl chain alcohols affords good candidates for our purpose. In this study, we focus on the swelling structure of dPMMA films in the presence of methanol, ethanol, 1-propanol, and

r 2010 American Chemical Society

1-butanol, which are all nonsolvents for dPMMA, obtained by neutron reflectivity (NR). There are three goals in this work, to find the amount of alcohol absorbed, the interfacial width between the dPMMA and alcohol phases, and the excess amount of alcohol at the substrate interface. Such information should lead to a better understanding of the unusual interface of the two-phase region composed of a small-molecule liquid and a polymer liquid. Figure 1a shows the scattering vector (q) dependence of NR curves for the dPMMA films contacted with air, methanol, ethanol, 1-propanol, and 1-butanol. The open symbols denote experimental NR. For clarity, each data set is offset by a decade. The different periodicity of the fringes as a function of q among the measurements is simply due to differences in the film thicknesses. After contact with the alcohol, the critical q, at which the reflectivity started to depart from unity, shifted to a lower value. This means that the scattering length density (b/V)of the film decreased. Also, the fringe periodicity became smaller, indicating that the film became thicker. However, it should be noted that it is difficult to make direct comparisons among measurements because the original film thicknesses before the alcohol contact were not all identical. These two changes observed after alcohol contact can be easily understood by taking into account that alcohol molecules penetrated into the film. Furthermore, when the periodicity became more diffuse, the fringe amplitude also became Received Date: January 13, 2010 Accepted Date: February 3, 2010 Published on Web Date: February 17, 2010

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DOI: 10.1021/jz100041h |J. Phys. Chem. Lett. 2010, 1, 881–885

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Figure 1. (a) Neutron reflectivity for dPMMA films in contact with air, methanol, ethanol, 1-propanol, and 1-butanol. Open symbols denote experimental data, and solid lines are reflectivity calculated on the basis of the scattering length density profiles shown in (b). For clarity, each data set in panel (a) is offset by a decade. (b) Scattering length density profiles for dPMMA films in contact with alcohols. The abscissa is normalized by the original thickness in air. (c) A model for a dPMMA film on quartz in an alcohol to fit the experimental reflectivity in panel (a). The model involves three fitting parameters, the width of the alcohol-dPMMA interface (σ), the alcohol content in the internal region of the film (φ), and the decay length at the substrate interface (ξ). Table 1. Parameter Values Used to Fit the Experimental Reflectivities Shown in Figure 1aa tair/nm

t/nm

t/tair

(b/V) 104/nm-2

alcohol content (bulk)/%

σ/nm

ξ/nm

χ

χSP

methanol

67.8

96.8

1.43

4.77

26.5

1.9

2.3

1.110

3.23

ethanol

67.7

80.3

1.19

5.79

12.0

1.6

1.8

1.60

4.83

1-propanol 1-butanol

63.5 63.1

70.6 68.0

1.11 1.08

6.27 6.31

5.0 4.4

1.5 1.4

1.4 1.0

2.25 2.36

6.60 7.98

a

tair: film thickness under air, t: film thickness under alcohol, σ: interfacial thickness, ξ: decay length.

smaller in the high q region, especially >0.8 nm-1. This implies that the interface between dPMMA and alcohol became diffused in comparison with the intact surface before the alcohol contact. To make the discussion of the extent to which the dPMMA film is swollen by an alcohol quantitative, the experimental NR curves were fitted to those calculated from model (b/V) profiles. The solid curves in Figure 1a, calculated from the (b/V) profiles in Figure 1b, are the best fits to the experimental results. The model shown in panel (b) is composed of three layers, alcohol, dPMMA, and quartz. Two interfaces between alcohol and dPMMA and between dPMMA and the quartz substrate are, respectively, expressed by error and exponential decay functions. Thus, three parameters, the interfacial roughness between alcohol and dPMMA (σ), the alcohol content in the internal part of the dPMMA film (φ), and the decay length of the excess amount of alcohol at the substrate interface (ξ), are used to fit the experimental data. This is schematically shown in panel (c). Since the calculated NR curves are in good accord with the corresponding experiments, the (b/V) profiles shown in the panel (b) reflect well the swollen structure of the dPMMA films in alcohol. The abscissa in Figure 1b is normalized by each original thickness in air (tair) so that all profiles can be directly compared in terms of the thickness increment caused by the alcohol swelling. Since the alcohol molecules were not deuterated, the (b/V) value at a depth in the film decreased if they penetrated into the films. Also, at the same time, the film became thicker to conserve the mass of PMMA. The swelling extent is in the order of methanol>ethanol>1-propanol>


1-butanol. This order seems to be quite reasonable because methanol possesses the smallest size among them and also forms the highest number density of hydrogen bonds with side chains of dPMMA. The parameter values used for the fitting are collected in Table 1. The interfacial width of dPMMA with alcohol and the excess amount of alcohol at the substrate interface also varied in the same order as the swelling extent, to be discussed later. The φ value in the internal bulk region of the dPMMA films was constant. Since this profile was not time-dependent, it seems reasonable to assume that the system was in a quasiequilibrium state. Thus, the system of dPMMA and alcohol considered here can be regarded as a phase-separated polymer solution. According to Flory's framework, the free energy (F) of the system is given as follows F=kT ¼

φdPMMA ln φdPMMA þ φalcohol ln φalcohol þ χφdPMMA φalcohol N ð1Þ

where χ, φ, and N are the interaction parameter, volume fraction, and degree of polymerization for dPMMA, respectively, and k and T are the Boltzmann constant and the absolute temperature.10 Figure 2 illustrates an example of the relation between φdPMMA and F. In this case, the χ parameter was set to be 1.1. The energy curve possesses two minima, and the broken line shows the common tangent between them. The two contact points between the curve and tangent correspond to stable fractions of each phase-separated component.

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Figure 3. Relation between χ-1/2 and σ. A solid line with the slope of 2b 6-1/2, where a b value of 0.69 was taken, is superimposed onto the data.

Figure 2. The composition dependence of free energy (F) for a binary mixture of dPMMA and alcohol. The curve corresponds to eq 1 with NdPMMA=2740, Nalcohol=1, and χ=1.1. The broken line denotes a common tangent line.

In our experiment, the χ parameters for the combination of dPMMA and alcohols are unknown. Instead, the alcohol fraction in the dPMMA films listed in Table 1 was experimentally obtained. This corresponds to the composition for the phase-separated dPMMA rich phase. Also, the composition for the phase-separated alcohol phase can be regarded as unity because the dPMMA used would not dissolve in the alcohol at all. Thus, constructing a free-energy curve that is consistent with the compositions for phase-separated phases allows the χ parameters for the current systems to be extracted. The values are also given in Table 1. The χ parameter can be also calculated on the basis of solubility parameters (δ) as follows V ð2Þ ðδPMMA -δalcohol Þ2 þ 0:34 χSP ¼ RT

Figure 4. Relation between Valcohol and ξ. The solid line is a guide to the eye.

experimental data and mean-field prediction is meaningful. However, there is no doubt that the interfacial width for the combination between polymer and nonsolvent is a function of the interaction χ parameter. It should be also noted that the alcohols penetrate deeply into the film and segregate at the substrate interface. The volume fraction of alcohol at the substrate interface (φalcohol,i) exponentially decays to the bulk value, namely φalcohol, with increasing distance from the substrate. The decay length is defined as a length, at which (φalcohol,i - φalcohol) becomes 1/e. The ξ values for the systems of dPMMA and alcohol were successfully determined by NR and are collected in Table 1. If the interfacial segregation of alcohol is enthalpically driven, the interaction between alcohol and quartz should be crucial. However, the surface free energy of methanol, ethanol, 1-propanol, and 1-butanol at 293 K are all similar at 22.6, 22.3, 23.7, and 24.6 mJ 3 m-2,15,16 respectively, and the magnitudes are not in the same order as the ξ values. This implies that the interfacial segregation of alcohol is controlled by something other than the enthalpic factor. Figure 4 shows that the molar volume of the alcohol (Valcohol) and ξ are inversely related. The Valcohol values are imported from the literature.17 That is, the interfacial segregation became more pronounced with decreasing molecular size. Translational entropy, or packing entropy, at the substrate interface increases with decreasing molecular size. Thus, it is conceivable that the segregation of alcohol in the dPMMA films at the substrate interface is entropically driven. The similar phenomenon of the segregation of one component at the surface of a miscible binary polymer mixture

Here, V, R, T, and δ are the molar volume, gas constant, temperature, and solubility parameter for a component, respectively.11 The χ parameter determined in this way was expressed as χSP to distinguish it from the χ obtained above from the free-energy curve, and its values are also collected in Table 1. Both χ and χSP increased with increasing alkyl length of the alcohol molecules. Although the absolute values for χ and the χSP do not agree, the trend with the alkyl length of the alcohol molecules does agree. In following discussion, the χ obtained by experiment is used. Next, we discuss the interfacial width between dPMMA and alcohol phases. The σ value decreased with increased alkyl length of the alcohol. Therefore, it should be in some way related to the χ parameter. Since the χ values obtained here are quite large, the current experimental systems should be far away from the critical point and thus be in the strong segregation limit. Using a mean field approximation, the σ value for two phases in the strong-segregation limit is given by12,13 2b σ ¼ pffiffiffiffiffiffi ð3Þ 6χ where b is the segment length. Figure 3 shows the χ-1/2 dependence of σ. Although χ and σ are independently determined, they exhibit a linear relation with each other. A solid line with a slope of 2b 6-1/2, where the b value of 0.69 nm14 was taken, is superimposed on the experimental data. Taking into account that the depth resolution of NR is 0.5 nm, it is not clear whether the comparison between


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has been widely studied.18-21 The behavior can be controlled by either an enthalpic or entropic factor, depending on the architecture of the polymer chains. However, such studies have been experimentally and theoretically carried out in the onephase region of a binary mixture. On the other hand, in this study, the segregation in one phase, the dPMMA film, of a twophase region is discussed. This is a unique feature of this work. In conclusion, aggregation states of thin dPMMA films in the alcohols, methanol, ethanol, 1-propanol, and 1-butanol were examined by NR. Although the alcohols used were typical nonsolvents for dPMMA, they discernibly penetrated into the films. The swelling extent was strongly dependent on the carbon number of the molecule. By applying the Flory equation to the volume fraction of alcohol in the film, the interaction χ parameter for the dPMMA/alcohol systems was successfully extracted. The interfacial width between dPMMA and alcohol phases was larger than that between the original surface and air, and the trend related to the carbon number could be quantitatively explained in terms of the χ parameter. Interestingly, the alcohol was segregated at the substrate interface. However, the extent of segregation was determined by the molecular size of the alcohol rather than the χ parameter. We interpret this to mean that the aggregation states at the substrate interface are mainly controlled by an entropic factor.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Tel: þ81-92-8022872. Fax: þ81-92-802-2880. E-mail: [email protected].

ACKNOWLEDGMENT This research was partly supported by an Industrial Technology Research Grant Program in 2006 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan and by Grant-in-Aid for Young Scientists A (No. 21685013) for Science Research in a Priority Area “Soft Matter Physics” (No. 21015022) and for Scientific Research on Innovative Areas “Molecular Soft-Interface Science” (No. 21106516) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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EXPERIMENTAL SECTION Monodisperse dPMMA with a number average molecular weight (Mn) of 296k was used. Methanol, ethanol, 1-propanol, and 1-butanol were used as typical nonsolvents for PMMA. All alcohols except 1-propanol were HPLC-grade, and 1-propanol with the purity of 99.5% was used. Films of dPMMA were prepared from a toluene solution onto quartz blocks with a size of 60 60 8 mm by spin-coating. The films were annealed for 24 h at 423 K in vacuum to relax the film preparation history. The dry thickness of the films measured by ellipsometry was in the range of 63-68 nm. The films were soaked in the alcohols for 2 h prior to the measurements. Density profiles of the dPMMA films along the direction normal to the surface under the alcohols were examined by NR measurements using the multilayer interferometer of neutrons (MINE) at the Institute for Solid-State Physics, the University of Tokyo22 and an apparatus for surface and interface investigations with reflection of neutrons (SUIREN) at the Japan Atomic Energy Agency.23 The wavelengths were 0.88 and 0.38 nm, respectively. The neutron beam was guided into the dPMMA film from the quartz side, and the reflected beam was detected under the specular condition. Reflectivity was acquired as a function of q = (4π/λ) sin θ, where λ and θ are the wavelength and the incident angle of the neutrons, respectively. It took approximately 21 h to obtain a reflectivity curve. Reflectivity was also calculated on the basis of the (b/V) profile along the depth direction by means of Parratt32 software, which is a freeware program from the Hahn-Meitner Institute.24 The (b/V) values of SiO2, dPMMA, methanol, ethanol, 1-propanol, and 1-butanol used for the calculations were 3.48 10-4, 6.62 10-4, -3.75 10-5, -3.44 10-5, -3.37 10-5, and -3.29 10-5 nm-2, respectively.


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Swelling Structure of Thin Poly(methyl methacrylate) Films in Various

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