Adsorption of Pluronic Surfactants in Alkylene Carbonates on Silica

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Adsorption of Pluronic Surfactants in Alkylene Carbonates on Silica Masaki Hanzawa, Hidekazu Oohinata, Shin-ichi Kawano, Masaaki Akamatsu, Kenichi Sakai, and Hideki Sakai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02543 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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Adsorption of Pluronic® Surfactants in Alkylene Carbonates on Silica Masaki Hanzawa,† Hidekazu Oohinata,‡ Shin-ichi Kawano,‡ Masaaki Akamatsu,† Kenichi Sakai,*,†,§ and Hideki Sakai†,§

†Department §Research

of Pure and Applied Chemistry, Faculty of Science and Technology, and

Institute for Science and Technology, Tokyo University of Science, 2641

Yamazaki, Noda, Chiba 278-8510, Japan. ‡Nomura

Micro Science Co., Ltd., 2-4-37 Okata, Atsugi, Kanagawa 243-0021, Japan.

Corresponding Author * Corresponding author: [email protected]

KEYWORDS. Pluronic® surfactant; Alkylene carbonate; Silica; Adsorption; Polymer brush; Atomic force microcopy

ACS Paragon Plus Environment

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ABSTRACT

Adsorption of triblock Pluronic® surfactants bearing poly(ethylene oxide) (PEO) chains of different lengths, L-62 (5 EO groups on each end), L-64 (13 EO groups on each end), and F-68 (79 EO groups on each end), on silica has been characterized using atomic force microscopy (AFM). The solvent used herein was a mixture of ethylene carbonate (EC) and propylene carbonate (PC). The three Pluronic® surfactants were dissolved in the mixed solvent, with the PEO chain acting as a solvophilic group and the poly(propylene oxide) (PPO) chain acting as a solvophobic group. The approaching force curve measurements for the three Pluronics® (at 10 mmol dm-3) revealed repulsive forces from an apparent separation of 20–30 nm. The most solvophilic Pluronic® surfactant with the longest PEO chain (F-68) showed continuously increasing repulsive interaction with decreasing separation. The MilnerWitten-Cates (MWC) theory described the repulsive force curve data of F-68, suggesting that F-68 forms a polymer brush on the silica surface. The retracting force curve measurements detected stretching forces for the three Pluronic® systems. These stretching forces were observed more frequently for the L-62 system than for the F-68 system, but the pull-off distance was shorter for L-62 than for F-68.


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INTRODUCTION Alkylene carbonates (sometimes called cyclic acid esters) are commercially available polar aprotic solvents for many industrial applications.1-3 Ethylene carbonate (EC) and propylene carbonate (PC) are typical examples of these solvents. Based on their chemical and biological properties, such as high polarity, high boiling point, high flash point, low volatility, low odor, low toxicity, and biodegradability,1,2 these solvents have been used for cleaning, degreasing, paint stripping, and textile dyeing.4 Recently, PC has also been utilized as a green solvent in agriculture, medications, and cosmetics.2,3 EC and PC found application as photoresist stripping agents because of their excellent miscibilities in water and organic solvents.1,5 Photoresist is used for forming fine patterns on semiconductor devices by a photolithography technique, followed by stripping the resist from the substrate. An EC/PC mixed stripping agent is less corrosive to the substrate and is less toxic compared with an amine-type agent.5 However, the resist stripped from the substrate can be readily readsorbed onto the surface via water rinsing. To prevent this readsorption, amphiphilic materials are usually added. Pluronic® surfactants [poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), PEOxPPOyPEOx triblock copolymers] have been used in many industrial applications, including detergents, emulsions, dispersions, and lubrication.6 The interfacial properties can be controlled by changing the content ratio and length of


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the PEO and PPO chains. Adsorption of Pluronics® has been researched for a number of systems including hydrophilic7-9 and hydrophobic9-11 surfaces in water. Selfassembled monolayers (SAMs) covered with Pluronics® have been reported to prevent adsorption of human plasma proteins.12 Adsorption of Pluronics®13 or PEO homopolymers14,15 has also been studied in ionic liquids (ILs). Atkin et al. reported that P-65 (PEO19PPO30PEO19) and L-81 (PEO3PPO43PEO3) form brush structures in ethylamine nitrate (EAN) on silica, and L-81 forms a polymer network between the silica surface and an atomic force microscope (AFM) cantilever tip.13 In the presence of such polymer bridges, characteristic attractive forces are detected in the retraction force curve due to stretching and breaking of the polymer bridges. Stretching forces and bridging distances of PEO homopolymers in EAN increased with increasing molecular weight.14 The EC/PC mixed solvent should act as a good solvent for the PEO block and as a poor solvent for the PPO block due to the high dipole moment and dielectric constant of the EC/PC mixture.1 This is similar to Pluronic® systems in water and EAN, even though water and EAN are protic solvents, whereas EC and PC are aprotic solvents. Herein we have characterized the adsorption of Pluronic® surfactants on silica in mixed EC/PC solvent, considering both the similarities and differences in the solvent properties. We have also evaluated the effect of PEO chain length on the adsorption property. The purpose of this study from industrial aspect is physicochemical understanding of the adsorption property of the Pluronic®


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surfactants in the EC/PC mixture and providing a possible mechanism regarding the anti-readsorption of photoresist onto solid substrate. EXPERIMENTAL SECTION Materials The Pluronic® surfactants used in this study (L-62 (PEO5PPO30PEO5), L-64 (PEO13PPO30 PEO13), and F-68 (PEO79PPO30PEO79), with different PEO chain lengths) were obtained from ADEKA and were used without further purification (see Table 1). Table 1. Characteristics of the Pluronic® surfactant samples used herein

Pluronics ®

Structure

Total molar mass (kg mol-1)

Molar PPO

mass

(kg mol-1)

of

PEO content a

Contour length b

(mass%)

(nm)

L-62

PEO5PPO30PEO5

2.50

1.75

20

11

L-64

PEO13PPO30PEO13

2.90

1.75

40

17

F-68

PEO79PPO30PEO79

8.35

1.75

80

63

a

PEO content is a percentage of the molar mass of PEO against the total molar mass.

Contour length is a maximum extended length of the Pluronics®, calculated from the lengths of the EO unit (0.35 nm)9,13 and the PO unit (0.25 nm)13,16. b

EC and PC used in this study were obtained from Kanto Chemical Co. The weight ratio of the EC/PC mixed solvent was fixed at 70/30. EC is a solid (freezing point: 36 °C)17 and PC is a liquid (freezing point: −49 °C)17 at room temperature. Practically, EC is a better solvent than PC for removing photoresist from solid substrate, so it is required to increase the EC content in the mixed solvent. Since the freezing point of


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the 70/30 mixture is 15 °C, the mixture is a liquid at room temperature. The purities of EC and PC are estimated as >98 % based on the information from the supplier, and we used these materials without further purification. Flat silica plates used for AFM were prepared from silicon wafers (Nilaco) by chemical oxidation.18 The water used in this study was filtered using a Millipore membrane filter (0.1 µm in pore size) after deionization using a Barnstead NANO pure DIamond UV system. Measurements AFM measurements were performed using a Hitachi High-Tech Science AFM 5200S/5000II. The nominal spring constants of triangular silicon nitride cantilevers (Olympus OMCL-TR800PSA) are 0.15 and 0.57 N m-1, and the nominal radius for these cantilever tips is 15 nm. The cantilevers were immersed in ethanol for 10 min, and then cleaned by UV irradiation using a Bioforce Nanosciences UV/ozone ProCleaner to remove organic contaminants. All the experiments were conducted 10 min after the immersion of the silica plate in the Pluronic® solution at room temperature. RESULTS AND DISCUSSION Force curves measured in the absence of Pluronics® Before discussing the force curve results obtained for Pluronic® solutions, the interaction between the pristine silica surface and the AFM tips in the EC/PC mixed


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solvent must be understood. The force curves measured in the absence of Pluronics® are shown in Figure 1. In these measurements, two types of cantilevers were used, the spring constants (k) of which were 0.15 and 0.57 N m-1. In the absence of Pluronics®, very weak attractive forces were detected, and then the cantilevers experienced “jump-in” to the substrate. This “jump-in” distance is generally dependent on the spring constant of each cantilever.19 In our current result, the distance was longer for the cantilever having smaller spring constant, as expected. The theoretical van der Waals (vdW) interaction curve is also presented in Figure 1. The Hamaker constant was calculated as 6.0 × 10-21 J, according to the procedure mentioned in Supporting Information. This value is smaller than the Hamaker constant in water (2.0 × 10-20 J), suggesting that the vdW attractive interaction in the EC/PC mixture is weaker than that in water. The resultant force curve data are in good consistency with the calculated data, suggesting that the attractive force results from vdW interactions. Here we note that the vdW curve was calculated using the nominal radius of the cantilvers (15 nm).


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Figure 1. Approaching force curves measured in the EC/PC mixed solvent without Pluronics®.

Inward force curves measured with Pluronics® Approaching force curves in the presence of L-62, L-64, and F-68 (10 mmol dm-3) were obtained using a cantilever having a spring constant of 0.15 N m-1 (Figure 2). For all of the Pluronic® systems investigated here, a weak repulsive interaction was observed from 20–30 nm. The shapes of the force curves obtained herein for the L62 and L-64 systems resemble that for the adsorption system of L-81 at an EAN/silica interface.9 Since the Pluronics® used herein have a given molecular weight of the PPO unit, the differences arise from the PEO chain length. Assuming that the Pluronics® form brush structures on the silica surface, their PEO blocks must be extended to the solution phase, whereas their PPO blocks must act as an anchor for the adsorption. This assumption is rationalized by the fact that the range of the repulsion increased with increasing PEO chain length (L-62 < L-64 < F-68) in Figure 2.


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At shorter separations, the force curves obtained for L-62 and L-64 experienced “pushing-through” into their adsorption layers. This force-instability reflects (i) the conformational change in the PEO chains (in other words, the compression of the PEO chains facing to the solution phase) and/or (ii) the partial desorption of the Pluronic® molecules weakly adsorbed on the silica surface, under the pressure applied by the cantilever. Again, this is supported by the fact that the force-instability occurred at shorter separation for L-62 than for L-64 and was not observed (or was negligibly small) for the longer-PEO analogue (F-68).

Figure 2. Approaching force curves measured in Pluronic® solutions (10 mmol dm-3) using a cantilever having a spring constant of 0.15 N m-1. For better clarity, each baseline (= 0 nN) is shown as dashed line.


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Compression force curve data obtained for adsorbed polymers with brush structures can be described by the Milner-Witten-Cates (MWC) theory.9,13.15,20,21 The MWC theory enables realistic and precise analysis using the parabolic segment density distribution profile.9,21 It can be applied to the force curve results under the Derjaguin approximation. The MWC theory also enables estimation of the following parameters by fitting the experimental force curves into equation 1: the thickness of the uncompressed brush layer (L), the average distance between the adsorption points (s), and the surface excess (Γ).

[ ( ) ( ) ]

2𝐿0 𝐹(𝐷) 𝐷 = 4𝜋𝑃0 + 𝑅 2𝐿0 𝐷

2

𝐷 ― 2𝐿0

5

―

9 5

(1)

where 𝑘𝐵𝑇𝑁 𝜋2 𝑃0 = 2 12

( )

1/3 4/3

𝑎

(2)

𝑠10/3

where L0 is the equilibrium brush thickness (L0 is assumed to be 1.3L22), D is the surface separation distance, R is the radius of cantilever tip (= 15 nm, nominal value), kB is the Boltzmann’s constant, T is the absolute temperature, N is the number of segments in a polymer chain, and a is the segment length. The average distance (s) can be calculated as s = 2(σ/π)1/2, where σ is the occupied area per polymer chain adsorbed on the solid surface, being directly relevant to the surface excess (Γ). Figure 3 shows the inward semi-log-format force curve data obtained for the F-68 system, as well as the MWC fitting curve. The MWC fitting curve is in good consistency with the experimental data, indicating that F-68 forms a brush layer on the silica surface. The uncompressed layer thickness (L) was estimated as 18 nm, shorter than the contour length of the EO79 block (0.35 nm9,13 ×79 = 28 nm). This suggests that the


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PEO chain of F-68 extends into the bulk solution with a certain degree of flexibility and the PPO chain anchors to the silica surface. The other fitted parameters were calculated as follows: s = 5.3 nm and Γ = 0.39 mg m-2. Here we calculated the fitting parameters using the nominal radius of the cantilever tip (15 nm). However, the wear of the cantilever due to scanning and/or the adsorption of F-68 onto the cantilever will result in a larger radius than the nominal value used here. In this case, the calculation yields larger average distance between the adsorption points (s) and smaller surface excess (Γ), although the brush thickness (L) is unchanged. It seems likely, therefore, that the Γ value calculated here is the maximum amount estimated from the force curve result. F-108 (PEO133PPO50PEO133, PEO chain length: 43 nm, PPO chain length: 14 nm) is known to form a brush layer in its aqueous solution (0.1 wt%) on a hydrophobic surface (L = 11 nm, s = 3.9 nm).9 Interestingly, the L value calculated herein is larger than that reported for the aqueous F-108 system, although the F-108 has longer PEO and PPO chains than F-68. This indicates that the PEO chains can extend more significantly in the EC/PC mixed solvent than in water. This is rationalized by the fact that EC or PC is a better solvent than water for the PEO chains, which is supported by the Hansen solubility parameters (see Supporting Information). Our preliminary experiments revealed that the solubility of Pluronics® in the EC/PC mixed solvent decreased with decreasing PEO content and with increasing PPO molecular weight. Particularly, Pluronics® having (i) PEO content of 20% or less and


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(ii) PPO molar mass of 2.05 kg mol-1 or more were insoluble in the EC/PC mixed solvent. These observations confirmed that the PEO chain acts as a solvophilic group and the PPO chain acts as a solvophobic group in the EC/PC mixed solvent. This situation is consistent with that in water, as mentioned in Introduction. Here the Pluronic® surfactants are dissolved in water (a protic solvent) via hydrogen bonding between the PEO chains and water molecules, whereas are dissolved in the EC/PC mixed solvent (an aprotic solvent) due to the polarity of the mixed solvent being close to that of the PEO chains. The inward force curve data conclude that the Pluronic® surfactants (in particular, the longer-PEO analogue, F-68) can form a brush structure on silica in the EC/PC mixed solvent. This brush layer will prevent the readsorption of photoresist from the EC/PC solution phase, as a result of steric effect and/or low affinity of the PEO chains against photoresist.


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Figure 3. A fit curve based on the MWC model (dashed line) and the approaching force curve data in the presence of 10 mmol dm-3 F-68.

Outward force curves measured with Pluronics® The retracting force curve data provide further insight about the polymer brush structure formed at the silica/solvent interface upon adsorption. The retraction force curve data obtained for homopolymers or Pluronics® in good solvents take on a sawtooth form,13,14 due to the stretching of a polymer chain adsorbed on the cantilever tip and/or the solid substrate. When the tip is separated from the substrate, the effect of stretching of the bridging polymer chains becomes significant. As the probe moves away from the surface and the separation reaches the fully elongated chain length, the stretching force increases markedly, followed by detachment of the polymer from the surface. Outward force curve data for the L-62 and F-68 systems are presented in Figures 4 and 5, respectively. The force curves measured in the 10 mmol dm-3 L-62 solution were classified into three types, as shown in Figure 4. The adhesion force was always observed at the apparent separation = 0 nm. It should be recalled that the cantilever can penetrate into the L-62 adsorption layer in the inward process, as suggested by the force curve result shown in Figure 2. Hence we assume that the interaction between the cantilever tip and the substrate primarily contributes to the observed adhesion force. In the force curve pattern shown in Figure 4a, we only observed the adhesion force. On the other hand, additional stretching forces were observed in


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Figure 4b and 4c. When considering the polymer chain length (Table 1), the stretching force observed at 5.3 nm (Figure 4b) was likely caused by the breaking of individual L-62 chains. In Figure 4c, multiple stretching forces were observed at greater separations (17 nm and 27 nm), suggesting the formation of polymer chain networks between the substrate and the cantilever tip.13 We analyzed 180 retraction force curves and found the probability as 56 % (pattern 4a), 32 % (pattern 4b), and 12 % (pattern 4c), respectively.

Figure 4. Retracting force curves measured in 10 mmol dm-3 L-62 solution in the 70/30 EC/PC mixture.

Interestingly, the adhesion force was not detected at the apparent separation = 0 nm for the F-68 system (Figure 5). It is noted that the “pushing-through” into the F68 adsorption layer was not observed in the inward process (Figure 2). Hence the


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adsorption layer still remained between the cantilever tip and the substrate, leading to the absence of the adhesion force at the apparent separation = 0 nm. Furthermore, no significant stretching force was detected in Figure 5a, while single stretching event was observed in Figure 5b. Since the contour length of F-68 is 63 nm, the single stretching was possibly caused by the breaking of the F-68 single chain. Again we analyzed 180 retraction force curve data and found the probability as 88 % (pattern 5a) and 11 % (pattern 5b). When comparing the probabilities in Figures 4 and 5, L-62 (shorter-PEO analogue) yields stretching forces more significantly than F-68 (longer-PEO analogue). It seems likely that the penetration of the cantilever into the L-62 adsorption layer occurred in the inward process.

Figure 5. Retracting force curves measured in 10 mmol dm-3 L-68 solution in the 70/30 EC/PC mixture.

The “pull-off distance” of the polymer brush was evaluated based on the data obtained from the 180 retraction force curves, to obtain variation in each measurement.14,23,24 Figure 6 shows the frequency of the pull-off distance of each system. Here we excluded the adhesion force observed at the apparent separation =


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0 nm (if present) from the counting. The contour length of L-62 (11 nm) is shorter than the highest-frequency distance (12–16 nm), suggesting that multiple L-62 polymers are attached to the two solid surfaces. The L-64 contour length (17 nm) is slightly longer than the highest-frequency distance in this system (12–16 nm), while the F-68 contour length (63 nm) is considerably longer. It has been reported that the pull-off distance of PEO homopolymers adsorbed at the EAN/silica interface increases

with

increasing

molecular

weight

(or

contour

length)

of

the

homopolymers.14 This behavior is consistent with our results; the pull-off distance was larger for the F-68 system than for the L-62 and L-64 systems, although the probability of stretching events was observed to be much smaller for the F-68 system. Finally, we summarize the inward and outward force events, as schematically shown in Figure 7.


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Figure 6. Histograms of pull-off distances obtained from the data of 180 force curves. The pull-off distance was estimated from the apparent separation where stretching events occurred. In the case of multiple stretching events, all of the pull-off distances were categorized in the histogram.


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Figure 7. Schematic figure of the inward and outward processes for the (a) L-62 or L-64 and (b) F-68 systems.

CONCLUSIONS In this study, we have characterized the adsorption of Pluronics® on silica in a mixed solvent of EC and PC using AFM. In the absence of Pluronics®, an attractive vdW interaction was detected between a silica surface and an AFM cantilever tip. This attractive interaction was changed into a repulsive interaction in the presence of 10 mmol dm-3 Pluronics®. When the AFM tip approached to the surface, the repulsion increased continuously for F-68. The approaching force profile could be fitted to the MWC theory, suggesting that F-68 forms a polymer brush. The retracting force curve


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measurements detected stretching forces for the three Pluronic® systems. These stretching forces were observed more frequently for the L-62 system than for the F68 system, but the pull-off distance was shorter for L-62 than for F-68. We demonstrated that the Pluronics® formed brush layers on a silica surface upon adsorption, which should prevent the readsorption of photoresists in industry.

SUPPORTING INFORMATION The calculation procedures for the vdW interaction and the Hansen solubility parameter are shown in Supporting Information. This information is available free of charge via Internet.

ACKNOWLEDGMENT The Pluronic® surfactants used in this study were kindly supplied from ADEKA Corporation.


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REFERENCES (1) Shaikh, A.-A. G.; Sivaram, S. Organic Carbonates. Chem. Rev. 1996, 96, 951–976. (2) Clements, J. H. Reactive Applications of Cyclic Alkylene Carbonates. Ind. Eng. Chem. Res. 2003, 42, 663–674. (3) Chernyak, Y. Dielectric Constant, Dipole Moment, and Solubility Parameters of Some Cyclic Acid Esters. J. Chem. Eng. Data 2006, 51, 416–418. (4) Palazzo, G.; Fiorentino, D.; Colafemmina, G.; Ceglie, A.; Carretti, E.; Dei, L.; Baglioni, P. Nanostructured Fluids Based on Propylene Carbonate/Water Mixtures. Langmuir 2005, 21, 6717–6725. (5) Ota, H.; Otsubo, H.; Yanagi, M.; Fujii, H.; Kamimoto, Y.; A New Eco-Friendly Photo Resist Stripping Technology Using. IEICE TRANS. Electron. 2010, E93-C, 1607–1611. (6) Schmolka, I. R. A Review of Block Polymer Surfactants. J. Am. Oil Chem. Soc. 1977, 54, 110–116. (7) Malmsten, M.; Linse, P.; Cosgrove, T. Adsorption of PEO-PPO-PEO Block Copolymers at Silica Macromolecules 1992, 25, 2474–2481. (8) Shar, J. A.; Obey, T. M.; Cosgrove, T. Adsorption Studies of Polyethers: Part Ⅱ: Adsorption onto Hydrophilic Surfaces. Colloids Surfaces A 1999, 150, 15–23. (9) McLean, S. C.; Lioe, H.; Meagher, L.; Craig, V. S. J.; Gee, M. L. Atomic Force Microscopy Study of the Interaction between Adsorbed Poly(Ethylene Oxide) Layers:


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TABLE OF CONTENTS/ABSTRACT GRAPHIC


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Adsorption of Pluronic Surfactants in Alkylene Carbonates on Silica

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