Aggregation Behaviors of PEO-PPO-ph-PPO-PEO ... - ACS Publications

Sep 6, 2012 - Technology Research Department, State Key Laboratory of Offshore Oil Exploitation, CNOOC Research Center,Beijing 100027, P. R. China...
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Aggregation Behaviors of PEO-PPO-ph-PPO-PEO and PPO-PEO-phPEO-PPO at an Air/Water Interface: Experimental Study and Molecular Dynamics Simulation Houjian Gong,†,‡ Guiying Xu,*,† Teng Liu,† Long Xu,† and Xueru Zhai† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, P. R. China College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, P. R. China

‡

Jian Zhang and Xin Lv Technology Research Department, State Key Laboratory of Offshore Oil Exploitation, CNOOC Research Center,Beijing 100027, P. R. China S Supporting Information *

ABSTRACT: The block polyethers PEO-PPO-ph-PPO-PEO (BPE) and PPO-PEO-ph-PEO-PPO (BEP) are synthesized by anionic polymerization using bisphenol A as initiator. Compared with Pluronic P123, the aggregation behaviors of BPE and BEP at an air/water interface are investigated by the surface tension and dilational viscoelasticity. The molecular construction can influence the efficiency and effectiveness of block polyethers in decreasing surface tension. BPE has the most efficient ability to decrease surface tension of water among the three block polyethers. The maximum surface excess concentration (Γmax) of BPE is larger than that of BEP or P123. Moreover, the dilational modulus of BPE is almost the same as that of P123, but much larger than that of BEP. The molecular dynamics simulation provides the conformational variations of block polyethers at the air/water interface.

1. INTRODUCTION Numerous researches have been carried out to understand the amphiphilic polyethers, such as poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO-PPO-PEO) triblock polyethers.1,2 The PEO-PPO-PEO block polyethers have rich aggregation behaviors not only in bulk solution3−5 but also at air/water interface.6−8 In bulk solution, PEO-PPO-PEO block polyethers dissolve in water as individual molecules at the low concentration and low temperature. The process of selfassociation can be induced by increasing block polyether concentration to above critical micelle concentration (cmc) or adjusting temperature to exceed the critical micelle temperature (cmt). Moreover, special attention has been paid to the films formed by PEO-PPO-PEO triblock polyethers at an air/water interface.6,7,9−11 The static properties of the spread and adsorbed films at the air/water interface have been studied in detail by surface tension,4,5,10,12 ellipsometry,10,13 Brewster angle microscopy,10 neutron reflectivity,11,14 and dilational viscoelasticity measurements.7,8,15,16 It has been shown that PEO-PPO-PEO triblock polyethers adopt different conformations with adsorption increasing. The layer structure changes from a two-dimensional structure with both PEO and PPO segments lying flat on the surface to a brush-like structure where the PEO segments are protruding into the aqueous solution. These changes of conformation at an air/water © 2012 American Chemical Society

interface have been proved by the classic brush model or the formation of loop and tail segments.7 The properties and practical applications of PEO-PPO-PEO polyethers depend greatly upon their molecular constructions, which can vary with the numbers of EO and PO blocks or the ratio of hydrophilic (PEO) to hydrophobic (PPO) blocks. A series of amphiphilic polyethers with branch structures have been studied in our laboratory, and many advantages in practical applications have been shown.4,5,17−21 However, the branched block polyethers are hard to synthesize and have broad molecular weight distribution because of high molecular weight and complex structure. Benzene ring has hydrophobicity, and the aromatic ring is a weak H-bond acceptor,22 causing benzene to have a certain level off water solubility in comparison to cyclohexane.23 Wang et al.24 have studied a family of nonionic surfactants poly(oxyethylene) glycol alkyl ethers containing benzene ring group. They have found that comparing with the nonionic surfactant with a methylene hydrophobic chain, the surfactants with benzene ring and adamantane groups have larger cmc values. Meanwhile, the cmc values increase with the size of the groups. Furthermore, Received: June 25, 2012 Revised: September 4, 2012 Published: September 6, 2012 13590

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Figure 1. Synthesis process of BPE and BEP.

Table 1. Molecular Composition of Block Polyethers Containing Bisphenol A Moiety

a

polyethers

δ3.65 ppm

δ1.14 ppm

EO/POa

EO/POb

NPPO

NPEO

mol wt (g mol−1)

BPE BEP

6.63 7.38

3.00 3.00

0.91 1.10

0.89 1.11

36 36

32 40

3720 4070

From 1H NMR spectra. bFrom molecular design.

had better dispersion ability than the commercial Pluronic block polyethers for dispersing carbon nanotubes.31 In this work, we synthesized the block polyethers PEO-PPO-ph-PPOPEO and PPO-PEO-ph-PEO-PPO by anionic polymerization using bisphenol A as initiator, which are abbreviated to BPE and BEP, respectively. The surface tension, dilational viscoelasticity, and dynamic simulation methods are used to investigate their aggregation behaviors and conformational variations at an air/water interface. In order to compare, commercially available PEO-PPO-PEO triblock copolymers of the Pluronic P123 were also selected. It is found that the aggregations of the block polyethers with bisphenol A moiety have peculiar properties.

moving phenyl group from the terminal of hydrophobic chain to the neighbor of hydrophilic headgroup leads to the decreased cmc. Sohn et al.25−27 investigated the conformational transitions of the hydrophobically modified poly(ethylene oxide) (PEO) in bulk solution and at the air/water interface. In bulk solution, the hydrophobic alkyl chains on both ends change from trans to gauche with increasing temperature, while the PEO backbone is either crystalline or amorphous (lessordered crystalline) depending on temperature. At air/water interface, conformational transitions vary among pancake, mushroom, and brush states of the hydrophilic backbone due to intermolecular interactions among hydrophobic chains.27 Molecular dynamics simulation may provide a possibility to understand the microscopic properties of a monolayer formed at an interface within a reasonable computer time. The dynamic properties and aggregation behaviors of alkyl glycol ethers at air/water interface have been carried out by the molecular simulation methods. The results show that the simulations can reproduce the experimental results and be used to study the atomic level details of the structure and dynamics of the adsorbed monolayer films.28−30 Until now, the investigations have mostly focused on the aggregation behaviors of PEO-PPO-PEO block polyethers with different EO and PO numbers or EO/PO mass ratio. However, we have found that the block polyethers with the benzene ring

2. EXPERIMENTAL SECTION 2.1. Materials. The block polyethers PEO-PPO-ph-PPO-PEO (BPE) and PPO-PEO-ph-PEO-PPO (BEP) were synthesized by anionic polymerization in our laboratory. The chemical reagents, such as potassium hydroxide, ethylene oxide, propylene oxide, and bisphenol A, were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). Pluronic P123 ((EO)20(PO)70(EO)20, Mw = 5800 g mol−1) was provided as a gift from BASF Corp. Water used in the experiments was triply distilled by a quartz water purification system. 2.2. Methods. 2.2.1. Synthesis and Characterization of Block Polyethers. Both BPE and BEP were synthesized by anionic polymerization, which was present in our previous work.4,5,31 The 13591

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synthesis process is shown in Figure 1. The products were dissolved in distilled water, and acetic acid was added to completely neutralize potassium hydroxide. The mixed products were filtered through 0.45 μm filter membrane, extracted with dichloromethane, separated via rotary evaporation, and followed by drying in a vacuum oven at 60 °C for 24 h. The compositions of both BPE and BEP were validated by 1H NMR quantitative analysis (Bruker AV-400 NMR spectrometer at 400 MHz and room temperature). The 1H NMR spectra of both BPE and BEP are shown in Figure S1. In the 1H NMR spectra, the peak area is proportional to the number of protons. Thus, the ratio of EO and PO group numbers could be calculated from the peak area at 3.65 and 1.14 ppm. The parameters of both BPE and BEP are shown in Table 1. The results indicate that the contents of EO and PO groups in the synthesized polyethers were close to the designed values. 2.2.2. Surface Tension Measurement. The surface tension measurements of polyether aqueous solutions of different concentrations were performed on a K12 processor tensiometer (Germany, Krüss Co.; the precise degree of measurement is 0.01 mN m−1) using the Wilhelmy plate. 2.2.3. Surface Dilational Viscoelasticity Measurement. Surface viscoelasticity was measured using an oscillating bubble rheometer (Tracker, Teclis Corporation, France). The method is the same as that used to investigate the surface viscoelasticity of polymers.32,33 The air/ water interface was created by injecting a known volume of air into an inverted stainless steel needle attached to an airtight syringe. The tip of the needle was placed in a quartz cuvette containing the solution. The image of the drop formed at the needle tip was captured by a CCD camera and analyzed by software employing the Laplace equation to obtain the surface tension at the air/solution surface. The surface rheological measurements were performed by oscillating the bubble volume to a maximum change of 10% of the original drop volume. The oscillating frequency varied from 0.005 to 0.1 Hz. Data for all oscillation cycles were collected and averaged. All experiments were performed at 25.0 ± 0.1 °C. 2.2.4. Simulation Details. Molecular dynamics (MD) simulation was carried out through the discover module, using the condensedphase optimized molecular potentials for atomistic simulation studies (COMPASS) force field,34 which is an ab initio force field, and its nonbond interaction energy is given by eq 1:

Enonbond =

⎡ ⎛ 0 ⎞9 ⎛ r 0 ⎞6 ⎤ ⎢ ⎜ rij ⎟ ⎜ ij ⎟ ⎥ + ε − 2 3 ∑ ij⎢ ⎜ ⎟ ⎜r ⎟ ⎥ r ⎝ ij ⎠ ⎦ i,j ⎣ ⎝ ij ⎠

∑ i,j

Figure 2. Reduced molecular conformation of different block polyethers: (A) P123, (B) BPE, (C) BEP. In the scheme, the balls with color of gray, red, and white represent carbon, oxygen, and hydrogen atoms, respectively. thermostat with relaxation time of 0.2 ps.40,44 The simulations were performed at room temperature 298 K and 2000 ps. MD production was run to obtain the dynamic information. The results were analyzed from the full structures, which were saved every 0.02 ps. The cutoff distance of the van der Waals interaction was 0.95 nm. All the simulations were run with the software Materials Studio 4.3.

3. RESULTS AND DISCUSSION 3.1. Surface Activities of the Polyethers. The surface tension isotherms of block polyethers are shown in Figure 3.

qiqj rij

(1)

Figure 3. Surface tension isotherms of block polyethers at 25 °C.

is the where εij is the energy parameter between atoms i and j, dimension parameter between particles i and j, rij is the distance between particles i and j, and qi and qj are the charges of i and j atoms, respectively. On the basis of the method to establish surfactant monolayer from Tarek et al.,35 two monolayers were constructed on opposite sides of a slab of aqueous solution, and the thickness of the slab is enough to make sure that the two monolayers are effectively isolated. If the distance between the isolated monolayers in the z-coordinate of simulation cell is enough long to ignore the electrostatic interaction, the Ewald summation method can be used to calculate the electrostatic interaction.36−38 According to our previous work,39,40 1000 water molecules and 9 polyether molecules are added into the simulation box with a size of 3.14 × 3.14 × 17.00 nm3 to simulate the system. The simple point charge (SPC) model is adopted for the water molecule. The MD simulation methods are suitable to simulate the surfactant with low molecular weight; however, the molecular weight of block polyether is higher than 1000. In order to obtain the relative information on aggregation behavior of block polyethers with different structure, the reduced molecular conformations representing P123, BPE, and BEP with the molecular weights of 900 are shown in Figure 2. All the simulations were carried out in the constant volume and temperature (NVT) canonical ensemble and a time step of 0.001 ps.41−43 The temperature was controlled using the Hoover−Nose r0ij

The surface tensions of the block polyethers decrease with the increase of concentration, and there are two breaks in the isotherms. The appearance of the breaks, which is discussed in our previous work,4 shows that the block polyethers containing bisphenol A moiety have similar property to PEO-PPO-PEO molecules. When the concentrations are lower than 1 × 10−3 g L−1 or larger than 0.03 g L−1, the surface tension of BPE is lower than that of P123. When the concentration is from 1 × 10−3 to 0.03 g L−1, the surface tension of P123 is lower than that of BPE. Considering that the molecular weights of P123 and BPE are 5800 and 3720 g mol−1, the introduction of bisphenol A moiety can enhance the surface activity of block polyether. Meanwhile, both the cmc and the surface tension at cmc (γcmc) of BPE solutions are lower than those of BEP solutions. When the concentration is lower than cmc, the surface tension of BPE is much lower than that of BEP solution, indicating that the change of molecular construction can influence the efficiency and effectiveness of block polyethers to lower surface tension. The maximum surface excess concentration (Γmax) and minimum occupied area per molecule at the air/water interface (Amin) can be obtained by Gibbs adsorption equations:45 13592

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Γmax =

∂γ −1 2.303nRT ∂(log C)

(2)

A min =

1018 N Γmax

(3) −1

bisphenol A moiety at the air/water interface are different from that of P123. 3.2. Dilational Viscoelasticity of Block Polyethers. The interfacial dilational viscoelasticity is associated with microscopic relaxation processes at the air/water or liquid/liquid interface and near the interface. Therefore, the dilational viscoelasticity can provide the information about molecular interactions and structures in the interfacial layer. The variations of dilational modulus of P123, BPE, and BEP as a function of concentration at different dilational frequency are shown in Figure 4A−C. The dilational modulus passes through a maximum value and then decreases with the increase of concentration. The increase of bulk concentration has two aspects of influence on the dilational modulus. The one is that the concentration of BPE at the air/water interface increases with the increase of bulk concentration, which can enhance the surface tension gradient, resulting in the increase of dilational modulus. And the other is that the ability of molecules diffusing from bulk to air/solution surface increases with the increase of bulk concentration, which can induce the decrease of dilational modulus. When the concentration is low, the increase of concentration mainly affects the surface adsorption; therefore, the dilational modulus increases with the increase of concentration. With further increase of concentration, the transfer of block polyether molecules from bulk to air/water interface plays a leading role, which results in the decrease of modulus.47,48 The dilational modulus values of block polyethers all increase with the increase of frequency (shown in Figure S2). This is because that when the working frequency is higher, the time for the surface adsorbed layer to resume the balance becomes shorter, and more energy is stored in the system.49

−1

where R = 8.314 J mol K , T = 298 K, n is taken as 1 for nonionic surfactant, N is Avogadro’s number, and ∂γ/∂ (log C) is the slope of the low concentration in the surface tension isotherms in Figure 3. The values of Γmax and Amin are shown in Table 2. The order of the Γmax values of three block polyethers Table 2. Surface-Active Parameters of the Block Polyethers polyethers

cmc/g L−1

γcmc/mN m−1

Γmax × 106/mol m−2

Amin/nm2

P123 BPE BEP

0.2 0.1 2.0

36.6 35.4 36.0

0.53 1.21 0.93

3.10 1.37 1.78

are BPE > BEP > P123. Taking into the similar molecular weight and ratio of EO to PO groups between BPE and BEP molecules, the hydrophobicity of BPE is stronger than that of BEP. Both γcmc and Amin values of block polyethers containing the similar EO group number decrease with the increase of PO group number in the molecules.46 P123 has a similar EO group number but more PO groups than BPE, but both the γcmc and Amin values of P123 are larger than that of BPE. Therefore, the introduction of bisphenol A moiety in the PEO-PPO-PEO molecules can enhance the surface activity and make the molecules easier to adsorb at the air/water interface. Meanwhile, the order of Amin value is P123 > BEP > BPE, implying that the molecular arrangements of polyethers containing

Figure 4. Effect of concentration on the dilational modulus of block polyethers at different frequency: (A) P123; (B) BPE; (C) BEP. (D) Effect of concentration on the dilational modulus of block polyethers at the frequency of 0.1 Hz. 13593

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Figure 5. Effect of concentration on the phase angle of block polyethers at different frequency: (A) P123; (B) BPE; (C) BEP. (D) Effect of concentration on the phase angle of block polyethers at the frequency of 0.1 Hz.

Figure 6. Effect of concentration on the dilational elasticity of block polyethers at different frequency: (A) P123; (B) BPE; (C) BEP. (D) Effect of concentration on the dilational elasticity of block polyethers at the frequency of 0.1 Hz.

The influence of concentration on the dilational modulus of different block polyethers at 0.1 Hz is shown in Figure 4D. The

dilational modulus of BPE is almost the same as that of P123, except for the concentrations lower than 1 × 10−2 g L−1, where 13594

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Figure 7. Effect of concentration on the dilational viscosity of block polyethers at different frequency: (A) P123; (B) BPE; (C) BEP. (D) Effect of concentration on the dilational viscosity of block polyethers at the frequency of 0.1 Hz.

but larger than the value of BEP at the concentration lower than 1 × 10−4 g L−1. Therefore, the structure of block polyethers can influence not only the dilational modulus but also the θ. However, the θ value reflects only the relative contribution of viscous and elastic part to the dilational modulus. The elastic and viscous contribution components of dilational modulus are investigated and discussed in detail as follows. The effects of concentration on the dilational elasticity of different block polyethers are shown in Figure 6A−C. The dilational elasticity first increases and then decreases with the increase of concentration. Meanwhile, the dilational elasticity increases with the increase of frequency. Dilational elasticity, which is known as storage modulus, is caused by the energy change of the molecules at the interface deviating from the equilibrium due to the disturbance. The dilational elasticity is closely correlated with the intermolecular interactions.51 When the concentration is high, the molecules at the surface are in the steady state. The energy change of the molecules at surface deviating from equilibrium is low, so the dilational elasticity is low. With the increase of frequency, the disturbance is enhanced and leads to the increase of energy change. Therefore, the dilational elasticity increases with the increase of frequency. The effects of concentration on the dilational elasticity of block polyethers at the frequency of 0.1 Hz are shown in Figure 5D. The dilational elasticity of BPE is near to that of P123 and much larger than that of BEP at the concentration lower than 1 × 10−2 g L−1. Georgieva et al.52 found that the dilational elasticity of PEO-PPO-PEO block polyether increased with the increase of the PO numbers. Here the molecular weight of BPE is lower than that of P123, but the dilational elasticity of BPE is almost the same as that of P123.

the dilational modulus of BPE is much larger than that of BEP. ́ et al. found that the dilational modulus increased when Ramirez the molecular weight increased.50 Here, BPE has a lower molecular weight than P123, while both of them have the same values of dilational modulus. This results show that the introduction of bisphenol A moiety plays an important role in enhancing the dilational modulus of block polyethers. The dilational modulus of BPE is very different from that of BEP because of the conformational difference between BPE and BEP molecules, caused by the variations of block order. The surface dilational modulus includes elastic and viscous parts. The relationship between them is shown in eq 4 E = |E| cos θ + |E| sin θ = E′ + iE″

(4)

where E′ and E″ are dilational elasticity and viscosity, respectively. The phase angle, θ, which is the quantitative characterization of dilational viscoelasticity for the interfacial film, reflects the ratio of viscous to elastic contributions. The effect of concentration on the θ of block polyethers is shown in Figure 5A−C. The trend of θ value of BPE with variation of its concentration is almost the same as that of P123, but different from that of BEP. When the concentrations of BPE are less than 1 × 10−3 g L−1, the θ values change little. When the concentration is larger than 1 × 10−3 g L−1, the θ value increases greatly. This means that the dilational elasticity is the major contribution for dilational modulus, and the contribution of viscous part becomes increasingly large with the increase of concentration. The θ value increases greatly with the increase of concentration but changes little when the concentration is larger than 1 × 10−2 g L−1. The effect of concentration on the θ value of block polyethers at the frequency of 0.1 Hz is shown in Figure 5D. Obviously, BPE has almost the same θ value as P123 13595

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Figure 8. Density profiles of different components in a direction normal to the plane of the interface (i.e., z-direction): (A) P123, (B) BPE, (C) BEP; the origin of coordinates is the center of the simulation cell.

Figure 9. Density profiles of each PO and EO group in a direction normal to the plane of the interface (i.e., z-direction): (A, B) P123; (C, D) BPE; (E, F) BEP; the origin of coordinates is the center of the simulation cell. Because the reduced molecules of block polyethers are symmetrical, the center groups in P123, BPE, and BEP molecules are labeled as Pc, B, and B, and then the PO group and EO groups from center to the both chain sides are labeled as P1, P2, P3, P4 and E1, E2, E3, E4, respectively.

air/water interface because the hydrophobic bisphenol A moiety and PO groups are connected together and will come back to the stable conformation once the disturbance disappears. However, hydrophilic EO groups seclude the

Thereby, the introduction of bisphenol A moiety can greatly affect the dilational elasticity of polyethers. Moreover, the block order in polyether molecule causes the difference of dilational elasticity. The BPE molecule has the stable conformation at the 13596

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Figure 10. Schematic diagram of the conformational change of the block polyethers with different structures; (A) to (C) represent the systems of 123, BPE, and BEP. The colors of black, blue, and red represent the groups of bisphenol A moiety, PO, and EO, respectively.

concentration is larger than 1 × 10−4 g L−1. The appearance of the maximum in the dilational viscosity curve is due to the conformational change of block polyether at the air/water interface.16,54,55 3.3. Conformation of Block Polyethers at the Air/ Water Interface. The MD simulation can provide the microscopic information about the aggregation behavior and conformation of amphiphile.56−59 In order to investigate the conformation of block polyether at the air/water interface, the MD simulation was carried out. The density profiles of different components in block polyethers with various structures in a direction normal to the plane of the surface (i.e., z-direction) are investigated. The density profiles are symmetrical to the origin of coordinates (i.e., the center of simulation cell) due to the arrangement of block polyether at the both side of water box to maintain the periodic boundary conditions. As shown in Figure 8A, the hydrophobic PO groups distribute in the positions of 15 Å; i.e., the PO groups locate at the air/water interface. The peak of the PO groups is higher than that of EO groups, indicating that the PO groups are mainly at the air/ water interface. The hydrophilic EO groups distribute in the position of 8 Å, which is in the water layer away from the surface. To make the polyether conformation clearer, the density profiles of each group are investigated and shown in Figure 9A,B. The center group in P123 molecules is labeled as Pc, and then the PO and EO groups from center to the both chain sides are labeled as P1, P2, P3, P4 and E1, E2, E3, E4,

bisphenol A moiety and PO chain in BEP molecule, leading to more than one stable conformation of BEP at the air/water interface. Once the disturbance appears, the conformation of BEP molecule will change to another stable one. Therefore, the energy change of BPE during disturbance is much larger than that of BEP. The dilational viscosity, also named loss modulus, is correlated with the relaxation process, such as molecular exchange between the surface and subsurface as well as the molecular arrangement at the air/water interface.51 The investigation on the dilational viscosity is of great importance to research the molecular diffusion and the change of molecular conformation at the air/water interface.53 The effect of concentration on the dilational viscosity of block polyethers at different frequency is shown in Figure 7A−C. The dilational viscosity values of block polyethers are lower than the dilational elasticity and first increase then decrease with the increase of concentration. Meanwhile, the dilational viscosity of BEP increases, while the values of BPE and P123 decrease with the increase of frequency. This means that the conformations of block polyethers with various structures are different at the air/ water interface. The effect of concentration on the dilational viscosity of block polyethers at the frequency of 0.1 Hz is shown in Figure 7D. BPE has a larger dilational viscosity than P123. When the concentration is lower than 1 × 10−4 g L−1, the dilational viscosity of BEP is less than that of BPE, while the dilational viscosity of BEP is larger than that of BPE as the 13597

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the interaction between P123 molecules at the air/water interface lead to the curl of PO chains. When the concentration P123 is higher than its cmc, the P123 molecules begin to form micelles in the bulk solution in addition to occupying the air/ water interface. The molecular exchange between the sublayer and surface is enhanced, leading to the decrease of dilational modulus with the increase of P123 concentration. The schematic diagrams of the conformational change of BPE are shown in Figure 10B. The conformational change of BPE is similar to that of P123. However, the bisphenol A moiety group in BPE molecules can interact with PO chains and cause the curl of PO chains. Therefore, the interaction between BPE molecules is stronger than that of P123, and the dilational modulus of BPE is slightly higher than that of P123. In BEP molecules, hydrophilic EO groups separate the hydrophobic PO groups and bisphenol A moiety, so that the molecular conformation at the air/water interface is different from those of P123 and BPE. The schematic diagrams of the conformational change of BEP are shown in Figure 10C. When the concentration of BEP is low, the distance between molecules at the air/water interface is large, and the molecular interaction is weak, which leads to the lower dilational modulus than that of P123 and BPE. With the increase of concentration, the extending EO chains start bending toward the aqueous phase and the hydrophobic interactions increase as the molecules close to each other. The result is that the density of adsorbed layer decreases and the dilational modulus lowers. When the concentration of BEP is high, the BEP molecules appear not only at the air/water interface but also at the sublayer of surface. The molecular exchange between the sublayer and surface causes the further decrease of dilational modulus with the increase of BEP concentration.

respectively. The groups of Pc, P1, and P2 all locate at 15 Å, and the groups P4, which are connected with EO groups, locate at 10 Å. The locations of EO groups are gradually away from the air/water interface. Therefore, the conformation of P123 at air/water interface is inverted U shape, which is consistent with the report.60 Figure 8B shows the density profiles of different components of BPE in a direction normal to the plane of the surface (i.e., zdirection). Compared with P123, the PO and EO groups in BPE are further away from the center of water box and trend to locate at the air/water interface. This is due to the substantial increase of the hydrophobicity after the introduction of bisphenol A moiety. The detailed location variation of each group in BPE is shown in Figure 9C,D. The hydrophobic PO chains bend and distribute at the air/water interface, while EO chains extend to the aqueous phase. This shows that the introduction of bisphenol A moiety mainly causes the conformation change of PO chains. Moreover, it can be seen from the discussion of surface tension of block polyethers that the introduction of bisphenol A moiety can enhance the adsorption excess and decrease the minimum occupied area per molecule at the air/water interface. Here the results of MD simulation indicate that the conformation of block polyether turns to bend from tile state, which will cause the decrease of the minimum occupied area per molecule. Therefore, the change of the conformation can be used to explain the variation of surface tension. The density profiles of different components of BEP in a direction normal to the plane of the surface are shown in Figure 8C. The PO groups exist at the air/water interface and EO groups extend to the aqueous phase, while bisphenol A moiety exists between PO and EO groups. The hydrophobicity of BEP decreases due to the separation of hydrophilic EO groups between PO chains and bisphenol A moiety, so that the BEP molecules trend to exist below the surface. The detailed location variation of each group in BEP is shown in Figure 9E,F. The EO chains of BEP molecules are still in the bending state, while PO groups can not only be toward the outer of air/ water interface but also have the possibility of extending to the water phase, which is different from that of BPE. This means that the hydrophobic interaction between the PO groups and bisphenol A moiety causes the conformational changes at the air/water interface. Because of computing power limitations, the conformation during dilation at the air/water interface cannot be studied now. However, we can predict the conformational change from the results of dilational rheology. The schematic diagrams of the conformational change of the block polyether P123 are shown in Figure 10A; the PO groups of P123 molecules spread at the air/water interface at a low concentration, which is proved by the MD simulation. The numbers of P123 at the air/ water interface increase, and the interaction between P123 molecules is enhanced with the increase of concentration. The molecules at the air/water interface close to each other and gradually form a layer covering the surface, which leads to the increase of dilational modulus with the increase of concentration. When P123 molecules cover the entire surface, the dilational modulus reaches the highest value. Then with the increase of concentration, the EO chains begin to stretch into water phase and to form loop and tail structures. This variation can break the integrity of the surface layer and lead to the decrease of dilational modulus. As the further increase of concentration, the increases of both the molecular number and

4. CONCLUSIONS The block polyethers PEO-PPO-ph-PPO-PEO (BPE) and PPO-PEO-ph-PEO-PPO (BEP) containing bisphenol A moiety are synthesized by anionic polymerization methods. Compared with linear block polyether P123, the aggregation behaviors of BPE and BEP are investigated by the measurements of surface tension and surface dilational viscoelasticity. Moreover, the conformational change of different block polyethers at the air/ water interface is probed by the MD simulations. The conclusions are presented as follows. The introduction of bisphenol A moiety in the block polyether molecule can enhance the adsorption capacity and significantly increase the efficiency of decreasing surface tension. The efficiency of BPE in decreasing surface tension is larger than that of BEP because the aggregation characteristics of block polyethers containing bisphenol A moiety vary with block order. The introduction of bisphenol A moiety in the polyether molecule and the structure of polyethers can influence the dilational viscoelasticity. The MD simulations prove that block polyethers with different structures have various microscopic conformations at the air/water interface. The research on the aggregation behavior of block polyethers containing bisphenol A moiety can enrich the properties of block polyether and provide important information for the application of block polyethers. 13598

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of the both copolymer in D2O at 298 K and effect of frequency on the dilational modulus of block polyethers at different concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-531-88365436; Fax +86-531-88564750; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (20873077) and the Special Program for Major Research of the Science and Technology, China (Grant 2011ZX05024-004-08).

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REFERENCES

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