Direct Characterization of In-Plane Phase Separation in Polystyrene

Article pubs.acs.org/Macromolecules

Direct Characterization of In-Plane Phase Separation in Polystyrene Brush/Cyclohexane System Daiki Murakami,†,‡ Yuki Norizoe,† Yuji Higaki,†,‡ Atsushi Takahara,*,†,‡ and Hiroshi Jinnai*,†,‡,§ †

Japan Science and Technology Agency (JST), ERATO, Takahara Soft Interfaces Project, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, CE41, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan S Supporting Information *

ABSTRACT: The phase behavior of polystyrene (PS) brushes in cyclohexane (CHX) was investigated, for the first time, by environmental atomic force microscopy as a function of the graft density and temperature. The polystyrene brushes of three different graft densities exhibited island-, bicontinuous-, and hole-shape microdomains in the direction parallel to the substrate. The size of such “in-plane” microdomains is close to the end-to-end distance of PS brush chain due to the anchoring of one of the chain ends of PS brushes to the substrate. The microdomain structure disappeared as the temperature increased, and new structure with same morphological features reappeared by lowering temperature. This reversible temperature response corresponds to the in-plane phase separation of the PS brush/CHX system. The UCST type binodal line shifted toward slightly lower temperature in the PS brush/CHX system compared to that of the corresponding nongrafted polymer solution, i.e., PS/ CHX system, in excellent agreement with our previous Monte Carlo simulation study.

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were also investigated by AFM11,12 and simulated works.13 Similar phase-separated brushes were also examined in the ternary brush system in a recent article.14 In addition, the microphase separation of A−B diblock copolymer brushes was also studied.15 Various types of the phase structure appeared as the result of the thermal and solvent annealing procedures. In contrast to the above brush systems where the lateral structure is due to the density distribution of the graft-point itself or immiscibility of two or three components, we recently simulated a simple polymer brush system which is composed of one monomer component homogeneously grafted on a planar substrate and immersed in a solvent.16 This is the simplest and most basic brush system that is suitable to determine the universal phase behavior of the brushes because the phase

INTRODUCTION Polymers whose chain ends are chemically grafted onto substrate surfaces are referred to as polymer brushes.1,2 Polymer brushes are widely investigated and utilized for their prominent application in the field of wetting,3−5 friction,6,7 adhesion,8,9 and so on. It is a well-received concept that the properties of polymer brushes are highly affected by vertical structures in the polymer brush layer. Furthermore, several recent studies have focused on the lateral structures in some polymer brush systems. The atomic force microscopy (AFM) study of a micropatterned polymer brush of poly(n-isopropylacrylamide) (PNIPAm) successfully revealed the lateral texture of polymer brush in water.10 The micropatterned polymer brush was observed both in the highly solvated and in the collapsed conditions, and the thickness of the brush layer showed the drastic thermosensitivity. The lateral phase separation of a brush with two kinds of homopolymer chains randomly and irreversibly grafted onto substrates in solvents © 2016 American Chemical Society

Received: January 22, 2016 Revised: May 13, 2016 Published: June 23, 2016 4862

DOI: 10.1021/acs.macromol.6b00151 Macromolecules 2016, 49, 4862−4866

Article

Macromolecules Table 1. Thickness L and the Graft Densities σ of Synthesized Polystyrene Brushesa

behavior of sophisticated polymer brushes significantly depends on unique characteristics of each system, e.g., the molecular architecture and substrate shape. Simulated phase behavior of a polymer brush system was qualitatively similar to that of nongrafted polymer solution. When the chemical and physical parameters of the system are set outside the binodal line of the nongrafted polymer solution, the grafted polymer chains are well solvated to form a homogeneous layer. If the parameters are changed and the system is brought to the region within the binodal line (e.g., by changing temperature, pressure, and graft density), the solvent quality becomes poor and the polymer chains prefer to segregate each other. However, the locational constraint of polymers by the fixed graft point hinders the macrophase separation and generates the various types of microphase separation depending on the parameters. The results are similar to those for the microphase separation of bulk systems of nongrafted diblock copolymer melts. The polystyrene (PS)/cyclohexane (CHX) system is one of the most famous and classical experimental systems with an upper critical solution temperature (UCST) type phase behavior.17 Utilizing the PS brush/CHX system, here we experimentally show the microphase separation of the singlecomponent homopolymers by environmental AFM. A variety of concomitant microdomain patterns are revealed on the substrate in the in-plane direction. The present experimental results are quantitatively consistent with our theoretical prediction.16

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entry

BHE/HHE ratio

L (nm)

σ (chains/nm2)

PSB-I PSB-II PSB-III PSB-IV

1/0 1/47 1/29 1/9

84.1 4.5 6.0 12.2

0.38 0.020 0.027 0.055

a

The L values were determined by ellipsometry in dry conditions (the experimental error was within ±0.5 nm), and the σ were calculated by eq 1.

An atomic force microscope (AFM, Cypher) equipped with an environmental cell (Oxford Inst. Inc.) was employed to observe inplane morphologies of the PS brushes in CHX. The morphological observations were carried out under the atmospheric pressure with a cantilever AC40TS-C2 (Olympus Co., spring constant ∼ 0.1 N/m) over the temperature range from 10 to 40 °C. Tapping scan in the slightly repulsive region was used. Before the measurements, the PS brushes were kept in CHX at high enough temperature above the binodal point of the system, i.e., 50 °C more than 10 min to erase the structure hysteresis.

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RESULTS AND DISCUSSION Figure 1 shows the topographic in-plane images obtained from the AFM observations of the PS brushes/CHX at 10 °C. Practically flat interface (the RMS roughness was less than 0.4 nm) was observed for the PSB-I/CHX system, while the PS brushes with low graft densities (PSB-II ∼ IV) exhibited a

EXPERIMENTAL SECTION

The surface initiator, (2-bromo-2-methyl)propionyloxyhexyltriethoxysilane (BHE), was mixed with 2-methylpropionyloxyhexyltriethoxysilane (HHE) to reduce the graft density of PS brushes. The mixture of BHE and HHE was dissolved in a mixed solvent of ethanol and NH3 aqueous solution (20%) (BHE−HHE mixture/ ethanol/NH3(aq): 1/89/10). Silicon wafers were immersed in the solution for 4 h to immobilize the mixed surface initiators. Then the styrene monomers (52 mmol) were polymerized through atom transfer radical polymerization method with CuBr (0.031 mmol)/ Tris[2-(dimethylamino)ethyl]amine catalyst (0.062 mmol) in the presence of the initiator immobilized silicon wafers and sacrificial nonimmobilized initiators (0.032 mmol) at 75 °C for 19 h.18 Five types of polymer samples, i.e., four PS brushes with different densities and nongrafted PS, were synthesized at once within a vessel, and the molecular weight (Mn = 139 000 g/mol) and the molecular weight distribution (Mw/Mn = 1.07) of brush polymers were determined by the gel permeation chromatography of the nongrafted polymers. The graft densities of the polymer brushes σ were calculated as19 σ = dLNA /M n

(1)

Here d is the density of PS, L is the thickness of the polymer brush layer determined by ellipsometry in dry conditions, and NA is Avogadro’s number. While the above calculation is not the direct way to characterize the graft density, the linearity of L against Mn and the valid approximation of Mn of grafted polymers by those of nongrafted polymers were well confirmed.19,20 The σ values of four PS brushes are listed in Table 1. The phase separation behavior of the nongrafted PS/CHX solution was examined by the turbidity measurement. Temperature of a small amount of polymer solution in a glass cell (ϕ = 7 mm, thickness = 1 mm) was precisely controlled to a target temperature at which the turbidity was measured. A thermocontroller FP90 with microscopy cell (FP84HT) (Mettler-Toledo International Inc.) was used for temperature control. The binodal temperature at a given polymer concentration was set to the temperature where abrupt increase of turbidity was observed.

Figure 1. AFM images of the PS brushes in CHX at 10 °C: (a) PSB-I (σ = 0.38 chains/nm2), (b) PSB-II (σ = 0.020 chains/nm2), (c) PSBIII (σ = 0.027 chains/nm2), (d) PSB-IV (σ = 0.055 chains/nm2). The height profiles were obtained along the white straight lines. The insets represent the in-plane density distribution predicted in previous simulation study (ref 16). 4863

DOI: 10.1021/acs.macromol.6b00151 Macromolecules 2016, 49, 4862−4866

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Macromolecules variety of in-plane structures. PSB-II (σ = 0.020 chains/nm2) showed a number of isolated islands (Figure 1b). We consider that these islands are PS-rich domains within PS-poor matrix based on our previous simulation work.16 The PS-rich domains are namely the segregation of PS brush in the CHX. In the PSpoor matrix, a small number of PS brushes protruding into solvent should exist but are not observed because they are well solvated and highly soft to detect by AFM. Thus, we may observe the bottom of polymer brush in polymer-poor domains. The size and spacing of the microdomains were 20−30 nm, which is close to the end-to-end distance of PS as expected by our simulated work. (The mean square of the endto-end distance ⟨Re2⟩ was defined as ⟨Re2⟩ = 6KMn. In the theta solvent, the constant K was estimated as K1/2 = 2.75 × 10−9 cm.21 Thus, Re was ca. 25 nm for PS of Mn = 139 000 g/mol). Similar microdomain structures were observed in PSB-III (σ = 0.027 chains/nm2) except for the ratio of the islands to the whole substrate being increased compared with that of PSB-II. Moreover, the islands were rather connected. Thus, the microdomains may be regarded as the in-plane bicontinuous structure rather than the isolated island structures. The dense polymer brush (PSB-IV with σ = 0.055 chains/nm2), in contrast, showed inverted island microdomains, i.e., low density (hole) domains in PS brush rich matrix. Figure 2 shows the temperature dependence of AFM images for the PSB-II/CHX system. As the temperature increased, the in-plane microdomain (island) structures at 10 °C gradually disappeared. The RMS roughness decreased from 2.1 nm at 10 °C to 0.70 nm at 20 °C. At 30 °C, the PS brush became essentially flat (RMS = 0.55 nm). Subsequently, as the temperature was lowered again to 10 °C, the microdomains reappeared. The processes can be repeated any number of times and were likewise observed in PSB-III/CHX and PSBIV/CHX systems. It was also found that the microdomain structures were different at each time in the heating−cooling cycles, while the morphological features were essentially the same (see Supporting Information for details). Note that the AFM observations were made at the same position of the Si substrate. Therefore, the observed microdomain structures at 10 °C (Figure 1) are not due to the intrinsic in-plane inhomogeneity of the polymer brushes (i.e., the nonuniform density distribution of surface initiators). In our previous work, the Monte Carlo simulation of polymer brush/solvent system has been carried out. The coarse-grained model of a simple polymer brush system which is composed of one monomer component (not A−B binary polymers or A−B copolymer) with linear chain structure was used to calculate microdomain structures of polymer brushes.16 The simulations revealed that the inhomogeneous lateral density profiles of polymer brush, i.e., in-plane microdomain structures, including island, lamellar, and hole microdomains, depend on the temperature and graft density. The topographical images obtained in the present study (Figure 1) agreed well with the predicted ones (see the insets in Figure 1, or Figure 4 in ref 16). Furthermore, the reversible morphological change, i.e., appearance and disappearance of the microdomains, corresponds to the phase transition phenomena across the binodal line in the PS brush/CHX system. Thus, it is obvious that our experimental observations are the in-plane phase separation of the polymer brushes, just as predicted in the Monte Carlo simulation. The small domain size is due to the spatial restriction caused by anchoring of the single chain-end of polymers onto the surface. This feature is

Figure 2. Temperature dependence of the PSB-II (σ = 0.020 chains/ nm2)/CHX interface. The heating/cooling cycle was in the order of (a) 10, (b) 20, (c) 30, (d) 10, and (e) 30 °C. The height profiles were obtained along the white straight lines.

similar to the microphase separation of block copolymers as their chemical junctions locate only at the microdomain interfaces that limit further segregation of block chains.22,23 Figure 3a displays AFM images of the PSB-II−IV/CHX system in the cooling process with small temperature step (2 °C). In the case of the PSB-II/CHX system, a flat surface, i.e., the homogeneous state, was observed at high temperature (25 °C). As temperature decreased, the roughness monotonically increased until clear in-plane microdomains of PS brushes appeared at the lowest temperature (10 °C). Because the AFM images, as well as the roughness plots (see Supporting Information) and the Fourier transformed images, had not changed steeply but rather gradually over the temperature range of a few degrees, it was not easy to precisely determine the binodal temperature. Nonetheless, the microdomains of PS brush appeared within the temperature range between 16 and 20 °C. Similarly, the hole structure (see Figure 1d) appeared in the PS matrix in the case of PSB-IV/CHX system around 18 °C. The PSB-III/CHX system showed an AFM image with 4864

DOI: 10.1021/acs.macromol.6b00151 Macromolecules 2016, 49, 4862−4866

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Macromolecules

Figure 3. (a) Temperature dependence of the interfacial structures for PSB-II−IV/CHX systems. One AFM image is 1 μm × 1 μm. The magnified images are available online. (b) Phase diagrams for the PS/CHX solution (filled circles) and the PS brush/CHX interface (open circles). The solid and dashed lines are guides for the eyes.

faint undulations even at the highest temperature. As the temperature decreased, the amplitude became larger, suggesting the formation of microdomains. Though it is again difficult to locate the exact position of the binodal point at this graft density, it seems like the microdomains appeared around 20 °C. The binodal temperatures of the PS brush/CHX system at different graft densities are plotted in Figure 3b together with the phase diagram of the corresponding PS/CHX system, a solution of PS with the same molecular weight (Mn = 139 000 g/mol), constructed by the turbidity measurement. The abscissa was normalized by the density (concentration) at the critical point both in the polymer brush and in the polymer solution systems. The critical point of the polymer brush system was assumed to be at the density of PSB-III (σ = 0.027 chains/nm2). It is evident that the PS brush/CHX system showed UCST type phase behaviors just like the CHX solution of the nongrafted PS counterpart. The binodal line of the PS brush system was lower than that of the PS solution as expected from the simulation study. In the simulation study, the type of the phase diagram, UCST type in this case, is common, but the binodal curve of the polymer brush/solvent system shifts toward lower temperature compared to the corresponding nongrafted polymer solution.16 We found exactly the same trend in our PS brush/CHX system. The shrink of miscibility gap in the polymer brush system is again due to the anchoring of single chain-end of polymers; i.e., the spatial restriction of polymer brush chains prevented them from segregating each other in the brush layer, while they can freely move and segregate in the polymer solution. Finally, we should consider how the interaction between grafted polymer and substrate could affect the phase behavior. In our previous simulated work,16 any specific interaction between the grafted polymer and substrate was not considered. Nevertheless, the microphase separations observed in the present study were in agreement with the simulation results, indicating that the polymer−substrate interaction is minor. The influence of the polymer−substrate on the phase separation of polymer brush on the basis of the present work is a fascinating subject that will be discussed in the future.

nm, and the height was around 10 nm. These microdomains got obscure and finally disappeared by heating and reappeared reversibly by cooling the system. These reversible behaviors are attributed to the phase transition across the binodal line in the PS brush/CHX system. The phase diagram of the PS brush/ CHX system showed UCST-type behavior as well as that of the PS/CHX solution. The binodal line shifted toward slightly lower temperature compared to that of the corresponding nongrafted polymer solution. The finite domain size and shrink of the miscibility gap in the polymer brush/solvent system may attributed to the spatial restriction of brush polymers due to the grafting of a single chain end onto the substrate and the interaction between polymer and substrate. The excellent agreement between the theoretical and experimental studies exactly demonstrated the presence of in-plane phase separation in the polymer brush/solvent system that should be essential but have never been perceived until now.

CONCLUSIONS The in-plane microdomain structures of the PS brushes in CHX were examined by environmental atomic force microscopy. The island-, bicontinuous-, and hole-shape microdomains were observed at 10 °C, depending on the graft density of the polymer brushes. The lateral size of the domains was 20−30

ACKNOWLEDGMENTS This work was supported in part by the Management Expenses Grants for National Universities Corporations from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Young Scientists B (Grant

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00151. (i) Report of the morphological features of the microdomains in the heating−cooling cycles; (ii) the magnified images of Figure 3a; (iii) the interfacial roughness vs temperature plots of polystyrene brush/ cyclohexane interfaces (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.J.). *E-mail [email protected] (A.T.). Present Address

Y.N.: National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8561, Japan. Notes

The authors declare no competing financial interest.

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15K17914). This work is partially supported by JSPS KAKENHI 16H02288 and 16K14001.

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DOI: 10.1021/acs.macromol.6b00151 Macromolecules 2016, 49, 4862−4866