Effects of Spreading Conditions on the Aggregation Behavior of a

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Interface-Rich Materials and Assemblies

Effects of Spreading Conditions on the Aggregation Behavior of a Symmetric Diblock Copolymer PS-b-PMMA at the Air/Water Interface Mingming Gao, Gangyao Wen, and Liang Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01649 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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Effects of Spreading Conditions on the Aggregation Behavior of a Symmetric Diblock Copolymer PS-b-PMMA at the Air/Water Interface Mingming Gao, Gangyao Wen,* and Liang Wang Department of Polymer Materials and Engineering, College of Material Science and Engineering, Harbin University of Science and Technology, 4 Linyuan Road, Harbin 150040, People’s Republic of China

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Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Langmuir monolayers and Langmuir−Blodgett (LB) films of a symmetric diblock copolymer polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) were characterized by the film balance technique and tapping mode atomic force microscopy (AFM), respectively. Effects of both the spreading solution concentration and the surface concentration on the aggregation behavior of PS-b-PMMA at the air/water interface and the morphologies of its LB films were studied in detail. When the monolayers spread in different concentrations (≤0.50 mg/mL), all their initial morphologies exhibit tiny circular micelles due to the long hydrophilic PMMA block in the copolymer. The initial tiny circular micelles form spontaneously, and then aggregate into small ones upon compression, which can further coalesce into rodlike aggregates or large micelles depending on the spreading concentrations. The LB films of PS-b-PMMA usually exhibit various mixed structures of rodlike aggregates and circular micelles, which can further transform into labyrinth patterns under some special spreading conditions. Besides spreading concentration and volume, we discover that the detailed spreading process should also be responsible for the initial and final morphologies of the LB films. Furthermore, the LB films prepared under different spreading conditions can be regarded as in the equilibrium or nonequilibrium structures due to the kinetic effect difference resulting from the different PS chain entanglement degrees.

KEYWORDS: LB film; block copolymer; PS-b-PMMA; spreading concentration; spreading volume

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INTRODUCTION It is well known that amphiphilic diblock copolymers can usually form the typically circular, rodlike, and planar aggregates at the air/water interface.1−25 Much extensive studies have been carried out on the systems of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP),1−4 polystyrene-block-poly(ethylene

oxide) (PS-b-P2VP),13−15

polystyrene-block-poly(2-vinylpyridine)

(PS-b-PEO),5−12 and

polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA).16−22 There are mainly four mechanisms proposed to describe the aggregation behaviors of various diblock copolymers at the air/water interface: spontaneous surface aggregation,1−11 compression-induced surface aggregation,16,17 simple deposition of solution micelles,12 and our suggested reversal of solution micelles.14 Furthermore, the occasionally mentioned dewetting mechanism at the air/water interface4,10 can actually be included in the above detailed mechanisms based on the relative contributions of various repulsive and attractive interactions among the hydrophilic and hydrophobic blocks and water molecules. Just as we know, various dewetting behaviors of polymer films on solid substrates can be mainly interpreted with the spinodal dewetting (SD) and the nucleation and growth (NG) mechanisms.22,26,27 Aggregation morphologies of block copolymers at the air/water interface can be affected by many factors such as nature of the core-forming and corona-forming blocks, copolymer composition, nature of spreading solvent, and subphase condition.14 Furthermore, spreading concentration and total block copolymer molecular weight also have large effects on the morphologies of the Langmuir−Blodgett (LB) films of diblock copolymers, which were

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generally studied with relatively low hydrophilic block contents and high spreading concentrations.5,6,9,10 Cheyne and Moffitt found the marked spreading concentration dependencies of compression isotherms and LB films of two highly hydrophobic PS-b-PEO systems with high molecular weights, and suggested a qualitative model showing the difference in copolymer packing densities within the aggregates at different spreading concentrations.10 For PS-b-PEO with less than 7 wt% PEO, it was reported by Baker et al. that the thin film morphology also depended on spreading concentration, and three types of features were observed in various proportions: dots (circular aggregates), spaghetti (rodlike aggregates), and continents (planar aggregates).6 Hosoi et al. simulated the concentration dependent morphology changes of PS-b-PEO based on a mathematical model incorporating the effects of surface tension gradients, entanglement or vitrification, and diffusion.28 More recently, Perepichka et al. distinguished equilibrium/near-equilibrium and nonequilibrium structures of a wide range of PS-b-P4VP at the air/water interface concerning the spreading concentration effect, and pointed out that the greater the nanodot order and size uniformity, the closer it is to equilibrium.4 As mentioned above, most previous work mainly focused on the relatively high concentration region (as high as 1.0, 2.0 and 5.0 mg/mL),5,6,9,10 which really exhibited various nonequilibrium structures due to the strong chain entanglements. However, less work was done in the relatively low concentration region (≤0.50 mg/mL)9,10 with the relatively low chain entanglement degrees, which deserves us to explore for different block copolymer systems. The interfacial aggregation behavior of PS-b-PMMA is quite interesting because both

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hydrophobic PS block and weakly hydrophilic PMMA block are expected to stay on the water surface. The PMMA conformation in the copolymer aggregates and the interfacial aggregation mechanisms of PS-b-PMMA suggested in the previous references were not very consistent. Seo et al. once studied the temperature and molecular weight effects on the aggregate behavior of PS-b-PMMA, and concluded that surface micelles formed upon compression

and

further

assembled

at

high

surface

pressure,

which

was

the

compression-induced surface aggregation mechanism.16,17 More recently, Destri et al. showed that

the

hydrophilic

PMMA

blocks

adopted

an

expanded

conformation

when

thermodynamically stable nanodots formed, whereas a coiled conformation was observed when the block copolymer formed kinetically frozen nanostrands, which were interpreted in terms of the positive and negative spreading parameters, respectively.18 Furthermore, PMMA homopolymer forms a condensed monolayer29 and PS homopolymers form inhomogenous circular or strand-like aggregates,30 while PS-b-PMMA diblock copolymers usually form homogenous surface micelles with PS cores and PMMA corona. It should be noted that there is a hydrophilic ester group in each repeating unit of PMMA which is well known to spread as a monolayer with its repeating units adsorbing on the water surface.21 In this sense, PMMA is two-dimensional (2D) hydrophilic as its per segment is “water surface soluble”,31 although PMMA is usually reported as surface-active but water-insoluble.16,17 In this work, we studied the effects of both the spreading concentration (≤0.50 mg/mL) and the surface concentration on the aggregation behavior of a symmetric PS-b-PMMA at the air/water interface. Nonselective solvent chloroform was used to spread the Langmuir

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monolayers of PS-b-PMMA. Spreading concentration and surface concentration in our system show significant effects on the isotherms of the Langmuir monolayers and the morphologies of the LB films. According to our results, we confirm that tiny circular micelles of PS-b-PMMA form spontaneously instead of being the compression-induced aggregation. As far as we know, seldom work except ours32 considered the polymer surface/interface concentration effect on the aggregation behavior of block copolymers by modulating spreading volume. Moreover, we noted Kim and Kim studied the spreading area effect (with the same spreading concentration and volume) on the aggregation behaviors of PS-b-P2VP at the air/water interface.33 Unfortunately, we found that their structures formed at small spreading areas could not be strict monolayer structures because their corresponding surface pressures were much lower than those through the conventional deposition/compression process. EXPERIMENTAL SECTION Materials. A symmetric diblock copolymer PS-b-PMMA (SMMA51K, Mn(PS) = 25 000, Mn(PMMA) = 26 000, Mw/Mn = 1.09) was purchased from Polymer Source Inc. and used as received. Nonselective solvent chloroform (HPLC grade, Mallinckrodt) was used to prepare the 1.00 mg/mL stock solution of SMMA51K, which was further diluted to 0.50, 0.25 and 0.10 mg/mL. All solutions were kept in a refrigerator overnight prior to use to allow for equilibration. Isotherm Experiments. Surface pressure−molecular area (π−A) isotherms of the SMMA51K monolayers under different spreading conditions (Table 1) were characterized

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with a KSV minitrough (Finland) with an effective trough area of 324 × 75 mm2. The subphase water with a resistivity of 18.2 MΩ cm used for the isotherm experiments was purified and deionized with a water purification system (Molecular 1810C, China). The isotherm experiments were performed at 25 oC by using a refrigerated bath circulator (THD-0510, China). A platinum Wilhelmy plate with a perimeter of 39.24 mm was used to measure surface pressure, which was previously soaked in a concentrated HNO3/H2SO4 mixture (1:1 in volume) and washed thoroughly with ultrapure water to ensure cleanliness. In Table 1, πini and Aini represent the initial surface pressure and mean molecular area (mma) after the spreading of a solution, respectively. According to our previous similar experiment design,32 spreading conditions I−III were with different concentrations but with the same volume of 20 µL, and spreading conditions III−V were with different concentrations and volumes but with the same polymer amount (N) of 1.96 × 10-10 mol. From Table 1, it can be seen that the Aini of conditions III−V are the same (206 nm2), and much smaller than those of conditions I (1029 nm2) and II (412 nm2). Table 1. Spreading Conditions of the SMMA51K Monolayersa

a

Spreading conditions

I

II

III

IV

V

c (mg/mL) V (µL) N (× 10-10 mol) πini (mN/m) Aini (nm2)

0.10 20 0.39 0.009 1029

0.25 20 0.98 0.015 412

0.50 20 1.96 0.016 206

0.25 40 1.96 0.026 206

0.10 100 1.96 0.003 206

πini and Aini represent the initial surface pressure and mma after the spreading of a solution, respectively.

The spreading solutions were carefully deposited on the water as uniformly as possible by using 50 (conditions I−IV) or 100 µL (condition V) gastight microliter syringes with sharp ends (Anting, China). Figure 1 shows the schematic diagram of an ideal solution spreading

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method on the water surface, which exhibits the droplet positions and the spreading sequence. In our experiments, ∼18 and ∼36 (two cycles) droplets were usually adopted for the 20 and 40 µL solution spreading, respectively, by adjusting droplet size, and larger droplets and more cycles were adopted for the 100 µL solution spreading. There was a small decrease of surface pressure after the spreading till a stable value πini (shown in Table 1) after ∼15 min, which indicates the total evaporation of spreading solvent before a compression initiated. The trough area was robotically controlled by symmetrically compressing the monolayers with two barriers at a relative rate of 10 mm/min, and the molecule compression rates for conditions I to V were evaluated as 32, 13, 7, 7, and 7 nm2/min, respectively. Complete experiments were usually performed two or three times till the isotherms were almost superimposed for each condition.

Figure 1. Schematic diagram of an ideal solution spreading method on the water surface. The numbers represent the spreading sequence and the droplet positions. The red rod represents the position of platinum plate.

Preparation and AFM Characterization of LB Films. The KSV minitrough was also used to prepare the LB films onto silicon wafers which were cleaned by mainly using a mixture of deionized water, ammonia−water, and hydrogen peroxide (5:1:1 in volume), and a mixture of deionized water, hydrochloric acid, and hydrogen peroxide (6:1:1) according to the modified RCA cleaning procedure described in our previous paper.32 Two or three LB

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films were usually prepared for each condition. In a transfer process, a substrate was dipped into water before the solution spreading and withdrawn vertically through the monolayer at a speed of 2 mm/min after it was compressed to 0.2 or 7 mN/m and kept for 20 min. In order to explore the initial morphologies, the LB films were also prepared at ∼0 mN/m after each barrier moved only 5 or 10 mm to obtain the relatively stable initial structures following the solution spreading and total solvent evaporation, whose transfer ratios could not be evaluated. Furthermore, the typical transfer ratios for conditions III to V at 0.2 and 7 mN/m were about 1.02−1.07, while those for conditions I and II were 1.39/0.31 and 0.83/1.06, respectively, which is attributed to their surface concentration difference. In our opinion, the transfer ratios of block copolymer monolayers with less compacted structures are likely to deviate from 1.00 due to the possible relaxation of their long hydrophilic blocks during the transfer, which is different from the typical transfer ratios of ∼1.00 for most organic small amphiphiles transferred under very high pressures. Although the two transfer ratios of 0.31 and 0.83 seem a little low, their corresponding LB films exhibit uniform structures (shown later), indicating the formation of relatively stable monolayers at the air/water interface. Therefore, it is acceptable for us to assume that the morphologies of the LB films are almost identical to those of their corresponding Langmuir monolayers.14 According to the isotherms shown later, the detailed transfer conditions of the LB films are given in Table 2. From Table 2, it can be seen that the large mma variation ratios (∆A2/Aini and ∆A3/Aini) compressing to the corresponding transfer surface pressures markedly decrease from conditions I to III, while those for conditions III−V are relatively close. Furthermore, those transferred at ~0 mN/m

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(∆A1/Aini) are only 3.4% to 6.3% and are much smaller than the others, which indicate their corresponding LB films can be regarded as in their initial structures. Table 2. Detailed Transfer Conditions of the LB Filmsa Spreading conditions 2

Aini (nm ) A1 (nm2) A2 (nm2) A3 (nm2) ∆A1/Aini (%) ∆A2/Aini (%) ∆A3/Aini (%)

I

II

III

IV

V

1029 966 335 96 6.1 67.4 90.7

412 386 141 61 6.3 65.8 85.2

206 193 93 48 6.3 54.9 76.7

206 193 92 50 6.3 55.3 75.7

206 199 98 53 3.4 52.4 74.3

a

A1, A2 and A3 represent the mma’s at the transfer surface pressures of 0, 0.2 and 7 mN/m, respectively. ∆Ai (= Aini−Ai) represents the mma variation from the initial state to the corresponding transfer surface pressure.

The LB films were then characterized with a tapping mode AFM (SPA-300, Japan) at room temperature. A scanner of 150 µm was used, and the spring constant of the etched single crystal silicon probes was 40 N/m, and the scanning areas were all 2 × 2 µm2. Each LB film was scanned for two or three times at different locations and far from the edges to ensure reproducibility.34 RESULTS AND DISCUSSION Isotherms. Figure 2 shows the π−A isotherms of the SMMA51K monolayers spread under different conditions at 25 oC. From Figure 2, it can be seen that the compression isotherms of SMMA51K are nearly featureless and surface pressures remain just above 0 mN/m in a very wide range due to the weak surface activity of PMMA blocks, resulting in their low stretching degree and the formation of the condensed-type monolayers.17 With the evaporation of solvent, PS blocks tend to leave the water surface due to their strong repulsion from both water and PMMA blocks, and aggregate into micelle cores attributed to their van der Waals interactions.

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The isotherms of the SMMA51K monolayers (I−III) apparently shift to small areas with the increase of spreading concentration, which is consistent with the increase of the chain entanglement in the spreading solutions and confirmed by the following AFM images. The limiting mma (A0) are 117, 58, and 41 nm2, respectively, showing the transformation from a certain condensed monolayer (condition I) to a much condensed one (condition III). The A0 was determined by extrapolating the steepest linear region to zero surface pressure. At the same time, the maximum pressures (πmax) of the monolayers spread under conditions I and II are, respectively, ~12 and ~24 mN/m when the barriers move close to platinum plate, which are much lower than the collapse pressure (πc ≈ 57 mN/m) of the monolayer spread under condition III due to the limited polymer amount for the former two conditions. With the increase of the spreading volume, which increases the total amount of polymer added at the air/water interface, the isotherms of the SMMA51K monolayers also shift to small areas (I to V, and II to IV), showing a marked surface concentration effect on the isotherms. On the other hand, the isotherms of the monolayers (III to V) are almost identical because they were spread with the same polymer amount on the water surface (i.e., the same surface concentration). The A0 and πc of the monolayers spread under conditions IV and V are about 43 and 47 nm2, and 54 and 53 mN/m, respectively, which are close to those under condition III. However, their monolayer structures are quite different according to the AFM images of their LB films (shown later) due to their “local” surface concentration difference concerning with the detailed spreading process.

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Figure 2. π−A isotherms of the SMMA51K monolayers spread under conditions I−V at 25 oC. The arrows represent the corresponding transfer mma’s at ~0 mN/m under different conditions.

AFM Images. Figure 3 shows the AFM height images of the LB films of SMMA51K prepared under different spreading conditions. Several luminant domains in panels IIa and IIIa marked in red arrows and cycles can be attributed to some dust or silicon oddment introduced during the preparation of LB films.23 The raised bright domains represent the aggregated PS blocks,32 and the surrounded PMMA blocks can’t be distinguished due to their extended/coiled conformation with the negligible height difference. As for conditions I−III, all their initial morphologies exhibit tiny circular micelles although the PS cores in panels Ia and IIa are less apparent than those in IIIa whose diameters are ∼10 nm, which is consistent with our symmetric SMMA51K. Due to the long hydrophilic PMMA block in the copolymer, the tiny circular micelles are likely to form spontaneously by the first mechanism mentioned in

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the Introduction section. It is different from other PS-b-PMMA systems with very high molecular weights in which very few aggregates could be observed just after spreading16,17 may be due to their very small aggregation number with the increase of the hydrophilic block length.14 As for conditions IV and V, their initial morphologies are quite strange and different from the above three panels and exhibit a significant mixed structure of predominantly large and small circular micelles (IVa) and a mixed structure of predominantly rodlike aggregates and a few circular micelles (Va), respectively. With the increase of spreading volume, we had to spread the solutions with two (condition IV) or more cycles (condition V). After the first spreading cycle, the monolayers exhibit the tiny circular micelles like panels Ia and IIa, and the raised PS cores and the surrounded PMMA corona make the monolayer surfaces become hydrophobic and hydrophilic at their corresponding domains. During the second and the following cycles, the PS and PMMA blocks in the droplets tend to aggregate to the former cores and interact with the former corona, respectively, which prohibits the molecules from spreading out fully and results in the significant local surface concentration difference. In other words, the spreading parameter of the polymer solution over water (SPs/w) at the first cycle is larger than those of the polymer solution over polymer monolayer (SPs/pm) at the second and the following cycles. Therefore, their initial LB films exhibit the above nonuniform mixed structures with significantly large micelles or rodlike aggregates. It can be concluded that the detailed spreading process should also be responsible for the initial and final morphologies of the LB films besides spreading concentration and volume.

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Figure 3. AFM height images (2 × 2 µm2) of the LB films of SMMA51K prepared under spreading conditions I−V. Transfer surface pressures are 0 (a), 0.2 (b) and 7 mN/m (c), respectively. The detailed transfer mma’s are listed in Table 2, and those at 0 mN/m are also marked with arrows in Figure 2. Several luminant domains in panels IIa and IIIa marked in red arrows and cycles represent dust or silicon oddment.

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Upon compression to 0.2 mN/m, the LB film under condition I (panel Ib) exhibits a loosely mixed structure of predominantly rodlike aggregates and a few circular micelles, and the image is not fully occupied by the aggregates due to the limited polymer amount on the water surface. Upon compression, the initial tiny circular micelles aggregate into small ones which can further coalesce into rodlike aggregates. The mixed structures of rodlike aggregates and circular micelles we actually see is surprising, this observation can perhaps be explained by the large mma variation ratio (67.4%, see Table 2) needed to attain the pressure of 0.2 mN/m, resulting in more rodlike aggregates. We believe the formation of rodlike aggregates in panel Ib should be attributed to the coalescence of some small neighboring micelles, resulting from the very few PMMA blocks within the micelle coronas are pushed aside upon compression. The coalescence behavior is confirmed according to their same diameter/width (40 nm) and height (5 nm) of the small circular and the rodlike cores, which can be further confirmed under other spreading conditions according to the statistical information of the measureable nanoaggregates listed in Table 3. Upon further compression to 7 mN/m, the LB film (panel Ic) becomes homogeneous. Furthermore, the average distances between the cores in the above two LB films are larger than those in the others due to the relatively extended PMMA blocks under condition I. Table 3. Statistical Information of the Measureable Nanoaggregates in Figure 3 Panels

Ib

Ic

IIb

IIc

IIIb

IIIc

IVa

IVb

IVc

Va

Vb

Vc

Droda (nm) hrodb (nm) Dcirclec (nm) hcircled (nm) e

40 5 40−50 5 1.77

40 5 40−50 5 1.73

40 8 40−50 8 1.95

40 5 40−50 5 1.60

− − 40−60 10 2.93

30 5 30−60 5−10 2.97

30 6 30−60 6−11 2.46

30 6 30−60 6−11 2.92

30 6 30−70 6−11 2.67

30 1.5 30−60 1.5 0.68

30 1.5 30−60 1.5 0.48

40 1.5 40−70 1.5−3 0.65

Rough (nm) a,b

Represent the width and height of the rodlike cores, respectively. c,d Represent the diameter and height of

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the circular cores, respectively. e Represents the root mean square (RMS) roughness values of the LB films.

With the increase of spreading concentration, the LB film transferred at 0.2 mN/m (panel IIb) exhibits a densely mixed structure with more circular micelles and fewer rodlike aggregates compared to panel Ib, which is attributed to the higher local surface concentration. Similarly, upon compression, the initial tiny circular micelles aggregate into small ones which can further coalesce into rodlike aggregates. Upon further compression, it exhibits a very homogeneous labyrinth pattern (panel IIc), which is similar to what we observed in a symmetric PS-b-P2VP system spread with the PS-selective solvent tetrachloride carbon, which can both be interpreted as the stochastic coalescence among the rodlike aggregates and circular micelles.14 Furthermore, the LB films prepared under conditions I and II can be regarded as in the equilibrium (or near-equilibrium) structures according to their great size uniformity with relatively small root mean square (RMS) roughness values (1.77/1.73 and 1.95/1.60 nm, see Table 3). With the further increase of spreading concentration, the LB film transferred at 0.2 mN/m (panel IIIb) exhibits a regular structure of mostly large isolated circular micelles (core diameter of 60 nm). Upon compression, the initial tiny circular micelles aggregate into small ones, however, which can only further coalesce into large ones instead of rodlike aggregates. This is due to the very high local density of PS blocks within the cores (i.e., the local surface concentration) related to the increased PS chain entanglements according to the viewpoint of Cheyne and Moffitt on the concentrated spreading of PS-b-PEO.9,10 The above circular micelle structure is quite similar to our previous result for the same sample transferred at 2 mN/m,22 and the average core distance in the latter is smaller than that in the former. Upon

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further compression to 7 mN/m, the LB film (panel IIIc) exhibits a mixed structure of predominantly circular micelles and a few rodlike aggregates. The large amount of extended PMMA blocks around the large PS micelle cores prohibits the further coalescence of the latter, which is why we do not see any wider rodlike aggregates. With the increase of the spreading volume at the middle and low spreading concentrations, the initial mixed structures (panels IVa and Va) of the LB films of SMMA51K become denser upon compression to 0.2 mN/m (IVb and Vb), respectively. Compared with panel IIb, many large circular micelles appear in IVb due to the second spreading cycle resulting in the different local surface concentration, which is neither as homogeneous as that spread in the higher concentration and smaller volume (IIIa). Compared with panel Ib, panel Vb exhibits the densely packed structure with smaller cores (30 and 1.5 nm in diameter/width and height, respectively) due to the smaller mma variation ratio (52.4%, see Table 2) needed to attain the pressure of 0.2 mN/m. The PMMA blocks take the coiled conformation in the densely packed structures (Vb and Vc) due to the kinetic hindrance, which is similar to those in the kinetically frozen copolymer nanostrands.18 It is worth noting that the increase of spreading volume for the dilute spreading exhibits less apparent effect on the aggregate size than that for the middle spreading concentration. Upon further compression, the LB films transform into a mixed structure of predominantly large circular micelles and a few long rodlike aggregates (IVc) and another labyrinth pattern with some large circular micelles (Vc), respectively, which are also attributed to the coalescence of some small neighboring micelles and the stochastic coalescence among the rodlike aggregates and circular micelles, respectively. Furthermore,

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the LB films prepared under conditions III and IV can be regarded as in the nonequilibrium structures according to their relatively low size uniformity with the relatively large RMS roughness values (2.93/2.97 and 2.46/2.92/2.67 nm, see Table 3). It can be attributed to the PS chain entanglements similar to the kinetically trapped nonequilibrium structures observed in the PS-b-PEO systems.10 However, the LB films prepared under condition V can be regarded as in the equilibrium structures, due to the dilute spreading condition, according to their great size uniformity with the minimum RMS roughness values of 0.68/0.48/0.56 nm. CONCLUSIONS With the increase of spreading concentration or the amount of polymer added at the interface, the isotherms of the symmetric SMMA51K monolayers shift to small areas. Although the isotherms of the monolayers spread in the same surface concentration are almost identical (conditions III to V), their structures are quite different due to their local surface concentration differences in the spreading conditions. When the monolayers spread in different concentrations (conditions I to III), all their initial morphologies exhibit tiny circular micelles (~10 nm in diameter) due to the long hydrophilic PMMA block in the copolymer. The initial tiny circular micelles form spontaneously, and then aggregate into small ones upon compression, which can further coalesce into rodlike aggregates or large micelles depending on the spreading concentrations. The LB films of SMMA51K usually exhibit various mixed structures of rodlike aggregates and circular micelles, which can further transform into labyrinth patterns under some special spreading conditions. As far as we know, seldom work except ours32 considered the polymer surface/interface concentration effect on the aggregation

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behavior of block copolymers by modulating spreading volume. Besides spreading concentration and volume, we discover that the detailed spreading process should also be responsible for the initial and final morphologies of the LB films. In our opinion, the extended or coiled conformation of PMMA blocks taken, the strong repulsion of PS blocks from both water and PMMA blocks, the attractive interaction between PS blocks, together with the local surface concentration, can be used to interpret the isotherms and the AFM images. Furthermore, the LB films prepared under different spreading conditions can be regarded as in the equilibrium or nonequilibrium structures due to the kinetic effect difference resulting from the different PS chain entanglement degrees.

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AUTHOR INFORMATION Corresponding Author *Joint first author. http://wengangyao.polymer.cn/.

E-mail:

[email protected].

Homepage:

ORCID Gangyao Wen: 0000-0003-1025-8147 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Natural Science Fund of Heilongjiang Province (No. B2015023).

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Table of Contents Graphic

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Effects of Spreading Conditions on the Aggregation Behavior of a

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