Effects of Subphase pH and Temperature on the Aggregation

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Effects of Subphase pH and Temperature on the Aggregation Behavior of Poly(lauryl acrylate)-blockPoly(N-isopropylacrylamide) at the Air/Water Interface Kun You, Gangyao Wen, Athanasios Skandalis, Stergios Pispas, Shicheng Yang, Hongfei Li, and Zhao-Yan Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01320 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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The Journal of Physical Chemistry

Effects of Subphase pH and Temperature on the Aggregation Behavior of Poly(lauryl acrylate)-block-Poly(N-isopropylacrylamide) at the Air/Water Interface Kun You,a Gangyao Wen,a, Athanasios Skandalis,b Stergios Pispas,b Shicheng Yang,a Hongfei Li,c and Zhaoyan Sunc a

Department of Polymer Materials and Engineering, College of Material Science and

Engineering, Harbin University of Science and Technology, 4 Linyuan Road, Harbin 150040, PR China b

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation,

48 Vassileos Constantinou Ave., Athens 11635, Greece c

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China

*

Corresponding author. E-mail: [email protected] (G. Wen); Website: http://wengangyao.polymer.cn/. 1

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ABSTRACT: Effects of subphase pH and temperature on the aggregation behavior of a thermosensitive amphiphilic diblock copolymer poly(lauryl acrylate)-block-poly(Nisopropylacrylamide) (PLA-b-PNIPAM)

at

the air/water interface

and

the

morphologies of its LangmuirBlodgett (LB) films were characterized with the Langmuir film balance technique and atomic force microscopy, respectively. The surface pressuremolecular area isotherms shift positively with the increase of subphase pH and there exist two quasi-plateaus under acidic condition but only one under neutral or alkaline conditions. The lower and upper plateaus under acidic condition are attributed to immersion into water for the protonated amide groups and the rest of PNIPAM blocks, respectively. The plateau pressures gradually decrease with the elevation of temperature due to promotion of protonation and solubility of PNIPAM blocks. On the contrary, those under neutral and alkaline conditions gradually increase but exhibit a lower critical solution temperature behavior which is consistent with that of PNIPAM-containing polymers in aqueous solutions. The initial LB films of PLA-bPNIPAM transferred from different subphases exhibit tiny isolated circular micelles which coalesce and transform into large dense ones upon compression. Furthermore, PLA cores usually coalesce with the elevation of temperature due to the increased molecular thermal mobility as a result of their low glass transition temperature.

2


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INTRODUCTION Amphiphilic block copolymers comprising hydrophobic and hydrophilic segments have attracted extensive attention because they can self-assemble into well-defined structures at the air/water interface.117 Most systems such as polystyrene-blockpoly(ethylene oxide) (PS-b-PEO),17 polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA),812 and polystyrene-block-poly(vinyl pyridine) (PS-b-PVP)1317 have been widely investigated. There are mainly four mechanisms proposed to describe the aggregation phenomena of the amphiphilic block copolymers at the air/water interface:10 spontaneous surface aggregation,1−4 compression-induced surface aggregation,8,9 simple deposition of solution micelles,7 and reversal of solution micelles.13 Furthermore, the Langmuir film balance technique and atomic force microscopy (AFM) were usually combined to investigate the aggregation behaviors of amphiphilic block copolymers at the air/water interface. Three types of features were usually observed: dots (circular micelles), spaghetti (rodlike aggregates), and continents (planar aggregates) mainly attributed to different block compositions.3,13 For example, Glagola et al. studied a series of PS-b-PEO samples and found that the LangmuirBlodgett (LB) films of the highly hydrophobic samples (∼7% PEO or less) exhibited the mixtures of dots, spaghetti and continent-like structures, whereas those of the predominantly hydrophilic samples ( 20% PEO) exhibited dots.3 Poly(N-isopropylacrylamide) (PNIPAM) is an interesting thermosensitive polymer and its aqueous solution properties were investigated by many researchers.1825 Most importantly, PNIPAM has a lower critical solution temperature (LCST) of 32 oC in 3



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aqueous solution,18 and it takes coil conformation below 32 oC whereas it transforms into globule conformation above it. The critical temperature of PNIPAM is close to body temperature of human beings, and hence it has potential applications in drug release and drug carriers.19,20 Graziano pointed that the coil-to-globule collapse of PNIPAM above the Θ-point temperature is a first-order entropy-driven phase transition.21 In recent years, the aggregation behaviors of block copolymers containing PNIPAM at the air/water interface have attracted much attention.2633 Liu et al. explored the interfacial conformation properties of PS-b-PNIPAM and found that the dependencies of surface pressure on temperature and compression rate were strongly influenced by the loop or trail conformations of PNIPAM blocks, however, it has no dependency when they took the train conformation.27 Furthermore, subphase pH is also an important factor to affect the aggregation behaviors of block polyelectrolytes at the air/water interface. Claro et al. studied the interfacial

aggregation

poly[(dimethylamino)ethyl

behavior

of

methacrylate]

poly(benzyl

methacrylate)-block-

(PBzMA-b-PDMAEMA)

and

the

morphologies of its LB films.34 They found that at low pH the morphologies of the LB films transformed from dot-like domains into an island-like structure and then into a continent-like structure upon compression. Conversely, at high pH there existed a structural evolution from circular and quasi-hexagonally packed micelles, followed by a worm-like structure that collapses into a homogeneous film. A similar system, PS-bP2VP, was studied by Chung et al. and their results showed that the LB films of PS-bP2VP exhibited isolated circular micelles at high pH, whereas a laced network of 4


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circular micelles appeared at low pH.15 Recently, we studied the effects of subphase pH, temperature, and ionic strength on the Langmuir monolayers and LB films of poly(nbutylacrylate)-b-poly(acrylic

acid)

(PnBA-b-PAA)

and

found

that

ringlike

nanostructures exhibited in the LB films prepared from alkaline subphase with medium ionic strength.35 As mentioned above, people mainly studied aqueous solution properties of PNIPAM and paid much attention to its temperature sensitivity. Furthermore, there was a lack of systematic research on the interfacial aggregation behavior of PNIPAM-containing copolymers, and especially subphase pH sensitivity was neglected. In this work, the interfacial

aggregation

behavior

of

poly(lauryl

acrylate)-block-poly(N-

isopropylacrylamide) (PLA-b-PNIPAM) was investigated by the Langmuir film balance technique and AFM. Effects of subphase pH and temperature on the Langmuir monolayers and LB films were systematically studied. We believe this work will contribute to a better understanding of the physics underlying the formation of polyelectrolyte monolayers under various conditions, which should also bring some new insights on their aqueous solution properties.

EXPERIMENTAL SECTION Materials and Reagents. The reversible addition fragmentation chain transfer (RAFT) polymerization methodology was used to synthesize the PLA-b-PNIPAM sample which was further characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR). For more details on the block copolymer synthesis 5



by RAFT, see the following references.3638 In SEC experiment, solvent tetrahydrofuran containing 5% v/v triethylamine was used as the mobile phase and the system was calibrated using a series of monodisperse linear polystyrene standards. Deuterated chloroform (CDCl3) was used as solvent in NMR experiment. The weight-average molecular weights of PLA and PNIPAM blocks were 6,000 and 12,000, respectively. Polydispersity index (Mw/Mn) of the copolymer was 1.55 and its number-average molecular weight (Mn = 11,600) was used to calculate mean molecular area (mma) in the following isothermal experiments. The chemical structure of the PLA-b-PNIPAM sample is shown in Scheme 1. It should be noted that a pH responsive –COOH group is attached at the end of the hydrophobic PLA block with a low glass transition temperature (Tg = 18 oC), while a hydrophobic C12H25 group is attached to the hydrophilic PNIPAM block. Taking into account the relatively low molecular weight of the copolymer, the presence of the particular end groups may influence its selfassembly behavior at the air/water interface.

Scheme 1. Chemical structure of the PLA-b-PNIPAM sample.

Stable spreading solution of PLA-b-PNIPAM (0.50 mg/mL) was prepared with HPLC grade chloroform whose nonselectivity resulting in the random coiled 6


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conformation of copolymer chains. Ultrapure water with resistivity of 18.25 M cm used as neutral subphase (pH 7) was prepared with a water purification system (Molecular 1810C, China). Acidic (pH 3) and alkaline subphases (pH 9) were adjusted with H2SO4 and NaOH, respectively. πA isothermal Experiments. A KSV minitrough (Finland) secured inside a dust shield was used to measure the surface pressuremolecular area (πA) compression isotherms of Langmuir monolayers of PLA-b-PNIPAM. Effective area of the trough was 324  75 mm2. Surface pressure was recorded in real time with a rectangular filter paper (25 × 10 mm2), and subphase temperatures were maintained at 10, 20, 30, and 35 o

C with a water bath (THD-0510, China). A spreading solution of 20 μL was carefully

dropped onto the subphase with a gastight Hamilton mini-syringe according to our suggested ideal spreading method,10 and a 15-min interval was remained for the solvent to evaporate completely. The Langmuir monolayer on the subphase was symmetrically compressed with two mobile barriers at a constant relative speed of 10 mm/min. Each compression isotherm was run at least two times with good reproducibility till they were almost/totally overlapped. With proficient skills, we usually carried out the hysteresis experiment (compressionexpansion cycle) when most of a compression isotherm was almost overlapped with the former one (see Supporting Information, Figures S1 and S2). When mma reached 5 nm2, the barriers were stopped and kept for 30 s to allow the monolayer to relax, and then the expansion process began. Preparation and AFM Characterization of LB Films. The corresponding LB films were usually prepared by transferring the Langmuir monolayers onto silicon wafers at 7



10, 25, and 30 mN/m. Before the transfer, the silicon wafers were pretreated to be hydrophilic according to the modified RCA cleaning procedure described by us.39 After a transfer pressure was maintained for 20 min, a monolayer LB film was prepared by vertically pulling a silicon wafer through the monolayer at 2 mm/min. Furthermore, the LB films with initial morphologies were transferred at the initial mma of 46 nm2 (~0.1 mN/m) without compression. Transfer ratios of the LB films transferred at the initial mma could not be evaluated because the barrier position remained unchanged during the deposition process. Furthermore, the transfer ratios of the LB films under other conditions were about 0.751.70. The morphologies of the LB films were characterized with a tapping mode AFM (AFM5100N, Hitachi, Japan). Scan area and speed in the AFM measurements were 2 × 2 μm2 and 1.0 Hz, respectively. Spring constant of the rotated monolithic silicon probes of Multi75Al (Budget Sensors) was 3 N/m and the amplitude was ~1.00 V. The samples were stored in a refrigerator and scanned within two weeks. Two samples were usually scanned for each condition. For an LB film, the AFM images were usually taken two times and far from the edges to make sure of good reproducibility.

RESULTS AND DISCUSSION Subphase pH Effect on the Isotherms. At the air/water interface, amphiphilic PLAb-PNIPAM with long PNIPAM block tends to form isolated circular micelles composed of PLA cores and PNIPAM coronas just like other typical systems such as PS-b-P2VP.13 When the solution is dropped at the air/water interface, hydrophobic PLA blocks tend 8


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to leave the water surface because of their repulsion from both water and PNIPAM blocks and aggregate into micelle cores attributed to their van der Waals interactions. At the same time, PNIPAM blocks spread at the interface and form micelle coronas adopting the extended conformation. Figure 1 displays the πA isotherms of PLA-bPNIPAM monolayers on different subphases at 20 °C. Under acidic condition, in the region of large mma ( 25 nm2), surface pressure remains almost unchanged because the concentration of the pancake-type surface micelles is too low for them to contact with each other.35 In the region of 2513 nm2, surface pressure increases gradually upon compression because of repulsion between PNIPAM coronas. Upon further compression, there exist two quasi-plateaus at 1310 and 84 nm2 (shown in the panel) which are attributed to immersion into water for the protonated portion and the rest of PNIPAM blocks, respectively. That is, protonation of the nitrogen of secondary amine groups40 and/or the oxygen of carbonyl groups41 in PNIPAM blocks takes place under acidic condition and thus amide groups preferentially immerse into subphase, which corresponds to the lower plateau. Then the rest of PNIPAM blocks gradually immerses into subphase and finally form the extended brush-type array,35 which corresponds to the upper plateau. After the pancake-to-brush transition of PNIPAM coronas ( 4 nm2), the relatively soft hydrophobic PLA cores begin to connect with each other resulting in the steep rise in surface pressure. According to the above two plateaus, it is evident that even under acidic condition the whole PNIPAM blocks are first adsorbed at the air/water interface due to the existence of the hydrophobic isopropyl and C12H25 groups and then gradually submerge into aqueous subphase upon compression. It is quite 9



different from the absence of plateaus for PS-b-P2VP under acidic condition15 and PnBA-b-PAA under alkaline condition35 due to ionization of their hydrophilic blocks which totally immerse into water prior to compression. Furthermore, Busse et al. reported that in the isotherms of triblock copolymers composed of long PEO middle blocks and poly((perfluorohexyl)ethyl methacrylate) (PFMA) end blocks also exhibited two pseudoplateaus which were attributed to immersion into water of PEO blocks (lower plateau) and the horizontal to vertical rearrangement of the whole PFMA chains (upper plateau) at the air/water interface according to x-ray reflectivity results.42 The isotherms under neutral and alkaline conditions are similar and absent of the lower plateau, and their single quasi-plateaus both appear at ~114 nm2 (plateau width A = 7 nm2), which indicates protonation degrees of PNIPAM blocks are quite low and negligible and different from that under acidic condition. At the same time, the total width of the two quasi-plateaus (A = 3 + 4 = 7 nm2) of the isotherm under acidic condition is identical to those of their single plateaus under neutral and alkaline conditions, which further confirms that the protonated amide groups of PNIPAM blocks preferentially immerse into subphase at the lower plateau region. It is worth noting that there exists a less apparent turn under neutral and alkaline conditions at 15 and 17 nm2, respectively, suggesting a conformational rearrangement of PNIPAM blocks. Upon compression, hydrophobic isopropyl groups in PNIPAM blocks probably transform from the irregular orientations to vertically towards the air. Furthermore, the isotherms shift to large mma with the increase of subphase pH, which is because of the decreased protonation degree and the resulting stretch degree increase of PNIPAM blocks at the 10


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air/water interface.

Figure 1. πA isotherms of PLA-b-PNIPAM monolayers on different subphases at 20 °C. The panel shows that the two quasi-plateaus under acidic condition are at 1310 and 84 nm2. Subphase pH Effect on the Hysteresis Curves. To further study the effect of subphase pH on the aggregation behaviors of PLA-b-PNIPAM at the air/water interface, the hysteresis curves of PLA-b-PNIPAM monolayers on different subphases at 20 °C were measured and shown in Figure 2. From Figure 2, it can be observed that all the monolayers show hysteresis phenomena due to the possible entanglements and hydration of the immerged PNIPAM blocks. Hysteresis degree can be expressed by A0 = A0p  A0p, where A0p and A0p are the pancake limiting areas in the compression and expansion isotherms, respectively.4 A0p and A0p were obtained by extrapolation of the 11



linear region (515 mN/m) in the isotherms to π = 0 and the A0 are 5, 4, and 3 nm2 at pH 3, 7, and 9, respectively. Hysteresis degrees of the monolayers under different conditions are almost close due to the negligible solubility difference of PNIPAM blocks when they are all compressed to 5 nm2 and all PNIPAM blocks immerse into water. Nevertheless, hysteresis degree under acidic condition is slightly larger than the others, which is attributed to protonation of PNIPAM blocks.

Figure 2. Hysteresis curves of PLA-b-PNIPAM monolayers on different subphases at 20 °C. Subphase Temperature Effect on the Isotherms. The πA isotherms of PLA-bPNIPAM monolayers on different subphases at various temperatures are presented in Figure 3. From Figure 3a, it can be observed that the isotherms under acidic condition at different temperatures are similar and almost overlap below ~15 mN/m but their 12


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transitional plateau pressures gradually decrease with the elevation of temperature in the region of 154 nm2. Generally, the proton binding process is endothermic and the driving force is entropic in nature,43 which promotes protonation and solubility of PNIPAM blocks at high temperature and results in the decrease of plateau pressures.

Figure 3. πA isotherms of PLA-b-PNIPAM monolayers on different subphases at various temperatures. It is evident that temperature effects on the isotherms under neutral and alkaline conditions are quite different from that under acidic condition. From panels 3b and 3c, it can be observed that the isotherms below ~19 mN/m slightly deviate to large mma and their transitional plateau pressures gradually increase with the elevation of temperature up to 30 °C. This means that adsorption of PNIPAM segments at the interface is favored at high temperature, which is due to the decrease of hydrogen bonding between nitrogen of PNIPAM and water molecules.27 Furthermore, the 13



increased interfacial adsorption of PNIPAM blocks enhances their repulsion, leading to the increase of plateau pressure. However, when subphase temperature reaches 35 °C, the transitional plateau pressures become lower than or close to instead of being higher than those at 30 °C, which is consistent with the coil-to-globule conformational transition of PNIPAM in aqueous solution when the temperature is higher than its LCST of 32 °C.18 Subphase pH Effect on the LB Films. Figure 4 shows the AFM height images of the LB films transferred from different subphases at 20 °C. Several relatively large luminous domains in panels 4c and 4d marked in red are impurities which may be attributed to some dust or silicon oddment introduced during the preparation of LB films.35 The raised bright domains are PLA cores due to their tendency to leave the water surface before transfer, and the surrounded PNIPAM blocks adsorbing on the silicon wafers cannot be distinguished because of their extended/coiled conformation with the negligible height difference. The cores in each panel were classified into two levels with small and large sizes, and then diameters of two typical cores at each level were measured (just as shown in panel 4a), and finally the core diameter range was provided according to their minimum and maximum. From Figure 4, all the initial LB films transferred from different subphases exhibit uniform tiny circular micelles and core diameters are about 2026 nm except for some large ones of ~48 nm in panel 4a labeled in red because of the possibly high local concentration. The initial LB films exhibit tiny circular micelles due to the long hydrophilic PNIPAM blocks which spread at the interface and prevent PLA cores from connecting with each other. 14


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Figure 4. AFM height images of the LB films of PLA-b-PNIPAM transferred from acidic (ad), neutral (eh), and alkaline subphases (il) at 20 oC. Transfer pressures: ~0.1 (a,e,i, mma = 46 nm2), 10 (b,f,j), 25 (c,g,k), and 30 mN/m (d,h,l). Several typical micelle core diameters are labelled in panel 4a, and several relatively large luminous domains are marked in panels 4c and 4d. Upon compression, the morphologies of the LB films transferred at 10 mN/m do not exhibit significant variation compared with their initial ones due to the strong repulsive interaction among long PNIPAM blocks, and the micelle core diameters are also about 2026 nm. Under acidic condition, the LB film transferred at 25 mN/m (panel 4c) exhibits dense circular micelles and core diameters are about 3051 nm, which is larger 15



than those transferred at low pressures due to the further coalescence of some adjoining PLA cores. The low Tg of PLA blocks facilitates this kind of coalescence. Upon further compression to 30 mN/m, the LB film (panel 4d) also displays dense circular micelles but their core diameters decrease to 3045 nm, which is due to the split effect of the large cores.39,44 It is noting that there are some conjoint micelles in panels 4c and 4d due to the high pressure overcoming the low electrostatic repulsion35 among protonated PNIPAM blocks. Under neutral and alkaline conditions, the LB films transferred at 25 mN/m both display dense circular micelles whose core diameters are 3045 (panel 4g) and 2230 nm (panel 4k), respectively. It is evident that the core diameter decreases with the increase of subphase pH due to the gradually extended PNIPAM blocks at the interface and the resulting decreased aggregation number. Upon further compression to 30 mN/m, the LB films under neutral and alkaline conditions (panels 4h and 4l) both display dense circular micelles with some conjoint ones, and the typical core diameters are about 3050 and 3046 nm, respectively. Subphase Temperature Effect on the LB Films. Figure 5 displays the AFM height images of the LB films of PLA-b-PNIPAM transferred from different subphases at 10 mN/m and different temperatures. All the LB films display tiny/small circular micelles and the core diameter ranges are given in Table 1. Under acidic condition, the difference among the LB films transferred at different temperatures can be negligible although there is a small change in the core sizes. Under neutral condition, core diameters in the LB films transferred at 10 and 20 °C are similar but those at 35 °C increase slightly due 16


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to the relatively higher molecular thermal mobility and the resulted micelle coalescence. Under alkaline condition, micelle amounts and core diameters in the LB films transferred at 20 and 35 °C are both smaller than those at 10 °C due to their relatively extended PNIPAM blocks and the resulting decreased aggregation number.

Figure 5. AFM height images of the LB films of PLA-b-PNIPAM transferred from acidic (ac), neutral (df), and alkaline subphases (gi) at 10 mN/m and 10 (a,d,g), 20 (b,e,h), and 35 oC (c,f,i).

17



Table 1 Core Diameter Ranges (in nm) in the LB Films of PLA-b-PNIPAM in Figure 5 Conditions 10 °C 20 °C 35 °C pH 3 2029 2026 2233 pH 7 2026 2026 2639 pH 9 2232 2026 2032 Figure 6 displays the AFM height images of the LB films of PLA-b-PNIPAM transferred from different subphases at 25 mN/m and different temperatures. Obviously, circular micelles in all the LB films transferred at 25 mN/m are denser than those at 10 mN/m. Under acidic condition, the LB films (panels 6a6c) exhibit dense circular micelles, and the core diameter ranges at 10, 20, and 35 °C are about 3047, 3051, and 3273 nm, respectively. Some large aggregates appearing at high temperature is due to the relatively higher molecular thermal mobility and the increased attractive interactions between PNIPAM coronas, which is benefit for micelle coalescence. Under neutral conditions, the LB films (panels 6d6f) also exhibit dense circular micelles, and the core diameter ranges are respectively about 2949, 3045, and 3174 nm with the rise in temperature and most of them are 4547 nm which is consistent with the overlapped region around 25 mN/m in the isotherms (Figure 3b). Under alkaline condition, the micelles are much sparser than those under acidic and neutral conditions due to the relatively extended PNIPAM blocks at the interface which can hinder contact of PLA cores. The LB films transferred at 10 and 35 °C (panels 6g and 6i) exhibit isolated circular micelles and the core diameter ranges are about 2448 and 2451 nm, respectively, which are larger than those of dense circular micelles at 20 °C (2230 nm, panel 6h) due to the possible micelle coalescence in the formers.

18


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Figure 6. AFM height images of the LB films of PLA-b-PNIPAM transferred from acidic (ac), neutral (df), and alkaline subphases (gi) at 25 mN/m and 10 (a,d,g), 20 (b,e,h), and 35 °C (c,f,i).

CONCLUSIONS In summary, the Langmuir film balance technique and AFM were used to investigate the effects of subphase pH and temperature on the aggregation behavior of the 19



amphiphilic copolymer PLA-b-PNIPAM at the air/water interface. The isotherms shift positively with the increase of subphase pH. There exist two quasi-plateaus in the isotherms under acidic conditions, which are attributed to immersion into water for the protonated portion and the rest of PNIPAM blocks. It is quite different from the absence of plateaus for PS-b-P2VP under acidic condition15 and PnBA-b-PAA under alkaline condition.35 Furthermore, the isotherms under neutral and alkaline conditions are similar and absent of the lower quasi-plateau, indicating protonation degrees of PNIPAM blocks can be negligible. Under acidic condition, the transitional plateau pressures in the isotherms gradually decrease with the elevation of temperature due to promotion of protonation and solubility of PNIPAM blocks. Under neutral and alkaline conditions, the transitional plateau pressures at 35 °C are lower than or close to instead of being higher than those at 30 °C, which is consistent with the LCST behavior of PNIPAM.18 All the initial LB films transferred from different subphases exhibit the tiny/small circular micelles which form spontaneously. Upon compression to 25 mN/m, the LB films transferred from acidic and neutral subphases at different temperatures display dense circular micelles. Whereas the LB films transferred from alkaline subphase at 10 and 35 °C exhibit isolated circular micelles which are larger than those dense circular micelles at 20 °C due to the possible micelle coalescence in the formers. Furthermore, core diameters decrease with the increase of subphase pH due to the gradually extended PNIPAM blocks and the resulting decreased aggregation number, facilitated also by the low Tg of PLA cores. All the LB films transferred from different subphases at 30 mN/m 20


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exhibit dense circular micelles with some conjoint ones. Usually, PLA cores coalesce with the elevation of temperature due to the relatively higher molecular thermal mobility and their low Tg.

ACKNOWLEDGEMENTS This work was supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and the Natural Science Fund of Heilongjiang Province (no. B2015023).

Supporting Information. Figures S1 and S2 showing the almost/totally overlapped compression isotherms.

REFERENCES 1. Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Controlled Patterning of Diblock Copolymers by Monolayer

LangmuirBlodgett Deposition.

Macromolecules 2000, 33, 5432−5436. 2. Cheyne, R. B.; Moffitt, M. G. Self-Assembly of Polystyrene-block-Poly(ethylene oxide) Copolymers at the AirWater Interface: Is Dewetting the Genesis of Surface Aggregate Formation? Langmuir 2006, 22, 8387−8396. 3. Glagola, C. P.; Miceli, L. M.; Milchak, M. A.; Halle, E. H.; Logan, J. L. Polystyrene-Poly(ethylene oxide) Diblock Copolymer: The Effect of Polystyrene and Spreading Concentration at the Air/Water Interface. Langmuir 2012, 21



28, 5048−5058. 4. Zhu, J.; Eisenberg, A.; Lennox, R. B. Interfacial Behavior of Block Polyelectrolytes. 5. Effect of Varying Block Lengths on the Properties of Surface Micelles. Macromolecules 1992, 25, 6547−6555. 5. Harirchian-Saei, S.; Wang, M. C. P.; Gates, B. D.; Moffitt, M. G. Patterning Block Copolymer Aggregates via Langmuir−Blodgett Transfer to Microcontact-Printed Substrates. Langmuir 2010, 26, 5998–6008. 6. de Vos, W. M.; de Keizer, A.; Kleijn, J. M.; Stuart, M. A. C. The Production of PEO Polymer Brushes via Langmuir-Blodgett and Langmuir-Schaeffer Methods: Incomplete Transfer and Its Consequences. Langmuir 2009, 25, 4490–4497. 7. da Silva, A. M. G.; Gamboa, A. L. S.; Martinho, J. M. G. Aggregation of Polystyrene-Poly(ethylene oxide) Diblock Copolymer Monolayers at the Air Water Interface. Langmuir 1998, 14, 5327−5330. 8. Seo, Y.; Cho, C. Y.; Hwangbo, M.; Choi, H. J.; Hong, S. M. Effect of Temperature on the Interfacial Behavior of a Polystyrene-b-Poly(methyl methacrylate) Diblock Copolymer at the Air/Water Interface. Langmuir 2008, 24, 2381−2386. 9. Lamarre, S. S.; Yockell-Lelièvre, H.; Ritcey, A. M. Assembly of polystyrenecoated gold nanoparticles at the air–water interface. J. Colloid Interface Sci. 2015, 443, 131–136. 10. Gao, M.; Wen, G.; Wang, L. Effects of Spreading Conditions on the Aggregation Behavior of a Symmetric Diblock Copolymer Polystyrene-block-poly(methyl methacrylate) at the Air/Water Interface. Langmuir 2018, 34, 9272–9278. 22


Page 22 of 28

Page 23 of 28 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


11. Seo, Y.; Paeng, K.; Park, S. Molecular Weight Effect on the Behaviors of Polystyrene-block-Poly(methyl methacrylate) Diblock Copolymers at Air/Water Interface. Macromolecules 2001, 34, 8735−8744. 12. Destri, G. L.; Gasperini, A. A. M.; Konovalov, O. The Link Between SelfAssembly and Molecular Conformation of Amphiphilic Block Copolymers Monolayers at the Air/Water Interface: The Spreading Parameter. Langmuir 2015, 31, 8856−8864. 13. Wen, G.; Chung, B.; Chang, T. Effect of spreading solvents on Langmuir monolayers and Langmuir–Blodgett films of PS-b-P2VP. Polymer 2006, 47, 85758582. 14. Perepichka, I. I.; Lu, Q.; Badia, A.; Bazuin, C. G. Understanding and Controlling Morphology Formation in Langmuir-Blodgett Block Copolymer Films Using PSP4VP and PS-P4VP/PDP. Langmuir 2013, 29, 4502−4519. 15. Chung, B.; Choi, M.; Ree, M.; Jung, J. C.; Zin, W. C.; Chang, T. Subphase pH Effect on Surface Micelle of Polystyrene-b-poly(2-vinylpyridine) Diblock Copolymers at the AirWater Interface. Macromolecules 2006, 39, 684−689. 16. Choi, M.; Chung, B.; Chun, B.; Chang. T. Surface micelle formation of polystyrene-b-poly(2-vinylpyridine) diblock copolymer at air-water interface. Macromol. Res. 2004, 12, 127−133. 17. Zhu, J.; Lennox, R. B.; Eisenberg, A. Interfacial Behavior of Block Polyelectrolytes. 2. Aggregation Numbers of Surface Micelles. Langmuir 1991, 7, 1579–1584. 18. Schild, H. G. Poly(N-isopropylacrylamide): experiment, theory and application. 23



Prog. Polym. Sci. 1992, 17, 163249. 19. Wu, C.; Ying, A.; Ren, S. Fabrication of polymeric micelles with core–shell–corona structure for applications in controlled drug release. Colloid Polym. Sci. 2013, 291, 827834. 20. Wei, H.; Cheng, S.; Zhang, X.; Zhuo, R. Thermo-sensitive polymeric micelles based on poly(N-isopropylacrylamide) as drug carriers. Prog. Polym. Sci. 2009, 34, 893910. 21. Graziano, G. On the temperature-induced coil to globule transition of poly-Nisopropylacrylamide in dilute aqueous solutions. Int. J. Biol. Macromol. 2000, 27, 8997. 22. Binkert, T.; Oberreich, J.; Meewes, M.; Nyffenegger, R.; Rička, J. Coil-globule transition of poly(N-isopropylacrylamide): a study of segment mobility by fluorescence depolarization. Macromolecules 1991, 24, 5806–5810. 23. Yang, M.; Zhao, K. Cononsolvency of poly(N-isopropylacrylamide) in methanol aqueous solution—insight by dielectric spectroscopy. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 12271234. 24. Zhu, P. Particle formation and aggregation–collapse behavior of poly(Nisopropylacrylamide) and poly(ethylene glycol) block copolymers in the presence of cross-linking agent. J. Mater. Sci-Mater. M. 2004, 15, 567573. 25. Gong, X.; Wu, C.; Ngai, T. Surface interaction forces mediated by poly(Nisopropylacrylamide) (PNIPAM) polymers: effects of concentration and temperature. Colloid Polym. Sci. 2010, 288, 11671172. 24


Page 24 of 28

Page 25 of 28 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


26. Yokoi, K.; Kawaguchi, M. Effect of temperature on surface pressure–area isotherms and surface dilational moduli of poly(N-isopropyl acrylamide) monolayers spread at air–water interface. Colloids Surf. A: Physicochem. Eng. Asp. 2014, 457, 469475. 27. Liu,

G.; Yang,

S.;

Zhang,

G.

Conformational

Changes

of

poly(N-

isopropylacrylamide) Chains at Air/Water Interface: Effects of Temperature, Compression Rate, and Packing Density. J. Phys. Chem. B 2007, 111, 3633–3639. 28. Deshmukh, O. S.; Maestro, A.; Duits, M. H. G.; van den Ende, D.; Stuart, M. C.; Mugele, F. Equation of state and adsorption dynamics of soft microgel particles at an air–water interface. Soft Matter 2014, 10, 70457050. 29. Matsuoka, H.; Uda, K. Nanostructure of poly(N-isopropylacrylamide) Brush at the Air/Water

Interface

and

Its

Responsivity

to

Temperature

and

Salt.

Langmuir 2016, 32, 8383–8391. 30. Okumura, Y.; Kawaguchi, M. Surface pressure–area isotherms and surface dilational moduli of poly (N-isopropyl acrylamide) monolayers spread at air–water interface. Colloids Surf. A: Physicochem. Eng. Asp. 2014, 441, 275280. 31. Zielińska, K.; Sun, H.; Campbell, R. A.; Zarbakhsh, A.; Resmini, M. Smart nanogels at the air/water interface: structural studies by neutron reflectivity. Nanoscale 2016, 8, 49514960. 32. Gonçalves da Silva, A. M.; Lopes, S. I.; Brogueira, P.; Prazeres, T. J.; Beija, M.; Martinho, J. M. Thermo-responsiveness of poly(N,N-diethylacrylamide) polymers at the air–water interface: The effect of a hydrophobic block. J. Colloid Interface 25



Page 26 of 28

Sci. 2008, 327, 129137. 33. Cohin, Y.; Fisson, M.; Jourdé, K.; Fuller, G. G.; Sanson, N.; Talini, L.; Monteux, C. Tracking the interfacial dynamics of PNiPAM soft microgels particles adsorbed at the air–water interface and in thin liquid films. Rheol. Acta 2013, 52, 445454. 34. dos Santos Claro, P. C.; Coustet, M. E.; Diaz, C.; Maza, E.; Cortizo, M. S.; Requejo, F. G.; Pietrasanta, L. I.; Ceolín, M.; Azzaroni, O. Self-assembly of PBzMA-bPDMAEMA diblock copolymer films at the air–water interface and deposition on solid substrates via Langmuir–Blodgett transfer. Soft Matter 2013, 9, 1089910912. 35. Wang, Y.; Wen, G.; Pispas, S.; Yang, S.; You, K. Effects of subphase pH, temperature and ionic strength on the aggregation behavior of PnBA-b-PAA at the air/water interface. J. Colloid Interface Sci. 2018, 512, 862870. 36. Papagiannopoulos, A.; Meristoudi, A.; Pispas, S.; Radulescu, A. Micelles from HOOC-PnBA-b-PAA-C12H15 Diblock Amphiphilic Polyelectrolytes as Protein Nanocarriers. Biomacromolecules 2016, 17, 3816–3827. 37. Skandalis, A.; Pispas, S. PLMA-b-POEGMA amphiphilic block copolymers: Synthesis and self-assembly in aqueous media. J. Polym. Sci. Part A: Polym. Chem. 2017, 55, 155–163. 38. Škvarla, J.; Zedník, J.; Šlouf, M.; Pispas, S.; Štěpánek, M. Poly(N-isopropyl acrylamide)-block-poly(n-butyl

acrylate)

thermoresponsive

amphiphilic

copolymers: Synthesis, characterization and self-assembly behavior in aqueous solutions. Eur. Polym. J. 2014, 61, 124132. 39. Wen, G. Network Structure Control of Binary Mixed Langmuir Monolayers of 26


Page 27 of 28 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


Homo-PS and PS-b-P2VP. J. Phys. Chem. B 2010, 114, 38273832. 40. Bush, M. F.; Forbes, M. W.; Jockusch, R. A.; Oomens, J.; Polfer, N. C.; Saykally, R. J.; Williams, E. R. Infrared spectroscopy of cationized lysine and epsilon-Nmethyllysine in the gas phase: Effects of alkali-metal ion size and proton affinity on zwitterion stability. J. Phys. Chem. A 2007, 111, 77537760. 41. Li, G.; Song, S.; Guo, L.; Ma, S. Self-assembly of thermo- and pH-Responsive poly(acrylic acid)-b-poly(N-isopropylacrylamide) micelles for drug delivery. J. Polym. Sci. Pol. Chem. 2008, 46, 50285035. 42. Busse, K.; Peetla, C.; Kressler, J. Water surface covering of fluorinated amphiphilic triblock copolymers: Surface pressure-area and x-ray reflectivity investigations. Langmuir 2007, 23, 6975–6982. 43. Bretti, C.; Stefano, C. D.; Lando, G.; Majlesi, K.; Sammartano, S. Thermodynamics (Solubility and Protonation Constants) of Risedronic Acid in Different Media and Temperatures (283.15–318.15 K). J. Solution Chem. 2017, 46, 19031927. 44. Wang, X.; Wen, G.; Huang, C.; Wang, Z.; Shi, Y. Aggregation behavior of the blends of PS-b-PEO-b-PS and PS-b-PMMA at the air/water interface. RSC Adv. 2014, 4, 4921949227.

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Effects of Subphase pH and Temperature on the Aggregation

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