Self-Assembly of Patterned Porous Films from Cyclic Polystyrenes via

Jan 31, 2018 - The architecture of polymers plays a crucial role in self-assembly processes. However ... It is the first time to introduce cyclic poly...
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Article Cite This: J. Phys. Chem. C 2018, 122, 3926−3933

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Self-Assembly of Patterned Porous Films from Cyclic Polystyrenes via the Breath Figure Method Bai-Heng Wu,†,‡ Lian-Wei Wu,†,‡ Kang Gao,†,‡ Sen-He Chen,†,‡ Zhi-Kang Xu,†,‡ and Ling-Shu Wan†,‡,* †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, Hangzhou 310027, China ‡

S Supporting Information *

ABSTRACT: The architecture of polymers plays a crucial role in self-assembly processes. However, the effect of end groups is still not understood thoroughly in many processes including the breath figure method, which is effective for preparing patterned porous films. Herein, we synthesized a series of well-defined cyclic polystyrenes by the combination of atom transfer radical polymerization and azide−alkyne click cyclization and investigated the self-assembly behaviors. The corresponding linear polymers were also compared. Results indicate that polar end groups dramatically improve the regularity of the self-assembled films. For cyclic polystyrenes without any end group, the ability to form patterned porous films is between two groups of linear samples. On the one hand, the results confirm the effect of polar end groups in the breath figure method. On the other hand, the impact of cyclic topology is also verified. This work reveals the nature of stabilization of templating droplets via the polymers by the measurement of the precipitation rate. It is the first time to introduce cyclic polymers to the breath figure process, and the results enlighten us on the molecular architecture design and provide insights into the self-assembly mechanism.

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property,20 type of substrates,21 and most importantly, the polymer architecture.22−25 Widawski et al. found that starshaped polystyrene can form honeycomb films first in 1994.1 It seems that only star-shaped polymers can generate regular honeycomb films in early research studies.26,27 Actually, some of the star-shaped polymers are aggregated by rod-coil diblock copolymers because of different solubilities of each block.1,28,29 Conjugated blocks with poor solubility in a nonpolar solvent will aggregate as a core, whereas soluble blocks surround as the shell. On the basis of the findings, empirical knowledge is summarized that a high segment density is necessary for polymers to form honeycomb-patterned films because it is beneficial to a rapid precipitation and can help with stabilizing water droplets.22 This has been preliminarily confirmed by surface tension measurement.30 As a result, other polymer

INTRODUCTION Hexagonal arrays with micrometer or submicrometer dimensions fascinate scientists and artists to no end because of their aesthetic appeal, ubiquitous nature, and promising potential in material design. Numerous techniques have been developed to fabricate well-ordered hexagonal arrays. Compared with lithography approaches, self-assembly methods draw great attention for their simplicity and efficiency. Among them, the breath figure method1 has received attention in recent years. In this method, uniform water droplets serve as templates via nucleation, growth, and arrangement driven by the evaporation of a volatile solvent. 2 After solidification and further evaporation, exquisite honeycomb-patterned porous films can be prepared and utilized as microreactors,3,4 separation membranes,5,6 cell culture substrates,7,8 superhydrophobic materials,9−11 lithography templates,12,13 and others.14−17 The morphology of honeycomb films has received particular interest. A series of experimental factors can influence the final structure, including relative humidity,18 airflow rate,19 solvent © 2018 American Chemical Society

Received: December 14, 2017 Revised: January 29, 2018 Published: January 31, 2018 3926

DOI: 10.1021/acs.jpcc.7b12286 J. Phys. Chem. C 2018, 122, 3926−3933

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The Journal of Physical Chemistry C

Topologically distinct cyclic polymers possess no end group and show many interesting unique physical properties compared with linear polymers,48 viscosity,49,50 density,51 glass transition temperature,52 thermal phase transition,53,54 and surface properties.55 The difference from linear polymers can be attributed to the lack of chain end and the restricted motion of segments. Moreover, the self-assembly of cyclic polymers is unique in many ways.56 As far as we know, for the first time, we propose investigating cyclic polymers by the breath figure method aiming to study the self-assembly behaviors. On the one hand, this distinct topology could help us with understanding the effect of end groups. On the other hand, the cyclic polymer has relatively higher segment density than its linear precursor, and in this way, it can facilitate understanding of the influence of segment density.

architectures with high segment density can also be used to prepare honeycomb films, including comb-like polymers,31−33 dendritic polymers,34−36 and hyperbranched polymers.37,38 In addition, some other polymers without high segment density have been investigated, and the results indicate that they can also stabilize water droplets quite efficiently. For example, monocarboxylated end-functionalized linear polystyrenes yielded high-quality honeycomb films.39,40 However, totally hydrophobic linear polystyrenes without any polar groups can only generate regular honeycomb films under very specific conditions.41 It should be noticed that end-functionalized polymers can be easily synthesized by anionic polymerization or controlled radical polymerizations. Also, excellent control of molecular weight and polydispersity can be reached at the same time. Thanks to these versatile techniques, we investigated the influence of different end groups systematically.42,43 The morphology of films varies along with the polarities of end groups, and polar end groups are beneficial for widening the experimental conditions for regular honeycomb films. It seems that the end groups govern the robustness of optimal conditions for the breath figure. Also, the influence of end groups is quite universal, and even for a star-shaped polymer with high segment density, the difference in end groups will change the pore size and regularity.26 Besides, the introduction of polar groups to the middle of a polymer chain is also verified effective to produce highly ordered porous honeycomb films.44 Hence, from these perspectives, it is quite necessary for a linear polymer to possess polar groups to promise the regularity. Amphiphilic linear polymer is another good candidate which does not demand high chain density. Shimomura and coworkers found that amphiphilic copolymers and amphiphilic polyion complexes can be employed in the breath figure method45 because these amphiphilic polymers serve as surfactants to maintain the stability of water droplets. In their later work, they reported the importance of both hydrophile− lipophile balance values and interfacial tensions, which are directly linked to the stability of water droplets.46 Our group synthesized a series of well-defined polystyrene-block-poly(N,Ndimethylaminoethyl methacrylate) (PS-b-PDMAEMA) and discovered that for a certain molecular weight and concentration, there is an optimal block length ratio to tailor appropriate amphiphilicity to inhibit coalescence during the breath figure process.47 A certain range of interfacial tension is indispensable, and this conclusion has been verified by other block copolymers. Two mechanisms are probably responsible for the formation of honeycomb-patterned porous films from polymers with different architectures. One is fast precipitation of polymers with high segment density to make envelops stop coalescence, and the other is working as surfactants to decrease the interfacial tension. The morphology of honeycomb films is based on a combination of contribution of the two mechanisms from the point of both dynamics and thermodynamics. Till now, we cannot tell exactly what happens during the breath figure process. Every polymer we have ever studied by the breath figure method possesses end groups as well as segment density. It is quite difficult to study the two effects separately. Furthermore, all the research studies focusing on the end group effect has been short of a control polymer sample without end groups. Besides, the use of polymers without end groups can fill the gap of the research on polymer architectures and help us further unveil the mechanism of the breath figure process.

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EXPERIMENTAL SECTION Materials. All materials were purchased from Sinopharm Chemical Reagent Co. unless otherwise stated. Styrene (St) was distilled under reduced pressure before use. 3-(Trimethylsilyl)-2-propyn-1-ol, propynol ethoxylate, 2-bromoisobutyryl bromide, and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) were purchased from Aldrich and used without further purification. Tetrabutylammonium fluoride and phenylacetylene were purchased from Energy Chemical and used as received. Triethylamine (TEA) and tetrahydrofuran (THF) were refluxed with CaH2 for 4 h prior to use. Copper(I) bromide (CuBr) was first stirred in 2% glacial acetic acid aqueous solution for 2 h and then washed with acetone for 3 times. A poly(ethylene terephthalate) (PET) film was kindly provided by Hangzhou Tape Factory and cleaned with acetone for 5 minutes before use. All other chemicals were analytical grade and used as received. Synthesis of Initiators. The atom transfer radical polymerization (ATRP) initiators were prepared by the acetylation reaction of alkynol with 2-bromo-2-methylpropanoyl bromide in the presence of TEA. A typical procedure was as follows. A 250 mL round-bottom flask was charged with 2-bromo-2methylpropanoyl bromide (2.05 g, 8.93 mmol) and THF (20 mL). The reaction mixture was cooled to 0 °C in an ice-water bath, and a solution of 3-(trimethylsilyl)-2-propyn-1-ol (776.7 mg, 6.06 mmol), TEA (905.3 mg, 8.95 mmol), and THF (40 mL) was added dropwise over a period of 30 min under magnetic stirring. After the addition was completed, the reaction mixture was stirred at 0 °C for 1 h and then at room temperature for 12 h. After the insoluble filtration, the filtrate was concentrated by rotary evaporation and redissolved in dichloromethane. Then, the solution was further washed with aqueous sodium hydrogen carbonate and water, each for three times. After the solvents were removed by rotary evaporation, the sample was further purified by silica gel column chromatography using n-hexane and ethyl acetate (15:1) as the eluent. After the rotary evaporation and vacuum drying, a colorless liquid was obtained. Polymerization of l-PS-Br. ATRP was employed to control molecular weight and polydispersity. In a typical procedure, styrene (5 mL, 44 mmol), 3-(trimethylsilyl)prop-2yn-1-yl 2-bromo-2-methylpropanoate (60 mg, 0.22 mmol), and PMDETA (91 μL, 44 mmol) were added into 50 mL Schlenk flasks. After two freezing−evacuation−thawing cycles, CuBr (30 mg, 0.22 mmol) was added, and then two more freezing− evacuation−thawing cycles were performed. The reaction mixture was placed in a preheated 110 °C oil bath and allowed 3927

DOI: 10.1021/acs.jpcc.7b12286 J. Phys. Chem. C 2018, 122, 3926−3933

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Scheme 1. Synthetic Routes for the Preparation of Well-Defined Cyclic Polystyrenes via the Combination of ATRP and Azide− Alkyne Click Reaction

of water droplets on the solution surface, and thus, the transparent solution turned turbid rapidly. After solidification, the film was dried at room temperature. Characterization. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker (Advance DMX500) NMR instrument with tetramethylsilane as the internal standard. The molecular weight and molecular weight distribution were measured by a PL 220 gel permeation chromatography (GPC) instrument at 40 °C, which was equipped with a Waters 510 HPLC pump, three Waters Ultrastyragel columns (500, 103, and 105 Å), and a Waters 410 DRI detector. THF was used as the eluent at a flow rate of 1.0 mL/min. The calibration of the molecular weights was based on PS standards. A field emission scanning electron microscope (Sirion-100, FEI) was used to observe the surface morphologies of films after being sputtered with gold using an ion sputter JFC-1100. Interfacial tension was measured using the pendent drop method by a Drop-Meter OSA200-E1 contact angle system (Ningbo NB Scientific Instruments Co., Ltd.) at room temperature. The average values were calculated from at least three parallel measurements. The pore size was analyzed with Image Pro Plus software. Cloud points were determined by titration of a nonsolvent into the polymer solution. The transparency transition was recorded on a Shimadzu UV-2450 spectrophotometer.

to magnetically stir under nitrogen for 35 minutes before quenching in liquid nitrogen. After precipitation in methanol and washing several times, white solid pure products were dried in vacuum oven overnight. Preparation of l-PS-N3. l-PS-Br (0.46 g, 0.08 mmol) was added into a 50 mL round-bottomed flask. The polymer was dissolved by 10 mL of dimethylformamide (DMF), and then 26 mg (0.40 mmol) sodium azide was added. The solution was allowed to stir for 72 h at room temperature before purification by precipitation into methanol. The white solid sample was dried and stored properly. Deprotection of l-PS-N3. Trimethylsilyl alkyne-functionalized l-PS-N3 (0.29 g, 0.05 mmol) was dissolved by 5 mL THF in a 50 mL round-bottomed flask, and then, tetrabutylammonium fluoride THF solution (1.0 mol/L, 0.50 mL) was added. The mixture was stirred for 72 h at room temperature. The polymer was purified by precipitation in methanol, yielding a white solid which was dried under vacuum. Cyclization of c-PS. DMF (115 mL) was added to a 250 mL Schlenk flask and was degassed using two freezing− evacuation−thawing cycles. CuBr (0.16 g, 1.11 mmol) and bipyridyl (0.34 g, 2.20 mmol) were added to the frozen DMF. Another freezing−evacuation−thawing cycle was performed and sealed to be used. A separate flask containing l-PS-N3 (0.058 g, 0.011 mmol) dissolved in 5 mL DMF was degassed through two freezing−evacuation−thawing cycles, which was then injected into CuBr/bipyridyl catalyst solution at 120 °C via a syringe pump at a rate of 0.2 mL/h (0.4 μmol/h). Once the adding procedure was finished, the reaction was allowed to proceed at 120 °C for another hour before quenching by liquid nitrogen. The crude polymer was precipitated in methanol and washed several times to give a white solid, and then the sample was sealed for further use. Preparation of l-PS-triazole. Trimethylsilyl-protected lPS-N3 (0.058 g, 0.011 mmol), phenylacetylene (0.056 g, 0.55 mmol), and bipyridyl (0.068 g, 0.44 mmol) were dissolved by 5 mL of DMF in a 50 mL Schlenk flask. After two freezing− evacuation−thawing cycles, CuBr (30 mg, 0.22 mmol) was added, and two more freezing−evacuation−thawing cycles were performed to degas. The reaction was proceeded in a preheated 120 °C oil bath and allowed to magnetically stir overnight before quenching in liquid nitrogen. After precipitation in methanol, the product was dissolved in THF for a deprotection step as described above. Fabrication of Honeycomb-Patterned Porous Films. The breath figure method was employed to fabricate honeycomb films. An aliquot of 50 μL polymer carbon disulfide solution was cast onto a piece of PET substrate placed under a 2 L/min humid airflow (25 °C and ∼80% RH) at different concentrations. The evaporation of solvents results in temperature decrease, leading to nucleation, growth, and arrangement

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RESULTS AND DISCUSSION Synthesis of Polystyrenes with Linear and Cyclic Architectures. Cyclic polymers have received great attention in recent years because of their unique topology-related properties. In general, the synthetic routes can be divided into two categories: cyclization of linear precursors and ringexpansion polymerization. Copper-catalyzed Huisgen 1,3dipolar cycloaddition click reaction has been used extensively in cyclization of linear polymers because of the extremely high coupling yields and broad functional group tolerance. Also, to control the molecular weight while guaranteeing the end group fidelity, ATRP was employed to synthesize linear polystyrene coupled with copper-catalyzed azide−alkyne click (CuAAC) cyclization in this work. As shown in Scheme 1, alkyne end groups with different polarities are introduced to initiators, and a series of α-end-functionalized polystyrenes were synthesized by ATRP. Then, a nucleophilic substitution reaction provides an azide group to the ω-end of the polymers. After deprotection, the α,ω-end-functionalized polystyrene is cyclized with the aid of a copper catalyst in high dilution. Our previous work has revealed the benefits of polar end groups in the breath figure process.42 In this work, we synthesized two initiators by the acetylation reaction of 2bromo-2-methylpropanoyl bromide and alkynol with a different number of ether bonds to adjust the polarity. 1H NMR spectra 3928

DOI: 10.1021/acs.jpcc.7b12286 J. Phys. Chem. C 2018, 122, 3926−3933

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The Journal of Physical Chemistry C of the initiators in the Supporting Information confirm the successful synthesis (Figure S1). The peak of the characteristic methylene proton next to the oxygen shows signals from 3.7 to 4.8 ppm according to specific chemical environments. The signal at 1.95 ppm is assigned to methyl protons, serving as a reference. The integration of each peak suggests high purity of the products. In the bulk ATRP process, the feed ratio and reaction time are controlled to tailor the molecular weight. Polymerization is quenched at low conversion to ensure high fidelity of the ω-end-group. GPC curves in Figure S2 show the molecular weights and molecular weight distribution. For each series, three linear precursors are synthesized with Mn of 2700, 5700, and 8600 g/mol (Table 1). Figure 1. Overlay of GPC curves of l-PS55-I1 and c-PS55-I1.

Table 1. Results of Linear Polystyrenes Prepared via ATRP a

entry

X

l-PS27-I1 l-PS55-I1 l-PS82-I1 l-PS25-I2 l-PS54-I2 l-PS83-I2

350 350 350 295 295 295

b

c

Mn,GPC 2750 5750 8550 2600 5750 8650

time (min)

conv. %

Mn,th

18 35 53 18 35 53

4.8 14.7 21.1 6.2 15.8 25.1

1800 5400 7700 1900 4850 7700

d

d

PDI

1.08 1.07 1.05 1.09 1.07 1.07

a

Polymerization conditions: [St]0/[I]0/[PMEDTA]0/[CuBr]0 = X/1/ 2/1, 110 °C. bCalculated by the gravimetric method. Conv. (%) = Wp/ WSt, where Wp and WSt are weights of the resultant polymer and styrene in feed, respectively. cTheoretical number-average molecular weight, Mn,th, was calculated according to Mn,th = [St] × MSt × conv./ [I] + MI. dGPC using differential refractive index detection vs linear polystyrene standards. Figure 2. MALDI-TOF mass spectra for l-PS55-I1 and c-PS55-I1.

After polymerization, azidation of the ω-end-group was carried out with sodium azide in DMF. For the polymer initiated by I1, a deprotection step is necessary. The changes of functional groups were characterized by NMR and Fourier transform infrared (FTIR). In a typical 1H NMR spectrum of lPS55-I1, the signals from 4.3 to 4.6 ppm are assigned to the proton next to the bromine. After azidation, this peak shifts to 3.7−4.1 ppm and is difficult to be resolved. The strong peak at 0.2 ppm is due to the protective group which is cleaved by tetrabutylammonium fluoride during the deprotection step. After that, the signal at 0.2 ppm vanishes, and that of alkynyl proton shows up at 3.4 ppm (Figure S3). Also, the emergence of the characteristic signal of the azide group at 2100 cm−1 as well as the alkyne group at 3300 cm−1 in FTIR spectra (Figure S4) further confirms successful synthesis of α,ω-end-functionalized linear polystyrenes. To suppress possible condensation reaction, cyclization was carried out in a highly dilute solution at an extremely slow reaction rate by employing a continuous addition technique.57 GPC, FTIR, and MALDI-TOF were performed to verify the cyclization. In Figure 1, GPC curves demonstrate that the macrocycle polymer is successfully obtained. A slight delay in elution time is observed because of the decreasing hydrodynamic volume after cyclization. The two nearly overlapping peaks suggest a conversion ratio of cyclization over 95%, which is also confirmed by the integration of the peaks. Another evidence is provided by MALDI-TOF-MS (Figure 2). The molecular weight of the cyclic polymer is unchanged after the click reaction. It should be pointed out that for azide endfunctionalized linear polystyrene, both parent ions and metastable ions (including in-source and post-source metastable ions) are generated during laser desorption. In general,

expulsion of N2 from the azide-functionalized polymer gives rise to metastable ions 28 mass units less than the mass of parent ions. In this observation, a mass loss of less than 28 mass units shows predominant intensity. This is attributed to the mode we chose in our measurement, in which the mass of the metastable ions will be some intermediate values between that of the parent mass and the in-source fragment mass.58 In FTIR spectra, the disappearance of alkyne (3300 cm−1) and azide (2100 cm−1) peaks provides further evidence to support triazole formation (Figure S5). All the six linear polystyrene precursors are cyclized by a similar procedure. Self-Assembly of Linear and Cyclic Polystyrenes by the Breath Figure Process. It is quite conventional that there is an optimal concentration window to be applied for highly ordered honeycomb films. Here, a wide concentration range was investigated, including 0.5, 1.0, 2.0, 5.0, and 10 mg/mL. A typical dynamic breath figure process was conducted with all the end-functionalized linear and cyclic polystyrenes. The prepared films are shown in the Supporting Information (Figure S12 and S13), and some typical images are organized in Figure 3. Increasing the molecular weight and concentration helps with optimizing the regularity of honeycomb films. Comparing polymers with similar molecular weights but different initiators and topologies, as shown in Figure 3, is quite interesting. The regularity of honeycomb films increases following a sequence of l-PS-I2 > c-PS-I2 > c-PS-I1 > l-PS-I1. Further analysis is carried out by fast Fourier transform (FFT) and Voronoi tessellation techniques. The FFT images show an obvious tendency of increasing periodicity along the sequence, and the Voronoi diagrams provide much more detailed 3929

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Figure 3. (a) SEM images of honeycomb films, (b) FFT images, and (c) Voronoi diagrams of SEM images. (d) Statistic analysis of polygons on the Voronoi diagrams.

quantitatively analysis.59 Conformational entropy can be calculated by eq 1. S = −Pn∑ ln Pn

(1)

where Pn is the probability of pores with n-nearest neighbors. A larger S value indicates higher irregularity. In this way, the order of the ability in stabilizing water droplets is verified. It should be noticed that the triazole group is also a polar group. To investigate the influence of such a triazole group, linear polystyrene with a triazole group was synthesized by CuAAC reaction with phenylacetylene according to a similar procedure. The characterization and the self-assembly results can be seen in the Supporting Information. It seems that the triazole group has no remarkable effect on the breath figure process (Scheme S1 and Figures S6−S9 and S12 in the Supporting Information). The breath figure method is a self-assembly process happening at the interface. It is easy to understand that a linear polymer with polar groups has higher interfacial activity. However, when it comes to cyclic polymers, the effects are complicated and depend on the end groups of linear precursors. For propargyl end-functionalized linear polystyrenes (without polar groups), the transition from linear topology to cyclic topology is beneficial for stabilizing the templating water droplets (Figure S12). As for propargyloxyethyl end-functionalized linear polystyrenes (with polar groups), cyclic topology will deteriorate the regularity slightly (Figure S13). To understand the film formation of different topologies, interfacial tension was measured by a pendent drop method (Figure 4). Attributing to the ether group in I2, l-PS54-I2 has a lower interfacial tension than l-PS55-I1. However, for the two cyclic polymers, it is quite confusing that they even have a lower interfacial tension than the linear polystyrene with a polar end group. The low interfacial tension can be related to the small hydrodynamic volume of cyclic topology. The volume of the cyclic polymer is considerably smaller than that of the corresponding linear precursor. The radius of gyration of a linear polymer ⟨s2⟩l is related to its mean square end-to-end

Figure 4. Interfacial tensions of polymers with different architectures at 2 mg/mL measured by a pendent drop method.

distance ⟨r2⟩l, adopting random-coil conformation obeying Gaussian statistics by eq 2.60

⟨s 2⟩l = ⟨r 2⟩l /6

(2)

The ratio of the radius of gyration of a cyclic polymer to that of the corresponding linear polymer at θ conditions has been shown theoretically to be as follows.

⟨s 2⟩l ⟨s 2⟩c

=2 (3)

This prediction has been confirmed experimentally by neutron scattering.61 A smaller hydrodynamic volume endows cyclic polymers with higher interfacial activity, and that is exactly what the breath figure process needs to make water droplets stable. As a result, cyclic polystyrenes form more regular honeycomb films than the linear polystyrene without the polar end group (c-PS-I1). The decrease of interfacial tension caused by cyclic topology is universal and has been verified under different molecular weights in Figure S10 (Supporting Information). In addition, the pore size is a factor for evaluating the ability of polymers to stabilize water droplets. 3930

DOI: 10.1021/acs.jpcc.7b12286 J. Phys. Chem. C 2018, 122, 3926−3933

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The Journal of Physical Chemistry C The pore size of c-PS54-I2 (1.28 ± 0.12 μm) is smaller than that of l-PS54-I2 (2.30 ± 0.19 μm), which indicates that the former polymer possesses stronger ability to stabilize water droplets. Also, it is consistent with the interfacial tension results. However, because of the constraint of cyclic topology, the contribution of the polar group (oxyethyl group) is restricted, and even though the cyclic topology is favorable for the breath figure, the final film prepared by c-PS-I2 is less regular than that of l-PS-I2. Therefore, from these perspectives, the results demonstrate that polar end groups possess more promising significance than cyclic topology in preserving film regularity. The difference in film regularity between two groups of cyclic polymers can be attributed to the polar oxyethyl group of c-PS-I2, which endows the polymer with a certain amphiphilicity. On the basis of the thermodynamics equilibrium state, interfacial tension shows how stable water droplets can be under a certain static condition. However, the breath figure method is a dynamic process. How polymers stabilize water droplets is a major problem. We studied the precipitation process of polymers by titrating polymer solutions with a nonsolvent. For two immiscible liquids, that is, carbon disulfide and water, precipitation happening at a micro-sized water droplet surface is difficult to monitor. Thus, a precipitation process was studied in toluene titrated by a miscible nonsolvent methanol. As shown in Figure 5, the cloud point is monitored

corresponding linear polymers. It can be concluded that a cyclic nonpolar polymer with a smaller hydrodynamic volume and higher chain density possesses much higher interfacial activity than its linear precursor.

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CONCLUSIONS In summary, a series of well-defined linear and cyclic polystyrenes were synthesized herein. For the first time, selfassembly of a cyclic polymer by the breath figure method was systematically investigated. Linear polystyrenes with polar end groups show significant improvement in the film preparation property than the linear polymer without polar end groups. As for cyclic polymers without end groups, the regularity of honeycomb films is something in between. Cyclic topology with high segment density endows polymers with high interfacial activity, which is favorable in the breath figure process to make water droplets stable. At the same time, the influence of end groups is suppressed because of the constraint of flexibility in cyclic topology. Polymers with polar moieties stabilize water droplets thermodynamically, but polymers with high segment density work kinetically based on interfacial tension and precipitation rate results. Cyclic polystyrene provides additional strategy to control the final morphology of honeycomb films. Also, more importantly, this work discussed the effect of end groups, established direct contact between segment density as well as molecular polarity, and provided important new insights into the mechanism of the breath figure method.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12286. GPC, NMR, and FT-IR results of polymers and images of typical honeycomb films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: +86-571-87953763. ORCID

Figure 5. Cloud points of polymers with different architectures measured by UV−Vis spectroscopy (λ = 600 nm).

Zhi-Kang Xu: 0000-0002-2261-7162 Ling-Shu Wan: 0000-0003-2570-5202 Notes

The authors declare no competing financial interest.

by measuring light transmission using UV−Vis spectroscopy, while the nonsolvent is titrated into the polymer solution (the initial concentration is 2 mg/mL). The precipitation process is revealed dynamically along with the decreasing transmittance. The major difference is the precipitation rate, for example, the slope of curve l-PS55-I1 is three times that of l-PS54-I2, which is similar to those of cyclic polymers. However, it should be noted that a fast precipitation is not sufficient for the formation of ordered honeycomb films, which is different from the traditional understanding of the breath figure process.22 The precipitation rate demonstrates kinetics of stabilizing water droplets. Polymers l-PS54-I2, c-PS54-I1, and c-PS55-I1 possess higher interfacial activity than the linear polystyrene without the polar end group (l-PS54-I1). The decline sequence in the solvent/nonsolvent ratio shows obvious dependence on molecular weights (Figure S11). Essentially, the delay of precipitation of cyclic polymers is attributed to the smaller hydrodynamic volume and higher chain density than the

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51522305 and 21374100) and the Fundamental Research Funds for the Central Universities (2015XZZX004-26) is gratefully acknowledged.

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REFERENCES

(1) Widawski, G.; Rawiso, M.; François, B. Self-Organized Honeycomb Morphology of Star-Polymer Polystyrene Films. Nature 1994, 369, 387−389. (2) Bunz, U. H. F. Breath Figures as a Dynamic Templating Method for Polymers and Nanomaterials. Adv. Mater. 2006, 18, 973−989. (3) Li, X.; Zhang, L.; Wang, Y.; Yang, X.; Zhao, N.; Zhang, X. L.; Xu, J. A Bottom-Up Approach to Fabricate Patterned Surfaces with Asymmetrical TiO2 Microparticles Trapped in the Holes of Honeycomblike Polymer Film. J. Am. Chem. Soc. 2011, 133, 3736−3739.

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DOI: 10.1021/acs.jpcc.7b12286 J. Phys. Chem. C 2018, 122, 3926−3933

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