ORIGINAL CONTRIBUTION

Synthesis of star-shaped polyhedral oligomeric silsesquioxane(POSS) fluorinated acrylates for hydrophobic honeycombporous film application

Xiu Qiang & Xiaoyan Ma & Zhiguang Li & Xianbing Hou

Received: 22 October 2013 /Revised: 5 December 2013 /Accepted: 30 December 2013# Springer-Verlag Berlin Heidelberg 2014

Abstract Two different eight-arm star-shaped polyhedraloligomeric silsesquioxane (POSS) fluorinated acrylates weresynthesized through atom transfer radical polymerization(ATRP) and applied for hydrophobic honeycomb-patternedporous films through the breath figure (BF) method. Thestructure of polymers was characterized by Fourier transforminfrared (FT-IR) spectroscopy, nuclear magnetic resonance(NMR), and gel permeation chromatography (GPC), and sur-face analysis was featured by X-ray photoelectron spectros-copy (XPS). Depending on the influences of polymer ar-chitectures, solvents utilized, and solution concentrations,honeycomb-patterned porous films were obtained. It couldbe found that the introduction of fluorine components was afavorable condition for BF formation and chloroform(CHCl3) utilized as solvent with an appropriate concentra-tion of 30 mg/mL was the best condition for these hydro-phobic honeycomb-patterned porous films. Meanwhile, theobtained honeycomb films could be retained after long-time preservation in an acid-base condition, which showsa great potential in filtration, cell culture, tissue engineer-ing, and marine antifouling applications.

Keywords Star polymers . POSS . Fluorinated acrylates .

Breath figure . Hydrophobic honeycomb porous film

Introduction

Incorporation of an inorganic component into an organicpolymer matrix is expected to enhance a variety of physicalproperties, like processability, toughness, and thermal andoxidative stability, which have received a considerable at-tention [1]. Polyhedral oligomeric silsesquioxanes (POSS)consist of a rigid silica-like core that is perfectly definedspatially (0.5–0.7 nm) and some readily modified R-Si frag-ment, with the general formula (RSiO1.5)2n, where R can be ahydrogen atom or an organic group [2]. These cubic siloxanecages with readily functionalized substituents or groups aresuitable for polymerization, grafting, surface bonding, orother transformations [3–5], and the obtained polymers andcopolymers containing covalently bonded POSS moietiesare an area of research that gained tremendous popularityin recent years. Among numerous POSS molecules, octa(γ-chloropropylsilsesquioxane) (OCP-POSS) has all its eightcorners modified by γ-chloropropyl substituents and be-comes an effective multi-arm star-like initiator. Wang et al.[6] reported the successful synthesis of the octa-armed star-shaped PMMA-b-PS block copolymers which were grownfrom OCP-POSS by atom transfer radical polymerization(ATRP).

High-performance fluoropolymers are attractive specialtypolymers due to a number of unique properties includingthermal and oxidative stability, optical transparency, solventcompatibility, and environmental stability and have been ex-tensively applied in many different fields [7–9]. Comparedwith usual carbon-hydrogen-oxygen-containing polymers,these unique properties originate from the incorporation offluorocarbon functionality. To date, much more studies offluoropolymers focused on their high performance in surfaceengineering. Xiang et al. [10] synthesized a series of blockcopolymers with semifluorinated monodendron side groups,and the surface of the block copolymer films was largelymade

X. Qiang :X. Ma (*) : Z. Li :X. HouKey Laboratory of Polymer Science and Technology, ShaanxiProvince, School of Science, Northwestern Polytechnical University,Xi’an 710129, Chinae-mail: m_xiao_yana@nwpu.edu.cn

X. Qiang :X. Ma : Z. Li :X. HouKey Laboratory of Space Applied Physics and Chemistry, Ministryof Education, School of Science, Northwestern PolytechnicalUniversity, Xi’an 710072, China

Colloid Polym SciDOI 10.1007/s00396-013-3157-9

up of a uniform –CF3 layer and typically has surface energiesof only 7~9 mJ/m2. Wei et al. [11] prepared a fluoropolymersuperhydrophobic polymer-coated surface under an ambientatmosphere, and the contact angle and sliding angle of the filmwere measured as 152.3° and 9.2°, respectively, demonstrat-ing excellent superhydrophobic property and stability. Similarwork was also investigated by numerous research groups suchas Luo [12], Theato [13], and Hussain [14].

To date, more and more fluorinated copolymers were cho-sen for hydrophobic material applications, but the hydropho-bic and oleophobic performance still has much room forimprovement. As Wenzel [15] and Cassie [16] proposed,surface properties are not only related to the nature of polymermatrix but also relevant to the surface structures of mem-branes. Here, a water-driven template-free method—thebreath figure (BF) method—is considered. Yabu et al. [17]prepared a superhydrophobic microporous surface of a fluo-rinated polymer by casting from a solution under humidconditions. The water contact angle was over 150°, whichwas much higher than 117° of the related flat film. The BFprocess involves the evaporation of a volatile solvent from apolymer solution in the presence of atmospheric humidity,wherein the size of the pores appears to be 0.2~10 μm indiameter and the thickness of the film is 10~30 μm. Since itsproduction in 1994 [18], the BF method has been widelyexploited due to its versatility and robustness, and to date,similar films have beenmade from a variety of polymers, suchas comb [19], dendritic [20], and even organic-inorganichybrid copolymers [21]. Overall, a spherical-shaped polymeris required to support a high regularity, and among thesenonlinear structure polymers, star-shaped polymers with line-ar chains having different chemical compositions connected tothe identical central core have important theoretical and prac-tical significance [22]. For example, Connal et al. [23] syn-thesized a series of star-microgels via the arm-first approachutilizing ATRP, which has been shown to be suitable forproducing highly ordered porous honeycomb films; the porediameters decrease with increasing number of PMMA armsand with increasing molecular weight of the star-microgel.Also, a highly branched star polymer with polydimethylsilo-xane (PDMS) functionality was used as a honeycomb precur-sor, and highly ordered honeycomb morphology was coated[24]. However, the exact mechanism of this process is stillunder investigation, which needs further exploring and attractsthe interest of industrial or academic researchers.

Taking into account the excellent surface properties of bothPOSS and fluorine components, we designed a series of star-shaped POSS fluorinated acrylate copolymers. This kind ofmacromolecules with POSS as a core and fluoropolymers orfluorate block copolymers as arms has been reported scarcely,and breath figure formation for honeycomb porous films hasnot been mentioned yet, from which an excellent surface

property could be expected for micro/nanoscale porous films.Based on this conception, we presented the synthesis of OCP-POSS with an intact cage structure by hydrolytic condensa-tion, and using it as an ATRP initiator, a star-shaped polymercomposed of OCP-POSS as a core and poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) as arms was synthe-sized; in addition, a star-shaped hybrid copolymer with armsof poly(methyl methacrylate-block-2,2,2-trifluoroethyl meth-acrylate) (PMMA-b-PTFEMA) was prepared via sequentialATRP under a two-step-one-pot procedure. Moreover, thefabrication and characterization of BFs from these two hybridpolymers had been investigated, and both of the hybrid poly-mers showed a great potential for highly honeycomb-structured porous membranes due to the introduction of fluo-rine components. Relevant results could greatly expand theselectivity of materials used for BF films and provide a certaintheoretical basis for the BF mechanism.

Materials and methods

Materials

Methyl methacrylate (MMA; Tianjin Fuyu Fine Chemics CoLtd, China) was washed with sodium hydroxide (5 wt%) threetimes to remove the inhibitor, followed by washing withdeionized water until neutralization and then dried over anhy-drous magnesium sulfate, distilled under reduced pressure,and stored in a refrigerator prior to use. Toluene, obtainedfrom Tianjin Fuyu Fine Chemics Co Ltd, was dried for 24 hover a molecular sieve of size 4 Å and distilled under reducedpressure. Cuprous chloride (CuCl), obtained from TianjinBASF Chemical Reagents Co Ltd, was purified in acetic acid,washed with ethanol , and dried under vacuum.Pentamethyldiethylenetriamine (PMDETA) and trifluoroethylmethacrylate (TFEMA) were purchased from Aldrich Co,USA, and used as received. γ-Chloropropyl trimethoxysilanewas obtained from Nanjing Xiangqian Chemical Reagents CoLtd, China. Alumina (Al2O3, 100–200 mesh) was obtainedfrom Tianjin Fuyu Fine Chemics Co Ltd, China. Unlessspecifically indicated, other reagents were of chemically puregrade, obtained from Tianjin Hongyan Chemical Reagents CoLtd, and used as received without further purification.

Synthesis of the OCP-POSS initiator

Octa(γ-chloropropyl) silsesquioxane (OCP-POSS) was syn-thesized via the hydrolytic condensation according to theliterature [25, 26], and the synthesis route was shown inScheme 1. The hydrolysis of γ-chloropropyl trimethoxysilane(Cl(CH2)3Si(OCH3)3, 10 mL) was performed in methanol(200 mL) in the presence of concentrated hydrochloric acid

Colloid Polym Sci

(8 mL). After reacting for 5 days in a 65 °C oil bath underrapid stirring, a white solid was obtained. Then the headproduct was washed with methanol for three times and driedunder vacuum at 40 °C for 2 days to get the final product.

Synthesis of star-shaped fluorinated acrylate polymers

Synthesis of POSS-(PTFEMA)8

The ATRP process was carried out using –CH2CH2CH2Clbonded on a POSS cage as the initiation group and under anitrogen environment (Scheme 2). In a typical run, oxygenin the flask in ice bath was first removed by continuous drynitrogen and then CuCl (0.010 g, 0.1 mmol), OCP-POSS

(0.10 g, 0.1 mmol), PMDETA (0.06 mL, 0.3 mmol), tolu-ene (10 mL), and TFEMA (10 mL) were added into theflask equipped with a magnetic stir bar. The system wasevacuated three times, filled with dry nitrogen, and placedin an oil bath warmed at 110 °C. After 24 h, the polymer-ization was terminated by cooling the flask in ice water.Then the mixture was diluted with tetrahydrofuran (THF),filtered over an alumina column to remove the catalyst, andpoured into a tenfold methanol-water solution. The productwas obtained after filtration and drying at 50 °C in a vacuumovernight.

Synthesis of POSS-(PMMA-b-PTFEMA)8

In order to produce excellent yields and reduce the post-processing, here we employed a one-pot reaction (Scheme 2).At first, a tailor-made flask in ice bath maintained an over-pressure of dry nitrogen to remove oxygen totally. Then CuCl(0.010 g, 0.1 mmol), OCP-POSS (0.10 g, 0.1 mmol),PMDETA (0.06 mL, 0.3 mmol), toluene (10 mL), andMMA (10 mL) were added into the flask equipped with amagnetic stir bar. The systemwas evacuated three times, filledwith dry nitrogen, and placed in an oil bath warmed at 110 °C.After 24 h, the same amount of TFEMA (10 mL) was added

Scheme 1 Synthesis route of OCP-POSS

Scheme 2 Synthesis route of star hybrid fluorinated acrylate polymers with POSS core

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into the system and reacted for another 24 h. The polymeri-zation was terminated by cooling the flask in ice water. Thenthe mixture was diluted with THF, filtered over an aluminacolumn to remove the catalyst, and poured into tenfold meth-anol. The product was obtained after filtration and drying at50 °C in a vacuum overnight.

Characterization

Fourier transform infrared (FT-IR) spectrum was determinedfrom the coating method of the WQF-310 FT-IR spectrometerat room temperature. Nuclear magnetic resonance (NMR) spec-tra were obtained on a Bruker AVANCF-300 NMR spectro-meter. Samples for 29Si NMR, 1H NMR, and 13C NMR wereprepared in CDCl3, and tetramethylsilane (TMS) was used asan internal standard. DIONEX BJ/U3000 gel permeation chro-matography (GPC) was used to determine the weight-average(Mw) and number-average (Mn) molecular weights, withShodex OHpak SB-803 HQ (300×8 mm) as chromatographiccolumns. It was carried out at 298 K with THF as solvent(0.5 mL/min) and polystyrene as calibration. X-ray photoelec-tron spectroscopy (XPS) data were acquired using a ThermoScientific K-Alpha system (Thermo Fisher Scientific, EastGrinstead, UK), equipped with an Al Kα excitation radiation(hν=1,486.6 eV). The peak positions in the spectra were deter-mined using the least squares fitting of data to the Gaussian lineshape. For charge referencing, an adventitious C 1s peak set at284.6 eV was used. Thermal stabilities of samples were inves-tigated using a TA Q500 TGA instrument operated at a rate of20 °C/min from 30 to 600 °C under a nitrogen flow.

Preparation and characterization of honeycomb films

Honeycomb porous membranes were prepared via the breathfigure method. POSS-(PTFEMA)8 or POSS-(PMMA-b-PTFEMA)8 was dissolved by dichloromethane, chloroform,benzene, and THF, respectively, and then cast onto a siliconwafer and dried under a flow ofmoist air. The moist air flow inthis method can be easily generated by bubbling air through awater-filled washing flask before blowing it through a nozzleover the solution of the polymer. The airflow was verticallyblown over the solution surface. For comparison, nonporousfilms were also cast from a chloroform solution ofPOSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8 driedunder a flow of nonhumidified air. Scanning electron micro-scope (SEM) images were used to observe apparent morphol-ogies of the films, which was carried on VEGA 3 LMH(Česko TESCAN) with an accelerating voltage of 10 kV.Contact angle measurements were measured by pendant dropmethod with a JC2000D4 Powereach tensiometer made byShanghai Zhongchen Company. The copolymer solutions(30 mg/mL in CHCl3) were cast on glass slides (43×17×

16 mm3) and the wetting liquid used was water. Amicrosyringe was used to deliver water to the film surface.For each angle reported, at least seven sample readings fromdifferent surface locations were averaged.

Results and discussion

Structure and properties of hybrid copolymers

Hybrid copolymers in which fluoropolymers were graftedon OCP-POSS by a covalent bond are expected to integratethe advantages of fluoropolymers and POSS together, andwe are hopeful to obtain novel organic-inorganic hybridpolymers with an excellent surface property and remark-able thermal property. Based on this view, we designed thesynthesis of a series of eight-arm star-shaped hybrid poly-mers with POSS as the core and PTFEMA or PMMA-b-PTFEMA as arm structures. The synthesis procedure wascarried out using –CH2CH2CH2Cl bonded on a POSS cageas the ATRP initiation group and CuCl/PMDETA as thecatalytic system, with monomers added into a one-pot-two-step polymerization. To prove the successful preparation ofthe ideal star-shaped polymers, FT-IR and NMR technolo-gies were applied for the molecular structure characteriza-tion. And the molecular weight distribution was also re-corded by the GPC method. The fluorine contents withinpolymers were also measured by XPS, and in addition, thethermal properties of hybrid polymers were investigatedthrough thermal gravimetric analysis (TGA).

Structural analysis

The s ta r - shaped hybr id POSS-(PTFEMA)8 andPOSS-(PMMA-b-PTFEMA)8 copolymers were preparedfrom the OCP-POSS initiator. And the structural integrity ofthe POSS cage was a prerequisite for the star topologysynthesis.

The synthesis route of the OCP-POSS is outlined inScheme 1, via the hydrolytic condensation of γ-chloropropyltrimethoxysilane according to the literature [25, 26]. The typ-ical 1H NMR spectrum of octa(γ-chloropropyl) POSS isshown in Fig. 1c. There were three peaks (centered at0.81, 1.86, and 3.58 ppm) corresponding to the protonpeaks of –SiCH2CH2CH2Cl, –SiCH2CH2CH2Cl, and –SiCH2CH2CH2Cl, respectively. Relevant peaks (47.03,26.27, and 9.36 ppm) were also found in 13C NMR inFig. 1b. In addition, the 29Si NMR spectrum of octa(γ-chloropropyl) POSS was present in Fig. 1a. There was onlyone single resonance at −67.05 ppm, which indicated that theeight silicon atoms were magnetically equivalent. Meanwhile,this peak occurred between −50 and −80 ppm, in the range of

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T-Si NMR chemical shifts, which demonstrated that the atomsof Si were coordinated by a carbon atom and three oxygenatoms. Therefore, a perfect cage core of octa(γ-chloropropyl)POSS was successfully prepared. The FT-IR spectrum isshown in Fig. 2: ν (Si-O-Si)=1,136 cm−1, ν (C-Cl)=750 cm−1, ν (C-H)=2,840–2,950 cm−1, δ (O-Si-O)=538 cm−1. The results also proved the structure of OCP-POSS, which agreed with the results of NMR.

Using octa(γ-chloropropyl) POSS as initiator, radical poly-merization of TFEMAwas carried out at 110 °Cwith themolarratio in feed: TFEMA/OCP-POSS/CuCl/ligand, to be about870:1:1:3. As PMDETA is a Lewis base which could donate apair of electrons to a Lewis acid (Cu(I) or Cu(II)) to form acomplex, but do no damage to the POSS core, it was applied asa ligand for ATRP polymerization. Meanwhile, fluorinatedsolvents were commonly used as reaction media in the poly-merization of fluorine-containing monomers, but here toluenewas chosen, as it was inexpensive, inert, and commerciallyavailable. The products were characterized by FT-IR and 1HNMR, which were respectively shown in Figs. 2 and 3. Fromthe FT-IR spectrum of POSS-(PTFEMA)8 in Fig. 2, it can beseen that a new peak appeared at 1,760 cm−1, whichcorresponded to the stretching vibrations of the C=O groupof the PTFEMA segments; a new peak also appeared at1,280 cm−1, which corresponded to the C-F group of the

PTFEMA segments, and the stretching vibrations of the C-O-C group appeared at 1,160 and 1,320 cm−1. In the 1H NMRspectrum of POSS-(PTFEMA)8, except for the overlappingregion of the proton signals about from 0.69 to 2.27 ppm, theother signal peaks were easy to be identified. The signal atδ=2.35 ppm was assigned to the methyl protons (–COOCH3).And the signal of the characteristic protons derived fromCF3CH2– was shown obviously at δ=4.34 ppm. As a whole,the measurement results proved that the star hybrid polymerPOSS-(PTFEMA)8 was successfully synthesized.

Although fluoropolymers possess high performance suchas excellent thermal and oxidative stability, optical transpar-ency, and environmental stability, they are always difficult toprocess and expensive for wide application [27]. To date,making fluoro-monomer copolymerize with others andadjusting the ratios to balance the cost and properties havebeen put forward and proved to be an appropriate way.Methylmethacrylate (MMA) has a similar chemical structure to thatof TFEMA but a much lower price than that of TFEMA. Fromthe perspective of cost reduction, we designed to introduceMMA segments to the arms of a POSS-based hybrid polymerand successfully prepared POSS-(PMMA-b-PTFEMA)8.Compared with that of POSS-(PTFEMA)8, a significant en-hancement in the FT-IR spectrum could be seen at 1,160,1,320, and 1,760 cm−1 due to the induction of MMA(Fig. 2). In the 1H NMR spectrum (Fig. 4), besides the signalof CF3CH2– at δ=4.34 ppm, a new peak at δ=3.63 ppm

Fig. 1 29Si NMR (a), 13C NMR (b), and 1H NMR (c) spectra of OCP-POSS in CDCl3

Fig. 2 FT-IR spectra of OCP-POSS and star hybrid fluorinated acrylatepolymers Fig. 3 1H NMR spectra of POSS-(PTFEMA)8

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occurred related to CH3O–, which confirmed that the poly-merization was completed (Fig. 4). Meanwhile, this processreacted via sequential ATRP by a one-pot-two-step procedure.Because of the low possibility of thermo-polymerization ofMMA, a middle product of POSS-(PMMA-Cl)8 could besuccessfully prepared by an ATRP route and further initiateTFEMA for polymerization, which finally got the ideal star-shaped hybrid copolymer with block arms of PMMA-b-PTFEMA, and the integral ratio of CF3CH2– and CH3O– inthe 1H NMR spectrum was about 1:1.61.

GPC analysis

Figure 5 presents the unimodal GPC traces ofPOSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8, andthe related molecular weights and polydispersity indexes(PDI; Mw/Mn) are shown in Table 1.

The molecular weights of star-shaped POSS-(PTFEMA)8and POSS-(PMMA-b-PTFEMA)8 were much higher than thatof the OCP-POSS initiator with a complete cage structure(Mn≈1,037); as the arm structures changed from PTFEMAto PMMA-b-PTFEMA, an increase of the molecular weightcould be seen. All these results meant the successful polymer-ization initiated by OCP-POSS. However, the PDI of thehybrid copolymer were relatively broader than those of relatedlinear polymers [28], whichmay be due to the steric hindranceeffects of this kind of topological hybrid copolymers.

XPS analysis

The presence of the fluorinated segments with low surfaceenergy causes it to migrate toward the surface of the films,which may dramatically affect the formation of breath figurepatterns [29]. Thus, exploring the content of fluorine elementsin each sample in advance is necessary to our study, and hereX-ray photoelectron spectroscopy was used.

The broad scan of the binding energy (BE) spectrum forPOSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8 is

shown in Fig. 6a, b, respectively. Both of the two polymerswere comprised of three strong and one weak peaks, whichwere at approximately 687, 532, 285, and 102 eV, correspond-ing to direct photoionization from F 1s, O 1s, C 1s, and Si 2pcore levels, respectively. XPS characterization showed thecompositions of polymers, and the fluorine content ofPOSS-(PMMA-b-PTFEMA)8 was about 16.04 %, whichwas much lower than that of POSS-(PTFEMA)8 (29.39 %).And this great difference in fluorine content should mainlyowe to the added polymerization of the MMA monomer.Figure 6c, d shows the high-resolution spectra of C 1s forPOSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8, whichconsisted of five different Gaussian fitting peaks, in a descend-ing order, –CF3 (292.5 eV), –O–C=O (288.6 eV), –C=O(286.8 eV), and –CHn (284.6 eV). No other carbon bondingpatterns in this scan spectrum existed, which also presented astrong evidence for well-defined polymerization.

Thermal stability analysis

For fluorinated copolymer materials, the thermal property isan important parameter for that it determines the application ofhybrid copolymers. Figure 7 displays the TGA curves ofOCP-POSS, POSS-(PTFEMA)8, and POSS-(PMMA-b-PTFEMA)8, and the values in the onset of decompositiontemperatures based upon 5 % mass loss can be measured,which were 349.77, 220.32, and 283.56 °C, respectively.

OCP-POSS displayed the highest initial decompositiontemperature and largest weight retention, probably resultingfrom fragmentation of the Si-O-Si core, which was in confor-mity with relevant researches [2]. And the thermal stabilitiesof POSS-(PMMA-b-PTFEMA)8 and POSS-(PTFEMA)8showed a significant reduction, which is mainly due to theintroduction of long side chains and the decreasing compo-nents of the inorganic core. Meanwhile, thermogravimetricFig. 4 1H NMR spectra of POSS-(PMMA-b-PTFEMA)8

Fig. 5 GPC curves of POSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8

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analysis of the POSS-(PMMA-b-PTFEMA)8 copolymers un-der nitrogen showed a 63 °C improvement relative to aPOSS-(PTFEMA)8 sample, which was believed to result fromthe higher molecular weight and the decreasing defects ofthe Cl terminal in star polymers; as PMMA segments wereintroduced into polymers, the relative content of the Clterminal decreased, leading to lower adverse effects onthermal stability [30]. Also, the restricted motion ofPOSS-(PMMA-b-PTFEMA)8 side chains may providemore possibility of chain tangles and result to enhancedinitial decomposition temperature.

In addition, the hybrid materials showed excellent thermalstability and were much more stable than copolymers ofPTFEMA-PMMA and homo-PTFEMA according to reportsin the literatures [28, 31, 32]. Results showed a significantenhancement with the incorporation of the POSS units, whichwas consistent with the literature recorded before [2].

Hence, we could conclude that the thermal stabilities ofPOSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8 wereboth excellent, and POSS-(PMMA-b-PTFEMA)8 hadgreater potential to be applied as a heat-resistant materialwith more excellent thermal property but lower cost thanPOSS-(PTFEMA)8.

Self-organized honeycomb morphology

The breath figure (BF) uses water droplets as a dynamictemplate and involves the evaporation of a volatile solventfrom a polymer solution in the presence of atmospheric hu-midity. And upon complete solvent and water evaporation,more or less regular patterns were obtained. In this process,varying the initial conditions, such as humidity, temperaturesolvent and solution concentrations, the morphology of filmswas modified [33, 34].

Table 1 Synthesis of star hybridfluorinated acrylate polymers

aMeasured by GPCbCalculated accordingto the equation Mn,POSS/Mn

a )

Polymer Mna Mw

a PDIa Yield POSS contentb

(g/mol) (g/mol) (%) (%)

POSS-(PTFEMA)8 46,430 102,260 2.20 43.19 2.22

POSS-(PMMA-b-PTFEMA)8 63,910 112,640 1.76 27.16 1.61

Fig. 6 XPS spectra (a, b) and C 1s XPS spectra (c, d) of hybrid polymers: a, c POSS-(PTFEMA)8 and b, d POSS-(PMMA-b-PTFEMA)8

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Star polymers have lower viscosities than the corre-sponding linear polymers of similar molecular weight[35], which are likely to increase the mobility of chainsegments and prevent their aggregation. In this article,self-organized honeycomb morphologies of star hybridfluorinated acrylate polymers covalent with POSS corewere studied, together with an investigation of differentinitial condition effects, including molecular architecture,solvents utilized, and solution concentration.

Dependence on molecular architecture

Af t e r c a s t i n g POSS - ( PTFEMA) 8 /CHC l 3 a n dPOSS-(PMMA-b-PTFEMA)8/CHCl3 solutions with a con-centration of 30 mg/mL on silicon wafers in the wateratmospheres, respectively, the transparent polymer solutionquickly became turbid due to the formation of watermicrodroplets. When the solvent volatilized completely, anopaque and cream-colored film was left. The top-view SEMimages of the films revealed that both POSS-(PTFEMA)8 andPOSS-(PMMA-b-PTFEMA)8 could form hexagonally or-dered BF arrays, as shown in Fig. 8b, c. In this specificpreparation, most of the pores were surrounded by six otherbubbles (which are shown by white lines), but defects existed

in this “lattice” (yellow arrows); and at these defect sites, aspecific pore was surrounded either by five or by seven othernext neighbors. For comparison, the BF film of OCP-POSS/CHCl3 under the same water atmosphere was also preparedand demonstrated in Fig. 8a. The differences of BF arraysunder these three kinds of molecular structures are clear at aglance.

Peng et al. [36] theorized that viscosity is a key parameter inBF formation, which is directly related to the Mw of polymersutilized. It was likely that the solution viscosity of low Mw

OCP-POSS was too low for effective water droplet stabiliza-tion, which resulted in no pore structures in Fig. 8a. And starpolymers with high Mw (Mw>100,000 g/mol) were good can-didates for honeycomb film formation, like POSS-(PTFEMA)8and POSS-(PMMA-b-PTFEMA)8 in Fig. 8b, c.

We also analyzed the quality of ordering and geometricparameters of pore structures further. Obviously, the quality ofordering in Fig. 8b was higher than that in Fig. 8c, whichindicated that POSS-(PTFEMA)8 showed a higher ability toself-assemble for highly structured honeycomb films, imply-ing that the PTFEMA segment may be a key factor and even agreat power for BF formation.

For the Mw effect on pore sizes, there were two oppositeviewpoints. Connal et al. [23] reported that the pore diametersdecreased as the molecular weight increased; the increase inmolecular weight would increase the density of the polymer,which is expected to increase the precipitation rate, hencecausing a reduced pore size. While, some researchers [37,38] suggested that increasing the molecular weight of poly-mers resulted in larger, less uniform shaped pores. And forPOSS-(PTFEMA)8 and POSS-(PMMA-b-PTFEMA)8 poly-mers, the results were in accordance to this. The Mw of thehybrid polymer POSS-(PMMA-b-PTFEMA)8 was about112,640 g/mol, larger than that of POSS-(PTFEMA)8(102,260 g/mol), but POSS-(PMMA-b-PTFEMA)8 BF filmshad pore sizes about 0.869±0.071 μm, which were muchlarger than that of POSS-(PTFEMA)8 (about 0. 647±0.079 μm), and we believed that it is caused by the trend offluorine migration to the surface [28]. Due to the high density

Fig. 7 Thermal analyses of (a) OCP-POSS and the hybrid copolymers((b) POSS-(PMMA-b-PTFEMA)8 and (c) POSS-(PTFEMA)8)

Fig. 8 SEMmicrographs of porous films prepared from 30mg/mL hybrid material solutions using CHCl3 as solvent. The hybrid materials utilized wereaOCP-POSS, b POSS-(PTFEMA)8, and c POSS-(PMMA-b-PTFEMA)8. d The cross section of the POSS-(PTFEMA)8 film

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of chloroform, pore structures were generated with a largerdiameter on the top and a smaller diameter on the bottom [39,40]. And with the significantly increasing effect of fluorinemigration to the surface, a higher deposit of water dropletsoccurred, which correspondingly leads to larger pore sizes.For the same reason, the rim width of POSS-(PTFEMA)8 wasabout 0.712±0.124 μm, which was smaller than that ofPOSS-(PMMA-b-PTFEMA)8 (about 0.817±0.197 μm).

Figure 8d shows a relatively thin film of POSS-(PTFEMA)8,about 6.81μm.As can be seen from themicrographs of the crosssection of the membrane, the pores were not through structure,which was due to the low surface tension corresponding toprevious research results [39].

Dependence on solvents utilized

As the solvent used to dissolve the polymer is expected to beof utmost importance, playing a role in almost all the phe-nomena involved in BF generation [41], four different sol-vents had been used to analyze solvent effects on patternformation, as shown in Fig. 9. The micrographs of differentsolvents highlighted that chloroform appeared to be the mostrobust candidate for the formation of our two hybrid starpolymers, due to its low boiling point and low solubility inwater. When dichloromethane as casting solvent was utilized,ordered porous BF films were also formed, but the quality ofpore ordering decreased obviously, due to a very fast solidifi-cation of the polymer film that occurred, favored by therelatively low boiling point and high vapor pressure of dichlo-romethane. No or poorly ordered BF morphologies wereobtained using THF and benzene, and the reasons betweenthem could differ a lot. Low water solubility in the solvent is arequirement for BF formation [41]; however, THF was mis-cible in water, which hindered droplet formation and resultedto poorly ordered BF films. Although benzene was one of themost common reagents for BF film preparation, it did notallow the formation of porous structures, as shown inFig. 9d, h, which was due to the low solubility of hybridcopolymers in benzene. Interestingly, after solvent evapora-tion, the surface of the POSS-(PTFEMA)8 sample preparedfrom benzene was characterized by the presence of polymermicrospheres rather than porous polymer films, as can be seenin Fig. 9h.

Dependence on the solution concentration

In this part, we mainly discuss how the solution concentra-tion affected the honeycomb morphology. Figure 10 showsthe SEM micrographs of POSS-(PTFEMA)8 andPOSS-(PMMA-b-PTFEMA)8 with different concentra-tions, from which we summarized that POSS-(PTFEMA)8and POSS-(PMMA-b-PTFEMA)8 were both capable of BF

formation with a large range of solution concentrations, andthe sizes of the honeycomb pores can be regulated between0.52 and 1.24 μm.

Comparing the SEM micrographs in Fig. 10, it is clear thata pronounced reduction of pore sizes occurred. Here, weselected a 19.3 μm×19.3 μm measurement area and gaugedall the pores in this area to calculate the mean values of poresize and rim width, which is shown in Fig. 11.

The viscosity of the solution increased with the polymerconcentration, which results in the slower growth of droplets,a faster polymer precipitation at the water droplet interface,and a lower pore size [42]. The qualitative formula R=K/c byStenzel [38] was applied to this phenomenon, where R standsfor the pore size, c stands for the solution concentration, and Kis a constant related to the polymer material used. This equa-tion was still applied for the polymers in this article. Mean-while, the rim width had the same trend of becoming thickerwith increasing concentration (almost linear variation), whichcould be easily understood by the increasing compounds ofthe polymer.

Moreover, in the wake of increasing solution concentra-tions, the number of polymers in each area increased, that is,the viscosity increased, which hindered the coalescence ofwater droplets, with a consequence that high quality of order-ing pore arrays is obtained. However, the solution concentra-tion has a tipping point. When the concentration exceeded themaximum value, an unexpected drop of the pore orderingquality could be found, which is observed as a wrinkled filmin Fig. 10f.

Also, compared with those of linear polymers of similarmolecular weight in a previous study, pore sizes of star poly-mers presented in this work were less sensitive toward con-centration, due to their lower tendency to entangle [43].

From the three-part discussions of varying formation con-ditions above, we could come up with the following rules offormation for ordered honeycomb porous films: differencesbetween molecular architectures and concentrations affectedthe viscosity of polymer solutions and further made effects onthe ability of the package of droplets; the difference betweensolvents, especially for volatility nature, could affect stabili-zation and coalescence time of water droplets; and under jointaction, the self-organized honeycomb morphology of BFfilms showed certain differences, including pore sizes, rimwidths, quality of pore ordering, and so on.

Properties of honeycomb porous films

The star hybrid fluorinated acrylate polymers covalent withPOSS core had been proved having great potential for thepreparation of self-organized honeycomb films in previousparts, which was expected to have a good solvent resistanceand hydrophobic property. To confirm our conjectures, these

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properties of honeycomb porous films were under investiga-tion in this section.

Acid-base resistance of honeycomb porous films

To analyze the acid-base resistance property of the porous filmbased on the fluorinated star hybrid polymer containingPOSS, we chose POSS-PTFEMA as an example and firstprepared the polymer porous film with regular pore arrays.

The film was then immerged in 10 % w/w hydrochloric acidsolution for 30 h at room temperature and washed with dis-tilled water until neutral, then dried naturally in air. Scanningelectron microscopy was used for observing the apparentmorphology, as shown in Fig. 12a. The alkali treatment wasunder the same handling method in 10 % w/w sodium hydrox-ide solution, and the SEM images are shown in Fig. 12b.

It was clearly shown that the porous polymer membranecan maintain honeycomb porous structures for large areas

Fig. 9 SEM micrographs of theporous films prepared from30 mg/mL polymer solutionsusing different solvents. Thesolvents were a, eCHCl3, b, fCH2Cl2, c, g THF, and d, hbenzene. a–d Prepared fromPOSS-(PTFEMA)8. e–h Preparedfrom POSS-(PMMA-b-PTFEMA)8

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even if immerged in strong acids and strong alkalis. And thegood resistance performance to acid-base of the membranematerials should be considered.

Hydrophobic property of honeycomb porous films

Fluorinated acrylate is an important chemical raw material;copolymers composed of it have been broadly applied andproved to possess good film forming property and excellentsurface performance [9]. And how the hydrophobic propertychanges according to the micro/nanopoles introduced in thefilm surface and whether superhydrophobic films could beobtained were our concerns. To solve these problems, flatfilms without porous structures were prepared and comparedwith related honeycomb porous films, and related contactangle results are shown in Fig. 13. Here, different polymersolutions of 30 mg/mL were first prepared and cast directly

onto precleaned glass substrates, and the flat films were thenwell prepared after solvent evaporation.

It was obvious that whether the films were flat or porous,the water contact angles (θ) were all larger than 90°, indicatingthat these hybrid polymers which were simultaneously cova-lent with POSS and TFEMA could be considered as hydro-phobic materials.

Compared with those of the flat films, the contact anglesof porous films based on POSS-(PTFEMA)8 andPOSS-(PMMA-b-PTFEMA)8 were improved to 113.7° withan up to 12.9° growth, which could be significantly seen inFig. 13, and it was induced from the micro/nanopoles in thefilm surface.

In addition, whether flat films or porous films,POSS-(PTFEMA)8 always possessed higher values of watercontact angles than POSS-(PMMA-b-PTFEMA)8, that is, thereduced hydrophobicity of materials. We believed that it

Fig. 10 SEM micrographs of the porous films prepared using different solution concentrations and CHCl3 as solvents. The concentrations of polymersolutions were a, d 15, b, e 30, and c, f 45 mg/mL. a–c Prepared from POSS-(PTFEMA)8. d–f Prepared from POSS-(PMMA-b-PTFEMA)8

Fig. 11 Concentration effects onpore sizes and rim width ofporous films: (left) prepared fromPOSS-(PTFEMA)8 and (right)from POSS-(PMMA-b-PTFEMA)8

Colloid Polym Sci

mainly resulted from the reduction of the fluorine component.From XPS data in a previous section, POSS-(PTFEMA)8possessed a much higher fluorine component thanPOSS-(PMMA-b-PTFEMA)8, and the more fluorine compo-nent with low surface energy the polymers had, the higherhydrophobicity the materials possessed.

Accordingly, we could draw a conclusion that the hydro-phobicity of materials came from the joint action of matrixproperties and surface structures. The porous structure couldcontribute to hydrophobicity, and the mechanism was thesame as that of the surface of a lotus leaf. Meanwhile, thiseffect has much more space for improvement for our study,which deserves further studies.

Conclusions

In summary, we have successfully synthesized two differentstar hybrid fluorinated acrylate polymers, POSS-(PTFEMA)8and POSS-(PMMA-b-PTFEMA)8, via ATRP. The polymerstructures were determined by Fourier transform infrared(FT-IR) spectroscopy and nuclear magnetic resonance(NMR). The fluorine content was featured by X-ray photo-electron spectroscopy (XPS), and the value was 29.39 and16.04 %, respectively. Gel permeation chromatography(GPC) results showed that the molecular weight distributionindexes were relatively low, and thermal gravimetric analysis

(TGA) proved the excellent thermal stability of the hybridpolymers. Also, we investigated the formation of hydrophobicbreath figure (BF) films based on these two hybrid copoly-mers under water atmospheres, and the influences of thepolymer architectures, polymer concentrations, and solventproperties on the film morphologies were investigated, whichwere mainly focused on pore sizes, rim widths, and the qualityof pore ordering.

It was found that hydrophobic honeycomb films with or-dered pores can be obtained with water droplets arrays as thetemplates based on both of the two polymers andPOSS-(PTFEMA)8 had greater potential for regular porousfilm formation due to the higher fluorine content and a corre-sponding higher water contact angle. It has also been demon-strated that the concentration of the polymer solution affectedthe surface morphologies. Pore size ranged from about 0.52 to1.24 μm, which almost linearly increased with the concentra-tions. And rim width ranged from about 0.51 to 1.17 μm,which becomes thicker with increasing concentration. Mean-while, the physical properties of different solvents influencedthe film morphologies, and we could draw a conclusion thatchloroform appeared to be the most robust solvent for our twohybrid star polymers. Furthermore, based on the excellentsurface properties (acid-base resistance and hydrophobicproperty) of the hybrid materials, it may be customized forspecific uses such as filtration, cell culture, tissue engineering,and marine antifouling.

Fig. 12 SEM micrographs ofPOSS-(PTFEMA)8 films aftera acid and b alkali treatment

Fig. 13 Water contact angles fordifferent films

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Acknowledgments We are grateful for the financial support from theNational Natural Science Foundation of Shaanxi Province (No.2009JZ004), Fundamental Research Foundation of NorthwesternPolytechnical University (Nos. JC201125 and JC201158), and GraduateStarting Seed Fund of Northwestern Polytechnical University (Nos.Z2013151 and Z2013150).

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