一步法合成聚苯胺/十二烷基苯磺酸钠超疏水复合材料

宋根萍 夏冬祥

(扬州大学化学化工学院,江苏扬州225002)

1 Introduction

In recent years,superhydrophobic surfaces have been attracted much attention due to their broad application prospects in the fields of industry,agriculture,and daily life.For example,they play an important role in metal anticorrosive,shipping antipollution,surface-tension-induced microfluidic manipulation,reduction of fluid flow resistance,light-driven motion of fluids and so on.Superhydrophobicity is an important property for most solid materials and is governed by both surface chemical composition(to lower surface free energy)and geometric structure(or surface morphology).In general,superhydrophobic surface should have a water contact angle(WCA)greater than 150°and can be obtained by decreasing the free energy of hydrophobic surfaces and controlling the topography of a rough surface(which is modified by covering with hydrophobic group).Superhydrophobicity of surfaces that mimic lotus leaves can be fabricated by various methods,including lithography,electro spinning techniques,hard-template,and self-organization.1-6

Polyaniline(PANI),which is easy to be synthesized,can be used as one of the most promising conducting polymers and have various possible applications in gas and chemical sensors,7antiseptic coating,cell of sun energy,etc.Its morphologies and conductivity can be controlled and can exhibit good environmental stability.However,the conducting behavior of PANI is easily influenced by weather conditions(for example,acid rain)and chemical contaminants in the natural environment.8As a result,static-charge accumulation may occur in PANI,which could be a disastrous drawback in its application.Therefore,how to prepare PANI composite material having both conductive and hydrophobic properties is important and interesting.We have previously developed a general chemical route to prepare conductive PANI with micro/nanostructure and multidimensional architectures.9,10

Recently,Jiang′s group11found that the water contact angle of PANI was increased from 0°to ＞150°after being doped with perfluorinated sebacic acid on the three-dimensional(3D)microstructures assembled from one-dimensional(1D)nanofibers of polyaniline.However,so far,the synthesis of surperhydrophobic of PANI via one step method is still challenging.

Surfactants have especial amphiphilic structure,and can be used to control both the material morphology and the surface chemical components.In this paper,we make use of hydrophobic group of surfactant to fabricate a novel PANI composite surface with low surface free energy and high roughness degree via one step method.The superhydrophobic material surface with high roughness degree is formed by the alkyl phenyl of SDBS and the aromatic group of PANI and exhibits two different structures:microstructured rods or sheet and nanostructured particles.In addition,this method is facile,mild,and cheap and no pollution from fluoride occurs.

2 Experimental

2.1 Materials

Aniline(An,99.5%,Shanghai Reagent Co.,China)was distilled under reduced pressure.Sodium dodecylbenzene sulfonate(SDBS,Aldrich).Ammonium peroxydisulfate(APS,98%,Shanghai Reagent Co.,China).Water used was deionized.

2.2 Synthesis of polyaniline/SDBS

superhydrophobic materials In a typical synthesis,0.58 g of aniline was placed in conical flasks first,and then,added 0.28 g of SDBS(dissolved in water)at 25°C under magnetic stirring.After 1 h,added adequate amounts of the aqueous solutions of initiatorAPS(the molar ratio n(An):n(APS)=1:1)and hydrochloric acid(as regulator of pH).Kept magnetic stirring for 24 h at(25±1)°C.The resulting product was washed with deionized water and ethanol,respectively,until the filtrate became colorless and then,dried in a vacuum at 60°C for 24 h.

2.3 Characterization

The contents of element S and the morphologies of the products were examined by a field-emission scanning electron microscopye(FESEM,S-4800,Hitachi Co.,Japan)equipped with an energy dispersive system(EDS).The EDS analysis of sample has typically been applied to quantitatively identify major elements based on relative peak intensities.12The FTIR spectra of the samples were recorded by a Fourier transform infrared spectrometer(TENSOR27,Bruker Co Germany).The X-ray diffraction patterns of the particles were recorded in a X-ray diffractometer(XRD,AXS D8,Brucker Co.Germany)with a Ni filter and Cu radiation(λ=0.1542 nm).The tube voltage was 50 kV and tube current was 180 mA.Water contact angle,were measured on a dataphysics OCA40 contact-angle system at ambient temperature.

3 Results and discussion

3.1 Surface morphology and surface properties of PANI/SDBS composites

The EDS spectrum of PANI/SDBS composites shows peaks corresponding to elements S(Fig.1),from which the content of element S in PANI/SDBS at pH=10(Fig.1A)is lower than that at pH=2(Fig.1B).This suggests that the existence of SO3-of SDBS in the superhydrophobic composites at pH=2 is more than that in PANI/SDBS at pH=10,indicating that PANI main chain may link with SDBS molecules by electrostatic force.

Fig.1 EDS spectra of PANI at pH being 10(A)and 2(B)

Fig.2 presents the FESEM images of PANI and PANI/SDBS composites,and the inset shows the water contact angle.By comparing Fig.2(a,b)with Fig.2(c,d),it can be seen that doping SDBS into PANI can make the surface of the product look like toad skin(Fig.2(c,d)).When the molar ratio of APS to aniline is 1:1 and the concentrations of SDBS and aniline are 0.016 and 0.1 mol·L-1respectively,a belt-like micro/nanostructured surface was formed,with a length of about 5-15 μm,a width of 2-3 μm,and a thickness of 200 nm.Furthermore,a mass of convex particles with an average size of about 100-200 nm was observed.The inset in Fig.2a shows that the water contact angle was 0°for PANI,and after doping SDBS into PANI,the water contact angle increased from 0°to 152°.Therefore,a surperhydrophobic PANI/SDBS composite is formed and the superhydrophobicity of PANI/SDBS composite is related to its morphology,following Cassie-Baxter equation.13

Fig.2 FESEM images of PANI synthesized at 25°C with SDBS

Fig.3 FTIR spectra of PANI synthesized at 25°C with SDBS concentrations of 0 mol·L-1(curve 1)and 0.016 mol·L-1(curve 2)

Fig.3 shows the FTIR spectra of PANI prepared in the absence and presence of SDBS.In curve 1 of Fig.3,the characteristic peaks at 1568 and 1481 cm-1are due to the stretching vibration of quinoid ring and benzenoid ring,respectively,and the bands at 1304 and 1246 cm-1can be attributed to C―H stretching vibration.The bands due to C―N stretching vibration of secondary amine in polymer main chain(1298 cm-1)and C―N stretching vibration in bipolaron structure(1245 cm-1)were overlapping by those at 1304 and 1246 cm-1,respectively.The absorption peak near 1140 cm-1is from the stretching modes of C＝N.The absorptions around 1042 and 750 cm-1should be assigned to the 1,2,4-trisubstituted aromatic ring.Therefore,the product should be cross-linked,which may cause the product showing a sheet-like structure(Fig.2(a,b)).

When the PANI is prepared in the presence of SDBS,the peaks at 1568,1481,and 1138 cm-1become much wider(curve 2 in Fig.3).The peak at 1160-1180 cm-1indicates the interaction between sulfonic acid group and N in sulphamide.14It has be well-known that anionic surfactant SDBS can dope into aniline oligomer via the electrostatic interaction with the anilinium cation and then form a composite structure,which indicates that SDBS molecules are linked with PANI main chain.Thus,SDBS molecules function as a soft-template in the preparation of PANI and at the same time,they are doped into PANI.The disappearance of the peak at 1042 cm-1further elucidates that SDBS can break the cross-linked bond in PANI and form PANI/SDBS composite material.As a result,the micro/nanostructure of PANI/SDBS composite material can grow in a more orderly way.

Three characteristic UV-Vis absorption peaks were observed for PANI/SDBS composites(Fig.4).15The peak at 340-350 nm is ascribed to the π-π*transition in the aromatic ring,and those at about 420 and 790 nm are ascribed to the polaron-π transition and π-polaron transition,respectively.By comparing curve 1 with curve 2,a red shift for the characteristic absorption can be found after the addition of SDBS due to a larger conjugate system.16

The result of XRD pattern of PANI shows an unusual high crystallinity with the main diffraction peaks centered at 2θ being 19.9°and 25.2°(Fig.5),which means that the introduction of SDBS units into PANI increases the intermolecular chain spacing and the orderliness of main chain,which is in agreement with the reported results.17

Fig.4 UV-Vis spectra of PANI synthesized at 25°C with SDBS concentrations of 0 mol·L-1(curve 1)and 0.016 mol·L-1(curve 2)

Moreover,the hydrophobic property of PANI/SDBS composite is influenced by the reaction condition,such as the concentration of SDBS,pH value of reaction solution,the molar ratio of initiator APS to aniline monomer,the reaction temperature and stirring rate.Fig.6 shows the relationship between pH and water contact angle for the PANI/SDBS composite material,from which it can be seen that the surperhydrophobic structure is stable with pH being 1-9.When we increase the pH value to 10,the water contact angle is decreased obviously.What should be mentioned here is that Feng et al.18also found that a doped PANI composite with hydrophobicity can not be prepared with pH higher than 9.Thus,the electrostatic interaction between―SO3-of SDBS and―NH+＝ of PANI should play a key role in the formation of the superhydrophobic composite material.Fig.7 shows the FTIR spectrum of the composite.Different from the case at pH=2(curve 1),a peak centered at 1034 cm-1was observed at pH=10(curve 2),which should be assigned to the 1,2,4-trisubstituted aromatic ring and suggest a cross-linked product at pH=10.Moreover,the morphology of the products at pH=10 is flake-like(Fig.8(a,b))and at pH=2,it is belt-like(Fig.8(c,d)).Fig.9 presents the SEM images of SDBS/PANI composites prepared with different inorganic acids(at pH=2).19By combing Fig.2c with Fig.9,it can be concluded that the superhydrophobic composite can be prepared no matter what inorganic acid is used.

Fig.5 XRD patterns of PANI synthesized at 25°C with SDBS concentrations of 0(curve 1)and 0.016 mol·L-1(curve 2)

Fig.6 WCAwith the pH of reaction system

Fig.7 FTIR spectra of PANI synthesized at pH 2(curve 1)and pH 10(curve 2)

Fig.8 FESEM images of PANI synthesized at pH 10(a,b)and pH 2(c,d)

Fig.9 FESEM and WCA(inset)images of PANI synthesized with H2SO4(a)and H3PO4(b)

Fig.10 shows the effect of SDBS concentration on the water contact angle,from which three stages were observed.When the concentration of SDBS is less than 0.008 mol·L-1,20WCA=0°(region A in Fig.10)and the morphology of the composite is bulky and sheet-like(Fig.2(a,b)).When the concentration of SDBS is appreciably more than 0.012 mol·L-1,the hydrophobic composites(WCA＞90°,region B in Fig.10)were obtained.When the concentration of SDBS is more than 0.016 mol·L-1,superhydrophobic composites(WCA＞150°,region C in Fig.10)can be formed,showing a belt-like structure(Fig.2(c,d)).With the increase of SDBS concentration,more hydrophobic chains can be located around the surface of the composite material,which may make the hydrophobic interaction increased and hence,result in a rough belt-like micro/nanostructure with low surface free energy.

3.2Formation mechanism

Fig.10 WCAwith the pH of reaction system

About 91%of aniline can be polymerized in the absence of SDBS,while the doping of SDBS makes 98%of aniline conversed.Regarding the formation mechanism of such composites micro/nanostructures,one main factor should be a cooperation between the electrostatic attraction(between the acid anion with amine cation)and non-covalent interaction(including hydrogen bonding,π-π stacking,intermolecular forces,and hydrophobic interaction as the driving forces).In this study,the micelles formed by SDBS/aniline monomer21can serve as the soft-template in the formation of self assembled micro/nanostructure of PANI/SDBS composite via one-step.As shown in Fig.11,under acidic condition,the electrostatic interaction or sulfamic bonding exists between―SO3-of SDBS and―NH+＝of PANI conjugate chain,and among PANI conjugate chains,there exist hydrogen bonds between nitrogen atoms and hydrogen atoms.22-25The electrostatic interaction and hydrogen bonding immobilize the hydrophilic head groups(―SO3-)of SDBS molecules around the PANI conjugate chain,which could make the hydrophobic chain of SDBS going out.

Then,due to the cooperative effect of hydrogen bonding,ππ stacking,intermolecular forces,and hydrophobic interaction,the hydrophobic chains may aggregate and form a nanoconvex structure on the surface of SDBS/PANI composites.As a result,the air/water interface area becomes bigger and hence,decreasing the free energy of the surface and forming the beltlike micro/nanostructured PANI/SDBS composite with superhydrophobicity.However,at pH＞9,there is no electrostatic interaction between―SO3-of SDBS and―N＝ of PANI,and therefore,superhydrophobic structure can not be formed.

Fig.11 Illustration for the formation of PANI/SDBS composites

4 Conclusions

The polyaniline/SDBS composite with a micro/nanostructure and superhydrophobicity was prepared by simple one-step chemical oxidation on a large scale at low cost,and the fluoride pollution usually existing in the other reports can be avoided.Moreover,the preparation conditions were optimized in the present study;the superhydrophobic composite material of PANI/SDBS can be synthesized when SDBS concentration is between 16 and 32 mmol·L-1and pH value between 1 and 9.

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