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Polymer 53 (2012) 2211e2216

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Polymer

journal homepage: www.elsevier .com/locate/polymer

Statically non-wetting electrospun perfluorocyclobutyl (PFCB) aryl ether polymerdoped with room temperature ionic liquid (RTIL)

Rajneesh Verma a, Neetu Tomar b, Stephen E. Creager b, Dennis W. Smith Jr. a,*aDepartment of Chemistry and The Alan G. MacDiramid NanoTech Institute, University of Texas at Dallas, Richardson, TX-75080, USAbDepartment of Chemistry and Center for Optical Material Science and Engineering (COMSET), Clemson University, Clemson S.C-29631, USA

a r t i c l e i n f o

Article history:Received 30 January 2012Received in revised form9 March 2012Accepted 15 March 2012Available online 23 March 2012

Keywords:ElectrospinningIonic liquidsPerfluorocyclobutyl aryl ether polymer

* Corresponding author. Tel.: þ1 864 207 0661.E-mail address: dwsmith@utdallas.edu (D.W. Smit

0032-3861/$ e see front matter � 2012 Published bydoi:10.1016/j.polymer.2012.03.033

a b s t r a c t

A statically non-wetting, electrospun surface of non volatile room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium hexafluorophosphate, (BMIM-PF6), hosted in a solution-processable, semi-fluori-nated perfluorocyclobutyl (BP-PFCB) aryl ether polymer was successfully prepared by electrospinning andcomparedwith a surface prepared by spin casting. The surface properties of undoped and BMIM-PF6 dopedsystems were analyzed by water contact angle (WCA) and atomic force microscopy (AFM). BMIM-PF6doped BP-PFCB surfaces prepared by spin casting showed a WCA of 90� while non-woven electrospunsurfaces with the same BMIM-PF6 concentration showed high degree of hydrophobicity with a WCAgreater than 150�. Morphologies of the electrospun surfaces were characterized by scanning electronmicroscopy (SEM). The surface compositionwas analyzed by energy-dispersive X-ray spectroscopy (EDXS)and attenuated total reflectance infrared spectroscopy (ATR-IR). Thermal analysis of the electrospun, non-woven surfaces of the doped and the undoped system of BP-PFCB were done by TGA.

� 2012 Published by Elsevier Ltd.

1. Introduction

Superhydrophobicity has attracted a significant amount ofattention frommany researchers seeking to exploit the potential ofthese surfaces for various applications ranging from water repel-lency, self-cleaning and anti-fouling coatings to microfluidicdevices [1,2]. Hydrophobic polymers (WCA � 90�) are useful invarious low function devices and environmental resistant coatingswhile superhydrophobic materials (WCA � 150�) are of specialinterest as self-cleaning and stain resistant surfaces [3]. However,the low surface tension and confined contact area at thesolideliquid interface associated with superhydrophobicity alwaysresult in a poor adhesion or dyeability, limiting many other appli-cations [4] Therefore, a surface which have a high static water CAand simultaneously provides a strong pinning of the water dropletto the surface is useful in many applications and studied by variousgroup [5,6].

Electrospinning has emerged as a useful processing technique toproduce non-woven mats consisting of sub-micron diameter fiberswith a large surface area to volume ratio.

A variety of electrospun polymer mats have been investigatedfor various potential applications such as functional membranes,

h).

Elsevier Ltd.

photocatalysts, electrical, optical, sensing applications and nano-electronics [7e14]. Perfluorocyclobutyl (PFCB) polymers are anemerging class of next-generation solution-processable, amor-phous, semi-fluorinated polymers that have proven their potentialin applications including integrated optics, proton exchangemembranes (PEMs) for fuel cells, light emissive materials forpolymer light emitting diodes (PLEDs), atomic oxygen resistantcoating materials, oleophobic coatings and hydrophobic hybridcomposites with polyhedral oligomeric silsesquioxanes (POSS) [15].

The hydrophobicity of a material depends upon the chemicalcomposition and the microstructure on the surface [16,17]. In thisstudy we are combining two widely known techniques of electro-spinning and spin casting to generate hydrophobicity. In general, thewater contact angle for spin cast films rarely exceeds 120� ascompared to electrospinning which ranges from 60� to 160� [17,18].In order to mimic the surface roughness and topography of a super-hydrophobic natural surface such as a lotus leaf, various approacheshave been reported [19e26]. These approaches, which includeaggressive chemical surface treatments, high temperature post-surface modification, or elaborate patterning, are limited by theirpropensity to create an accumulation of static charge which cancause a fire or explosion when used in practical applications underdry conditions [27]. Recently, non volatile room temperature ionicliquids (RTIL’s) have received great attention in the field of polymerscience including polymerization media, polymer electrolytes, andsuperhydrophobicity [28e31]. The unique properties of ionic liquids

Fig. 1. Structures of (a) BP-PFCB (b) BMIM-PF6.

R. Verma et al. / Polymer 53 (2012) 2211e22162212

include a very large liquidus range, almost no vapor pressure, awidewindow of electrochemical stability, good electrical conductivity,high ionic mobility and excellent chemical stability. The successfulapplication of RTIL’s in cellulose processing has further suggestedtheir potential as functional polymer additives [31].

Here, we are reporting on the fabrication of a novel staticallynon-wetting PFCB aryl ether polymer surfaces doped with BMIM-PF6, and the effect of the following processing techniques onhydrophobicity: spin casting and electrospinning. The hydrophobiccharacter of RTIL was recognized in our efforts to produce waterrepellent coatings of PFCB aryl ether polymers [32]. The ionic liquidin these kinds of surfaces may help to dissipate static charge in dryconditions.

Earlier we reported the hydrophobic surfaces of PFCB with POSSmodifiers and recently Meskini and coworkers made perfluorodecyl vinyl ether based hydrophobic surfaces [33,34].

2. Experimental

2.1. Materials

The biphenyl perfluorocyclobutyl polymer (BP-PFCB), (Mn ¼40,000 g mol�1) was obtained from Tetramer Technologies, L.L.Cand distributed through Oakwood chemicals, Inc. THF and BMIM-PF6 were purchased from SigmaeAldrich and used as acquired.

Fig. 2. WCA on spin cast (SC) and electrospun (ES) surfaces. (standard deviation � 2�).

2.2. Electrospinning set up

The electrospinning apparatus consisted of a syringe with a flat-end metallic needle, a syringe pump to control flow rates,a grounded aluminum foil wrapped board, and a high voltage DCpower supply (HAN IL TCM Co., (DC) Ltd.). In our electrospinningexperiment, BP-PFCB solutions were delivered to the tip of thesyringe needle by the syringe pump (74900, ColeeParmer Instru-ment Co., Ltd.) and electrospun polymer fibers were collected ontoaluminum foil using a point to plate assembly. Optimized pro-cessing parameters, evaluated in terms of contact angle, arereported here. Spin cast films were prepared on glass plates at1800e2000 rpm using model WS-650S-6NPP/LTTE Laurell Tech-nologies Co spin coater.

2.3. Characterization

Spin cast film thicknesses were approximately 1e2 mm asmeasured byMitutuyo, IP-65 model. Zygo NewView 6300 3D whitelight optical profiling system. Static water contact angles weremeasured by placing 1e3 mL drop of deionized water on thehydrophobic surfaces then capturing the image with FDS Data-physics contact angle analyzer. The reported static contact anglesare an average of six values measured on 7 different areas of theelectrospun and spin cast surfaces with an average standard devi-ation of �2�. The morphologies of the electrospun surfaces wereexamined by scanning electron microscopy using SEM-Hitachi

Table 1Various properties of BP-PFCB surfaces with and without BMIM-PF6 prepared byelectrospinning and spin casting.

Surface WCAa EDXS in wt. (%) AFM

C O F P Roughness(Ra,Rq) (nm)

BP-PFCBSpincast Surface(SC)

70� 62 11.3 20.7 e 0.19, 0.26

BP-PFCB þ BMIM-PF6Spincast Surface(SC)

90� 63 13.6 25.7 0.28 1.01, 1.16

BP-PFCBElectrospun Surface(ES)

143� 59 11.0 18.9 e 3.01, 3.39

BP-PFCB þ BMIM-PF6Electrospun Surface

(ES)

154� 58 12.6 21.2 0.21 11.70, 12.31

a Static contact angle with standard deviation � 2.0� .

Fig. 3. ATR-IR of (a) Neat BP-PFCB (b) Blended BP-PFCB with BMIM-PF6 (c) NeatBMIM-PF6.

R. Verma et al. / Polymer 53 (2012) 2211e2216 2213

S4800 operated at 20 kV in VP-mode. 8 different fibers were chosenfrom 6 different areas and an average image was presented.

Elemental mapping was performed using energy dissipation X-ray analysis (EDXS) with INCA software attached with SEM-HitachiS3400N at 10 kV.

Thermal Gravimetric Analysis (TGA) was performed on TAinstrument 2950. The temperature range for TGA is 0e600 �C. Theinvestigationwas carried out in a nitrogen and air atmosphere witha heating rate of 5� C/min.

Atomic Force Microscopy (AFM) was used to measure theaverage surface roughness (Ra) and the root mean square rough-ness (Rq) of electrospun fibers and spin coated films using Veecodimension 3100 in tapping mode with a set point ratio of 0.81.Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR)analyses of resulting films were performed on a Thermo-Nicolet

Fig. 4. EDXS fluorine mapping of (a) BP-PFCB (ES) fiber (b) BP-PFCB/BMIMPF

Magna 550 FTIR spectrophotometer with a high endurance dia-mond ATR attachment. The depth of penetration of the evanescentwave for the analysis was 2 mm.

3. Result and discussion

3.1. Optimization of electrospinning parameters and WCA

Solutions of BP-PFCB of concentration 11.2 w/v (%) in THF wereprepared. The chemical structures of the components of the blendsare shown in Fig. 1. The processing parameters of electrospinninginclude BP-PFCB polymer solution of concentration 11.2 w/v (%)(Mn ¼ 40,000 g mol�1) doped with 0.75 w/v (%) of BMIM-PF6 andelectrospun from a 0.6 mm metallic needle with an interelectrodedistance and interelectrode voltage of 12.2 cm and 25 kV, respec-tively. The flow rate is kept constant at 7.5 mL/h.

The BMIM-PF6 concentration strongly influenced the surfacemorphology of the fibers even at less than 1 wt (%) concentration.When the BMIM-PF6 content was increased to 1.5%, the fibers wereexpelled from the needle so rapidly that they were difficult tocollect onto the collecting screen.

Solutions with the same concentration of BP-PFCB and BMIM-PF6, as were used in electrospinning, were also spin casted ontoglass plates. The polymer coated substrate was dried in a vacuumoven at room temperature for 24 h prior to the thicknessmeasurement.

The comparison of the WCAs of the undoped BP-PFCB and theBMIM-PF6 doped samples, processed by two different techniquesi.e. electrospinning (ES) and spin casting (SC), are shown in Fig. 2.

The WCA of the electrospun surface of BP-PFCB doped withBMIM-PF6 was higher than that of the undoped BP-PFCB electro-spun surface. Both, the doped and the undoped systems of BP-PFCBelectrospinning resulted in a more hydrophobic surface than spincasting. The water contact angles indicated that the surfaces ofdoped and undoped BP-PFCB polymer, processed by two differenttechniques, had very different surface characteristics, (Fig. 2 and

6(ES) fiber (c) BP-PFCB (SC) surface (d) BP-PFCB/BMIM-PF6(SC) surface.

Fig. 5. SEMs of (a) BP-PFCB fiber, (b) BP-PFCB/BMIM-PF6 fiber.

R. Verma et al. / Polymer 53 (2012) 2211e22162214

Table 1). The BP-PFCB/BMIM-PF6 (ES) surface showed a very highhydrophobic character with a static water contact angle of 154�

while BP-PFCB (SC) showed a smaller degree of hydrophobicitywith a static water contact anglew90�. The addition of 0.75 w/v (%)of BMIM-PF6 produced a 29% increase in the WCA for the BP-PFCB/BMIM-PF6 (SC) surface while the same loading of BMIM-PF6produced an 11% increase in the WCA of BP-PFCB/BMIM-PF6 (ES)surface and resulted in a highly hydrophobic character. These

Fig. 6. AFM images of the surface of (a) BP-PFCB

polymer surfaces have comparable fluorine content except for theminor PF�6 counter ion from BMIM-PF6 which induced an increasein the WCA of the doped samples.

The addition of BMIM-PF6 to BP-PFCB resulted in a smallerpercentage increase in the WCA of electrospun surface than it doesin the spin cast surface, for which the addition of BMIM-PF6produced a noticeable percent increase in the WCA. It was evidenton the basis of the WCA of BP-PFCB (ES) and BP-PFCB/BMIM-PF6(ES) surfaces that the WCA is not only dependent upon the fluorinecontent but on another important factor as well i.e., surface texture.Controlling the degree of surface roughness can be one of the mosteffective ways to tune hydrophobicity.

3.2. Surface characterization and infra-red analysis

Fig. 3 shows the ATR-FTIR spectra of electrospun undoped BP-PFCB, BP-PFCB doped with.

BMIM-PF6 and neat BMIM-PF6. The intensive characteristic peakof BMIM-PF6, at 3172 cm�1 corresponding to C-H vibrations forcyclic BMIþ, can be seen in the BP-PFCB/BMIM-PF6 doped sub-micron fibers, confirming the presence of BMIM-PF6 in BP-PFCB/BMIM-PF6 (ES) fibers [35]. The presence of BMIM-PF6 was furtherconfirmed by EDXS elemental mapping which indicated excellentdispersion of the BMIM-PF6 within the BP-PFCB aryl ether polymermatrix (Fig. 4 and Table 1).

It was observed that the fluorine and phosphorus content werehigher in BP-PFCB/BMIM-PF6 (SC) and BP-PFCB/BMIM-PF6 (ES) thanit was in their non-doped analogues. No special hierarchicalstructures were seen in the spin cast films.

The increase in the fluorine content on the surface of the doped,spin coated sample may be attributed to the blooming of BMIM-PF6while the increase in the electrospun analoguemay be attributed tothe migration of BMIM-PF6 to the surface, under the influence ofthe electric field. SEM images of the undoped and doped fibersurfaces, generated through electrospinning, are shown in Fig. 5.

The morphology of electrospun BP-PFCB (ES) (Fig. 5 a) and BP-PFCB/BMIM-PF6 (ES) (Fig. 5 b) was examined by SEM. The SEMimage of fibers from the doped system showed hierarchical struc-tures, in the form of nano imprints on the surface, while the surfaceof the fibers from the non-doped system was smooth. No suchhierarchical structures were observed in the SEM images of spincast films (not shown here). Nanoscale and microscale hierarchicalstructures were also seen in the elecrospun surface of composites ofpolystyrene (PS) with BMIM-PF6, generated through electro-spinning, these structures raised the degree of hydrophobicity ofelectrospun surface of PS/BMIM-PF6 composite as compared to theneat PS elecrospun surface [35].

The AFM images (Fig. 6) of the electrospun fibers showed anincreased surface roughness in the doped systems. Surface

(ES) fiber (b) BP-PFCB/BMIM-PF6 (ES) fiber.

Fig. 7. TGA traces of spin cast films of neat, BP-PFCB, BP-PFCB/BMIM-PF6 and neatBMIM-PF6 in air.

R. Verma et al. / Polymer 53 (2012) 2211e2216 2215

roughness is the measure of the finer irregularities in surfacetexture and it plays a major role in depicting hydrophobic behavior[36e38]. The AFM analysis comparing the doped BP-PFCB fiberwith the un-doped BP-PFCB fiber (Fig. 6 a and Fig. 6 b) clearlydistinguishes the crinkled surface of the doped fiber from thesmooth surface of the undoped fiber. The undoped BP-PFCB fiberand the doped BP-PFCB fiber have an average surface roughness of3.01 � 0.20 nm and 11.70 � 0.30 nm respectively. The crinkledsurface of the doped fiber caused a marked increase in surfaceroughness. The spin cast system showed a marginal increase in theaverage surface roughness going from 0.19 nm (doped) to 1.01 nm(undoped) with a standard deviation of�0.05 nm (Fig S1). A carefulanalysis of the average roughness (Ra) and the root mean squareroughness (Rq) for electrospun and spin cast surfaces of neat BP-PFCB and BP-PFCB/BMIM-PF6 is given in Table 1. This kind ofnanoscale hierarchical structure (Fig. 5 and 6) as seen from AFMand SEM enhances the water repelling tendencies of the surface[39e43]. The electrospun fibers are rougher than the spin castfilms. Although the reason for the increased roughness isundoubtedly complex, a variety of instrumental parameters, suchas those affecting field induced perturbation of charged mobilespecies at the surface, may account for this increase.

In general, we can conclude that the BMIM-PF6 affected theoverall hydrophobicity by inducing nano structures on the surfaces,which in turn affect the behavior of the three phase contact line[44]. The increase in the elemental fluorine content is anotherfactor which contributed to the high hydrophobic behavior.

Fig. 8. TGA traces of spin cast films of BP-PFCB, BP-PFCB/BMIM-PF6 and neat BMIM-PF6in N2.

Thewater droplet remained pinned to the surface irrespective ofthe tilt angle. The electrospun surfaces of BP-PFCB and BP-PFCB/BMIM-PF6 are not oleophobic and completely wetted by hex-adecane. Low hexadecane contact angles have been reported forspin cast films of PFCBs [45]. Ionic liquid did not induce any degreeof crystallinity in the inherently amorphous BP-PFCB polymer.

3.3. Thermal analysis

Thermal stability of BMIM-PF6, BP-PFCB and BP-PFCB/BMIM-PF6nano fibers and films were carried out by TGA. The degradationpattern for BMIM-PF6, BP-PFCB and BP-PFCB/BMIM-PF6 filmsprepared, either from spin casting or electrospinning, were thesame. The TGA curves (spin cast films) in air and in N2 are shown inFig. 7 and Fig. 8.

All of the systems were moderately stable up to 300 �C,however, in the presence of BMIM-PF6 in N2 atmosphere earlyonset of weight loss occurred which may be due to BMIM-PF6catalyzed BP-PFCB degradation and is the subject of the ongoingstudy (Fig. 8). In air atmosphere, BP-PFCB shows two step degra-dation while BMIM-PF6 and BP-PFCB/BMIM-PF6 shows the samedegradation onsets, Fig. 7.

4. Conclusion

Statically non-wetting non-woven surfaces of BP-PFCB polymerwere successfully prepared by incorporating a room temperatureionic liquid i.e. BMIM-PF6 through electrospinning. The wettingbehavior of the BMIM-PF6 doped solid surfaces prepared by spincasting and electrospinning were studied and compared. The pres-ence of BMIM-PF6 in the surfaces of BP-PFCB generated throughelectrospinning and spin casting enhanced the degree of hydro-phobicity as compared to the surfaces of the undoped analoguesgenerated through the same technique. The electrospun surfacesshowed higher contact angles as compared to their spin castanalogues. The superior surface hydrophobicity, characterized bythe static contact angle exceeding 150�, was observed for electro-spun BP-PFCB containing less than 1 wt (%) concentration of BMIM-PF6, which is attributed mainly to the nanoscale hierarchical struc-tures on the electrospun BP-PFCB doped fibers. The solvent castedfilms had contact angles typical for surface enriched fluorinatedcompounds. We anticipate that the RTIL containing PFCB aryl etherpolymer may find wide potential applications in the fabrication ofcontrollable and functional surfaces such as biochips.

Acknowledgment

We gratefully acknowledge the financial support from Depart-ment of Energy (Grant No. DE-FG02-08ER46501), South CarolinaEPSCoR, Defense Advanced Research Projects Agency (DARPA), DoEBAS and the Robert. A. Welch foundation (AT-0041). We also thankIntel corporation for their partial financial support. We thank theCU microscope facility for technical support.

Appendix A. Supplementary information

Supplementary information related to this article can be foundonline at doi:10.1016/j.polymer.2012.03.033.

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