Electrospinning of Smart Thermochromic Nanofibers as...

Scientia Iranica F (2020) 27(6), 3447{3453

Sharif University of TechnologyScientia Iranica

Transactions F: Nanotechnologyhttp://scientiairanica.sharif.edu

Electrospinning of smart thermochromic nano�bers assensors

N. Eslahia;�, T. Fatemia, M. Varseia, and S. Bazgirb

a. Department of Textile Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.b. Department of Petroleum and Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.

Received 29 March 2020; received in revised form 16 August 2020; accepted 21 September 2020

KEYWORDSThermochromic;PDA;Nano�bers;Electrospinning;Sensors.

Abstract. Color-transition features of polydiacetylene (PDA) have gained attention inrecent years owing to its potential use in a wide range of sensors. This paper investi-gates electrospinning of PDA and polyvinylpyrolidone (PVP) to develop thermochromicnano�brous composites. Polymer mixtures at di�erent mass ratios and concentrations wereelectrospun and characterized afterwards. SEM results showed successful electrospinningof continuous and bead-free nano�bers with di�erent diameters depending on the massratio of PVP to PDA. The obtained �brous mats were then photo-polymerized under UVirradiation, which led to generation of a blue color showing the self-assembly of diacetylenemonomers. The colorimetric transition of the �bers was also investigated and a color changefrom blue to red occurred by heating the �bers up to 80�C because of the conformationalalterations in the PDA molecules. FTIR and DSC analysis validated these �ndings as well.As a result, the fabricated nano�brous composites are potentially appropriate candidatesfor sensing applications.

© 2020 Sharif University of Technology. All rights reserved.

1. Introduction

Recent advances in nanotechnology have led to anupsurge of interest in the development of sensorswith enhanced sensitivity and speci�city [1]. Variouselectrospun polymer nano�bers have been developed assensors owing to their intrinsically high aspect ratio,surface functionality, low cost, and simple fabrica-tion [2,3]. Electrospun nano�bers have been employedin a wide range of detection systems such as colorimet-ric, biological, and electrochemical applications [4]. Inparticular, interest has grown in colorimetric sensors

*. Corresponding author.E-mail address: [email protected] (N. Eslahi)

doi: 10.24200/sci.2020.55714.4369

owing to their portability, rapid response, accuracy,and cost-e�ectiveness [5].

Conjugated polymers have recently gained atten-tion in sensor development since they impart easysignal analysis, color transitions and facile diagnosis [6].Besides, owing to the electrical conductivity along thepolymer backbone, conjugated polymers can provide amore sensitive response than small molecules in varioussensing technologies [7]. Polydiacetylene (PDA) isan interesting conjugated polymer due to its uniqueblue-to-red color transition in response to externalstimuli (such as heat, pH, and organic solvents), whichcan be easily observed by the naked eye [8]. PDAcan be simply synthesized by polymerizing diacetylenemonomers under ultraviolet or gamma irradiation with-out the need for chemical initiators or catalysts [9].Thermochromism is a phenomenon in which a materialswitches color in response to a change in tempera-

3448 N. Eslahi et al./Scientia Iranica, Transactions F: Nanotechnology 27 (2020) 3447{3453

ture [10]. The colorimetric transition temperaturesof PDAs correspond with the melting points of thediacetylene monomers which usually occurs above roomtemperature (typically > 40�C) [11]. PDAs in variousforms such as �lms, powders, coatings, and �bers havebeen studied and used for sensing chemical or biologicaltargets [12,13].

For production of ultrasensitive sensors, electro-spinning is a promising fabrication technique for aproviding high surface area [14]. Electro spinninghas proven an e�cient method to produce polymeric�bers having a porous structure and good intercon-nectivity [15]. It can be employed to design self-assembled nanostructures with adjustable features fordeveloping multifunctional devices [2]. PDA electro-spun nano�bers are particularly promising for opticalsensing owing to simple functionalization, facile syn-thesis, and the adaptability to enable colorimetric or

uorescence signaling [16,17]. Because of the highcost and low spinnability of PDA, this conjugatedpolymer has been mixed with other polymers toproduce nano�brous composites [9]. According toYapor et al., PDA-containing nano�ber compositesexhibited profounder colorimetric responses than thoseof the pristine PDA [5]. Jeon et al. functionalizeddiacetylene monomers with trimethyl amine (PCDA-DMEDA) and mixed them with Poly (Ethylene Oxide)(PEO) into electrospun �bers for potential diagnosis ofHCl gas [18]. In another study, PDA was combinedwith PEO to produce electrospun nano�bers withoptical sensing ability for detection of volatile organiccompounds [16]. Moreover, Ali et al. fabricated PDA-poly "-caprolactone (PCL) electrospun �ber mats forcolorimetric detection of fake gasoline [19].

In this study, PDA and polyvinylpyrolidone(PVP) were used for the �rst time to develop nano�bercomposites via an electrospinning process. PVP is asynthetic polymer exhibiting proper physiological com-patibility, low chemical toxicity, transparency, goodsolubility in most organic solvents, suitable spinnabil-ity, antibacterial activity, and the ability to interactwith di�erent materials [20,21]. Electrospun PVP-based �bers have also been applied in di�erent chemicaland biological sensing [22,23]. The aim of this studyis to evaluate the feasible fabrication of PDA-PVPnano�brous composites as potential sensors. The pro-duced electrospun �bers were characterized afterwardsby SEM, FTIR, and DSC analysis, and the colorimetrictransition behavior was investigated by heating theproduced �bers.

2. Experimental

2.1. Materials10,12-pentacosadiynoic acid (PCDA, 98%) as PDAmonomer and polyvinylpyrolidone (PVP, MW 360000)

were purchased from Sigma-Aldrich (USA). Ethanoland other chemicals were of analytical grade andobtained from Merck (Germany).

2.2. MethodsMixture solutions of polymers in ethanol were preparedat two di�erent mass ratios of PVP to PCDA (3:1 and4:1) and at various polymer concentrations (4, 6, 8,and 10 wt%). The resulting solutions were then stirredovernight at room temperature and electrospun via anelectrospinning apparatus (Labscale ElectrospinningUnit, ESDP30, Iran). The polymer solution was fedinto the horizontally aligned syringe and an aluminiumfoil was used for covering the collector. Several pro-cessing parameters were tested in order to optimize theelectrospinning conditions and the optimal parameterswere chosen as follows: feeding rate of 0.5 mL/h,applied voltage of 15 kV, and tip-to-collector distanceof 15 cm. The electrospun mats were dried and kept inthe dark overnight before use.

Photopolymerization of PCDA was carried out byirradiatiing the �bers under UVc light (HPA Philips, 30W, Poland) for 5 min. After irradiation with UV-lightat 254 nm, the color of the white colored electrospun�bers turned blue validating the polymerization ofdiacetylene monomers. The colorimetric transition ofthe �bers was investigated by heating them at di�erenttemperatures (40, 60 and 80�C) for 15 min in anoven and the reectance spectra were measured by areectance spectrophotometer (Varian Cary 500, Aus-tralia). The dimension and morphology of the �berswere explored using a scanning electron microscope(SEM, Philips XL30, USA) at a voltage of 25 kV aftersputter coating with gold. The average �ber diameterwas measured for each sample (out of 30 random�bers) by employing image analysis software (ImageJ, National Institute of Health, Bethesda, MD). Tostudy the chemical structure of the obtained compos-ites, FTIR analysis was performed at 400{4000 cm�1using a Thermo Nicolet FTIR spectrometer (NEXUS870, USA). Di�erential scanning calorimetry (DSC 131SETARAM, France) was also utilized to evaluate thethermal transitions of the �bers at a heating rate of10�C/min in a nitrogen atmosphere.

3. Results and discussion

The SEM images of the electrospun �bers togetherwith their relative size distribution plots are presentedin Figure 1. The �brous mats resemble a spiderweb comprising of intermingled thin and thick �bers.Upon elongation of the electrically actuated polymerjet in the spinning region, the entanglement of polymerchains hampers them from disintegrating and bead-free, long, and continuous �bers are formed [24]. The�ber diameter varied depending on the polymers ratio

N. Eslahi et al./Scientia Iranica, Transactions F: Nanotechnology 27 (2020) 3447{3453 3449

Figure 1. SEM images and �ber diameter distribution plots of electrospun mats at di�erent polymer concentration andtwo mass ratios of PVP to PCDA.

and concentration. As can be seen, with decreasingpolymer concentration, the average �ber diameter isdeclined owing to lower electrospinning force and re-sults in a better stretching of �bers. Increasing polymerconcentration inhibits fast changes in the Taylor cone,leading to formation of thick �bers [8].

Furthermore, the 3:1 mass ratio of PVP to PCDApromoted more uniform and thinner �bers at all con-centrations, possibly owing to the suitable viscosityof PVP aiding �ber stretchability. Consequently,electrospun �bers at 3:1 mass ratio and 4% polymerconcentration are selected as the optimum specimenhaving the minimum average �ber diameter.

During solvent evaporation and �ber formationin the electrospinning process, PCDA self-assemblyoccurred, since the attractive forces between the DAmonomers were stronger than those between the PVPpolymer and DA monomers [18]. Polymerization ofPCDA was then conducted by irradiating the elec-trospun webs under 254 nm UV light for 5 min.A distinctive blue color after UV light irradiationcon�rmed successful photopolymerization in the �bersthrough conjugation, suggesting that the delocalized �-network and the conformational constraints within thepolymer chain were preserved in the electrospinningprocedure [16]. Besides, by heating the �bers up


to 80�C, a blue-to-red color transition occurred dueto the twisting of the conjugated backbone of thePDA. In fact, changes in temperature decrease theconjugation length and expand the energy band gapin PDA structures [25]. It was found that changes inPDA side chain length as well as PVP molecular weightcould inuence the thermochromic behavior of PDA-PVP nanocomposite �lms. There was a reduction incolor-transition temperature with a shortening of thePDA alkyl tail and increasing PVP molecular weightowing to the diminished dispersion interactions in thenanocomposite [26].

The colorimetric transition of the electrospun�bers was assessed by a reectance spectrophotometer.The obtained results in Figure 2 show a signi�cantdecline in the reectance of the UV irradiated sampleat 650 nm to 700 nm corresponding to the formationof a blue color. Moreover, no major morphologicalalterations were seen in the SEM images before andafter UV-irradiation (data not shown). To better showthe temperature sensitivity of the produced �bers, theywere heated at three di�erent temperatures and theresulting reectance spectra were measured. Uponheating the �bers, a reectance switch to a lowerwavelength was observed, resulting in a red color. Ascan be seen, by increasing temperature from 25�Cto 80�C, the minimum reectance wavelength (�min)shifts from 700 nm to 550 nm with an abrupt reductionin the whole reectance, validating the blue-to-redcolor change by an increase in temperature. Thisphenomenon (thermochromism) is the main reason forvarious applications of PDA as sensors. The obtainedresults also revealed that the matrix polymer in thePVP-PDA �ber composite had no negative e�ect onthe colorimetric transition of PDA. A literature review

Figure 2. Reectance spectra of electrospun �bers:White: before irradiation, blue: after UV irradiation, andred: after heating at 40, 60 and 80�C.

showed that the colorimetric transition temperatureof PDA is dependent on the melting point of thecorresponding DA monomer indicating conformationalchanges in the macromolecule [11].

To examine any possible changes in the chemicalstructure upon UV irradiation and heating, FTIRspectra of the �brous membranes were recorded. Fig-ure 3 illustrates the characteristic functional groups ofthe employed polymers. A broad absorption peak ataround 3500 cm�1 is associated with the stretchingvibration of O-H groups in PDA. In addition, for-mation of hydrogen bonds between carboxylic groupsof PDA and carbonyl group of PVP could occur inthe composite [26]. The peak in the range of 3000{2800 cm�1 belongs to the C-H stretching vibration ofmethylene groups. The observed peaks in 1675 cm�1and 1281 cm�1 correspond to the C=O stretching vi-bration of both polymers and C-N stretching vibrationof PVP, respectively [27]. The peak in 1433 cm�1is also attributed to CH2 bending vibration. As canbe seen, there is no signi�cant di�erences betweenthe samples before (white) and after UV irradiation(blue). However, after heating (red), the intensity ofthe peaks declined noticeably which might be due tothe conformational variations in PDA disrupting the� overlap and, as a result, changing the electroniccon�guration of the polymer [28].

The thermal transitions of the polymerized elec-trospun �bers were determined by DSC analysis, andthe melting temperature (Tm) and the correspondingenthalpy were calculated for each specimen. Accordingto Figure 4, the UV-irradiated nano�bers (blue sample)show an endothermic peak (Tm) at 74�C with enthalpyof 104.72 J/g. After heating (red sample), this peakshifted slightly to a lower temperature at 70�C, while

Figure 3. FTIR spectra of samples: White: beforeirradiation, blue: after UV irradiation, and red: afterheating.


Figure 4. DSC plots of PVP-PDA �bers: (a) Blue and(b) red.

the enthalpy increased to 146.3 J/g. It is worthmentioning that the melting temperature appears ataround 70�C, which is similar to the color transitiontemperature from blue to red upon heating. Theoptical properties of PDA depend on the structuralalignment of the polymer chains. With increasingtemperature, C-C bond rotation is expedited and theplanarity of the polymer con�rmation is disrupted,resulting in color change [8].

4. Conclusions

Polydiacetylene (PDA) is an attractive conjugated ma-terial due to its unique thermochromism in response toexternal stimuli. Mixtures of PDA and PVP solutionswere employed to develop nano�ber composites via asimple electrospinning process. The smooth and bead-free electrospun �bers were obtained and then photo-polymerized using UV-light irritation. SEM imagesshowed that the size and uniformity of the �bers weredependent on polymer concentration as well as theirratio in the mixture. The colorimetric transition wasevaluated by a reectance spectrophotometer and acolor change from blue to red was observed uponheating, with a pronounced color switch above 60�C.The FTIR and DSC analysis showed that the colortransition of the electrospun �bers was due to theconformational changes in PDA macromolecules, im-plying that addition of Polyvinylpyrolidone (PVP) inthe mixture did not inuence the optical properties ofPDA. Therefore, the fabricated mats could be used asthermochromic sensors.

Acknowledgements

We would like to express our gratitude to the membersof the Iranian Academic Center for Education, Cul-ture & Research (Amirkabir University of TechnologyBranch).

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Biographies

Niloofar Eslahi completed BS (2007), MS (2009)and PhD (2013) degrees with the highest placingfrom the Textile Engineering Department (TextileChemistry and Fiber Science) and admitted to theCenter of Exceptional Talents at Amirkabir Universityof Technology (AUT, Iran). She received an Allamehpost-doctoral fellowship (2014) from the Iran NationalElites Foundation to carry out research at theCenter for Nanostructured and Advanced Materials(CNAM) at the Department of Materials Scienceand Engineering, Sharif University of Technology(SUT). She was granted funding from the IranNational Science Foundation (INSF) for US Patent14/641,460 in 2015 and from the Iran Patent O�ce forProvisional US Patent Application 62512018 in 2017.Currently, Dr. Eslahi is Assistant Professor in theEngineering Department at the Science and ResearchBranch of the Islamic Azad University of Tehran anda professional member of the Iran NanotechnologyInitiative Council (INIC), Iranian Textile Associationof Science and Technology (ITAST), and YoungResearchers and Elite Club. Her research interestsare focused on medical textiles, smart biomaterials,electrospun nano�ber composites, and biocompatiblenanomaterials for tissue engineering applications.

Tara Fatemi was an MS graduate student in the De-partment of the Textile Engineering, Science and Re-search Branch of the Islamic Azad University, Tehran,Iran in 2018. Her research interests include electrospun


nano�bers, smart textiles and sensors.

Mehdi Varsei received MS (2004) and PhD (2012)degrees from Amirkabir University of Technology, andthe Science and Research Branch of the Islamic AzadUniversity, Tehran, Iran, respectively. He is currentlyPresident of ACECR, Amirkabir University of Tech-nology Branch, and also Assistant Professor of TextileEngineering at the Science and Research Branch ofthe Islamic Azad University, Tehran. During thepast 20 years, he has been a consultant for variousapparel and textile industries and delivered numerouslectures, seminars and workshops for both academicsand industry executives in the �eld. He is the author of

three books, journal articles, peer-reviewed conferencepapers and book chapters. His expertise includes tech-nical textiles, textile industry, smart textiles, textilemanagement, textile reinforced concrete (TRC), �brereinforced concrete (FRC) and composites.

Saeed Bazgir received MS and PhD degrees fromAmirkabir University of Technology, Tehran, Iran, in1997 and 2003, respectively. He is now Associate Pro-fessor in the Department of Polymer Engineering (Sci-ence and Research Branch) of the Islamic Azad Univer-sity, Tehran, Iran. His main research interests includepolymeric nanocomposites, nanoparticles, nano�bers,nano�brous membranes, and wastewater treatment.

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