Polymeric Composites Based on Carboxymethyl Cellulose Cryogel and Conductive Polymers: Synthesis and Characterization

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Article Polymeric Composites Based on Carboxymethyl Cellulose Cryogel and Conductive Polymers: Synthesis and Characterization

Sahin Demirci 1, S. Duygu Sutekin 2 and Nurettin Sahiner 1,3,4,* 1 Department of Chemistry & Nanoscience and Technology Research and Application Center, Canakkale Onsekiz Mart University Terzioglu Campus, Canakkale 17100, Turkey; [email protected] 2 Department of Chemistry, Hacettepe University, Beytepe, Ankara 06800, Turkey; [email protected] 3 Department of Chemical and Biomolecular Engineering, University of South Florida, Tampa, FL 33620, USA 4 Department of Ophthalmology, Morsani College of Medicine, University of South Florida, 12901 Bruce B. Downs Blvd, MDC21, Tampa, FL 33612, USA * Correspondence: [email protected]

Received: 7 March 2020; Accepted: 27 March 2020; Published: 29 March 2020

Abstract: In this study, a super porous polymeric network prepared from a natural polymer, carboxymethyl cellulose (CMC), was used as a scaﬀold in the preparation of conductive polymers such as poly(Aniline) (PANi), poly(Pyrrole) (PPy), and poly(Thiophene) (PTh). CMC–conductive polymer composites were characterized by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) techniques, and conductivity measurements. The highest conductivity was observed as 4.36 10 4 4.63 10 5 S cm 1 for CMC–PANi cryogel composite. The changes in conductivity × − ± × − · − of prepared CMC cryogel and its corresponding PAN, PPy, and PTh composites were tested against HCl and NH3 vapor. The changes in conductivity values of CMC cryogel upon HCl and NH3 vapor treatment were found to increase 1.5- and 2-fold, respectively, whereas CMC–PANi composites showed a 143-fold increase in conductivity upon HCl and a 12-fold decrease in conductivity upon NH3 treatment, suggesting the use of natural polymer–conductive polymer composites as sensor for these gases.

Keywords: CMC cryogel; natural polymer–conductive polymer cryogel composite; carboxymethyl cellulose cryogel composites; conductive natural polymer cryogel

1. Introduction Conductive polymers were ﬁrst synthesized as a new generation of organic materials in the mid-1970s. Although the electrical and optical properties of conductive polymers are similar to metals and organic semiconductors, they have attracted more attention owing to their properties, such as the ease of synthesis and easier processing than metals [1–3]. The synthesis methods for conductive polymers can be divided into two main groups, where the starting material is either a polymer or a monomer. Soluble conductive polymers can be deposited through direct polymer solution spraying, sieve, microcontact printing, probe-based deposition, photolithography, and spin coating. Alternatively, monomers can be polymerized by chemical, electrochemical, or plasma techniques [4–7]. Conductive polymers are widely used in microelectronics industries including photovoltaic devices, light-emitting diodes, electrochromic screens, sensors, and battery technology [8–10]. However, the studies in the 1980s demonstrated that conductive polymers are compatible with many biological molecules, causing a signiﬁcant increase in the use of polymers for biomedical applications. Many conductive polymers such as poly(Aniline) (PANi) [11,12], poly(Pyrrole) (PPy) [13,14], poly(thiophene) (PTh) [15], and their derivatives [16–18] have been reported as cell and tissue compatible, which was

J. Compos. Sci. 2020, 4, 33; doi:10.3390/jcs4020033 www.mdpi.com/journal/jcs J. Compos. Sci. 2020, 4, 33 2 of 12 supported by in vitro and in vivo studies. In addition, the chemical, physical, and electrical properties of conductive polymers can be adapted to particular needs by modification of antibodies, enzymes, and some other biological molecules [19–23]. Many events in the human body, such as neural communication, embryonic development, tissue repair after injury, and heartbeat, among others, are regulated by electrical signals [24]. The components that are electrically active in the human body are the nervous system, heart, and muscles, respectively [25,26]. While the electric is used during the transmission of information between neurons in the nervous system, the muscle cells in the heart produce heartbeats by generating electrical impulses that lead to muscle contraction circulating throughout the organ [25,26]. Besides, some tissues, such as bone marrow, have been shown to use conductivity to regenerate [27]. In this context, studies on the design of electrically conductive materials for biotechnological and biomedical applications have attracted a great deal of attention [28] Conductive polymers have become prominent candidates for materials that can be used in these biotechnological and biomedical fields such as biosensors [29,30], neural electrodes [31,32], bio-activators [33,34], wound healing and controlled release systems [35,36], and tissue scaffolds [37,38], owing to their electrical behavior and biocompatibility [39]. The utilization of conductive polymers in these applications can be augmented further by preparing conductive polymers within super-porous biocompatible cryogels. In this study, for the first time, in situ synthesis of conductive polymers within natural polymer CMC cryogels is reported. The synthesis of cryogels from natural carboxymethyl cellulose (CMC) polymer by employing cryopolymerization technique was accomplished. Then, the synthesized CMC cryogel was used as a template for in situ synthesis of conductive polymers, PANi, PPy, and PTh for the first time. The prepared CMC–conductive polymer cryogel composites were characterized by means of Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and conductivity measurements employing an electrometer. Moreover, the changes in the conductivity of CMC, CMC–PANi, CMC–PPy, and CMC–PTh cryogel composites were investigated for the detection of HCl and NH3 vapor as potential sensing materials.

2. Materials and Methods

2.1. Materials

Carboxymethyl cellulose sodium (CMC, Mw~250,000, degree of substitution: 1.2, Aldrich, St. Louis, MO, USA), divinyl sulfone (DVS, 98%, Merck, Kenilworth, NJ, USA), aniline (ANi, 99%, Sigma-Aldrich, St. Louis, MO, USA), pyrrole (Py, 98%, Aldrich), thiophene (Th, 99%, Aldrich), ammonium persulfate (APS, 98%, Sigma-Aldrich), Iron (III) chloride (FeCl3, 98%, Fluka), hydrochloric acid (HCl, 37%, VWR Chemicals, Radnor, PA, USA), sodium hydroxide (NaOH, 98.8%, VWR Chemicals), chloroform (CH3Cl, 99%, Riedel de Haen), ethanol (99.8%, Sigma-Aldrich), hydrochloric acid (HCl, 37%, VWR Chemicals), ammonium solution (NH3, 25%, Sigma Aldrich), and double distilled water (DDW, GFL2108) were used as received.

2.2. Synthesis of CMC Cryogel The synthesis of CMC cryogel was reported earlier [40]. In brief, 0.2 g of CMC was dissolved in 6 mL 0.2 M NaOH under constant stirring at 500 rpm for 4 h at ambient temperature, and then cooled in a freezer about 5 min. Then, DVS crosslinker, with a mole ratio of 100% relative to the CMC repeating unit, was added and quickly pipetted (0.8 cm diameter). The mixture was then placed in a freezer at 18 C for cryopolymerization reaction for 24 h to complete cryo-crosslinking. Prepared − ◦ CMC cryogels were cut in similar shapes and washed with DDW for puriﬁcation. Puriﬁed CMC cryogels were dried in a freeze-dryer and stored in closed tubes for further use. J. Compos. Sci. 2020, 4, 33 3 of 12

2.3. In Situ Synthesis of Conductive Polymers within CMC Cryogels For in situ synthesis of poly(Aniline) (PANi) within CMC cryogels, a similar procedure reported in the literature was followed [41,42]. In short, 0.1 g CMC cryogels were placed in 10 mL ANi monomer for 2 h to load ANi monomer into CMC cryogels at a 100 rpm mxing rate at an ambient temperature. After that, the ANi monomer containing CMC cryogels were weighed, and the amount of loaded ANi was gravimetrically calculated. Then, ANi-loaded CMC cryogels were transferred in 200 mL 1 M HCl containing APS at 1:1.25 APS/ANi mole ratio according to the amount of ANi loaded into the CMC cryogel and stirred at 250 rpm for 30 min. Then, the prepared CMC–PANi cryogel conductive polymer composites were washed with DDW and ethanol, and dried in an oven at 50 ◦C. In situ synthesis of poly(Pyrrole) (PPy) within CMC cryogels was also carried out in a similar procedure with some modification [41,42]. CMC cryogels weighing 0.1 g were placed in 10 mL Py monomer to load Py into CMC cryogels. Then, the Py-loaded CMC cryogels were placed into 0.05 M 250 mL FeCl3 solution in DDW and stirred at 250 rpm for 2 h at an ambient temperature for in situ polymerization of Py within CMC cryogels. Then, the prepared CMC-PPy cryogel conductive polymer composites were washed with DDW and ethanol, and dried in an oven at 50 ◦C. The synthesis procedure of poly(Thiophene) (PTh) within CMC cryogels was also done based on a previously reported method [41,42]. Briefly, CMC cryogels weighing 0.1 g were placed in 10 mL of Th monomer to load Th into CMC cryogels. After that, Th loaded CMC cryogels were placed into 50 mL 0.03 M FeCl3 solution in chloroform and placed in an oil bath at 65 ◦C with reflux system, and stirred at 250 rpm for 16 h for the polymerization of Th within CMC cryogels. Finally, the prepared CMC–PTh cryogel conductive polymer composites were washed with DDW and ethanol, and dried in an oven at 50 ◦C.

2.4. Characterization The characterization of synthesized CMC cryogels and conductive polymer composites was done using Fourier transform infrared (FT-IR) spectrometer (Nicolet iS10, Thermo, Waltham, MA, USA), thermogravimetric analyzer (TGA, SII 6300, Exstar, Stoneham, MA, USA), and an electrometer (2400 Source-meter, Keithley, Cleveland, OH, USA) for conductivity measurements.

2.5. Conductivity Measurements Conductivity of synthesized CMC cryogels and their corresponding PANi, PPy, and PTh composites was measured from the ohmic region of current–voltage (I–V) curves constructed from an electrometer using Equations (1) and (2): V = I R (1) × σ = (1/R) (l/A) (2) × where ‘V’ is the voltage, ‘I’ is the current, ‘R’ is the bulk resistance, ‘σ’ is the conductivity, ‘1/R’ is the resistivity, ‘l’ is the thickness, and ‘A’ is the cross-sectional area of the sample. In the conductivity measurements, small pieces of conductive carbon tapes (3 mm 4 mm) were × attached at the top and bottom sides of CMC cryogels and/or CMC–conductive polymer composites, and then the electrodes of the electrometer were contacted with these carbon tapes. Next, the I–V curves were obtained by applying voltages up to 100 V to determine the conductivity of the samples.

3. Results and Discussion

3.1. Synthesis and Characterization of In Situ Synthesized Conductive Polymer CMC Cryogel Composites The well-known cryo-crosslinking technique was used in the preparation of CMC cryogels having an interconnected super macroporous structure. Scanning Electron Microscopy (SEM) images of CMC cryogels synthesized using 100% DVS crosslinker based on the repeating unit of CMC were previously reported by our group reporting the super macroporous structure of CMC cryogels with a J. Compos. Sci. 2020, 4, 33 4 of 12 pore size in a range of 1–100 µm [40]. Recently, the synthesis of conductive polymers within cryogels was reported [41,42]. The schematic presentation of in situ synthesis of conductive polymers within CMCJ. Compos. cryogels Sci. 2020 is, 4 given, x in Figure1a–c for PANi, PPy, and PTh synthesis, respectively. Brieﬂy,4 of 11 0.1 g of puriﬁed cylindrical CMC cryogel pieces was placed into ANi, Py, and Th monomers separately separately about 2 h for loading of monomers into CMC cryogels. Then, depending on monomer, the aboutconductive 2 h for loadingmonomer-loaded of monomers CMC intocryogels CMC were cryogels. placed Then,into the depending appropriate on initiator monomer, solution, the conductive as monomer-loadeddescribed above. CMC cryogels were placed into the appropriate initiator solution, as described above.

FigureFigure 1. The1. The schemcatic schemcatic presentation presentation of inof situin synthesissitu synthesis of (a )of poly(Aniline) (a) poly(Aniline) (PANi), (PANi), (b) poly(Pyrrole) (b) (PPy),poly(Pyrrole) and (c) poly(Thiophene)(PPy), and (c) poly(Thiophene) (PTh) conductive (PTh) conductive polymers polymers within carboxymethyl within carboxymethyl cellulose cel- (CMC) cryogels.lulose (CMC) APS, ammoniumcryogels. APS, persulfate. ammonium persulfate.

AsAs a resulta result of of in in situ situ synthesissynthesis of of conductive conductive polymer, polymer, the theinterconnect interconnecteded super supermacro macropores of pores of CMCCMC cryogels cryogels were were filledfilled with PANi PANi or or PPy, PPy, or or PTh, PTh, as asshown shown in Figure in Figure 1. The1. amounts The amounts of loaded of loaded ANi, Py, and Th into CMC cryogels; the amounts of in situ synthesized conductive polymers within ANi, Py, and Th into CMC cryogels; the amounts of in situ synthesized conductive polymers within CMC cryogels; and in situ polymerization efficiency of conductive polymers within CMC cryogels CMC cryogels; and in situ polymerization efficiency of conductive polymers within CMC cryogels are are summarized in Table 1. The ANi, Py, and Th loaded CMC cryogels were weighed and the summarizedamount of incorporation in Table1. The of each ANi, ANi, Py, andPy, and Th Th loaded monomer CMC into cryogels CMC structure were weighed is calculated. and the Briefly, amount of incorporationcryogels of known of each weight ANi, Py,were and placed Th monomer in monomers into CMCand weighed structure again is calculated. to calculate Briefly, the loaded cryogels of knownamount weight of the werecorresponding placed in monomers. monomers and weighed again to calculate the loaded amount of the correspondingIn brief, monomers.the weight of known cryogel was placed into monomers of the desired conductive polymer and weighted again to calculate the loaded amount of the corresponding monomers. The amountsTable 1. ofThe in situ amount prepared of loaded conductive Aniline polymers (ANi), Pyrrole were (Py),then andcalculated Thiophene from (Th); the theweight amount of the of cry- in situ ogel–conductivesynthesized cryogel polymer composites; composites. and The in situ percent polymerization yield of in situ yield. polymerized CMC, carboxymethyl conductive cellulose.polymers was calculated from the amount of in situ polymerized conductive polymers and loaded amount of monomer.Materials (Cryogel Weigth of CMC Loaded Amount Amount of Composite Yield of In Situ Composite) Cryogels (g) of Monomer (g) Cryogel (g) Polymerization (%) CMC–PANiTable 1. The amount of 0.1 loaded0.01 Aniline (ANi), 1.98 Pyrrole0.02 (Py), and Thiophene 0.97 0.11 (Th); the amount46.2 of in 5.44 ± ± ± ± CMC–PPy 0.1 0.01 1.94 0.03 0.23 0.01 10.9 0.21 situ synthesized cryogel composites;± and in situ ±polymerization yield. CMC,± carboxymethyl cellu-± CMC–PTh 0.1 0.01 1.92 0.03 0.11 0.01 5.18 0.49 lose. ± ± ± ± Materials Weigth of Loaded Amount of Yield of In Situ In brief,(Cryogel the weight of knownCMC cryogelAmount was placed of into monomersComposite of the desired conductive polymer Polymerization (%) and weightedComposite) again to calculateCryogels the(g) loadedMonomer amount (g) of theCryogel corresponding (g) monomers. The amounts of in situ preparedCMC–PANi conductive 0.1 polymers ± 0.01 were1.98 then ± 0.02 calculated 0.97 from ± 0.11 the weight of46.2 the ± cryogel–conductive5.44 polymerCMC–PPy composites. The0.1 percent ± 0.01 yield of1.94 in ± situ0.03 polymerized0.23 ± 0.01 conductive polymers10.9 ± 0.21 was calculated from theCMC–PTh amount of in situ0.1 polymerized ± 0.01 conductive1.92 ± 0.03 polymers0.11 ± and 0.01 loaded amount5.18 ± 0.49 of monomer.

J.J. Compos.Compos. Sci. 2020,, 4,, 33x 55 of 1211

Upon polymerization of ANi, Py, and Th within CMC cryogels by placing into corresponding solutionsUpon for polymerization the in situ synthesis of ANi, of Py,conductive and Th withinPANi, CMCPPy, and cryogels PTh polymers by placing within into correspondingCMC cryogels, solutionsthe weights for of the conductive in situ synthesis PANi, ofPPy, conductive and PTh PANi,polymers PPy, were and PThcomputed polymers as 0.97 within ± 0.11, CMC 0.23 cryogels, ± 0.01, the weights of conductive PANi, PPy, and PTh polymers were computed as 0.97 0.11, 0.23 0.01, and 0.11 ± 0.01 g, respectively. The in situ polymerization efficiency of conductive± PANi, PPy,± and and 0.11 0.01 g, respectively. The in situ polymerization eﬃciency of conductive PANi, PPy, and PTh within± CMC cryogels was gravimetrically assessed as 46.21% ± 5.44%, 10.95% ± 0.21%, and PTh within CMC cryogels was gravimetrically assessed as 46.21% 5.44%, 10.95% 0.21%, and 5.18% ± 0.49%, respectively. ± ± 5.18% 0.49%, respectively. The± FT-IR spectra and TGA thermograms of CMC cryogel, as well as CMC–PANi, CMC–PPy, and CMC–PThThe FT-IR spectracomposites, and TGAare shown thermograms in Figure of 2a,b, CMC respectively. cryogel, as In well Figure as CMC–PANi, 2a, the FT-IR CMC–PPy, spectrum andof CMC CMC–PTh cryogels composites, gave characteristic are shown in peaks Figure as2a,b, wide respectively. band around In Figure 33502a, cm the−1 FT-IRassigned spectrum to –OH of 1 CMCstretching, cryogels asymmetric gave characteristic stretching peaks of –COO- as wide at band1592 aroundcm−1, symmetric 3350 cm− stretchingassigned toof –OH-COO- stretching, at 1423 1 1 asymmetriccm−1, and –CO stretching stretching of at –COO- 1047 cm at− 15921, respectively. cm− , symmetric On the other stretching hand, ofnew -COO- bands at appeared 1423 cm− at, 1544 and 1 1 –COcm−1 stretchingcorresponding at 1047 to benzenoic-quinonic cm− , respectively. nitrogen, On the other at 1494 hand, cm− new1 for bandsaromatic appeared C-C, at at1288 1544 cm cm−1 for− 1 1 correspondingaromatic amine to group, benzenoic-quinonic and at 1118 cm nitrogen,−1 for C-N-C at 1494 stretching cm− for aromatic in the FT-IR C-C, spectrum at 1288 cm of− CMC–PANifor aromatic 1 aminecryogel group, composites, and at which 1118 cmare− infor accordance C-N-C stretching with the inliterature the FT-IR [43]. spectrum The characteristic of CMC–PANi bands cryogel of PPy composites,were also observed which are in inthe accordance FT-IR spectrum with the of literature CMC–PPy [43 ].cryogel The characteristic composites at bands 1358 of cm PPy−1 corre- were 1 alsosponding observed to =C-H in the in-plane FT-IR spectrum vibrations, of CMC–PPy at 1276 cm cryogel−1 for N-C composites stretching at 1358vibrations, cm− corresponding and at 901 cm to−1 1 1 =assignedC-H in-plane to the vibrations, presence atof 1276polymerized cm− for pyrrole N-C stretching units [44]. vibrations, Moreover, and the at 901 peaks cm− observedassigned at to 703, the 1 presence1319, and of 1426 polymerized cm−1 from pyrrole the FT-IR units spectrum [44]. Moreover, of CMC–PTh the peaks cryogel observed composites at 703, were 1319, assigned and 1426 to cm α–−α fromconnection the FT-IR between spectrum thiophene of CMC–PTh molecules cryogel [45], compositesand C-H in-plane were assigned bending tovibrationsα–α connection of Th molecules, between thiophenerespectively molecules [46]. [45], and C-H in-plane bending vibrations of Th molecules, respectively [46].

CMC cryogel 1592, and (a) -1 1426 cm 1047 cm-1 3350 cm-1 CMC-PANi cryogel composite

1544 1118 cm-1 1494 1288 CMC-Pyrrole cryogel composite cm-1 cm-1 cm-1

1735 cm-1 1358 C=C peaks cm-1 CMC-PTh cryogel composite 1276 901 cm-1 cm-1 1426 -1 1319 703 cm -1 cm cm-1 4000 3500 3000 2500 2000 1500 1000 650 Wavenumbers (cm-1)

100 (b)

80 CMC CMC-PTh cryogel cryogel composite 60 TG % 40 CMC-PANi cryogel composite 20 CMC-PPy cryogel composite 0 100 200 300 400 500 600 700 800 T (oC)

FigureFigure 2.2. ((aa)) FourierFourier transformtransform infraredinfrared (FT-IR)(FT-IR) spectrumspectrum andand ((bb)) thermogravimetricthermogravimetric analysisanalysis (TGA)(TGA) thermogramsthermograms ofof CMCCMC cryogel,cryogel, CMC–PANi,CMC–PANi, CMC–PPy,CMC–PPy, and and CMC–PTh CMC–PTh cryogel cryogel composites. composites.

The thermal stability and degradation steps of CMC cryogel, CMC–PANi, CMC–PPy, The thermal stability and degradation steps of CMC cryogel, CMC–PANi, CMC–PPy, and and CMC–PTh cryogel composites were compared with TGA analysis and corresponding TGA CMC–PTh cryogel composites were compared with TGA analysis and corresponding TGA ther- thermograms are given in Figure2b. The ﬁrst degradation step of CMC cryogel was observed between mograms are given in Figure 2b. The first degradation step of CMC cryogel was observed between 260 and 308 C with 41.5% weight loss from the thermogram, the second step of degradation was 260 and 308 ◦°C with 41.5% weight loss from the thermogram, the second step of degradation was observed between 676 and 694 °C with 75.8% weight loss, and 78.8% weight loss was observed at 800

J. Compos. Sci. 2020, 4, 33 6 of 12

J. Compos. Sci. 2020, 4, x 6 of 11 observed between 676 and 694 ◦C with 75.8% weight loss, and 78.8% weight loss was observed at 800 C. On the other hand, it was deduced that the thermal stability of CMC cryogels decreased with °C. On◦ the other hand, it was deduced that the thermal stability of CMC cryogels decreased with the the in situ synthesis of PANi conductive polymer, where the ﬁrst degradation step was seen between in situ synthesis of PANi conductive polymer, where the first degradation step was seen between 137 and 208 C with 43.1% weight loss, the second step of degradation was observed between 425 and 137 and 208 °C◦ with 43.1% weight loss, the second step of degradation was observed between 425 607 C with 89.8% weight loss, and 91.2% weight loss was attained at 800 C from the TGA thermogram and 607◦ °C with 89.8% weight loss, and 91.2% weight loss was attained◦ at 800 °C from the TGA of the CMC–PANi cryogel composite. The one step degradation of CMC–PPy cryogel composites was thermogram of the CMC–PANi cryogel composite. The one step degradation of CMC–PPy cryogel noticed between 224 and 517 C with 91.2% weight loss, and 92.1% weight loss was observed at 800 C. composites was noticed between◦ 224 and 517 °C with 91.2% weight loss, and 92.1% weight loss was◦ Finally, the TGA thermogram of CMC–PTh cryogel composites also resulted in two degradation steps, observed at 800 °C. Finally, the TGA thermogram of CMC–PTh cryogel composites also resulted in starting between 222 and 278 C with 16.5% weight loss, between 373 and 414 C with 58.5% weight two degradation steps, starting◦ between 222 and 278 °C with 16.5% weight loss, ◦between 373 and 414 loss, and 63.8% weight loss was observed at 800 C. TGA thermograms of CMC cryogel, CMC–PANi, °C with 58.5% weight loss, and 63.8% weight lo◦ss was observed at 800 °C. TGA thermograms of CMC–PPy, and CMC–PTh cryogel composites clearly show that the thermal stability of CMC cryogel CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh cryogel composites clearly show that the decreased by in situ synthesis of PANi, and slightly decreased for the in situ synthesis of PPy and PTh thermal stability of CMC cryogel decreased by in situ synthesis of PANi, and slightly decreased for conductive polymers. the in situ synthesis of PPy and PTh conductive polymers. 3.2. Conductivity Measurements 3.2. Conductivity Measurements Although the reason behind the conductivity for conjugated organic molecules was initially Although the reason behind the conductivity for conjugated organic molecules was initially assumed to be derived from unﬁlled electronic valence and conduction bands, it was later shown assumed to be derived from unfilled electronic valence and conduction bands, it was later shown that the conductivity can be associated with spineless charge carriers such as soliton, polaron, and that the conductivity can be associated with spineless charge carriers such as soliton, polaron, and bipolaron, after the discovery of conductive polymers such as PANi, PPy, and PTh [47]. In this study, bipolaron, after the discovery of conductive polymers such as PANi, PPy, and PTh [47]. In this the conductivity of CMC cryogels, as well as the changes in conductivities of CMC–PANi, CMC–PPy, study, the conductivity of CMC cryogels, as well as the changes in conductivities of CMC–PANi, and CMC–PTh cryogel composites, were determined by using an electrometer from the ohmic region of CMC–PPy, and CMC–PTh cryogel composites, were determined by using an electrometer from the obtained I–V curves. In Figure3a, the digital camera images of CMC cryogel, CMC–PANi, CMC–PPy, ohmic region of obtained I–V curves. In Figure 3a, the digital camera images of CMC cryogel, CMC– and CMC–PTh cryogel composites are given. It was clearly seen that the white color of CMC cryogels PANi, CMC–PPy, and CMC–PTh cryogel composites are given. It was clearly seen that the white turned to black after in situ synthesis of PANi and PPy, and also turned to light brown upon in situ color of CMC cryogels turned to black after in situ synthesis of PANi and PPy, and also turned to synthesis of PTh conductive polymer. light brown upon in situ synthesis of PTh conductive polymer.

FigureFigure 3. 3. (a(a) )Digital Digital camera camera images images of synthesizedsynthesized CMC CMC cryogel, cryogel, CMC–PANi, CMC–PANi, CMC–PPy, CMC–PPy, and and CMC–PTh CMC– PThcryogel cryogel composites; composites; and and (b) schematic(b) schematic representation representation of electrometer of electrometer measurement. measurement.

TheThe schematic schematic representation representation of of electrometer electrometer me measurementasurement setup setup is is shown shown in in Figure Figure 3b.3b. As As can can bebe seen, seen, the the probes probes of of electrometer electrometer were were contacted contacted to to carbon carbon tapes tapes that that were were attached attached at at the the top top and and bottom sides of CMC-based cryogels and cryogel composites. The corresponding I–V curves of CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh are given in Figure 4a.

J. Compos. Sci. 2020, 4, 33 7 of 12 bottom sides of CMC-based cryogels and cryogel composites. The corresponding I–V curves of CMC cryogel,J. Compos. CMC–PANi, Sci. 2020, 4, CMC–PPy, x and CMC–PTh are given in Figure4a. 7 of 11

1×100 CMC cryogel (a) CMC-PANi cryogel composite -2 1×10 CMC-PPy cryogel composite CMC-PTh cryogel composite 1×10-4

1×10-6 Current (A) 1×10-8

1×10-10 1×100 1×101 1×102 Vo l t a g e ( V)

1×10-1 (b) Conductivity ) -1 1×10-3

1×10-5

1×10-7 Conductivity (S.cm

1×10-9 CMC CMC-PANi CMC-PPy CMC-PTh

FigureFigure 4. The 4. The comparison comparison of of ( a()a) current–voltage current–voltage (I–V) (I–V) curves curves obtained obtained from from electrometer; electrometer; and (b) and (b) calculatedcalculated conductivityconductivity values values of ofCMC CMC cryogel, cryogel, CMC–PANi, CMC–PANi, CMC–PPy, CMC–PPy, and CMC–PTh and CMC–PThcryogel cryogelcomposites. composites.

The conductivityThe conductivity values values were were calculated calculated from from I–V I–V curvescurves using Equations Equations (1) (1) and and (2), (2),and and are are illustratedillustrated in Figure in Figure4b for 4b CMC for CMC cryogel, cryogel, CMC–PANi, CMC–PANi, CMC–PPy, CMC–PPy, and and CMC–PTh CMC–PTh cryogel cryogel composites. composites. The conductivity of CMC cryogels was calculated as 2.21 × 10−9 ± 1.98 × 10−10 S·cm−1. An ap- The conductivity of CMC cryogels was calculated as 2.21 10 9 1.98 10 10 S cm 1. An approximately proximately 200 K-fold increase was observed in the× conductivity− ± ×of CMC− ·cryogels− by the in situ 200 K-foldsynthesis increase of PANi, was which observed was calculated in the conductivity as 4.58 × 10−4 ± of 5.68 CMC × 10− cryogels5 S·cm−1. The by conductivity the in situ synthesisof CMC– of 4 5 1 PANi,PPy which cryogel was composites calculated was as 4.58calculated10− as 5.025.68 × 10−510 ± −5.54S cm× 10−−6. S·cm The−1 conductivitywith approximately of CMC–PPy a 25 × ± 5 × 6· 1 cryogelK-fold composites increase was as compared calculated with as 5.02 the conductivity10− 5.54 of bare10− CMCS cm −cryogel.with approximatelyOn the other hand, a 25 the K-fold × ± × · increasedifference as compared between with conductivity the conductivity of CMC cryogels of bare and CMC CMC–PTh cryogel. cryogel On the composites other hand, was the insignifi- difference betweencant, conductivity where the calculated of CMC conductivity cryogels and value CMC–PTh of CMC–PTh cryogel cryogel composites composites was was insignificant, 2.28 × 10−9 ± 5.24 where the calculated× 10−10 S·cm conductivity−1. Among the value monomers of CMC–PTh used for in cryogel situ synt compositesheses of conductive was 2.28 polymers10 9 within5.24 CMC10 10 × − ± × − S cm 1cryogel,. Among Th the polymerization monomers usedstands for out in as situ the syntheses most difficult of conductive one. The results polymers summarized within CMCin Table cryogel, 1 · − Th polymerizationconfirm this phenomenon stands out asas thethe mostin situ di polymerizafficult one.tion The efficiency results summarizedfor PTh within in CMC Table cryogels1 confirm was this calculated as 5%. Therefore, the observation of some major differences in conductivity values among phenomenon as the in situ polymerization efficiency for PTh within CMC cryogels was calculated as the conductive polymers and CMC and CMC–PTh cryogel composites can be explained by a very 5%. Therefore,small amount the observationof PTh conductive of some polymer major in di CMCfferences cryogels. in conductivity values among the conductive polymers andThe CMChighest and increase CMC–PTh in conductivity cryogel compositeswas observed can after be in explained situ synthesis by a of very PANi small with amount a 200 of PTh conductiveK-fold increase, polymer following in CMC Py cryogels.with a 25 K-fold increase, which could be owing to their higher Theamounts highest within increase CMC in pores, conductivity allowing wasbetter observed conductivity. after in situ synthesis of PANi with a 200 K-fold increase, following Py with a 25 K-fold increase, which could be owing to their higher amounts within CMC pores, allowing better conductivity.

J. Compos. Sci. 2020, 4, 33 8 of 12

3.3. The Change in Conductivity Upon Exposure to HCl and NH3 Vapors J. Compos. Sci. 2020, 4, x 8 of 11 The potential sensor use of the prepared CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh cryogel3.3. composites The Change in for Conductivity HCl and NHUpon3 vaporsExposure was to HCl tested and NH by3 measuring Vapors the changes in conductivities of preparedThe CMC-based potential sensor cryogels use of bythe followingprepared CM literatureC cryogel, [41 CMC–PANi,]. In brief, CMC–PPy, CMC cryogel, and CMC–PTh CMC–PANi, CMC–PPy,cryogel and composites CMC–PTh for HCl cryogel and NH composites3 vapors was were tested treated by measuring with HCl the and changes NH3 invapors conductivities for 15 min by employingof prepared a simpleCMC-based experimental cryogels by setfollowing up [41 lite].rature Cryogel [41]. composites In brief, CMC were cryogel, exposed CMC–PANi, to HCl and NH3 vaporsCMC–PPy, using and aCMC–PTh simple set cryogel up. Incomposites brief, two were funnels treated were with connectedHCl and NH with3 vapors a glass for 15 fritted min by neck, whereemploying cryogel composites a simple experimental were placed set onup the[41]. topCryogel surface composites of the glasswere fritexposed connected to HCl withand NH another3 funnelvapors containing using a simple glass cotton set up. thatIn brief, is attached two funnels to HCl were or connected NH3 solutions with a withglass afritted valve neck, for thewhere release of vapors.cryogel Thecomposites conductivities were placed of CMCon the cryogel,top surface CMC–PANi, of the glass frit CMC–PPy, connected and with CMC–PTh another funnel cryogel compositescontaining were a calculatedglass cotton before that is and attached after gasto HCl exposures. or NH3 Thesolutions conductivity with a valve changes for the in CMCrelease cryogel, of vapors. The conductivities of CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh cryogel com- CMC–PANi, CMC–PPy, and CMC–PTh cryogel composites after treating with HCl vapor for 15 min posites were calculated before and after gas exposures. The conductivity changes in CMC cryogel, are demonstratedCMC–PANi, CMC–PPy, in Figure and5a, CMC–PTh and the values cryogel arecomposit summarizedes after treating in Table with2. HCl The vapor conductivity for 15 min of 9 10 1 9 10 CMCare cryogel demonstrated was increased in Figure from 5a, and 2.31 the values10− are2.19 summarized10− Sincm Table− to2. The 4.59 conductivity10− 6.33 of CMC10 − 1 × ± × · × ± × S cm−cryogelwith approximatelywas increased from a twofold 2.31 × 10 increase−9 ± 2.19 after× 10−10 15 S·cm min−1 to of 4.59 HCl × vapor 10−9 ± treatment.6.33 × 10−10 S·cm Moreover,−1 with the · 4 5 1 conductivityapproximately of CMC–PANi a twofold increase cryogel after composite 15 min increasedof HCl vapor from treatment. 4.36 Moreover,10− 4.63 the conductivity10− S cm− to 2 3 1 −4 ×−5 −±1 × −2 · 6.24 of10 CMC–PANi− 5.21 cryogel10− S compositecm− with increased approximately from 4.36 a× 150-fold10 ± 4.63 increase × 10 S·cm afterto 156.24 min × 10 of ± HCl 5.21 vapor× ×10−3 S·cm± −1 with× approximately· a 150-fold increase after 15 min of HCl vapor treatment.9 Further,9 the 1 treatment. Further, the conductivity of CMC–PTh was increased from 2.12 10− 8.36 10− S cm− conductivity8 of CMC–PTh9 was 1increased from 2.12 × 10−9 ± 8.36 × 10−9 S·cm−1× to 3.67 ±× 10−8 ±× 3.36 × 10−·9 to 3.67 10− 3.36 10− S cm− with approximately a 20-fold increase after 15 min of HCl vapor × −1 ± × · treatment.S·cm The with change approximately in conductivity a 20-fold of increase CMC–PPy after was15 min found of HCl to be vapor insigniﬁcant treatment. as The the change conductivity in conductivity of CMC–PPy was found to be insignificant as the conductivity was increased to 5.97 × was increased to 5.97 10 5 9.32 10 6 S.cm-1 from 4.98 10 5 3.57 10 6 S cm 1. Overall, the 10−5 ± 9.32 × 10−6 S.cm× -1 −from± 4.98 ×× 10−−5 ± 3.57 × 10−6 S·cm−1. Overall,× − ±the highest× −conductivity· − change highestwas conductivity observed for change CMC–PANi was observedcryogels with for CMC–PANia 150-fold increases cryogels after with HCl a vapor 150-fold treating increases for 15 aftermin. HCl vaporAs treating HCl was for also 15 min. used Asfor HCl the in was situ also synthesis used for of thePANi in within situ synthesis CMC cryogels, of PANi this within conductivity CMC cryogels, in- this conductivitycrease in the increase CMC–PANi in the cryogel CMC–PANi composite cryogel can be composite explained can by bethe explainedfurther doping by the effect further of HCl doping eﬀectvapor of HCl for vapor PANi forconductive PANi conductive polymers within polymers the CMC within superporous the CMC network. superporous network.

1×10-1 Before (a) After ) -1 1×10-3 15 min HCl vapor 1×10-5

1×10-7 Conductivity Conductivity (S.cm

1×10-9 CMC CMC-PANi CMC-PPy CMC-PTh

-1 1×10 Before (b)

) After -1 1×10-3 15 min 1×10-5 NH3 vapor

1×10-7 Conductivity (S.cm Conductivity

1×10-9 CMC CMC-PANi CMC-PPy CMC-PTh

Figure 5. The change in the conductivities of CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh

cryogel composites after 15 min (a) HCl and (b) NH3 vapor exposure. J. Compos. Sci. 2020, 4, 33 9 of 12

Table 2. The changes in the conductivities of CMC cryogel, CMC–PAN, CMC–PPY, and CMC–PTh

cryogel composites after 15 min HCl and NH3 vapor treatment.

Conductivity (S cm-1) · Materials HCl Vapor Change NH3 Vapor Change (fold) (fold) Before After Before After CMC 2.31 10 9 4.59 10 9 2 2.16 10 9 3.11 10 9 1.5 × − × − × − × − CMC–PANi 4.36 10 4 6.24 10 2 143 4.11 10 4 3.27 10 5 12 × − × − × − × − CMC–PPy 4.98 10 5 5.97 10 5 1.2 5.16 10 5 1.11 10 5 5 × − × − × − × − CMC–PTh 2.12 10 9 3.67 10 8 17 2.51 10 9 4.14 10 8 16 × − × − × − × −

The conductivity changes in CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh cryogel composites after treating with NH3 vapor for 15 min are shown in Figure5b. As shown in Table2, the conductivity changes in CMC cryogel, CMC–PANi, CMC–PPy, and CMC–PTh cryogel composites are from 2.16 10 9 3.71 10 10 S cm 1 to 3.11 10 9 5.95 10 10 S cm 1, from 4.11 10 4 5.32 × − ± × − · − × − ± × − · − × − ± × 10 5 S cm 1 to 3.27 10 5 2.56 10 5 S cm 1, from 5.16 10 5 4.38 10 6 S cm 1 to 1.11 10 5 − · − × − ± × − · − × − ± × − · − × − ± 8.73 10 6 S cm 1, and from 2.51 10 9 9.17 10 10 S cm 1 to 4.14 10 8 4.28 10 9 S cm 1, × − · − × − ± × − · − × − ± × − · − respectively. The highest conductivity change was observed for the CMC–PTh cryogel composite with a 15-fold increase upon 15 min NH3 vapor treatment. Besides, 12-fold and 5-fold decreases were observed in the conductivity of CMC–PANi and CMC–PPy cryogel composites, respectively. As a consequence, it can be assumed that the amounts of conductive polymer in CMC cryogel templates and the types of conductive polymer can aﬀect the potential sensor applications of composite systems against diﬀerent gases, for example, HCl and NH3 vapor.

4. Conclusions Herein, CMC cryogels from natural sources were utilized as a template for in situ PANi, PPy, and PTh conductive polymer synthesis for the first time in the literature. It was observed that the conductivity of CMC cryogels was increased by in situ preparation of PANi and PPy conductive polymers to 4.36 10 4 4.63 10-5 S cm 1 with a 200 K-fold increase, and 4.98 10 5 3.57 × − ± × · − × − ± × 10-6 S cm 1 with approximately a 25 K-fold increase according to conductivity of CMC cryogels, · − respectively. The potential sensor application of CMC-based PANi, PPy, and PTh cryogel composites was also tested against HCl and NH3 vapor exposure. The most significant conductivity difference was realized for CMC–PANi cryogel against HCl vapor by 15 min exposure time, with a 150-fold increase in the conductivity attributed to the doping of PANi conductive polymers with HCl vapor. Additionally, a 15-fold increase was observed in the conductivity of CMC–PTh, and a 12-fold decrease was observed in the conductivity of CMC–PANi against NH3 vapor exposure by 15 min exposure. Overall, it can be concluded that CMC–PANi, CMC–PPY, and CMC–PTh cryogel composites can be used as a sensor for HCl and NH3 vapors. Importantly, the innate biocompatibility of CMC cryogels, and sustainability of CMC with the capability of in situ prepared PANi, PPY, and PTh conductive polymers, make these natural–synthetic polymer composites promising materials for biotechnological and biomedical applications.

Author Contributions: Conceptualization, N.S.; Formal analysis, S.D. and S.D.S.; Funding acquisition, N.S.; Investigation, N.S. and S.D.; Methodology, N.S., S.D., and S.D.S.; Resources, N.S.; Supervision, N.S.; Writing—original draft, S.D. and S.D.S.; Writing—review & editing, N.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. J. Compos. Sci. 2020, 4, 33 10 of 12

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