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한국정밀공학회지 제 34 권 제 8 호 pp. 575-580J. Korean Soc. Precis. Eng., Vol. 34, No. 8, pp. 575-580August 2017 / 575https://doi.org/10.7736/KSPE.2017.34.8.575ISSN 1225-9071 (Print) / 2287-8769 (Online)폴리아크릴아미드/셀룰로오스 나노결정 복합체 하이드로젤의 광학 및전기기계적 특성 연구Optical and Electro-Active Properties of Polyacrylamide/CNC CompositeHydrogels김현찬1, 고소원1, 자야라무두티파바티니1, 강진모1, 김재환1,#Hyun Chan Kim1, Xiaoyuan Gao1, Tippabattini Jayaramudu1, Jinmo Kang1, and Jaehwan Kim1,#1 인하대학교 기계공학과 (Department of Mechanical Engineering, Inha University)# Corresponding Author / E-mail: [email protected], TEL: 82-32-860-7326, FAX: 82-32-832-7325KEYWORDS: Polyacrylamide (폴리아크릴아미드), Cellulose nanocrystal (셀룰로오스 나노결정), Hydrogel (하이드로젤),Dielectric property (유전특성), Electro-active (전기활성)Polyacrylamide (PAM) was used for matrix material to fabricate composite hydrogels reinforced with natural cellulosenanocrystal (CNC). Invoking in situ free-radical polymerization with different concentration of cellulose nanocrystal,polyacrylamide hydrogels were fabricated. The chemical structure, compression strength, morphology and dielectricproperties of the composite hydrogels were investigated. The CNC played a role as a reinforcing filler and a multifunctionalcross-linker in the hydrogel. The elastic modulus and dielectric property of the composite hydrogels increased as increasingthe CNC concentration. The electrical actuation test of the PAM/CNC hydrogel shows its possibility for soft electro-activematerials for active lens.Manuscript received: January 18, 2017 / Accepted: April 23, 2017responsive materials, so called electro-active polymers. Due to thisNOMENCLATUREε' Dielectric constantW1 The weights of the sample before swellingW2 The weights of the sample after swellingphenomenon, those materials can be applied to produce softactuators and artificial muscles.1,2 Most of them exhibit dimensionalchanges in an electrolyte solution or in solid-state like film, whilefew reports have been investigated on hydrogel state.Cellulose, most abundant natural and renewable polysaccharides,is a classical highly-biocompatible material. Cellulose is usuallybiosynthesized by diverse organisms and deposited in a continuouselementary micro fibrils, which can be changed from 100 nm to1. Introductionseveral μm in length and can be 5-10 nm in diameter depending onthe source of cellulose. The elementary cellulose fibrils are madePolymers which undergo shape change in response to variousup of amorphous and crystals parts. The crystalline parts areenvironmental stimuli can convert physical or chemical energy intoseparated by amorphous parts and existed in rod-shaped cellulosemechanical behavior directly. Some polymers such as polypyrrole,nanocrystals (CNCs) with dimension of about several to 20 nm inpolyaniline, cellulose have been reported this kind of stimuli-diameter and lengths up to 2 um. The cellulose nanocrystals can beCopyright The Korean Society for Precision EngineeringThis is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
한국정밀공학회지 제 34 권 제 8 호576 / August 2017obtained by hydrolysis under acidic controlled conditions.Hydrolysis consists in destruction of the amorphous partssurrounding and between the cellulose microfibrils, while thecrystalline segments remain intact. Finally, the obtained colloidalparticles commonly referred to as CNCs.3,4 Due to some propertiessuch as chemical resistance, high mechanical strength, high aspectratio, biocompatibility, high surface area, and excellent propertiesof surface derivatization, CNCs have attracted lots of attention forthe reinforcement of polymer matrix.5 However, a polymerFig. 1 Schematic of PAM/CNC composite hydrogel synthesishydrogel, in other words, a cross-linked polymer network reinforcedwith CNCs has not been studied enough. Considering excellentdispersion of CNCs in water, the molding, fabrication, andelectron microscopy (SEM). To investigate the electro-activeapplication of hydrogels containing CNCs have many advantagesbehavior of CNC in PAM hydrogels, an electric field was appliedcompared with other solid nanocomposites. It also shows goodacross its thickness. The electro-active behavior and its mechanismelectro-active response applicable for sensors, actuators andare also explained.biomedical devices.4,6Polyacrylamide (PAM), one of the most commonly availablepolymers for hydrogels, can be defined as lightly cross-linked2. Experimentalpolymer. It can swell dramatically in the presence of water butinsoluble due to their non-toxic and biologically inert long chain2.1 Materialslengths. It also has a capacity for preserving its shape andAcrylamide (AM), Ammonium persulfate (APS), N, N’-mechanical strength and for conveniently adjusting its mechanical,Methylenebisacrylamide (MBAAm), concentrated sulfuric acid7chemical and biophysical properties. Hence, PAM hydrogels have(H2SO4),various applications in agriculture,8 drilling fluids,9 tissueTetramethylethylenediamine (TEMED), Sodium hydroxide (NaOH),engineering,10 and waste treatments. Up to now, considerableCotton pulp powder. We used all chemicals without any furtherstudies have been conducted to improve mechanical and chemicalpurification as received. DI-water was used in the whole es of PAM hydrogels via adding nanofillers such aspolymer nanoparticles, inorganic clay, and metal nanoparticles112.2 Preparation of CNCinto PAM-based matrix. PAM can sense its physical, chemical andThe CNC was prepared by hydrolysis process. We put purebiological environment changes and respond to external stimuli in acotton pulp (20.0 g) in 60 wt% H2SO4 aqueous solution (200 mL)controllable way. PAM can show abrupt and vast volume changesunder mechanical stirring (2 h, 200 rpm) at 50 C. Prior to acidin response to various stimuli from a surrounding environment andhydrolysis, an alkaline treatment was also made on the cotton pulpcan be driven by electric field. For the past several decades, PAMto remove non-cellulosic components. The hydrolysis process12-14hydrogel has been developed very fast in this area.In addition, based on the excellent optical transparency, PAMcan be applied for active lens.This paper aims at developing PAM based electro-active hydrogelresulted in a suspension, which was further centrifuged successivelywith DI water to obtain neutrality, followed by homogenization(11000 rpm, 10 min) and dialysis. Finally, the CNC suspension wasobtained.that can be applied for active lens. This electro-active hydrogelshould be able to change volume in the presence of electric field2.3 Synthesis of PAM/CNC Composite Hydrogelswith maintaining optical transparency for lens application. WePAM hydrogels were synthesized by the standard free radiationintroduce PAM hydrogel as the base matrix and CNCs as the filler.copolymerization method. An aqueous solution of TEMED wasBy adding CNC filler in PAM matrix, its dielectric property can beprepared and used as a cross-linker in the polymerization process.15drastically increased without sacrificing the optical transparency ofAlso, ammonium persulfate solution and MBAAm were preparedPAM. Fig. 1 shows the synthesis scheme of PAM/CNC compositeand used as initiator and accelerator, respectively. The monomerhydrogels. The chemical structure, morphology and structuralsolution was stirred with a magnetic bar on a stirring plate until allproperty of the PAM/CNC composite hydrogels are determinedreactants were completely dissolved. After that, CNC aqueouswith Fourier transform infrared (FTIR) spectroscopy and scanningsolution was added to fabricate the PAM/CNC hydrogel. Following
한국정밀공학회지 제 34 권 제 8 호August 2017 / 577Table 1 Composition for PAM, PAM-CNC hydrogelsGel codeAAMCNC(1%)MBA(mM)APS(mM)TEMED(mM)PAM1g-0.648 ml2.19 ml0.862 mlPAMCNC11g1 ml0.648 ml2.19 ml0.862 mlPAMCNC21g2 ml0.648 ml2.19 ml0.862 mlPAMCNC31g3 ml0.648 ml2.19 ml0.862 mlthis, MBAAm was added. After stirring 10 mins, TEMED and APSFig. 2 Experimental setup for electro-active actuation testwere added to initiate polymerization at the same time. At last, itwas cast into a mold. The synthesized gel was dried at oven. Afterthat, dried gel was immersed in DI-water for three days, with water(LDV, OMETRON VS-100) which measured displacement. Inputchanges three times to remove any unreacted monomers. Finally,voltage was applied and amplified by using function generatorPAM/CNC hydrogel can be obtained. Table 1 shows different(33220A, Agilent) and high voltage amplifier (HP 8452A, TREK).composition of PAM/CNC hydrogels.Pulse analyzer and Personal computer (PC) were used for collectingdisplacement signal from LDV. Before conducting actuation test,the desired hydrogels were equilibrated in the DI water for 24 h.3. CharacterizationsThe thickness of the hydrogel was around 4 mm. After reachingthe equilibration, the swollen hydrogels were located between twoBy using VERTEX-80 FT-IR spectrometer (Bruker), we recordedFTIR spectra of CNC, PAM and PAM/CNC hydrogel samples atITO glasses as electrodes which applied electric signal. Fig. 2illustrates the experimental setup for actuation test.room temperature. We prepared the test specimen using the KBrdisc method and analyzed the specimen within range of 400-4000cm-1. FE-TEM micrographs were taken by using a TEM (JEOL,4. Results and DiscussionJEM 2100F). We took FE-SEM images of the specimen with thefield emission scanning electron microscope (FE-SEM, Hitachi S-4.1 Sample Morphology4300) to study the sample morphology. Samples were prepared byFig. 3(a) shows the AFM image of isolated CNCs. They exhibita freeze-dry method. Before putting in a freeze-dry machine, it wasrod like nanostructure with 25-40 nm diameter. For AFMfrozen by liquid nitrogen and we coated the samples with platinumobservation, the isolated CNCs were coated on a silicon wafer onby using a sputter before the SEM observation. Optical property ofwhich oxidized using piranha solution. Compact network structurethe samples was studied by using UV-visible spectrophotometerof CNCs shows the internal bonding present in it. Fig. 3(b) shows(HP 8452A). The optical transmittance spectra of the films werethe high resolution TEM image of the CNCs. The diameter ofrecorded within the range of 200-800 nm wavelengths.CNC has broad range of distribution but the length of great part ofMechanical properties of the samples were tested by usingthe ‘rod-like’ CNCs is within the range 250-380 nm.universal testing machine according to ASTM D-882-97. We putWe investigated the composite’s morphology by the SEM imagesthe samples between the two parallel plates and the upper plateof polyacrylamide with CNCs. Fig. 4 illustrates the SEM images ofpressed the sample. Test was performed under room temperaturea polyacrylamide/CNC hydrogel after freeze drying method. Thewith the compression rate of 0.005 mm/s. Dielectric constant andnanocomposite hydrogels with CNC loadings exhibit uniformdielectric loss of specimen were measured using the HP 4284Adispersion of CNCs within the polyacrylamide matrix, and alsoLCR meter which has frequency range of 20 Hz-1 MHz at roomuniform pore structure can be observed in the SEM images.temperature.To investigate the electro-active behavior of PAM/CNC4.2 UV-Transmittance, FTIR and Swelling Indexcomposite hydrogels, electrical actuation test was performed. TheFig. 5 illustrates the UV-visible spectra of PAM and PAM/CNCactuation test was conducted by using a laser Doppler vibrometercomposite hydrogels. The PAM hydrogel is known to be a
한국정밀공학회지 제 34 권 제 8 호578 / August 2017Fig. 3 Images of the isolated CNC: (a) AFM, (b) TEMFig. 6 FTIR spectra of pure PAM, CNC and PAM-CNC1 compositehydrogelFig. 4 SEM analysis of polyacrylamide/CNCFig. 7 Swelling ratio of PAM-CNC composite hydrogels withdifferent CNC concentrationand 1 wt% of CNC concentration is suggested for bettertransparency.Fig. 6 represents FTIR spectra of pure PAM, CNC and PAMCNC3 composite hydrogel. The spectrum of pure PAM showscharacteristic peaks at 3390, 3193, 1620 cm-1, attributed to theCONH2 group. The PAM–CNC3 shows characteristic peaks at1130 and 1062 cm-1 attributed to C-O bond. This indicates that CO bond is made between PAM and CNC.The swelling ratio of the PAM and PAM/CNC compositeFig. 5 Optical transparency of polyacrylamide and its compositehydrogeltransparent optical hydrogel as it shows 80% transmittance (at 480hydrogels swelled in DI-water was calculated using Eq. (1) andresults are shown in Fig. 7W2 – W1Swelling ratio ( % ) ----------------- 100W1(1)nm). The transmittance increases with the wavelength and saturateswhere W2 and W1 are a weights of the sample after and beforebeyond 400 nm within visible wavelength region. This observationswelling. When CNCs were dispersed in PAM hydrogels, signifi-hydrogels, in which CNCs are well dispersed in the compositecant changes in swelling ratio were observed. Structural characteri-hydrogels. However, as increasing the concentration of CNCs, itszation result indicates that the CNCs are strongly interacted withtransparency of composite hydrogel decreases. There is threshold ofthe polyacrylamide matrix through hydrogen bonding.CNC concentration in the transparency of the composite hydrogelsHydrogels can undergo large reversible deformations in response
한국정밀공학회지 제 34 권 제 8 호August 2017 / 579Fig. 8 Compressive stress-strain curves of different hydrogelsFig. 10 Electro-Active behavior of PAM-CNC composite hydrogelsconstant, ε' of the samples was measured within 10 Hz - 1 kHzfrequency band, at 25 C, 25% relative humidity, and the results areshown in Fig. 9. When the values are compared among PAM andcomposites, CNC itself has low dielectric constant. This is attributedto a motion of free charge carriers due to charges present on theCNC surface and interfacial polarization effects. Because thecharges of CNC surface counteract the charges of PAM chains, thisis why the value decreased with the CNC concentration increase.Electrical actuation test of the samples was performed. Fig. 10Fig. 9 Dielectric property of polyacrylamide and its compositehydrogelsshows the results. 400 V on 2 mm thickness of samples, whichcorresponds to 0.2 V/μm electrical field strength, was applied tothe samples with frequency range of 20-200 Hz. Displacement inthe thickness direction of samples was measured by using the LDVto water interaction.sensor. As increasing the actuation frequency, the displacementdecreases. Regarding the CNC concentration, PAM-CNC1 exhibited4.3 Mechanical, Dielectric and Electro-Active Properties1400 ppm at 20 Hz and its value increased to 1750 ppm for PAM-We investigated the mechanical compression properties of PAM-CNC2 and decreased to 1050 ppm for PAM-CNC3. There is aCNC composite hydrogels with three different concentrations,trade-off relation between elastic modulus and electro-activePAM-CNC1, PAM-CNC2, PAM-CNC3, as the stress-strain curvesbehavior associated with dielectric property. Elastic modulus andshown in the Fig. 8. The results show the CNC addition in thedielectric property of PAM-CNC composite hydrogels increasePAM hydrogel improves its mechanical property. Compared withwith the CNC concentration. As the elastic modulus increases, itsthe pure PAM hydrogel, PAM-CNC3 composite hydrogel exhibitsstiffness of hydrogels is so stiff that it is hard to deform in thesignificant advance in Young’s modulus as value increases 0.4,presence of electric field even though dielectric constant increases.0.48, 0.55 and 0.68 GPa for pure PAM, PAM-CNC1, PAM-CNC2Thus, PAM-CNC1 is an optimum concentration of CNC thatand PAM-CNC3, respectively. The high modulus is associated withproduces large electro-active strain. The actuation might bethe formation of multifunctional physical cross-links of CNCs withassociated with electro-static force associated with dielectricpolyacrylamide. Due to strong interaction of hydroxyl groups onproperty of the soft hydrogels.the surface of CNCs, they tend to have self-association, which isfavorable for a formation of load-bearing percolating networksinside of the host polymer matrix.5. ConclusionDielectric properties of the composite hydrogels are important tounderstand electro-active behavior of the hydrogels. DielectricPolyacrylamide was used as a matrix material to fabricate
580 / August 2017nanocomposite hydrogel reinforced with natural cellulosenanocrystal. The fabrication process of polyacrylamide hydrogelvia in situ free-radical polymerization with different concentrationof CNC was reported. The composite hydrogel’s chemical structure,elastic modulus, morphology, optical transparency and dielectricproperties were investigated. The CNCs played a role as areinforcing filler and a multifunctional cross-linker in the hydrogelsystem. The elastic modulus and dielectric property of the compositehydrogels increased as increasing the CNC concentration in thecomposite hydrogels. Finally, the actuation test for the compositehydrogels revealed 1750 ppm strain in thickness direction. There isa trade-off relation between elastic modulus and electro-activebehavior associated with dielectric property. Thus, PAM-CNC1was found to be optimum concentration of CNC that produceslarge electro-active strain. This composite hydrogels are applicablefor tunable active lens.ACKNOWLEDGEMENTSThis research was supported by National Research Foundationof Korea (No. NRF-2015R1A3A2066301).REFERENCES1. Tungkavet, T., Seetapan, N., Pattavarakorn, D., and Sirivat, A.,“Electromechanical Properties of Multi-Walled Carbon Nanotube/Gelatin Hydrogel Composites: Effects of Aspect Ratios, ElectricField, and Temperature,” Materials Science and Engineering: C,Vol. 46, pp. 281-289, 2015.2. Kim, K. J. and Tadokoro, S., “Electroactive Polymers for RoboticApplications,” Springer, pp. 121-152, 2007.3. Habibi, Y., Lucia, L. A., and Rojas, O. J., “Cellulose Nanocrystals:Chemistry, Self-Assembly, and Applications,” Chemical Reviews,Vol. 110, No. 6, pp. 3479-3500, 2010.4. Gao, X., Sadasivuni, K. K., Kim, H.-C., Min, S.-K., and Kim, J.,“Designing pH-Responsive and Dielectric Hydrogels fromCellulose Nanocrystals,” Journal of Chemical Sciences, Vol. 127,No. 6, pp. 1119-1125, 2015.5. Lam, E., Male, K. B., Chong, J. H., Leung, A. C., and Luong, J.H., “Applications of Functionalized and Nanoparticle-ModifiedNanocrystalline Cellulose,” Trends in Biotechnology, Vol. 30, No.5, pp. 283-290, 2012.6. Fox, J. D., Capadona, J. R., Marasco, P. D., and Rowan, S. J.,“Bioinspired Water-Enhanced Mechanical Gradient NanocompositeFilms that Mimic the Architecture and Properties of the SquidBeak,” Journal of the American Chemical Society, Vol. 135, No.13, pp. 5167-5174, 2013.한국정밀공학회지 제 34 권 제 8 호7. Xiang, Y., Liu, G., Zhang, C., and Liao, J., “Sulfoacetic AcidModifying Poly (Vinyl Alcohol) Hydrogel and ItsElectroresponsive Behavior under DC Electric Field,” SmartMaterials and Structures, Vol. 22, No. 1, pp. 1-7, 2012.8. Sojka, R., Bjorneberg, D., Entry, J., Lentz, R., and Orts, W.,“Polyacrylamide in Agriculture and Environmental LandManagement,” Advances in Agronomy, Vol. 92, No. 25, pp. 75162, 2007.9. Yao, L. and Krause, S., “Electromechanical Responses of StrongAcid Polymer Gels in DC Electric Fields,” Macromolecules, Vol.36, No. 6, pp. 2055-2065, 2003.10. Kai, D., Prabhakaran, M. P., Stahl, B., Eblenkamp, M.,Wintermantel, E., et al., “Mechanical Properties and in VitroBehavior of Nanofiber-Hydrogel Composites for TissueEngineering Applications,” Nanotechnology, Vol. 23, No. 9, pp.1-10, 2012.11. Guo, Y.-G., Hu, J.-S., Liang, H.-P., Wan, L.-J., and Bai, C.-L.,“Highly Dispersed Metal Nanoparticles in Porous Anodic AluminaFilms Prepared by a Breathing Process of PolyacrylamideHydrogel,” Chemistry of Materials, Vol. 15, No. 22, pp. 43324336, 2003.12. Dai, T., Qing, X., Wang, J., Shen, C., and Lu, Y., “InterfacialPolymerization to High-Quality Polyacrylamide/PolyanilineComposite Hydrogels,” Composites Science and Technology,Vol. 70, No. 3, pp. 498-503, 2010.13. Guan, T., Ceyssens, F., and Puers, R., “Fabrication and Testingof a Mems Platform for Characterization of Stimuli-SensitiveHydrogels,” Journal of Micromechanics and Microengineering,Vol. 22, No. 8, pp. 1-9, 2012.14. Khaleque, T., Abu-Salih, S., Saunders, J., and Moussa, W.,“Experimental Methods of Actuation, Characterization andPrototyping of Hydrogels for BioMEMS/NEMS Applications,”Journal of Nanoscience and Nanotechnology, Vol. 11, No. 3, pp.2470-2479, 2011.15. Pandey, M., Mohd Amin, M. C. I., Ahmad, N., and Abeer, M.M., “Rapid Synthesis of Superabsorbent Smart-Swelling BacterialCellulose/Acrylamide-Based Hydrogels for Drug Delivery,”International Journal of Polymer Science, No. 23, pp. 1-10, 2013.
dielectric loss of specimen were measured using the HP 4284A LCR meter which has frequency range of 20 Hz-1 MHz at room temperature. To investigate the electro-active behavior of PAM/CNC composite hydrogels, electrical actuation test was performed. The actuati
