Thermomechanical properties of alkali treated jute-polyester/nanoclay biocomposites fabricated by VARTM process

Thermomechanical properties of alkali treated jute-polyester/nanoclay biocomposites fabricated by VARTM process
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  Thermomechanical Properties of Alkali Treated Jute-Polyester/Nanoclay Biocomposites Fabricated by VARTM Process Mohammad Washim Dewan, 1 Mohammad Kamal Hossain, 1 Mahesh Hosur, 2 Shaik Jeelani 2 1 Mechanical Engineering Department, Tuskegee University, Tuskegee, Alabama 36088 2 Materials Science and Engineering Department, Tuskegee University, Tuskegee, Alabama 36088Correspondence to: M. K. Hossain (E-mail: ABSTRACT:  A systematic study was carried out to investigate the effect of alkali treatment and nanoclay on thermomechanical proper-ties of jute fabric reinforced polyester composites (JPC) fabricated by the vacuum-assisted resin transfer molding (VARTM) process.Using mechanical mixing and sonication process, 1% and 2% by weight montmorillonite K10 nanoclay were dispersed into B-440premium polyester resin to fabricate jute fabric reinforced polyester nanocomposites. The average fiber volume was determined to bearound 40% and void fraction was reduced due to the surface treatment as well as nanoclay infusion in these biocomposites.Dynamic mechanical analysis (DMA) revealed enhancement of dynamic elastic/plastic responses and glass transition temperature ( T  g  )in treated jute polyester composites (TJPC) and nanoclay infused TJPC compared with those of untreated jute polyester composites(UTJPC). Alkali treatment and nanoclay infusion also resulted in enhancement of mechanical properties of JPC. The maximumflexural, compression, and interlaminar shear strength (ILSS) properties were found in the 1 wt % nanoclay infused TJPC. Fouriertransform-infrared spectroscopy (FT-IR) revealed strong interaction between the organoclay and polyester that resulted in enhancedthermomechanical properties in the composites. Lower water absorption was also observed due to surface treatment and nanoclay infusion in the TJPC. V C  2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS:  biomaterials; composites; crosslinking; mechanical properties; thermal propertiesReceived 7 May 2012; accepted 22 September 2012; published online DOI: 10.1002/app.38641INTRODUCTION Composite materials made from synthetic fibers such as glassfiber and carbon fibers are already available for consumer andindustrial uses. Manufacturing of synthetic fiber composites notonly consume huge energy but also their disposal at the end of the life cycle is very difficult since there is virtually no recyclingoption. Stringent environmental legislation and consumerawareness have forced industries to develop new technology based on renewable feedstock that are independent of fossilfuels. Industrial crops grown for fiber have the potential to sup-ply enough renewable biomass for various bio-products includ-ing composites. The scope of possible uses of natural fibers isenormous. 1 Natural fiber reinforced composites are light weightand possess good thermal and acoustic insulating properties,higher specific properties, and higher resistance to fracture. 2–4 Lignocellulosic biofibers derived from various sources such asleaf, bast, fruit, grass, or cane contribute to the strength andstiffness of bio as well as synthetic polymer composites in vari-ous applications. 5 Jute fiber is known to have excellent tensilestrength and a high modulus among lignocellulosic fibers. 6 Different types of surface treatment procedures have been sug-gested to enhance the interaction between natural fiber and ma-trix. Alkali treatment is the easiest and most widely investigatedsurface treatment technique for natural fibers. The effect of alkali treatment on mechanical and thermal properties of com-posites has been studied by many researchers. 7–9 They found abetter adhesion of fibers with matrices due to the surface modi-fication by alkali treatment.Nanoscale materials offer the opportunity to explore new behavior beyond those established in conventional materials.Various types of nanoparticles, including carbon nanofiber, car-bon nanotube, nanoclay, and metal oxides have been used toimprove the performance of composites. It has been establishedthat the addition of a small amount of nanoparticle into a ma-trix can improve thermal and mechanical properties signifi-cantly without compromising the weight or processability of thecomposite. 10 For example, it has been observed that moisturebarrier, flame resistance, thermal, and mechanical properties of polymeric composites can be improved by adding a smallamount of nanoclay as filler particles. 11 The higher surface area V C  2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP  J. APPL. POLYM. SCI.  2012 , DOI: 10.1002/APP.38641  1  is one of the most promising characteristics of nanoparticlesdue to their ability in creating good bonding in composites.The dispersion of nanoparticles in the matrix is one of the mostimportant parameters in fabricating nanophased composites. Ithighly depends on processing techniques such as solutionblending, shear mixing,  in situ   polymerization, ultrasonic cavita-tion, and high pressure mixing. 12–14 Many researchers have used montmorillonite nanoclay as fillerin polymeric composites and their laminates for its low cost,availability, well-known intercalation/exfoliation chemistry, highsurface area, and high surface reactivity. The montmorillonitelayer aspect ratio can be as high as 1000 in the well dispersedstate without breaking of layers. Its surface area is in the rangeof 220 to 270 m 2 /g. Additionally, nanoclay has excellent physicaland thermal properties. Nanoclay reinforced polymer compo-sites and their laminates have excellent characteristics, includingimproved physical (dielectric, optical, permeability, and shrink-age), thermal (flammability, decomposition, coefficient of ther-mal expansion, and thermal stability), and mechanical (tough-ness, strength, and modulus) properties even at a very low fillerloading. Generally montmorillonite clay is hydrophilic in na-ture. The incompatibility of hydrophilic clay layers with hydro-phobic polymer chains makes the dispersion of clay withinpolymer matrix difficult and leads to weak interfacial interac-tions. Nanoclays are miscibile only with a few hydrophilic poly-mers such as poly(ethylene oxide) and poly(vinyl alcohol). 15,16 In order to achieve an enhanced compatibility to various poly-mer matrices, the clay surface is organically treated to makethem compatible with polymers by assisting in intergallery absorption. This modification is done by ion-exchange utilizingsuitable organic surfactants, including primary, secondary,tertiary, and quaternary alkylammonium or alkylphosphoniumcations. The most widely used surfactant in polymer clay nano-composite processing is quaternary ammonium salts due totheir high ion exchange efficiency. This organic surfactant cansignificantly lower the surface energy of the clay layers andmatch their surface polarity with polymer polarity. Hence, poly-mer chains can be more easily wetted on the layer surface gen-erating a larger interlayer distance. This larger interlayer distancewill facilitate the nanoclay intercalated and/or exfoliated intothe polymeric matrix that will result in enhanced properties inthe composite. Thus, the reinforced properties largely dependon the degree of dispersion of silicate platelets within a polymermatrix, which is a function of polymer-nanoclay compatibility.Basically, the degree of interaction between the surfactantmonomer and polymer is crucial to achieve nanoscale disper-sion of clay layers to obtain the ultimate properties of nano-composites. Hence, organically modified montmorillonite clay isused in this study. Incorporating a small amount of nanoclay can improve composite properties significantly. However, higherclay loading above a certain threshold value increases the viscos-ity of the matrix. A higher clay loading also increases theamount of air bubble during the mixing process. Therefore, anoptimum amount of nanoclay will provide better properties asit is uniformly dispersed into the composite.Jute fibers and polyester resins are low-cost materials for fabri-cating biocomposites. Several researchers worked on jute polyes-ter composites produced by various processing methods. 17–21 Alkali-treated jute roving polyester composites exhibited bettermechanical properties due to better fiber-matrix mechanicalinterlocking at the interface. 22 Higher storage modulus andthermal transition temperatures have been found in surfacetreated jute/polyester composites processed by hand lay-up. 18 Jiang et al. also found improvement in storage modulus in 5 wt% cellulose nanowhisker infused biopol composites prepared by solution casting and extrusion followed by injection moldingprocess. 23 Flexure strength, flexure modulus, and interlaminarshear strength were improved by 20%, 23%, and 19%, respec-tively, in the 4-h alkali-treated 35% jute fiber reinforced vinylester composites compared with those of untreated ones. 24 Jutefabric reinforced polyester composites produced by hand lay-uptechnique showed maximum compression strength, compressionmodulus, and interlaminar shear strength (ILSS) to be 45 MPa,2.1 GPa, and 10 MPa, respectively, at a fiber volume fraction of 45%. 2 Hwang et al. showed 45% improvement in the ILSS of  jute fiber reinforced polypropylene composites with 5% maleicanhydride coupler prepared by the compression molding pro-cess. 25 The ILSS of glass-epoxy composites manufactured by theVARTM process was improved by 31% due to the addition of 2% oxidized multiwalled carbon nanotubes. 26 Water absorptionis a problem with composites, particularly with natural fiber re-inforced composites. For example, synthetic fiber reinforcedcomposites absorb water around up to 4% depending on typeand amount of fiber, type and amount of resin, and environ-mental conditions 27,28 whereas water content of natural fiberreinforced composites varies between 5 and 15%. 29,30 For exam-ple, jute fiber reinforced polyester composites fabricated by theVARTM process exhibited 8.9% water gain at room temperaturehaving fiber volume fraction of about 40%. 31 Vilay et al. fabri-cated alkali treated and untreated baggage fiber reinforced poly-ester composites using the vacuum bagging process. They reported that 7% and 12% water were absorbed by the treatedand untreated fiber reinforced composites, respectively, underthe same condition. 32 Any natural fiber reinforced composite tobe effective needs to bring the water absorption rate down to4% or below.To the best of our knowledge, there is no study reported in theopen literature on jute based polyester nanophased compositethat was processed by the VARTM process. Hence, the objectivesof this study are to fabricate nanoclay-infused untreated/alkalitreated jute fiber reinforced polyester biocomposites using theVARTM process and explore their thermomechanical propertiesfor structural applications. Using mechanical mixing and soni-cation process, montmorillonite K10 nanoclays were dispersedinto B-440 premium polyester resin to fabricate jute fiber rein-forced polyester nanocomposites. Nanoclay content was variedto observe its effect on the performance of jute fiber compositesmanufactured by the cost-effective VARTM process. Variouspercentage of nanoclay loadings (1–4 wt %) were tried. How-ever, a higher percentage of nanoclay (above 2 wt %) results inconsiderable increase in the viscosity of the polyester resin thatinduces improper impregnation of the reinforcements. More-over, a higher clay content causes agglomeration in the sonica-tion mixing process that leads to easy material removal ARTICLE 2  J. APPL. POLYM. SCI.  2012 , DOI: 10.1002/APP.38641 WILEYONLINELIBRARY.COM/APP  compared to the lower clay content. 33–35 In addition, pot life of polyester resin is very short, usually 15 to 20 min. It was very difficult to infuse highly viscous resin within 15 to 20 min by the VARTM process. Hence, only 1 and 2 wt % nanoclay wereinfused in this study.Common solvent method or solution based processing techni-ques could be employed to solve this problem. However,increase of clay content will require an increasing amount of solvent solution for proper dispersion. This will require a largeramount of energy to remove the solvent that may cause thermaldegradation of the polyester. 35 It will also increase the fabrica-tion cost and is not feasible for industry application due to theuse of solvent. However, it is important to control the size of the clay agglomerates for obtaining a better dispersion of nano-clay in the VARTM process. Observations of the microstructuresof the nanocomposites suggest that ultrasonic dispersion pro-vides the best results in terms of size and dispersion. 36 Jute fiber reinforced composites have been studied for a numberof years. Researchers have found that composites having a fibervolume fraction of 30 to 40% provide optimum proper-ties. 31,37,38 Hence, in this study, we have decided to maintain asimilar fiber volume fraction in our composites. The thermome-chanical performances of these composites were evaluated usingDMA, flexure, interlaminar shear, and compression tests. Thefracture morphology of the flexure and compression tested sam-ples were analyzed using scanning electron microscope (SEM)and optical microscope (OM). Interaction between polyesterand organoclay was studied by FT-IR. The effects of alkali treat-ment and nanoclay on the water absorption of the jute polyestercomposites were also evaluated in this study. MATERIALS AND METHODS Materials Selection Commercially available B-440 premium polyester resin was pur-chased from U.S. Composites. Hessian jute fabrics (Natural ColorBurlap, Material: 100% Jute, Width: 47’’, 11 Oz.) were supplied by Organically modified montmorillonite K10nanoclay (surface area: 220–270 m 2 /g, thickness of each layer: 1nm and lateral dimension: several microns) was procured fromSigma-Aldrich. Ammonium based organic modifiers are generally used for the modification of montmorillonite nanoclay. 39 Polyes-ter, jute, and nanoclay were used as matrix, reinforcement, andnanofillers, respectively because of their good property values andlow cost. Polyester resin comes in two parts: part A (polyesterresin) and part B (methyl ethyl ketone peroxide, MEKP). For al-kali treatment of jute fibers, 5 wt % sodium hydroxide (NaOH)solution was used. Alkali Treatment Jute fibers were soaked with 5 wt % NaOH solution for 2 h at30  C. Then the fibers were rinsed with water several times toremove NaOH and dissolved impurities. After rinsing, the fiberswere dried in an oven at 100  C for 5 h. The alkaline treatmentincreases the surface roughness of natural fibers. It results in bet-ter mechanical interlocking properly. It also increases the amountof cellulose exposed on the fiber surface, thus increasing the num-ber of possible reaction sites. 40 The alkali treatment further resultsin a large number of –OH groups accessible on the surface of fibers. 41 It breaks down fiber bundles into single fibers andincreases effective surface area available for interacting withmatrix. Resin Preparation and Composites Fabrication Polyester resin has two parts. For nanoclay infused specimens,desired amount of nanoclay was first mixed with part A of theresin by a high-speed mechanical stirrer for 5 min followed by sonication for 60 min in a beaker. 42 Sonication was performedusing a high intensity ultrasonic irradiation (Ti-horn, 20 kHzSonics Vibra Cell, Sonics Mandmaterials, Inc.). The mixing pro-cess was carried out in a pulse mode: 20 s on and 20 s off andthe amplitude was 40% of the maximum value. The beaker wassubmerged in a continuously cooled water bath to maintain thetemperature at 25  C during the sonication process. After sonica-tion, the mixture was cooled in a water bath and degasified usinga vacuum oven. Once the bubbles were completely removed fromthe mixture, 0.7 wt % initiator (MEKP) was added and stirredusing a mechanical stirrer for about 2 to 3 min. The sample wasfurther degasified to remove the bubbles produced during the ini-tiator mixing. Composite panels were then fabricated using thevacuum-assisted resin transfer molding (VARTM) process. Themold was left for about 24 h at room temperature for curing theresin. After 24 h, the mold was opened and the panel was placedto an oven at 110  C for 3 h for post curing. After postcuring, testcoupons were prepared according to the ASTM standard fromdifferent sections of the panels. Coupons were randomly collectedfor each type of test.During infusion of nanoclay loaded resin into mold, there isalways a possibility of filtering of nanoparticles by the top layersof the fabric. Filtration effect depends on the type of fiber, typeand amount of nanofillers, and resin infusion techniques. This fil-tration effect generally occurs in nanoparticles infused compositesfabricated by the VARTM process due to the filtering role of thefabrics. 26,36,43 Hence, small variations of nanoclay content areexpected in top and bottom half of the composite panels. Experimental ProceduresVoid and Fiber Volume Fraction Calculation.  Void content of the jute biopol composites was determined according to theASTM D 2734-94 standard using composite mixing eqs. (1)through (4). V  v   ð % Þ¼ð q t    q e  Þ = q t    100 (1) q t   ¼ 1 = ð W   f   = q  f    þ W  m = q m þ W  n  = q n  Þ  (2) V   f    ¼ð W   f    = q  f   Þ = ð W   f   = q  f    þ W  m = q m þ W  n  = q n  Þ  (3) q e   ¼ W  s  = V  s   (4)Here,  V  v  ,  V   f   ,  q t  , and  q e   are the void fraction, fiber volume frac-tion (considering no void), theoretical density, and experimentaldensity of the composites, respectively.  W   f   ,  W  m , and  W  n   are theweight fractions and  q  f   ,  q m , and  q n  , are the densities of fiber, ma-trix, and nanoclay, respectively.  W  s   and  V  s   are the weight (g) andvolume (cm 3 ) of the specimens. Densities of polyester resin, jutefiber, and nanoclay were taken as 1.19 g/cm 3 , 1.4 g/cm 3 , and 2.35g/cm 3 , respectively. Experimental densities of composites were cal-culated using eq. (4). Three samples from each category werechosen to calculate composite experimental density. For void frac-tion calculation, average experimental density ( q e  ) was used. 44 ARTICLE WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP  J. APPL. POLYM. SCI.  2012 , DOI: 10.1002/APP.38641  3  Thermogravimetric Analysis (TGA).  To test the filtering effect,thermogravimetric analysis (TGA) tests were performed on ran-domly selected samples from top-half and bottom-half of thenanoclay-loaded composite panels. TGA was conducted with TAInstruments Q 500 setup fitted with nitrogen purge gas. Threesamples from each category were tested. Samples were kept in aplatinum sample pan, weighed, and heated to 700  C from roomtemperature at a heating rate of 5  C/min under nitrogen atmos-phere. The real time characteristic curves were generated by aUniversal Analysis-TA Instruments data acquisition system. Dynamic Mechanical Analysis (DMA).  Dynamic mechanicalanalysis of various jute polyester composites was carried out onTA instrument Q800 according to the ASTM D4065-01 stand-ard. 45 Nominal specimen dimensions were 60 mm    12 mm    3mm. The tests were run under a double cantilever beam modewith a frequency of 1 Hz and an amplitude of 15  l m. The tem-perature was ramped from 30  C to 180  C at a rate of 5  C/min.At least three samples from each category were tested to obtainthe average dynamic elastic response (storage modulus), dynamicplastic response (loss modulus), and the ratio of loss modulusand storage modulus (tan  d ). Flexure Test.  Flexural properties of jute-polyester compositeswith/without nanoclay were evaluated using a Zwick Roell testingunit under three point bending mode according to the ASTMD790-02 standard at a crosshead speed of 2.0 mm/min. Spanlength to thickness ratio of the specimen was 16 and the width of the specimen was 12 mm. Five samples from each category weretested to obtain the average result. Interlaminar Shear Strength (ILSS) Test.  The apparent inter-laminar shear strength (ILSS) of the jute polyester composites wasdetermined by the short beam shear (SBS) test. ILSS testing wascarried out using a Zwick-Roell testing unit under three pointbending mode according to the ASTM D 2344-00 standard at acrosshead speed of 1.3 mm/min. Span length to depth ratio of the specimen was 6 and the width of the specimen was double of the thickness. By using a short beam, it is assumed that the beamis short enough to minimize bending stresses resulting in aninterlaminar shear failure by cracking along a horizontal planebetween the laminae. The ILSS was calculated using the eq. (5).ILSS ¼ð 0 : 75  P  Þ = ð b   h  Þ  (5)where  P   is the breaking load (N),  b   and  h   are the width andthickness of the specimen, respectively, in mm. Three identicalspecimens from each category were tested and the average ILSSwas calculated. Quasi-Static Compression Test.  Quasi-static compression testsof the jute polyester composites were performed using a servo-hy-draulic MTS testing unit according to the ASTM D 695-02 stand-ard at a cross head speed of 1.2 mm/min. The dimensions of thespecimens were 12.5 mm    12.5 mm    5 mm and loaded in thefiber direction. The end friction between the specimen and themachine was minimized using a lubricant at the contact area.Contact surfaces were prepared parallel by grinding for the fullcontact and to eliminate potential bending moment. Five samplesfrom each category were tested to obtain the average result. Fracture Morphology Study.  Morphology of fractured speci-mens was studied by JEOL JSM5800 scanning electron micro-scope (SEM) and Olympus DP72 optical microscope (OM). Polyester-Organoclay Interaction Study by FT-IR.  A Fouriertransform-infrared (FT-IR) spectrophotometer was employed forthe study of chemical reaction between nanoclay and polyesterresin using Nicolet 6700 DX IR spectrophotometer with attenu-ated total reflectance (ATR) sampling. The crystal material for theATR was diamond. The background was taken after every 60 minand each spectrum was recorded by co-adding 32 scans at 4 cm  1 resolution within the range 4000 to 600 cm  1 . Three samplesfrom each category were selected randomly and tested. The back-ground spectrum of KBr pellet was subtracted from the samplespectra. Water Absorption Test.  In order to measure the water absorp-tion of jute polyester composites, five rectangular specimens fromeach category were prepared with dimensions of 80 mm    13mm    3.5 mm. The specimens were dried in an oven at 105  Cfor 2 h, cooled in a desiccator, and immediately weighed ( W  0 ).Samples weights were measured using a precision balance havingan accuracy of 0.0001 g. The samples were then immersed intodistilled water according to the ASTM D 570-99 standard for 24h. After removing the samples, the excess water on the surface of the specimens was removed using clothes. The final weight ( W  )of the specimens was then taken. The water absorption ( M  w  ) of the specimens was calculated using eq. (6). M  w   ð % Þ¼ð W    W  0 Þ = ð W  0 Þ 100 (6) RESULTS AND DISCUSSIONS ResultsVoid Fraction and Fiber Volume Fraction.  When the fill andwarp direction fiber cross each other, it allows voids to beformed in the composite system. The presence of trapped air orvolatile materials and incomplete wetting out of the fibers by the matrix also causes the void in the fiber reinforced compo-sites. 22,46 Theoretical density, experimental density, void Table I.  Density, Void Fraction, and Fiber Volume Fraction of JPC Theoretical density( q t ) (gm/cm 3 )Experimentaldensity ( q e ) (gm/cm 3 )Voidfraction (%)Fiber volumefraction ( V  f  )UTJPC 1.288 6  0.021 1.181 6 0.075 8.33 0.449 6 0.041TJPC 1.271 6  0.013 1.203 6 0.049 5.38 0.385 6 0.0341% TJPC 1.272 6  0.010 1.211 6 0.051 4.75 0.389 6 0.0352% TJPC 1.287 6  0.011 1.222 6 0.070 5.075 0.416 6 0.039ARTICLE 4  J. APPL. POLYM. SCI.  2012 , DOI: 10.1002/APP.38641 WILEYONLINELIBRARY.COM/APP  fraction, and fiber volume are presented in Table I. A higherpercentage of voids were found in the UTJPC compared withthe TJPC. In this study, there was a small variation in fiber vol-ume fraction due to limitations in the fabrication process. Tocompare the properties, the results were normalized by dividingexperimentally acquired properties by the respective fiber vol-ume fractions. Nanoclay infused treated jute fiber reinforcedcomposites demonstrated same trend even after normalization. Thermogravimetric Analysis (TGA) Test Results on Nanoclay Filtering Effect To test the filtering effect in our study thermogravimetric analy-sis (TGA) tests were performed. First, six samples (70 mm   12.5 mm    3 mm) from 1% and 2 % TJPC panels (three ineach category) were randomly selected. These samples were cutinto halves along the thickness with a diamond cutter, yielding12 top-half and bottom-half samples. Approximately 12 to 13mg portions from these 12 samples were prepared for the TGAtests by using a diamond cutter and grinder. From the TGAanalysis, it was observed that at about 650  C the ash residuefrom each sample reached an asymptotic value. The variation of ash content in top- and bottom-half of the 1 wt % nanoclay-loaded TJPC panel is presented in Figure 1. The average ashcontent for the 1% TJPC top-half samples was 0.21% higherthan that from the 1% TJPC bottom-half samples. For the 2%TJPC, the ash content was found to be 0.56% higher for thetop-half samples. Thus, these results lend support to our conjec-ture that the filtering effect is very low for the 1% TJPC andmoderate for the 2% TJPC. Dynamic Mechanical Analysis (DMA) Test Results.  The varia-tion in storage modulus of the jute polyester composite withsurface treatment and nanoclay loading is shown in Figure 2(a)as a function of temperature. The results indicate the increasein the storage modulus at room temperature with surface treat-ment and nanoclay loading. TJPC samples with 2 wt.% nano-clay showed the highest storage modulus followed by 1% nano-clay infused and TJPC. TJPC, 1% and 2% nanoclay TJPCshowed 16%, 18%, and 43% improvement in storage modulus,respectively, compared with UTJPC. In Figure 2(a), the sharpdrop in storage modulus indicates the glass transition tempera-ture ( T  g  ) of the composites. The entire region can be dividedinto two sections: below   T  g   (glassy plateau region) and above  T  g  (rubbery plateau region). The operating temperature of thecomposite should be below   T  g  . The flatter section above  T  g  indicates the rubbery region of the composites. The storage Figure 1.  TGA curves to show the filtering effect in 1 wt % nanoclay-loaded TJPC. [Color figure can be viewed in the online issue, which isavailable at] Figure 2.  DMA curves of JPC (a) storage modulus, (b) loss modulus, and(c) tan delta (tan  d ). ARTICLE WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP  J. APPL. POLYM. SCI.  2012 , DOI: 10.1002/APP.38641  5
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