Design and electrical conductivity of poly(acrylic acid-g-gelatin)/graphite conducting gel.
Recently, carbon conducting polymer composites have been developed, mainly concentrated on adding graphite (1), (2), carbon fibers (3), (4), carbon black (5), (6), and carbon nanotubes (7), (8) to polyethylene, nylon, esters, etc. These conducting composites are mostly solid state. There are few reports on the conducting materials with gel conductivity.
Gels are three-dimensional (3D) and hydrophilic polymer networks capable of imbibing large amounts of water or fluids (9). The gel structures crosslinked by physically or chemically are the results of covalent bonding, hydrogen bonds, and even van der Waals interactions. There are numerous applications of these gels, especially in the medical and pharmaceutical fields. Gels resemble natural living tissue because of their high water contents and soft touch. On the other hand, amphiphilic polymers such as poly(acrylic acid-g-gelatin) have two hydrophilic/hydrophobic (HI/HO) phases with a suitable degree of cross-linking (9), (10) have not been developed. It is able to absorb several times its own mass of water or inorganic-solvents to form a stable gel (11-15). Based on the electrical conductivity of graphite and the absorbency of the amphiphilic polymer, adding graphite nanoplatelets to poly (acrylic acid-g-gelatin) to form a composite with a better gel conductivity is important. A conducting gel could be used in fuel cells, supercapacitors, dye sensitive solar cells, and rechargeable lithium batteries (16-20), because of its better conductivity property, colloid stability, low cost, and simple preparation.
To exclude the effect of ionic conducion in aqueous solution, in this work, a novel conducting gel consisting of poly(acrylic acid-g-gelatin) and graphite nanoplatelets with an electrical conductivity of 3.18 mS [cm.sup.-1] and a percolation threshold of 3 wt% in cyclohexane is reported. Because of the lower dielectric constant (about 2.02 at 80[degrees]C), nonpolarity, nonionization, and insulation of cyclohexane. the electrical conductivity can be completely displayed and attributed to electronic conduction. The effects of synthesis parameters on the electrical conductivity of gel are discussed in detail and an appended network structure model of the poly(acrylic acid-g-gelatin)/graphite conducting gel is proposed.
Acrylic acid monomer was reduced distilled prior to use. Gelatin, a mixture of water-soluble macromolecular protein made from porcine skin (300 Bloom), was used as received. Potassium persulfate (KPS), as a radical initiator for the synthesis of poly(acrylic acid-g-gelatin)/graphite, was purified by recrystallization from 66 wt% ethanol/water solution. N,N'-methylene bisacrylamide (NMBA) was a crosslinker for preparation of the copolymer. Natural graphite powders with a specific surface area larger than 80 [m.sup.2] [g.sup.-1] and an average particle size smaller than 6 x [10.sup.-6] [m.sup.2] (Shanghai Colloid Chemical Industry, China) were exfoliated to nanoplatelets and used as conducting materials. Cyclohexane was used as a typical inorganic solvent and absorbed in the composite to form a gel. The above materials were all analysis reagents and purchased from Shanghai Chemical Reagents, China. All aqueous solutions were prepared in 18 M[OMEGA] water obtained by purification of deionized water with a Millipore Milli-Q system.
Exfoliation of Graphite Nanoplatelets
Natural graphite was oxided in concentrated [H.sub.2][SO.sub.4] and fuming [HNO.sub.3] (4:1, v/v) for 16 h under appropriate cooling and stirring. After being filtered and washed using deionized water until the pH of rinse water becomes neutral. The filtered product was dried at 100[degrees]C overnight, the resulted preoxidized graphite was subjected to a thermal shock at 1050[degrees]C for 20 s in a muffle furnace to form an expanded graphite.
One gram expanded graphite oxide was mixed and saturated with 300 ml alcohol solution consisting of alcohol and deionized water with a volume ratio 7:3 for 12 h. Then the dispersion was centrifugated and dried at 70[degrees]C in a vacuum oven after 10 h of sonication under a frequency of 59 KHz. The obtained graphite oxide nanoplatelets were reduced in hydrazimum hydroxide to graphite nanoplatelets and then dried at 105[degrees]C for 4 h prior prior to use.
Preparation of Poly(acrylic acid-g-gelatin)/Graphite Composite
Acrylic acid monomer and gelatin were dissolved in deionized water at 60[degrees]C to prepare a mixture solution. A predetermined amount of graphite nanoplatelets and crosslinker NMBA were dispersed in the above solution. Then, initiator KPS was added to the solution consisting of monomer, gelatin, graphite nanoplatelets. and NMBA. Under a nitrogen atmosphere, the reaction mixture was stirred and heated to 80[degrees]C in a water bath for 30 min. After completion of the copolymerization reaction, the half-product was washed with enough deionized water to remove any impurities. The product, a typically black jell, was dried under vacuum at 80[degrees]C for more than 3 h to constant weight and milled using a 40-mesh screen for conductivity measurement.
Measurement of Gel Conductivity
The powdered composite of 3 g was immersed in cyclohexane of 200 ml at room temperature for at least 24 h to reach swelling equilibrium, which resulted in the absorption of cyclohexane inside of the network of the composite and the formation of a conducting gel. The unadsorbed solvent was removed by filtration over a 40-mesh stainless steel screen and hanging up for 25 min. The swelling ratio in cyclohexane (SR, g/g) of the composite was obtained according to the Eq. 1 (21):
Swelling ratio (SR) = [[[W.sub.2] - [W.sub.1]]/[W.sub.1]] (1)
where [W.sub.1] was mass of dried composite (g), [W.sub.2] was mass of swollen gel (g). The electrical conductivity of the gel was measured by using a Pocket Conductivity Meter (HANNA8733) in a cylinder containing a saturated swollen sample of 30 g. Each experiment was repeated three times with errors of ~ [+ or -]2.5%, and an average value was recorded.
Measurement of Mechanical Strength
The mechanical properties of gels were measured using self-prepared equipment and a Dejie DXLL-20000. The test conditions were controlled as follows: temperature 25[degrees]C; the sample length of 80 mm; crosshead speed of 100 mm [min.sup.-1]. The cubic samples are elongated at a strain rate of 5% * [min.sup.-1]. The strain under stress was defined as the change in length relative to the initial length of the specimen. The tensile strength and modulus were calculated on the basis of the initial cross section. Strain relaxation test was performed by increasing a predetermined load on the gel sample in 10 sec and then keeping the predetermined load and allowing the strain of gel sample to relax for some times until the strain unchanged. The strain-time plots of the gel sample were recorded. The gel strength and strain were obtained according to Eqs. 2 and 3:
[sigma = [F/A] = [mg/A] (2)
[epsilon] = [[[l.sub.2] - [l.sub.1]]/[l.sub.1]] x 100% (3)
where [sigma] was tensile strength. A was cross-sectional area of zonal gels, m was mass of water and the cup; [epsilon] was strain: [l.sub.1] and [l.sub.2] were lengths of gel before and after extension.
The morphology of poly(acrylic acid-g-gelatin)/graphite samples was examined using a scanning electron microscope (Hitachi S-5200, JAPAN). Poly(acrylic acid-g-gelatin)/graphite sample was cut into a piece, and mounted on metal stub, and coated with gold, subsequently, its surface was observed and photographed by SEM (scanning electron microscope). The powdered sample was also identified by IR spectroscopy on a Nicolet Impact 410 FTIR spectrophotometer using KBr pellets.
RESULTS AND DISCUSSION
Acrylic acid and gelatin were simultaneously cross-linked in a homogeneous aqueous solution using KPS as a radical initiator and NMBA as a crosslinker. The [alpha]-C atom connecting--CO--or--NH--is easy to lose a hydrogen atom to form carbon free radical, which provides a feasibility to reactive with acrylic acid to form an acrylic acid-g-gelatin segment. The sulfate anion radical produced from thermal decomposition of KPS can abstract hydrogen from the C--H groups in amino acids to form the carbon free radical on the substrates. The graft copolymerization can be carried out in two ways (22): (a) the carbon radicals on the gelatin backbones result in active centers capable of initiating free radical reactions with NMBA to form a gel and (b) self-crosslinking of the free radicals onto acrylic acid-g-gelatin segments results in crosslink points to produce a gel (23).
Because of the hydrophobic property of gelatin, acrylic acid-g-gelatin segments can be integrated to surface of reduced graphite nanoplatelets via strong hydrophobic interactions, which is similar to the self-assembly of surfactants on graphite surface (24). In the presence of KPS and NMBA, the graft copolymerization proceeded and the graphite nanoplatelets connected each other to form conducing channels. Based on the mechanism, effects of synthesis parameters such as cross-linker, initiator, monomer concentration, reaction temperature, graphite content, gelatin content, and swelling ratio on the electrical conductivity of the gels were discussed. And an appended network structure model of the poly(acrylic acid-g-gelatin)/graphite conducting gel was proposed.
The SEM images of the poly(acrylic acid-g-gelatin)/ graphite gel are shown in Fig. 1a. It can be seen that graphite nanoplatelets are equably dispersed in poly (acrylic acid-g-gelatin) matrix and connect each other to form conducting channels. It is well known that ions are inert in organic solvent, such as cyclohexane, and thus the measured gel conductivity is a result of electron transfer along the graphite channels. For the ionic conduction hydrogels, the conductivity can be easily achieved through the absorption to electrolyte solutions. However, these hydrogels are usually unstable. So, we design and fabricate an amphiphilic poly(acrylic-g-gelatin) gel by electron conduction. There are many organic solvents can be used, such as methanol, ethanol, acetone, etc. In this work, we choose cyclohexane as a target solvent because of lower dielectric constant, nonpolarity, and nonionization. The tension morphology of the poly(acrylic acid-g-gelatin)/graphite gel is photographed by a digital camera. From the inset of Fig. 1a. the gel presents perfect tensile property and elasticity.
[FIGURE 1 OMITTED]
The conducting composites with high conductivity in the presence of low graphite dosages have been paid more attention (2). So, it is crucial to exfoliate graphite particles to nanoplatelets. From Fig. 1b, it can be seen that natural graphite powders have been expanded and exfoliated to nanoplatelets after oxidation, expansion, and ultrasonic irradiation.
The FTIR spectra of gelatin (a), poly(acrylic acid) (b), and poly(acrylic acid-g-gelatin)/graphite composite (c) are shown in Fig. 2. For gelatin, the absorption peak at 3311 [cm.sup.-1] is attributed to N--H stretching. 3063 [cm.sup.-1] and 2947 [cm.sup.-1] are attributed to C--H stretching in--[CH.sub.3] and--[CH.sub.2] groups, respectively. 1664 [cm.sup.-1] peak is for C=0 bending (amide I) and 1539 [cm.sup.-1] peak belongs to C--N stretching (amide II) (25). The absorption peaks at 1452, 1333, and 1253 [cm.sup.-1] are results of C--H bending deformation vibration of-[CH.sub.3], C--H bending, and C--N stretching (amide III), respectively. In the case of poly(acrylic acid), the absorption peak at 3356 cm.sup.-1] corresponds to O--H stretching in H--O--H, 1690 [cm.sup.-1] is the result of C=0 stretching, 1123 [cm.sup.-1] belongs to C--H bending. 1570 [cm.sup.-1] is attributed to C=0 bending in--COOH, 1180 [cm.sup.-1] is for C--O stretching and 1319 [cm.sup.-1] is for C--C stretching. In the case of poly(acrylic acid-g-gelatin)/graphite composite, the vibration absorption peaks at 3063 and 2947 [cm.sup.-1] have weakened or disappeared, the absorption peak at 1664 [cm.sup.-1] has transferred to 1717 [cm.sup.-1]. The transformations of peak intensity and location reveal the graft reaction of acrylic acid to [alpha]-C in gelatin.
[FIGURE 2 OMITTED]
Influence of Preparation Conditions on Gel Conductivity
The preparation condition and structure of the composite play an important role in the electrical conductivity of gel. From Fig. 3a, it is obvious that the gel conductivity increases with increasing the crosslinker dosage from 0.04 to 0.14 wt%. Beyond a crosslinker amount of 0.14 wt%, the gel conductivity decreases gradually.
[FIGURE 3 OMITTED]
The electrical conductivity of the poly(acrylic acid-g-gelatin)/graphite gel depends on the size of polymeric space network. Clearly, a lower concentration of cross-linker does not produce enough crosslink points to construct a 3D network to adsorb graphite nanoplatelets. Especially, when the crosslinker concentration is lower than 0.04 wt%, the polymer network does not form effectively (Fig. 3a). As a consequence, the graphite nanoplatelets are dispersed in solution and washed out in the preparation process. On the other hand, higher crosslinker concentration results in the generation of more crosslink points and the smaller network space of the polymer (26), (27), which is not enough to hold graphite nanoplatelets. In this case, the conducting channels will be broken and the gel conductivity will decrease. In our experiment, the crosslinker dosage of 0.14 wt% is better.
The effect of initiator on the electrical conductivity of the gel is similar to that of the crosslinker. Figure 3b shows that the gel conductivity increases with the increase of initiator amount from 0.4 to 1.2 wt%, but decreases when the initiator amount changes from 1.2 to 2.4 wt%. The reason maybe that the polymerization reaction takes place slowly and fewer polymer networks with a larger space volume are produced when the initiator concentration is lower. On the contrary, higher initiator dosage results in a faster reaction velocity and produces more polymer networks with a smaller space size. The above two cases cannot contain enough graphite nanoplatelets to form conducting channels, and result in the decline of the gel conductivity. In our experimental conditions, the initiator amount of 1.2 wt% is available.
The acrylic acid monomer concentration is another factor affecting the conductivity of poly(acrylic acid-g-gelatin)/graphite conducting gel. From Fig. 3c, we can see that the gel conductivity increases gradually in the range of 33.3-50 wt% of the monomer concentration and declines in the concentration of 50-70 wt%. The behavior can be explained according to the polymerization reaction shown in Eq. 4:
n(acrylic acid + gelatin)[right arrow]poly(acrylic acid-graft-gelatin)/graphite (4)
According to the rate law, under a higher monomer concentration, the reaction takes place faster, which causes an uneven dispersion of polymer networks and graphite nanoplatelets, and thus the connections between graphite nanoplatelets are interdicted. On the other hand, under a lower acrylic acid monomer concentration, the polymerization reaction proceeds slower, even cannot polymerize effectively, the acrylic acid exist in the presence of oligomer instead of polymer, the 3D network of polymer is incompact, graphite nanoplatelets cannot connect to form effective channels, which results in a lower gel conductivity. In our conditions, the reaction takes place effectively, and the gel possesses the highest electrical conductivity at a monomer concentration of 50 wt%.
From Fig. 3d, the conductivity increases with increasing of gelation amount from 1.67 to 20 wt% and decreased from 20 to 66.67 wt%. It is considered that graphite nanoplatelets are absorbed in copolymer by strong hydrophobic/hydrophilic interactions, and so the active site is important for the formation of graphite networks. Because [alpha] carbon is easier to lose hydrogen atom to form an active site, when the gelatin amount is 20 wt%, adequate active sites are produced to absorb graphite nanoplatelets. If the gelatin amount exceeds 20 wt%, much more active sites will result in the agglomeration of graphite, and thus conducting channels are interdicted and conductivity decreases under the same conditions.
As is shown in Fig. 3e, the gel conductivity increases with the increase of reaction temperature from 65 to 75[degrees]C in the preparation process of poly(acrylic acid-g-gelatin)/ graphite composite, but declines in the range of 75-90[degrees]C. As we know, the reaction takes place slowly under lower temperature and even do not occur at the temperature below 65 [degrees]C. Thus, the copolymer cannot be formed to adsorb graphite nanoplatelets effectively. On the other hand, the polymerization is an exothermic reaction (shown in Eq. 4), a higher temperature is disadvantageous for the formation of excellent copolymer. The acrylic acid, gelatin, and graphite nanoplatelets will disperse in the solution in the state of monomer, oligomer, or nanopowders, instead of the composite. So, reaction temperature at 75[degrees]C is available for the highest gel conductivity.
Figure 3f shows the effect of the graphite amount on the gel conductivity. It can be seen that the conductivity increases slowly with the increase of the graphite amount in the range of 0-3 wt% and sharply increases in the amount of 3-10 wt% both in cyclohexane and deionized water. When the graphite content is 15 wt%, the gel conductivity in cyclohexane is the highest with 2.13 mS [cm.sup.-1]. Beyond an amount of 15 wt%, the electrical conductivity of gel changes little.
Clearly, when the graphite content is lower, graphite nanoplatelets are dispersive and cannot form conducting channels for electron transfer. With the increase of the graphite amount, the connections between graphite nanoplatelets become better and better, which leads to the enhancement of the electrical conductivity. According to experimental results shown in Fig. 3f, the graphite content of 3 wt% is a critical value, more than 3 wt%, a conducting network can be formed. On the other hand, when the graphite amount exceeds 15 wt%, the graphite network has formed perfectly, the further increase of graphite amount offers less contributions to conducting channels, which results in a slight alteration of the gel conductivity.
Effect of Swelling Ratio on the Gel Conductivity
The swelling of gel has visible effect on gel conductivity. From Fig. 4. we can see that the gel conductivity decreases with the increase of the SR when the SR is larger than 6 g [g.sup.-1]. It is believed that the gel conductivity depends on the electron transfer along the conducting channels, which are formed by the connections of graphite nanoplatelets. The gel will swell with the increase of SR, which result in the disconnection of graphite networks and thus the gel conductivity decreases.
[FIGURE 4 OMITTED]
According to above discussions, an appended network structure model of the poly(acrylic acid-g-gelatin)/graphite conducting gel is proposed, and the sketch figure is shown in Fig. 5a. As we know that the poly(acrylic acid-g-gelatin) gel is an amphiphilic polymer, the graphite nanoplatelets can be adsorbed on the polymer network by strong hydrophobic interactions to form another appended network. In this case, the whole system becomes a polymer network adsorbed graphite nanoplatelets structure, the graphite nanoplatelets can connect each other, and some conducting channels are formed. Based on the appended network structure model, the effects of synthesis parameters such as crosslinker, initiator, monomer concentration, gelatin dosage, graphite content, and SR on the gel conductivity can be explained, which further confirms the rationality of the structure model.
[FIGURE 5 OMITTED]
To prove the availability of appended network structure model of the poly(acrylic acid-g-gelatin)/graphite conducting gel, a polyacrylate/sericite hydrogel has been synthesized, and its polarizing microscopic photograph is recorded and shown in Fig. 5b. Because of the special polarized light property for the sericite particles, the bright network belongs to the sericite network in Fig. 5b. Because under the polymerization reaction conditions, polyacrylate can form a network structure with chemical crosslinking and sericite particles cannot do by themselves. Therefore, the bright network structure is believed that the sericite particles are absorbed on the network of polyacrylate to form another appended network. In this case, whole system becomes a structure with polymer network adsorbed sericite particle.
Because of the unique properties of gels such as stimulus-responsive properties, penetrability, sorption, and elasticity, they have been used in a variety of applications in separation membranes, biosensors, artificial muscles, chemical valves, superabsorbents, and drug delivery devices. Despite this extensive use, the poor mechanical properties have limited the practical applications. Future gel applications will require significantly improved mechanical properties and increased compatibility with a range of chemically diverse functional groups.
The mechanical properties of the poly(acrylic acid-g-gelatin)/graphite gel were measured by using self-prepared equipment and referring to Ref. (28). The stress--strain curves of the gels with different gelatin dosages are shown in Fig. 6. From Fig. 6, it can be seen that the stress increases and strain decreases with increasing of gelatin dosages. The introduction of flexible gelatin chains will increase the van der Waals' force and hydrogen bonds of the gel which results in the increase of strain. Under a low gelatin dosage, the SR in cyclohexane is lower because of the decreased hydrophobic property. It is known that the high tensile strength and the elongation at break of the gel derive from the breakages of polymer chains, hydrogen bonds, physical entanglement, van der Waals forces, and chemical crosslinking. At a low SR, the polymer chains are closed and the network density is tight, the interaction between polymer chains is reinforced, and the mechanical strength of the gel is better.
[FIGURE 6 OMITTED]
Strain relaxation tests were performed by tensing the gels under a load until the gel reaching its largest displacement. For the same sample, loads play an important role in stretching of poly(acrylic acid-g-gelatin)/graphite conducting gel (see Fig. 7). The elongation ratios of gel are 1720% and 240%, respectively, after the maximum and minimum loads at 160 s. We observe that the action is irreversible if the loads are much higher, such as more than 200 g. It is clear that the strain in high loads is higher than that in low loads. It is well known that the viscoelastic properties of polymeric materials depend on the ease of molecular movement, which is determined by the number and strength of molecular interactions such as physical entanglement, electrostatic forces, hydrophobic/ hydrophilic interactions, and chemical crosslinking (28-30). The relative stiffness of the polymeric chains also plays an important role (31). Up to now, the low mechanical strength of gels has limited the further applications in many fields, and so, it is important and interesting to increase the strength of gels, including tensile strength, compressive strength or strains.
[FIGURE 7 OMITTED]
A novel poly(acrylic acid-g-gelatin)/graphite composite is prepared. The preparation conditions are optimized as crosslinker of 0.14 wt%, initiator of 1.2 wt%, graphite of 15 wt%, gelatin of 20 wt%, monomer concentration of 50 wt%, and a reaction temperature of 75[degrees]C. Based on the absorbency, when immersing the composite in cyclohexane, a gel with excellent mechanical strength and electrical conductivity of 3.18 mS [cm.sup.-1] is obtained. The effects of synthesis parameters such as crosslinker, initiator, monomer, gelatin, graphite, reaction temperature, and swelling ratio on the gel conductivity are detailedly investigated and an appended network structure model is proposed.
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Qunwei Tang, Xiaoming Sun, Jihuai Wu, Qinghua Li, Jianming Lin
The Key Laboratory for Functional Materials of Fujian Higher Education, Institute of Material Physical Chemistry, Huaqiao University, Quanzhou 362021, China
Correspondence to: J. Wu; e-mail: email@example.com
Contract grant sponsor: National High Technology Research and Development Program of China: contract grant number: SQ2008AA03Z2470974; contract grant sponsor: National Natural Science Foundation of China: contract grant numbers: 50572930, 50842027: contract grant sponsor: Specialized Projiect of Fujian Province: contract grant number: 2007HZ0001-3; contract grant sponsor: Specialized Research Fund for the Doctoral Program of Chinese Higher Education; contract grant number: 20060385001.