expanded graphite composites prepared via in situ polymerization

Materials Chemistry and Physics 111 (2008) 368–374

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Study on the interface of phenolic resin/expanded graphite composites prepared via in situ polymerization Xiali Zhang, Liang Shen, Xue Xia, Haitao Wang ∗ , Qiangguo Du ∗ Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China

a r t i c l e

i n f o

Article history: Received 6 March 2008 Accepted 13 April 2008 Keywords: Composite materials Surface properties Electrical conductivity

a b s t r a c t Phenolic resin/expanded graphite (EG) composites were synthesized via in situ condensation polymerization of the monomers in the presence of foliated graphite. SEM observation showed that the graphite flakes were well dispersed in the phenolic resin matrix. The electrical conductivity of the composites was investigated as a function of the foliated graphite fraction. The composites containing graphite sheets exhibited an electrical conductivity percolation threshold with 3.2 wt% graphite content in polymer matrix. Inverse gas chromatography (IGC) measurements were carried out to characterize the surface of the foliated graphite before and after condensation polymerization of phenolic resin using a series of both non-polar and polar acid–base probe gases. The data obtained indicated that the character of graphite surface changed after the polymerization of phenolic resin. The dispersive component of surface free energy decreased greatly. Before polymerization the graphite surface is predominantly acidic while the surface turns to basic after polymerization. The increased polarity of surface contributed to the stronger interactions between graphite and phenolic resin and the fine dispersion of expanded graphite in the matrix, and resulted in the low conductivity percolation threshold. © 2008 Published by Elsevier B.V.

1. Introduction Polymer composites with inorganic materials exhibit a significant increase in the properties of polymers and even generate certain new properties. Phenolic resins, notwithstanding the century-long history, are still attracting a great deal of research interests. It is produced by the reaction of phenol with aldehyde and is classified as resol and novolac by synthetic conditions and curing mechanism [1]. They are important technical materials and irreplaceable in many areas, especially in thermal insulation, coating, aeronautic utilities, electro-optical devices and sensors, and composite materials because of their thermal stability, high char yield, structural integrity, and solvent resistance. In recent years, there has been considerable interest in phenolic resin/layered silicate nanocomposites because of their unexpected improvement in thermal and mechanical properties, even with small silicate content [2–5]. However, it is very difficult for phenolic resin to intercalate into the silicate gallery to form a rigid composite with three-dimensional structure. To overcome these difficulties, using linear novolac resin, researchers tried to synthesize phenolic resin/layered silicates via melt intercalation and sol–gel process

∗ Corresponding authors. Tel.: +86 21 6564 2392; fax: +86 21 6564 0293. E-mail addresses: [email protected] (H. Wang), [email protected] (Q. Du). 0254-0584/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2008.04.028

[3–5]. The reports of composite system based on phenolic resin and expanded graphite, and in situ polymerization method still have not been well concerned. Natural flake graphite is a kind of layered filler with a caxis lattice constant of 0.66 nm which was used to prepare conducting polymer based composites. The crystalline lattice of graphite consists of grapheme layers formed by sp2 hybridized carbon atoms, while the carbon sheets are bounded by weak van der Waals forces with each other. Graphite has a good electrical conductivity in the range of 104 S m−1 at room temperature [6] and has been used as conducting plates in fuel cell and super capacity devices. Graphite–polymer composites have been found to possess outstanding physical [7], chemical [8], mechanical [9], and membrane-like properties [10]. The availability of graphite–polymer composites is an essential requirement for the construction of advanced magnetic, electronic, optical, electrooptical self-lubricating devices and sensors. Unlike several lamellar silicate solids [4,5] whose exfoliation can be achieved by ion exchange reactions, the exfoliation of graphite, due to its lack of any net charges, cannot be done in the same fashion. Natural graphite is first converted to intercalated or expandable graphite through chemical oxidation in the presence of some concentrated acid, such as H2 SO4 or HNO3 . Then expanded graphite is obtained by abrupt expansion and exfoliation of expandable graphite in a furnace around 800–1000 ◦ C. The EG keeps a lay-

X. Zhang et al. / Materials Chemistry and Physics 111 (2008) 368–374

ered structure similar to natural flake graphite but with large interlayer spacing [11]. It contains tremendous different size of pores and sheets which stick to each other. Some monomers, initiators and polymers are able to be absorbed into the pores and gallerids of EG by proper method to produce conductive polymer/graphite composites [12,13]. The previous reports indicated that the graphite sheets are often less than 100 nm in thickness and the graphite filled polymers can be considered as nanocomposites. The electrical conductivity of the polymer nanocomposites is greatly affected by the addition of EG, and the nanocomposites have lowered percolation threshold of electrical conductivity when graphite sheets are well dispersed in the polymer matrices. It is well established that the interface structures of filler/matrix are a key factor of the distribution of the fillers and the performance of the resulted composites. It is reported that the graphite sheets contained functional groups such as –OH, –COOH after acid and high temperature treatments. These functional groups are assumed to promote both physical and mechanical interaction between foliated graphite and the polymer (or monomer) [14,15]. However, unlike the fiber/matrix system, there is no proper method that can characterize the interface interactions directly between foliated graphite and matrix until now. Therefore, it is highly required and yet the challenging work to develop an appropriate method to quantitatively characterize the interface of EG and matrix. Inverse gas chromatography (IGC) is a powerful technique for investigating the characteristics of solid surfaces in powder form [16] and has been widely used to the characterizing of the surface properties of calcium carbonate, fibers, clays, polymers, fullerene, graphite, carbon black [17–21], and carbon nanotube [22], etc. In this work, the well-dispersed composites were synthesized via an in situ condensation polymerization of phenol and formaldehyde in the presence of foliated graphite which were prepared from the EG dispersed by ultrasonic irradiation. The electrical conductivity of synthesized EG/phenolic resin composites and its dependency on graphite concentration was measured. These results was used to estimate the dispersion of EG in the polymer matrix. Then we used IGC to determine the surface thermodynamic characteristics of the foliated graphite before and after condensation polymerization by using different probe molecules under the condition of near zero surface coverage of the test materials. And finally, the mechanism of improved dispersion of graphite sheets in polymer matrix prepared via in situ polymerization was discussed. 2. Experimental 2.1. Materials Expandable graphite (50 meshes) was generously provided by Tianhe Graphite Company, Limited (Qingdao, China). Phenol and formaldehyde (37 wt% aqueous solution) of analytical reagent grade were purchased from Shanghai Chemical Reagents Company. Cresol (the weight ratio of 3-hydroxy-1-methyl benzene to 4-hydroxy-1-methyl benzene is 0.8–1.0) was a purified industry-grade reagent obtained from the Wujing Coking and Chemical Manufacturing Company (Shanghai, China). Magnesium hydroxide (chemical reagent grade) was purchased from Shanghai Sihewei Chemical Co., Ltd. n-Pentane, n-hexane, n-heptane, n-octane, nonane, dichloromethane (DCM), ethyl acetate (EtAc), acetone (Acet), tetrahydrofuran (THF), and diethylether (DEE) (analytical reagent grade) used as probes in IGC measurements were purchased from Shanghai Chemical Reagents Company and Shanghai Lingfeng Chemical Reagent Co., Ltd. All reagents were used directly without further purification. 2.2. Preparation of the wormlike porous expanded graphite and foliated graphite The expandable graphite was heated at 1000 ◦ C for 30 s in a Muffle furnace to generate expanded graphite particles with a c-direction dimension about 300 times of the original one. Expanded graphite was dispersed in a water–ethanol mixture (1:1) and placed in an ultrasonic bath. The disper-

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Table 1 Physicochemical properties of the IGC probes used in the present study Probe

˚ 2 ˛(A)

LD (mJ m−2 )

n-Pentane n-Hexane n-Heptane n-Octane n-Nonane DCM EtAc Acet THF DEE

45.0 51.5 57.0 62.8 69.0 31.5 48.0 42.5 45.0 47.0

16.1 18.4 20.3 21.3 22.7 27.6 19.6 16.5 22.5 15.0

AN (kJ mol−1 )

DN (kJ mol−1 )

– – – –

– – – –

16.3 6.3 10.5 2.1 5.9

0 71.5 71.1 84.0 80.6

Specific character Neutral Neutral Neutral Neutral Neutral Acid Amphoteric Amphoteric Base Base

sion was sonicated for 4 h, and then filtered and dried to produce foliated graphite. 2.3. Synthesis of the phenolic resin and preparation of the phenolic resin/graphite composites via in situ condensation polymerization The phenolic resin/graphite composites were fabricated in a 500 mL, fournecked flask equipped with a mechanical stirrer, a temperature controller with a thermometer, and a reflux condenser. 146.0 g phenol (1.55 mol), 41.4 g (0.38 mol) cresol, and 136.2 g formaldehyde (37 wt% aqueous solution) were mixed in the reactor to form a monomer solution. The corresponding amount of foliated graphite was dispersed in the monomer solution with the help of ultrasonication for 30 min. Two grams of magnesium hydroxide powder were added as catalyst and the pH of the solution was adjusted to 6. The reaction was highly exothermic and violent, therefore it should be carefully controlled at 80–85 ◦ C for 1.5 h, and then proceeded to reflux for another 1.5 h at 96–98 ◦ C. The reaction product was dehydrated at 40–60 ◦ C under 700 mmHg of vacuum for 1.5 h. 20.0 g of the dehydrated material was filterer through filter parer. The black solid collected on the filter parer was dispersed in excessive ethyl alcohol and centrifugated. The floater was collected, and the wash process was repeated for five times. The sample was then dried overnight under vacuum before the IGC measurements were made. The rest of the dehydrated material was transferred to a mould of 7.8 cm × 5.0 cm × 1.2 cm and cured at 110 ◦ C for 2 h to produce the final products of cross-linked resol-type phenolic resin/expanded graphite composites. 2.4. Measurements The morphologies of the fracture surfaces of the phenolic resin and phenolic resin/graphite composites were observed with a TESCAN 5136 MM scanning electron microscope. Electrical conductivity of the phenolic resin and the composites was measured on an EST121 digital ohmmeter (Beijing Work Protect Institute Technology Development Company, China) under dc current 1000 V pressure. The conductivity range of the EST121 digital ohmmeter are 10−4 to 10−18 S. When the conductivity of specimens was larger than 10−4 , then the specimens were measured by multimeter. The size of the specimens was 7.8 cm × 5.0 cm × 1.2 cm and aluminum foil was stuck on both sides of sample. Then the aluminum foil on both sides was connected with both electrodes of the ohmmeter or multimeter. The electrical conductivity of samples was calculated. 2.5. Inverse gas chromatography Inverse gas chromatography measurements were carried out to characterize the surface of the graphite before and after prepolymerization using a series of both nonpolar and polar acid–base probe gases. Their physiochemical properties are listed in Table 1. The donor numbers (DNs) and acceptor numbers (ANs) of the probes were taken from literatures [23–25]. These probes are commonly used in IGC for solid surface characterization. The chromatographic experiments were performed with a gas chromatography (GC112A) equipped with a flame ionization detector. Retention times were recorded on a (N2000) workstation. Nitrogen of high purity (99.999%) was used as the carrier gas with a flow rate of about 30 mL min−1 . The flow rate of the carrier gas was measured at the column outlet with a soap bubble flow-meter and was corrected for the pressure drop and the temperature change in the column using the James–Martin factor. A stainless steel column (30 cm long, 2 mm i.d.), cleaned with methanol and acetone, was used in this study. About 0.2 g of sample was filled into the column by the aid of vacuum and mechanical vibration. The two ends of the column were plugged with silane-treated glass wool. The column was then stabilized in the GC

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Fig. 1. SEM of (a and b) expanded graphite; (c) sonicated graphite.

system at 57 ◦ C overnight with a nitrogen flow at the rate of 30 mL min−1 . In order to avoid contaminating the detector, the outlet of the column was not connected to the detector during this stabilization treatment period. Measurements were carried out in the temperature range of 37–57 ◦ C. In order to meet the requirement of adsorption at infinite dilution, corresponding to zero coverage and GC linearity, the probes were introduced into the column using a 1 ␮L syringe with injecting vapor volume less than 0.1 ␮L. Therefore, the solute–solute interaction is infinitesimal and can be neglected, and the retention on the solid surface can be regarded as governed solely by solid–probe interactions. For each measurement, at least three repeated injections were made to obtain reproducible results. Methane was used as the marker for the retention time correction and it was used to ensure the absence of dead volume when a new column was placed in the chromatographic system.

3. Results and discussion 3.1. SEM of EG and EG/phenolic resin Rapid heating of expandable graphite flakes to a sufficiently high temperature causes exfoliation, a sudden increase in the dimension perpendicular to the carbon layers of the expanded graphite. The expanded graphite is a loose and porous vermicular material as shown in Fig. 1a. It basically consists of parallel boards, which collapse and deform desultorily, resulting in many pores of different sizes ranging from 10 to 50 ␮m, see Fig. 1b.

X. Zhang et al. / Materials Chemistry and Physics 111 (2008) 368–374

Fig. 2. SEM photographs of phenolic resin/3.5 wt% graphite composite.

In our experiments the expanded graphite was sonificated in an alcohol solution to get further exfoliated graphite sheets, see Fig. 1c. The graphite sheets, like other kinds of nanoparticles, tend to attract each other and are difficult to be dispersed in polymers by traditional means such as blending. Sonication is an effective way to disperse graphite sheets in monomers solutions. It is conceivable that the monomers can intercalated into the space between the graphite sheets and polymerized in situ, and the welldispersed graphite sheets are fixed after the system is cured. In the present study, the graphite sheets were wetted and dispersed in the monomer solution via sonication for 30 min and followed by in situ condensation polymerization. Since macromolecules take more space than monomer molecules do, the graphite sheets can be further exfoliated and fixed during the curing process. This was proved by SEM images (Fig. 2) of the cross section of the composites, which demonstrated that graphite sheets were dispersed in the phenolic resin matrix. The composites prepared using our methods have excellent electrically conducting property. Fig. 3 shows the variation in the electrical conductivity of plates with respect to the graphite contents. The sharp increase in conductivity of the composites is closely

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related to the formation of a network of conducting graphite within the phenolic resin matrix. The order of electrical conductivity of the 4 wt% graphite sheets composite is 5 higher than that of neat resin, up to approximately 10−2 S cm−1 . The electrical conductivity keeps the rising trend for composites with the higher graphite sheets loading. And it also exhibits a percolation threshold at the graphite sheets loading around 3% (by weight), indicating the existence of a percolation path via connecting graphite sheets, whereas, in order to make a composite with satisfactory conductivity, loadings of conventional graphite filler are usually as high as 20 wt% or even higher with micron-scale [26]. The high loading often results in a material with poor mechanical properties and high density. The graphite sheets with high aspect-ratio have advantages to form the conducting network, leading to higher conductivity at a given loading level when compared to the conventional graphite flake filler, thus the percolation threshold can be reached at very low weight fraction of graphite. The high conductivity of our composites at low inorganic loading indicates that these expanded graphites do not aggregate obviously in phenolic resin matrix. The good dispersion of graphite sheets as shown in SEM image demonstrates that there are strong interactions between graphite surfaces and polymer matrix. Percolation theory defines an insulator–conductor transition and its corresponding threshold of the conducting filler concentration Pc , via the following equation,  = 0 (P − Pc )t

for P > Pc

(1)

where  0 is a constant, P the weight fraction of graphite, and t the critical exponent [27]. When the mass fraction exceeds the percolation threshold the conductivity increases sharply as conducting paths begin to form. The exponent t depends on sample dimensionality and its values are t ∼ 1.33 and t ∼ 2.00 for two and three dimensions, respectively [27]. For our system, best-fitted values based on the scaling law are 3.2 wt% for Pc , 1.2 × 10−3 S cm−1 for  0 , and 1.33 ± 0.21 for the t which was calculated from the solid straight line in Fig. 3. And the low value of t = 1.33 suggests that charge transport through a two-dimensional object, such as surface or interface. Taking into account the fact that the graphite sheets were formed by the sp2 hybridized carbon atoms and the charges can only transfer in the graphite sheet planes, therefore, the value of the resulted critical exponent is reasonable. There are reports about t values observed in other systems, for example, t = 1.36 for MWNTs–PmPV composite thin film [28], t ∼ 1.3 for polyaniline–PMMA [28,29], carbon black–polyethlene [30], and carbon nanotubes–epoxy [31] composites. These values that were lower than the universal values for three-dimensional percolating systems, indicated that those systems in which the percolation took place were not ideal classical random networks but networks with more “dead arms” [31]. When the content of graphite is higher than 4 wt%, the conductivity of the composite reaches 10−3 S cm−1 and beyond, at which a material is regarded as a conducting material useful for the electrical industry. Therefore, the conducting phenolic resin/graphite composites are potential for applications requiring good conductivity especially at high temperature. 3.2. IGC characterization of surface of foliated graphite before and after polymerization

Fig. 3. Dependence of the dc conductivity on the graphites weight fraction P. The inset shows the percolation scaling law between log  and log(P − Pc ) where the solid straight line corresponds to the best-fitted line.

There are some functional groups such as –OH and –COOH generated on surface of expanded graphite after acid and high temperature treatments [15]. It is suggested that these functional groups can promote the monomer and chain segments of phenolic resin into the interspace of graphite, which contributes to the dispersion of graphite sheets in the resin and the interaction of graphite and phenolic resin. These functional groups may react

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0.5

Fig. 4. RT ln VN versus ˛(LD ) plot for the adsorption of n-alkanes on foliated graphite after prepolymerization.

with the raw material or phenolic resin which further contributes to the graft on the surfaces of graphite during the condensation polymerization of phenolic resin. IGC data can provide information on the surface of expanded graphite before and after prepolymerization, and lead to insight on the interface of expanded graphite and penolic resin. 3.2.1. Dispersive component SD of the surface energy SD expresses the potential of a solid to undergo London or dispersive types of interactions. These interactions are also termed ‘non-specific’ because they will always take place whatever the partners in contact. SD is calculated from the slope of the straight line connecting retention volume to the surface energy of the series of n-alkanes injected in the GC column. RTC ln VN =

1/2 1/2 2N(SD ) ˛(LD )

+ Ct

(2)

where R is the gas constant, TC is the column absolute temperature, N is Avogadro’s number, ˛ is the surface area of the probe molecule, LD s the dispersive component of the surface energy of the probe, and Ct is a constant. The plot of RTC ln VN versus 1/2

1/2 2N˛(LD )

Fig. 5. The dispersive component of surface free energy SD (mJ m−2 ) of foliated graphite.

3.2.2. Specific acid–base interactions The interactions between polar probes and the substrate involve both dispersive and specific interactions. And the specific interactions include dipole–dipole and acid–base interactions (or the electron acceptor–donor effect), the latter involving much higher energy than the former. In fact, it is usually assumed that the specific contributions of the adsorption of polar probes are solely from acid–base interactions. Therefore, the adsorption energy includes two components: the dispersive (GD ) and the specific (GAB ) according to equation: G = GD + GAB

(3)

The free energy of adsorption, GAB , corresponding to acid–base interactions on the surface is related to VN by the following equation,



−RT ln

VN VNref



= GAB

(4)

where VN is the specific retention volume of a polar probe and VNref is the specific retention volume of a hypothetical reference n-alkane with the same 2N˛(LD )

1/2

value as the polar probe. An example of

should

give a straight line with the slope (SD ) . The result of this procedure is shown in Fig. 4. The excellent linearity of each isotherm confirms the applicability of this expression to stationary phases. From the slope of these lines, the dispersive component of the surface energy SD can be calculated. SD , of expanded graphite before and after polymerization are presented in Fig. 5. The measured value of the surface free energy is about 110 mJ m−2 for exfoliated graphite before prepolymerization at 318.2 K, which is closed to that for graphite A (105 mJ m−2 at 416 K) but lower than that for graphite B (279 mJ m−2 at 416 K) observed by Papirer et al. [21]. As we know the dispersive component of the surface energy of the solid provides the information of the force field of the high energy sites. The graphite sheets will attract each other and accumulate together if the surface energy is very high. It is observed that the surface free energy of exfoliated graphite decrease greatly to about 56 mJ m−2 at 318.2 K after the phenolic resin polymerization. This may be induced by some organic groups mask the highest energy sites on the surface after the condensation polymerization which results in lower SD values.

Fig. 6. Determination of specific free energy GAB for polar probes on foliated graphite after prepolymerization at 308.2 K.

X. Zhang et al. / Materials Chemistry and Physics 111 (2008) 368–374

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Fig. 8. Correlation between specific enthalpies HAB of adsorption of the chosen probes on graphite and the DN and AN values of these probes according to Eq. (6).

Fig. 7. The specific free energy of adsorption of polar probes on expanded graphite before (a) and after (b) polymerization at different temperatures.

the plot for foliated graphite after prepolymerization at 308.2 K is shown in Fig. 6. Fig. 7 shows the free energy of the specific interactions between the two samples and polar probes at different temperatures. It can be obviously observed that for the exfoliated graphite before polymerization all the probes exhibit low −GAB values and only amphoteric probe (Acet) shows higher −GAB values which suggests that the surface polarity is very weak. While for the graphite after polymerization the −GAB values of all the probes increase greatly which suggests the polarity of surface increases greatly. The free energy of adsorption, GAB , corresponding to the specific acid–base interactions is also related to the enthalpy of adsorption, GAB , by the following equation, GAB = H AB − TS AB

(5)

where SAB is the entropy of adsorption corresponding to the specific acid–base interactions. A plot of GAB /T versus 1/T should yield a straight line with slope HAB . The enthalpy of adsorption corresponding to the specific acid–base interaction, HAB , is related to the surface acceptor and donor parameters, KA and KD , of the substrates by the following expression, −H AB = KA DN + KD AN

(6)

where DN and AN are the numbers of donors and acceptors, respectively, of the acid–base probe as defined by Gutmann. A plot of

HAB /AN versus DN/AN should yield a straight line with slope KA and intercept KD . The values of the specific enthalpy of adsorption, −HAB , are shown in Table 2. It can be observed that the enthalpy of adsorption of basic and some amphoteric probes decrease for the exfoliated graphite after the polymerization reaction while those of acid probes increase to some extend. The phenomenon suggests that the surface polarity of graphite change to basic direction after the condensation polymerization reaction. Fig. 8 shows HAB /AN versus DN/AN. The linear relation is excellent, indicating that the Gutmann acid–base concept is applicable to these systems. The values of KA and KD were calculated according to the plot and listed in Table 3. The amphoteric characteristic of graphite is reflected in the KD /KA ratio. It can be seen from Table 3 that for the expanded graphite before polymerization the surface is predominantly acidic while the surface turns to basic after polymerization. The acidic surface of expandable graphites, detected by IGC tests, comes from the formation of carboxylic and other polar groups during the treatments. Phenol and phenolic resin will react with carboxylic groups on the surfaces of graphite sheets during the condensation polymerization, which may cause the polymer

Table 2 The specific enthalpy of adsorption (HAB ) of polar probes on expended graphite Probe

Acet EtAc THF DEE DCM

−HAB (kJ mol−1 ) Foliated graphite before prepolymerization

Foliated graphite after prepolymerization

7.41 9.46 9.82 8.45 2.86

9.37 5.68 6.57 7.20 3.78

Table 3 Surface acceptor and donor parameters KA and KD for expended graphite and graphite after polymerization Probe

Foliated graphite before prepolymerization

Foliated graphite after prepolymerization

KA KD KD /KA (R2 )*

0.115 0.055 0.478 0.993

0.072 0.248 3.444 0.989

*

Regressive coefficients.

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The IGC results indicated that the surface character of graphite changed after the condensation polymerization of phenolic resin. The dispersive component of surface free energy decreased and polarity of surface increases greatly, which contributed to the stronger interactions between graphite and phenolic resin and resulted in the fine dispersion of expanded graphite in the matrix. The excellent electrical conductivity of the composite was attributed to the fine dispersion of the graphite sheets through the entire composite, forming graphite network. Therefore, IGC is an effective method to evaluate the interfacial structures of composite materials, including phenolic resin/expanded graphite system. References

Fig. 9. FT-IR spectra of graphite before and after prepolymerization.

chains grafting on the surfaces of graphites. Therefore, proved by IGC results, the acidity of graphite surface decreases after the prepolymerization. Furthermore, the grafting of polymer chains on inorganic surfaces may improve the interfacial strength between graphite sheets and phenolic resin and result in the good dispersion of expanded graphites. This kind of nanocomposites is expected to have good electrical conductivity and other properties. We had performed FT-IR measurement on both samples (see Fig. 9). However, no difference on the spectra can be found because of inadequate sensitivity. IGC measurement provided more information of graphite surface that is difficult to be detected by other methods such as FT-IR. Therefore, IGC can be used as a powerful technique to effectively estimate the interfacial interactions between graphite sheets and phenolic resin, as well as other composite materials. 4. Conclusions The phenolic resin/graphite composites were synthesized via in situ condensation polymerization of phenol and formaldehyde in the presence of the exfoliated graphite. The composites containing graphite sheets exhibited an electrical conductivity percolation threshold with 3.2 wt% graphite content in polymer matrix.

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