Dispersion and Rheology of Multiwalled Carbon Nanotubes in

Article pubs.acs.org/Macromolecules

Dispersion and Rheology of Multiwalled Carbon Nanotubes in Unsaturated Polyester Resin Esteban E. Ureña-Benavides, Matthew J. Kayatin, and Virginia A. Davis* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849, United States S Supporting Information *

ABSTRACT: Dispersions of multiwalled carbon nanotubes (MWNT) in unsaturated polyester resin (UPR) were studied by rheology and optical microscopy. The experimentally determined percolation threshold of 0.097 vol % was very close to the theoretical value of 0.085 vol % expected for individually dispersed MWNT with an average aspect ratio of 590. Rheological analysis showed that the dispersions formed an open network with a fractal dimension of 1.28. The results were compared with singlewalled carbon nanotubes (SWNT) and polystyrene (PS)-modified MWNT (MWNT-PS). The SWNT formed stronger networks than MWNT, but the MWNT-PS networks were weaker than unmodified MWNT.

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INTRODUCTION The incorporation of carbon nanotubes (CNT) into polymers has the potential to create composites with enhanced mechanical, thermal, and electrical properties. However, the strong van der Waals attraction between nanotubes can result in the formation of large aggregates. Aggregation reduces the potential for property enhancement by reducing the interfacial surface area and creating nonuniformities that can result in a deterioration of mechanical properties. The incorporation of CNT into thermoset polymer resins has predominately been studied using epoxies.1−7 However, for the past 30 years unsaturated polyester resins (UPR) have had twice the domestic market share as epoxies.8 In fact, UPR comprises 80% of the worldwide thermoset resin market.9 From a manufacturing standpoint, UPR is typically chosen for its low viscosity, fast cure time, and low cost. UPR also has better hightemperature performance than epoxy resins. The continuous working temperature for an epoxy resin is typically 150 °C or less, but UPR has heat deflection temperatures as high as 205 °C, making it a better choice for high-temperature applications. 10 The major disadvantage to UPR is its mechanical properties. For cast UPR, tensile strength typically ranges from 34.5 to 103.5 MPa with a tensile modulus of 3.1− 3.45 GPa. On the other hand, cast epoxy resin can show tensile strength and modulus as high as 130 MPa and 4.1 GPa, respectively. CNT−UPR composites have the potential to significantly improve mechanical properties, but there have been relatively few investigations of CNT incorporation into UPR.11−16 Rheological analysis of CNT-UPR nanocomposites provides insight into their microstructure and the potential for property enhancement.13 However, UPR rheology is challenging given the rapid evaporation of the styrene solvent, present in most UPR, and undesired curing of the resin during testing.13,14 In our previous work, a methodology was developed for probing © 2013 American Chemical Society

the rheological properties of single-walled carbon nanotubes (SWNT) dispersed in UPR. In this paper, we describe the microstructure of 0.05−0.30 vol % multiwalled carbon nanotube (MWNT) networks dispersed in UPR. In addition, we describe grafting polystyrene (PS) chains to the MWNT surface with the goal of improving compatibility with the styrene present in uncured UPR. Herein, we examine the effects of surface modification on the microstructure and rheological properties of the uncured nanocomposite resin.

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EXPERIMENTAL SECTION

Materials. MWNT (Lot # MKBD4154V), synthesized by SouthWest NanoTechnologies Inc. (Norman, OK), with 98.7 wt % carbon were purchased from Sigma-Aldrich (St. Louis, MO) and used as received. The nanotubes had a length of ∼5000 nm and a diameter of ∼8.5 nm. The resin Polylite X31003-00 (Lot # 632889) was kindly provided without catalyst by Reichold, Inc. (Durham, NC); it contained 40 wt % styrene and 60 wt % proprietary isophthalic polyester. Lithium pieces and styrene were purchased from SigmaAldrich (St. Louis, MO), and anhydrous ammonia was purchased from Airgas, Inc. (Kennesaw, GA). Dispersion of MWNT in UPR. MWNT were dispersed using our previously published procedure.13 An Ace Glass (Vineland, NJ) Trubore mixer was used with a modified paddle design to increase the wall shear stress. The mixing was performed for three consecutive days in a water bath at 1000 rpm with an estimated minimum shear stress of 2 kPa. To reduce evaporation during mixing, the water was replenished as needed, and a high shear lubricant was added to the bearing. This method was used to prepare samples with concentrations ranging from 0.05 to 0.31 vol %. After mixing, the suspensions were allowed to rest for 1 day in the dark before rheological measurements and optical microscopy; this was done to allow the samples to relax while avoiding the potential for photoinduced curing during storage. Received: August 26, 2012 Revised: January 18, 2013 Published: February 8, 2013 1642

dx.doi.org/10.1021/ma3017844 | Macromolecules 2013, 46, 1642−1650

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Rheological Measurements. Steady shear and oscillatory shear experiments were performed with an Anton Paar GmbH (Ashland, VA) Physica MCR 301 rheometer. The measurements were done in a cone and plate geometry with a 49.997 mm diameter, 2.018° cone angle, and a truncation gap of 52 μm. All samples were run at 10 °C except otherwise noted; this temperature minimized evaporation of the styrene without being so low as to condense ambient moisture on the fixtures. The temperature was controlled with a Peltier lower plate and a Peltier hood enclosing the sample. To reduce evaporation, the bottom plate was surrounded by a trough filled with pure resin; the upper hood had a ridge that touched the resin in the dip, providing a seal around the sample and thus minimizing evaporation. Using this approach, the neat resin showed only a 4% increase in viscosity due to evaporation after continuous testing for 9600 s (over 1.5 h) at 10 °C. For all experiments, the samples were allowed to rest in the rheometer for 25 min after loading; this was determined to be enough time for the network to recover enough to enable a reasonable experimental error of 1, for the entire frequency range. All others showed a solidlike behavior, tan(δ) < 1, at low frequencies followed by a viscous region above a critical frequency. At ωc, tan(δ) = 1, and thus G′ = G″ = Gc. The crossover parameters were used to obtain a master curve of the viscoelastic properties.19,20 To obtain the reduced G′ and G″, each variable was multiplied by a vertical shift factor b = Gc,ref/Gc, while ω was multiplied by a horizontal shift factor a = ωc,ref/ωc. The reference concentration was taken as 0.10 vol % (the lowest concentration that showed a crossover point within the measurable range). The resultant master curve is shown in Figure 4. Based on this scaling theory, the factor b should scale linearly with a/ηs, where ηs is the viscosity of the resin. The inset in Figure 4 confirms this relation (R2 = 1.000). The shape of the master curve in Figure 4 suggests the presence of a long relaxation time on the order of (ωa)−1 ∼ 104 s, indicating a low-frequency end of the plateau region. A similar behavior has been observed for MWNT suspended in polydimethylsiloxane (PDMS), for which two crossover points were measured experimentally.21 In that case, the authors attributed the behavior to Brownian motions of the nanotubes in the semidilute regime. For the nanotubes used herein, the rotational Peclet number

Figure 2. Steady shear rheology curves of (a) viscosity and (b) shear stress as a function of shear rate; the continuous line represents the fit to the Herschel−Bulkley equation. The inset shows the yield stress vs nanotube concentration; the line represents a power law fit of the data. Symbols key: (blue ◆) 0.31, (red ■) 0.25, (black ∗) 0.20, (green ▲) 0.16, (orange ●) 0.10, (purple ◆) 0.05, and (light blue ■) 0.00 vol %.

Pe = πηsL3γ /{3 ̇ kBT[ln(L /d) − 0.8]}

where τ is the shear stress, τy is the yield stress, K is the consistency, γ̇ is the shear rate, and n is the flow index. As shown in the inset of Figure 2b, τy has a power law dependence with MWNT concentration ϕ described as τy ∼ ϕ1.17±0.93 (R2 = 0.88). As discussed later in this paper, the power law index can be related to the fractal dimension of the nanotube network.

(2)

is 270 for the lowest experimentally accessible shear rate of 0.01 s−1. In eq 2, ηs is the viscosity of the resin, L is the length of the MWNT, kB is the Boltzmann constant, T is the temperature, and d is the diameter of the MWNT. Pe indicates the relative importance of the hydrodynamic and Brownian forces, with 1644


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Figure 4. Time−concentration superposition of MWNT in UPR referenced to 0.10 vol %. The inset shows the linear relation of b as a function of a/ηs. Symbols key: (blue ◆) G′ 0.31 vol %, (red ■) G′ 0.25 vol %, (black ∗) G′ 0.20 vol %, (green ▲) G′ 0.16 vol %, (orange ●) G′ 0.10 vol %, (blue ◇) G″ 0.31 vol %, (red □) G″ 0.25 vol %, (black ×) G″ 0.20 vol %, (green △) G″ 0.16 vol %, and (orange ○) G″ 0.10 vol %.

Figure 5. Low-frequency storage modulus vs concentration of MWNT and SWNT. The solid lines represent the fits to eq 3; the concentration at G′0 = 0 indicates the percolation threshold. The inset shows the data on a shifted concentration scale ϕ − ϕper. Symbols key: (blue ◆) MWNT, (red ■) SWNT. Figure 3. Oscillatory shear rheology curves of (a) G′, (b) G″, and (c) tan(δ) as a function of frequency. Symbols key: (blue ◆) 0.31, (red ■) 0.25, (black ∗) 0.20, (green ▲) 0.16, (orange ●) 0.10, (purple ◆) 0.05, and (light blue ■) 0.00 vol %.

Nevertheless, the opposite trend is observed here, indicating that there are competing factors lowering the stiffness of the MWNT suspensions relative to SWNT. It has been reported that the stiffness of a nanotube increases slowly with its diameter until it reaches the limit of the graphite layer at diameters on the order of 1.2 nm and above; thus, the Young’s Modulus does not vary much with diameter.22−25 In fact, experimental and calculated Young’s modulus values for of SWNT and MWNT are close to each other, with values ranging from 0.320 to 1.47 TPa for SWNT and 0.270 to 1.28 TPa for MWNT.26−29 Estimates of the Young’s modulus of MWNT also suggest that the stiffness is not affected by the number of walls.23−25 Consistent with the aforementioned observations, it has been suggested that the stiffness of a carbon nanotube is mostly defined by the C−C intrawall bonds.23,25 It has also been reported that poor lateral load transfer between the layers within MWNT nanotubes, and in bundles, may reduce MWNT strength and modulus in tension.25 The load transfer between nanotubes is also of significant importance. However, we suggest that the lower storage modulus observed in the MWNT suspensions maybe due to a higher amount of imperfections of

high Pe indicating greater importance of hydrodynamic forces. The high Pe obtained indicates a non-Brownian behavior. We suggest that the relaxation time on the order of 104 s (referenced at ϕ = 0.10 vol %) arose from low rigidity of the MWNT network. Interestingly, data obtained for SWNT using the same mixing and characterization methods showed a welldefined low-frequency plateau,13 suggesting that UPR/SWNT has a more rigid structure than UPR/MWNT. In Figure 5, the low-frequency storage modulus G′0 of the MWNT suspensions is compared with the values measured for SWNT in our previous paper.13 It is evident that the SWNT bundles, with L/d ∼ 500, exhibit stiffer networks than the MWNT with slightly larger L/d ∼ 588. The storage modulus of the nanotubes networks is expected to increase with aspect ratio if all the other properties are equal (i.e., same rigidity, interparticle potential, concentration, network structure, etc.). 1645


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Figure 6. Power law fits of G′0 and γc vs ϕ for percolated samples of MWNT and SWNT. Symbols key: (blue ◆) G′0 MWNT, (blue ◇) γc MWNT, (red ■) G′0 SWNT, and (red □) γc SWNT.

the individual graphitic walls of the nanotubes. This would lower the stiffness of the individual MWNT. At the same time, it would reduce the intertube van der Waals interactions, thereby decreasing the connectivity, stress transfer between the tubes, and network stiffness. The data from Figure 5 were also used to calculate the percolation threshold for both MWNT and SWNT using G′0 ∼ (ϕ − ϕper)ν

Table 2. Structural Parameters for Carbon Nanotubes Suspended in Thermoset Resins

γc ∼ ϕ B = ϕ(d − β − 1)/(d − d f )

(5)

(6)

where x is the fractal dimension of the backbone and α is a parameter that ranges from 0 to 1. A value of α = 0 indicates a strong-link regime where the links between the flocs are stronger than the flocs themselves; the opposite occurs when α = 1 and the system is in the weak-link regime. The yield stress measured from the steady shear measurements can be defined as the stress at which the linear response breaks down; consequently34,35 τy ≈ G′0 γc ∼ ϕ(d − 1)/(d − d f )

df

m

α for 1.0 < x < 1.3

1.28 1.65 2.15

0.34 0.26 0.32

0.35−0.41b 0.06−0.14 0.30−0.40

indicate slow aggregating flocs that partially interpenetrate after collision. Various factors influence df which possesses complex interrelations with aspect ratio. In general, it is expected that sticky (fast-aggregating) flocs show more open networks than slow-aggregating flocs that interpenetrate before aggregating.36 A study performed on Brownian boehmite rods with aspect ratios ranging from 1 to 30 revealed df increased with L/d.37 Our data suggest an opposite trend for the non-Brownian CNT used here. The SWNT had df = 1.65, typical of fast aggregating colloids with a very open network, while the MWNT had an even lower df = 1.28. Such low values have been observed for gelatin gels38 and for confined suspensions of flexible MWNT in N-methyl-2-pyrrolidone (NMP).39 The MWNT result was also confirmed by fitting eq 7 to the steady shear measurements; this resulted in a nearly identical value df = 1.29. A recent study performed on much shorter MWNT (L/d ∼ 45) dispersed in epoxy gave df = 2.15 (Table 2).30 Generally, more open networks were observed for longer nanotubes; however, properties such as the rigidity of the nanotubes and the size of the flocs relative to the system geometry also affected the fractal dimension. The radius of gyration ξ of the flocs has a power law relation with shear rate, ξ ∝ γ̇−m, where for rigid flocs m = mrigid = 1/β, in which case 0.23 < mrigid < 0.29.30,31 For soft aggregates, 0.4 < msoft < 0.5.30,31 Therefore, for typical colloidal suspensions 0.23 < m < 0.50. In the case of SWNT in UPR, m = 0.26 indicates rigid clusters where noncentral interactions play a dominant role. For MWNT in UPR, m = 0.34, suggesting intermediate behavior. A very similar result was reported by Khalkhal and Carreau for MWNT dispersed in epoxy (Table 2).30 The results are consistent with the hypothesis that the apparent relaxation time is on the order of 104 s (Figure 4) arises from the low rigidity of the MWNT network. The SWNT, on the other hand, have very rigid network with very long relaxation times that could not be measured within the frequency range studied (even through time−concentration superposition). It is also interesting to notice the link regime for the different nanotube suspensions. The values of α in Table 2 were

where d is the Euclidean dimension (d = 3 for a 3D network), df is the fractal dimension, and β = (d − 2) + (x + 2)(1 − α)

B −0.55 −1.36 −1.38

From ref 30. bα is given for 1 < x < 1.28 since x cannot be higher than df = 1.28.

where ϕper is the percolation threshold and ν is the power law exponent. The measured percolation threshold for MWNT and SWNT are 0.097 and 0.115 vol %, respectively, consistent with the larger L/d for MWNT used in this research. It is also interesting to note that the expected percolation thresholds for individually dispersed nanotubes are 0.085 vol % for MWNT and 0.100 vol % for SWNT, very close to the experimental measurements. Calculating the aspect ratio of the nanotubes from the experimental percolation thresholds results in ⟨L/d⟩ = 515 and 435 for MWNT and SWNT, respectively. These results compare favorably to the estimated values of 588 and 500 based on AFM measurements. Fractal Structure. The dependence of G′0 and γc as a function of nanotube concentration has been related to the fractal nature of the flocs that are the building blocks of the networks.30−33 The following scaling relations apply:30,32,33 (4)

A 1.71 2.84 3.74

a

(3)

G′0 ∼ ϕ A = ϕ β / d − d f

sample MWNT/UPR SWNT/UPR MWNT/epoxya

(7)

The data in Figure 6 were fit to eqs 4 and 5 to obtain the exponents A and B and enable calculation of df (Table 2). Values of 1.7 < df < 1.8 indicate fast aggregating flocs that merge as soon as they collide. Higher values 2.0 < df < 2.2 1646


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calculated from eq 6 assuming 1 < x < 1.3, which is typical of colloidal gels. Hobbie and Fry also estimated x ∼ 1 for MWNT dispersed in low molecular-mass polyisobutylene.40 Assuming x = 1, α = 0.06 for SWNT in UPR and α = 0.35 for MWNT in UPR. Therefore, the SWNT dispersions are in the strong-link regime where the flocs break before the links between the flocs; the elasticity of the network is governed by the rigidity of the flocs. In contrast, the MWNT dispersions are in an intermediate regime where both the flocs and the links between them contribute to the elasticity of the network. As was the case of m, the values of α for MWNT in UPR are in excellent agreement with those reported by Khalkhal and Carreau (Table 2).30 Effect of Temperature. Rheological measurements were performed for two MWNT concentrations (0.25 and 0.15 vol %) at 10, 15, and 20 °C. The analysis was limited to a reduced temperature range due to the evaporation of styrene above 20 °C and condensation of ambient humidity below 10 °C during testing. At high frequency (short observation times), the UPR matrix dominated the behavior of the material, while at low frequency the nanotubes networks governed the measurements. At low frequencies, G′ increased with increasing temperature while at high frequency the opposite trend was observed (data shown in Supporting Information). Consequently at high frequency, the typical inverse behavior for moduli and temperature was observed. However, at low frequencies the moduli increase with temperature, suggesting an increase of the net attraction forces between MWNT. The differences at low frequency were more evident for the highest concentration tested. Other authors have observed similar behaviors in MWNT suspensions. For example, Abbasi et al. reported larger complex viscosities of MWNT dispersed in polycarbonate at higher temperatures; the differences were also more evident at higher MWNT loads.41 Khalkhal and Carreau found that G′/ G* increased with temperature throughout the entire frequency range for MWNT dispersed in epoxy; the normalization against G* as done to cancel the effect of the matrix.30 Interestingly, the aggregation of nanoparticles with increasing temperature has been widely reported and has been attributed to the reduction of repulsive interactions caused by the reduced solvent density.42−44 The same scaling theory used for the time−concentration superposition was used to obtain a time−concentration− temperature superposition (Figure 7); this enabled extending the data to almost 1 × 10−5 rad/s referenced to a sample with 0.10 vol % MWNT at 10 °C. The expressions for the shift factors a and b were used with 0.10 vol % at 10 °C as the reference condition. The inset shows the linear relation between b and a/ηs including the factors from the time− concentration superposition shown in Figure 4 (R2 = 0.9998). The results indicate that the relaxation mechanism is not significantly affected by either temperature or concentration within the range of conditions studied. MWNT Surface Modification. The surface of the MWNT was modified with the goal of sterically stabilizing the MWNT and improving compatibility with the styrene solvent present in the resin. The modification was performed through simultaneous polymerization and grafting of PS to the sidewall of the nanotubes in a reductive Li/NH3 medium. This method was also employed by our group to modify the surface of SWNT;45 however, only the results for MWNT are presented here. Figure 8 shows the IR spectra of polystyrene functionalized MWNT (MWNT-PS) and unmodified MWNT. The bottom trace

Figure 7. Time−temperature−concentration superposition of MWNT in UPR referenced to 0.10 vol % at 10 °C. The inset shows the linear relation of b as a function of a/ηs for all experiments, including those for the time−concentration superposition in Figure 4. Symbols key: (blue ◆) G′ 0.25 vol % at 10 °C, (red ■) G′ 0.25 vol % at 15 °C, (green ▲) G′ 0.25 vol % at 20 °C, (blue ◇) G″ 0.25 vol % at 10 °C, (red □) G″ 0.25 vol % at 15 °C, (green △) G″ 0.25 vol % at 20 °C, (ornage ●) G′ 0.15 vol % at 10 °C, (light blue ■) G′ 0.15 vol % at 15 °C, (black ∗) G′ 0.15 vol % at 20 °C, (orange ○) G″ 0.15 vol % at 10 °C, (light blue □) G″ 0.15 vol % at 15 °C, and (black ×) G″ 0.15 vol % at 20 °C. Inset key: (red ◇) 0.15 vol %, (blue □) 0.25 vol %, and (black △) time−concentration superposition.

Figure 8. FTIR spectra of PS-modified MWNT and unmodified MWNT.

reveals no significant absorptions in the range from 2500 to 3500 cm−1 for as-received MWNT. The upper trace (MWNTPS) shows the characteristic peaks present in PS: −CH− stretches within the aromatic ring slightly above 3000 cm−1 and alkyl −CH− and −CH2− stretches from the backbone of the polymer slightly below 3000 cm−1. The signals just below 3000 cm−1 demonstrate polymerization since, as opposed to polystyrene, the styrene monomer does not contain any alkyl groups. Repeated washing with toluene, water, and ethanol was performed to remove unbound PS. The remaining PS was determined by TGA in argon (Table 3). The unmodified MWNT lost negligible weight within the temperature range studied, while the rest of samples lost from 7.3 to 8.0%. Sample MWNT2 appeared to have the largest amount of PS; however, the differences were too small to be conclusive. Unlike SWNT, MWNT do not exhibit observable van Hove transitions by UV−vis−nIR adsorption/emission spectra. Therefore, in order to determine if the PS was bound to the surface, the thermal 1647


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with slightly denser networks. Another important difference noted is that at 0.30 vol % no large agglomerates are observed, as opposed to the micrograph in Figure 1 for 0.31 vol %. This structure was consistently observed throughout the entire sample and is an indication that the MWNT-PS were somewhat more compatible with the resin than the unmodified MWNT. Rheological analyses were also performed on the MWNT-PS suspensions (Figure 10). The MWNT-PS clearly had lower G′

Table 3. Characterization Results from TGA and Raman Spectroscopy of Unmodified and PS-Modified MWNT sample MWNT MWNT1 MWNT2 MWNT3 a

wt loss in argon (%)a 0.9 7.3 8.0 7.8

± ± ± ±

0.3 0.7 0.5 0.5

Td (°C) 571 511 502 501

± ± ± ±

2 2 2 2

D/G (514 nm) 1.10 1.23 1.26 1.18

± ± ± ±

0.03 0.01 0.03 0.08

D/G (785 nm) 2.01 2.17 2.22 2.18

± ± ± ±

0.04 0.02 0.03 0.01

The weight loss is measured from 150 to 500 °C.

decomposition temperature Td in air and Raman spectra were analyzed. As shown in Table 3, for unmodified MWNT Td = 571 °C. After modification, Td was reduced by 60 °C in the case of MWNT1 and 70 °C in the case of MWNT2 and MWNT3. The reduction in Td can be attributed to the introduction of defects into the structure of the nanotubes themselves and indicates that the PS chains are probably covalently bound to the sidewalls. Additional confirmation of covalent attachment was obtained by Raman spectroscopy. The intensity ratio between the D and G peaks in the Raman spectra indicates the relative amount of sp3-hybridized carbons with respect to sp2 carbons. The measured D/G ratios are shown in Table 3. The unmodified MWNT had the smallest values using both excitation wavelengths, while MWNT2 had the largest. The changes in D/G are relatively small, particularly compared to changes expected for SWNT. This is due to only the exterior wall of the MWNT being available for chemical modification. Moreover, the larger outer diameter of MWNT means the carbons are less strained, more stable, and therefore less reactive. The combination of the Raman data and thermal decomposition data provide confirmation of covalent attachment. Dispersion of Modified MWNT in UPR. The characterization results suggest that sample MWNT2 had the highest degree of modification; consequently, this sample was used to prepare a stock of MWNT-PS to be dispersed in UPR. Figure 9 shows a series of optical micrographs of the modified nanotubes in UPR. Uniformly distributed flocs were observed at all the three concentrations studied. The structure looks similar to the dispersions prepared from unmodified MWNT (Figure 1), but

Figure 10. Oscillatory shear rheology curves of (a) G′, (b) G″, and (c) tan(δ) as a function of frequency for MWNT and MWNT-PS. Symbols key: (orange ●) 0.30 vol % MWNT-PS, (blue ◆) 0.25 vol % MWNT-PS, (red ■) 0.20 vol % MWNT-PS, and (green ▲) 0.15 vol % MWNT-PS.

and G″ than unmodified MWNT (Figure 3) at similar concentrations. Moreover, tan(δ) is generally higher for the modified nanotubes. The results showed that MWNT-PS dispersions had a reduced elasticity and a higher percolation threshold ϕper. For the unmodified MWNT, ϕper = 0.097%, but for MWNT-PS even 0.15 vol % had tan(δ) > 1 for the entire

Figure 9. Optical photomicrographs of MWNT-PS suspensions in UPR. Scale bars are 100 μm. 1648


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alization was achieved through simultaneous polymerization and grafting of polystyrene to the sidewall of the nanotubes. Functionalization only slightly increased the D/G ratio, but this limited functionalization was sufficient to change the dispersion microstructure at a given concentration. For a given concentration, optical microscopy indicated better dispersion and rheological analysis showed clear changes in properties. An increase in the percolation threshold for PS-MWNT was partially attributed to increased repulsive interactions.

frequency range, and there was no evidence of a low frequency plateau. At 0.30 vol % there was some indication of a lowfrequency plateau, and there was a large increase in both moduli between 0.25 and 0.30 vol %. Therefore, for MWNTPS, 0.25 vol % < ϕper < 0.30 vol %. This large value made determination of the fractal characteristics of the MWNT-PS networks impractical since concentrations well above ϕper would be required for this purpose. However the reduced elasticity of the modified MWNT networks is evident. There are several possible reasons for the differences between the rheological behavior of unmodified MWNT and MWNT-PS. One possibility is functionalization reduced MWNT bending stiffness by creating defects. However, since only the outer shell was accessible and there was a relatively low degree of functionalization, this possibility is highly unlikely. On other hand, even limited functionalization would increase the diameter of an individual MWNT resulting in a decrease in aspect ratio. However, since ϕper ≈ D/(2L), there would have to be a 3-fold increase in MWNT diameter to account for the nearly 3-fold change in ϕper. A mean diameter of 8.3 ± 0.7 nm after PS functionalization was measured with AFM compared to 8.5 ± 0.8 nm of dodecane-modified MWNT. The collapsed dodecane chains add a maximum of ∼1.2 nm to the measured diameter. This result indicates no significant difference between the diameter of unmodified and polystyrene-modified MWNT. Even if the polystyrene groups were fully extended from the sidewalls in UPR, it is unlikely that the diameter of the MWNT increased to ∼22 nm. Most likely, the changes in rheological properties were due to a slight decrease in aspect ratio coupled with an increase in repulsion forces between MWNT. The elasticity of networks arises from the stiffness of the nanofillers and the connections between them. Adding a covalently bound PS layer around the MWNT should have improved the compatibility with the styrene containing resin. However, the increase in steric hindrance also hindered the connections between both MWNT−MWNT and MWNT−flocs and reduced the overall elasticity of the nanocomposite material. Interestingly, this result is consistent with the theory outlined by Surve et al. which predicts an increase of percolation threshold by introducing strongly adsorbing polymers on the surface of nanorods.46

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ASSOCIATED CONTENT

S Supporting Information *

Plots of G′ and G″ vs frequency at 10, 15, and 20 °C for samples with 0.15 and 0.25 vol % MWNT; TGA traces in air and argon of MWNT-PS; Raman spectra of modified and unmodified MWNT; and diameter measurements of dodecanemodified MWNT and MWNT-PS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Chris Kirschling from Reichhold for donating the resin. Funding for this research was provided by the Department of Defense, a Department of Education Graduate Assistantship in Area of National Need Grant and National Science Foundation Award 1158862.

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REFERENCES

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CONCLUSIONS The measured rheological percolation threshold of 0.097 vol % for unmodified MWNT dispersed in UPR using high shear mixing was very close to the value expected for individually dispersed nanotubes. Superposition of oscillatory rheology data onto a master curve revealed a relaxation time on the order of ∼104 s at the reference concentration of 0.10 vol %. This relaxation time was attributed to low rigidity of the MWNT network. According to scaling theories based on fractal networks, the MWNT used in this study formed more open and less rigid networks than SWNT. It was also determined that the SWNT in UPR were in the strong-link regime while the MWNT were in an intermediate state between strong-link and weak-link regimes. Increasing the temperature of the suspensions yielded higher G′ at low frequencies; this was attributed to smaller repulsive interactions between the MWNT possibly caused by a lower resin density. A master curve for the rheological behavior was also obtained via time−temperature− concentration superposition. Chemical modification of the MWNT with polystyrene was demonstrated in a reductive Li/NH3 medium. The function1649


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