A comparison of the mechanical strength and stiffness of MWNT-PMMA and MWNT-epoxy nanocomposites

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1 Composite Interfaces, Vol. 14, No. 4, pp (2007) VSP Also available online - A comparison of the mechanical strength and stiffness of MWNT-PMMA and MWNT-epoxy nanocomposites LU-QI LIU and H. DANIEL WAGNER Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Received 5 May 2006; accepted 18 August 2006 Abstract The surface of multi-wall carbon nanotubes (MWNTs) was functionalized by covalent linking of long alkyl chains. Such functionalization led to a much better tube dispersion in organic solvents than pristine nanotubes, favored the formation of homogenous nanocomposite films, and yielded good interfacial bonding between the nanotubes and two polymer matrices: a thermoset (Epon 828/T-403) and a thermoplastic (PMMA). Tensile tests indicated, however, that the reinforcement was greatly affected by the type of polymer matrix used. Relative to pure PMMA, a 32% improvement in tensile modulus and a 28% increase in tensile strength were observed in PMMA-based nanocomposites using 1.0 wt% nanotube filler. Contrasting with this, no improvement in mechanical properties was observed in epoxy-based nanocomposites. The poorer mechanical performance of the latter system can be explained by a decrease of the crosslinking density of the epoxy matrix in the nanocomposites, relative to pure epoxy. Indeed we demonstrate that the presence of nanotubes promotes an increase in the activation energy of the curing reaction in epoxy, and a decrease of the degree of curing. Keywords: Multi-wall carbon nanotube; functionalized carbon nanotube; nanocomposites; mechanical properties; activation energy. 1. INTRODUCTION An increasing number of studies reveal that to significantly improve the mechanical properties of polymer-based nanocomposites, carbon nanotube (CNT) dispersion and nanotube-polymer interfacial interaction must be adequately controlled and understood [1 10]. However, the effect of these parameters appears to vary depending on whether the polymer matrix is a thermosetting or a thermoplastic material. Generally speaking, improvements of the mechanical properties seem to be more easily obtained with thermoplastic polymer matrices than with thermosetting matrices. For example, significant nanocomposite stiffness increases arise with nylon-6 To whom correspondence should be addressed. Daniel.wagner@weizmann.ac.il

2 286 L.-Q. Liu and H. D. Wagner as a matrix [11, 12] whereas only marginal improvement or even a decrease in nanocomposite tensile modulus is observed with standard epoxy matrices [13 16]. The quasi-absence of reinforcing ability of nanotubes in epoxy is attributed to the poor dispersion and interfacial adhesion of nanotubes in the matrix. An effective solution to this double problem is to suitably functionalize the nanotube surface [7, 10, 17, 18]. In this paper, the surface of multi-wall carbon nanotubes was functionalized by covalent linking of long alkyl chains. Such functionalization led to a much better tube dispersion in organic solvents than the pristine nanotubes, and favored the formation of homogenous composite films [19, 20]. Moreover, the presence of chemical groups on the nanotubes surface was hypothesized to yield improved interfacial bonding between the nanotubes and two polymer matrices: a thermoset (Epon 828/T-403) and a thermoplastic (PMMA). As will be seen, unambiguous differences in reinforcing ability were observed following the introduction of nanotube in the two polymer matrix types. The poorer mechanical properties of the epoxy-based nanocomposite were linked to the increased activation energy of the curing reaction of epoxy, leading to a relatively lower crosslinking density. 2. EXPERIMENTAL 2.1. Materials The nanotubes used in the present work were multiwall carbon nanotubes (MWNT) from Sun Nanotech (Nanchang, China). They were functionalized (f -MWNT) according to a previously published procedure [19]. The nanotube diameter ranged between 30 and 70 nm. Previous elementary analysis (EA) and thermogravimetric analysis (TGA) measurements have indicated that the nanotube content in functionalized carbon nanotube samples is around 70 wt%. The epoxide resin used in this work was diglycidyl ether of bisphenol A (Epon 828) provided by Kidron Co. (Israel). Glycolitic polypropyleneoxide triamine (Jeffamine T-403) with a molecular weight of 440 was used as curing agent in the preparation of epoxy matrices. PMMA was purchased from Aldrich (M w = 350,000) Epoxy-based nanocomposites The pure epoxy system (Epon 828/T-403, weight ratio: 100/42) was prepared by stirring the liquid mixture for 15 min at room temperature to make it homogenous, followed by degassing in vacuum for 2 h. The mixture was then poured into a dog-bone shaped RTV mold, cured at 80 C for 2 h and postcured at 125 C for 3 h. Since nanocomposites based on this epoxy system must be prepared by dissolving the nanotubes in CH 2 Cl 2, we prepared pure epoxy control specimens by mixing the curing agent T-403 with this solvent, then evaporating it completely before mixing with Epon 828. We ensured that solvent evaporation was indeed complete

3 Strength of MWNT nanocomposites 287 by measuring the glass transition temperature (T g ) and mechanical properties. These showed no significant change, an indication that CH 2 Cl 2 had indeed been completely removed. To prepare 1.0 wt% nanotube-epoxy resin composites, a given amount of f -MWNT samples was first dispersed in CH 2 Cl 2 ; the T-403 curing agent was then added to the solution which was mildly sonicated for 1 h using a Hz bath sonicator. CH 2 Cl 2 was slowly evaporated by continuous stirring, and the mixture was put under vacuum at 40 C for several hours to remove all the residual solvent. A stoichiometric amount of Epon 828 was then added into the mixture, followed by the same steps as previously described to prepare the unreinforced Epon 828/T-403 matrix. The overall weight ratio of f -MWNT/Epon828/ T-403 was 1/25/75, which resulted in 1.0 wt% nanotube-epoxy composites. A similar procedure was used to prepare 0.1 wt% and 0.5 wt% nanotube based Epon 828/T-403 nanocomposites PMMA-based nanocomposites To prepare nanotube-pmma composites, a given amount of f -MWNT was first dispersed in CH 2 Cl 2, then mixed with a volume of PMMA solution in CH 2 Cl 2 and mildly sonicated for 1 h using a Hz bath sonicator. After slowly removing most of the solvent at high temperature using magnetic stirring, the concentrated composites solution was poured into a dish, where the solvent evaporated to leave a film. The film was heated at 40 C under vacuum to let its weight reach an equilibrium value. Pure PMMA specimens were also prepared using this procedure Morphological characterization The morphology of the nanotubes/epoxy composites was investigated using a high resolution scanning electron microscope (SEM, Philips XL-30, The Netherlands) at an accelerating voltage of 5 10 kv. Prior to observation, the fractured surfaces (following the tensile tests, see below) were sputtered-coated with chromium Quasi-static tensile tests Tensile tests of the polymer and nanocomposites were performed with a Rheometric Scientific Minimat tensile tester using a 200 N load cell at a testing speed of 1.0 mm/min. The typical size of the epoxy-based nanocomposites was 1 mm (thickness) 2 mm (width) 20 mm (gauge length). The dimensions of the PMMA-based nanocomposite films were µm (thickness) 2mm (width) 20 mm (gauge length). To ensure data accuracy and repeatability, seven samples of each type were tested Thermal analysis The curing behavior of Epon 828 with T-403 was monitored by differential scanning calorimetry (DSC) in the temperature range of C using a Mettler differential

4 288 L.-Q. Liu and H. D. Wagner scanning calorimeter (DSC 30 connected to a TC11 processor). A sample weight of 11 ± 2 mg was used. To determine the activation energy of the curing reaction of neat epoxy resin and nanotube based composites, dynamic DSC scans were recorded at four different heating rates (i.e. 2.5, 5, 7.5, 10 C/min). Moreover, DSC tests in isothermal mode were conducted at 100 C. Non-isothermal scans were then performed on similar samples to obtain the heat of reaction necessary to complete the cure of the reactive system. 3. RESULTS AND DISCUSSION 3.1. Mechanical properties Typical tensile stress strain curves for epoxy-based and PMMA-based nanocomposites are presented in Fig. 1. The Young s modulus and ultimate tensile strength of both types of nanocomposites are presented in Fig. 2, which demonstrate that (i) the mechanical properties of epoxy-based nanocomposites are not significantly affected, positively or negatively, by the presence of f -MWNT, except possibly for a slight decrease in the failure strain; (ii) the mechanical properties of PMMAbased nanocomposites are significantly and increasingly improved by the presence of f -MWNT. The behavior of epoxy-based nanocomposites containing functionalized nanotubes is in fact not different from previous work with epoxy-based nanocomposites with pristine nanotubes [13, 21]. The immediate conclusion is that the better dispersion of f -MWNT (in various organic solvents) has no effect on the mechanical properties of the present epoxy-based nanocomposites, and the presence of alkyl chains attached on the nanotube surface, which in principle induces improved interfacial bonding, is ineffective in these nanocomposites. This drastically contrasts with the results for PMMA-based nanocomposites, as seen, for which both the tensile modulus and the tensile strength significantly increase upon adding small amounts of f-mwnt, thus confirming earlier data [7, 10 12]. Cooper et al. [22] reported a 25% increase in Young s modulus in PMMA reinforced with 4 wt% (aligned) carbon nanofibril. The f -MWNT based composite studied here exhibits better mechanical performance, which may be attributed to the quality of the nanotube dispersion in the matrix and the better tube-matrix interfacial adhesion. What is the intrinsic cause of such differences in the mechanical properties of epoxy-based and PMMA-based nanocomposites? Why would a given batch of nanotubes, with good dispersion and adhesion capability, give rise to much improved properties in one type of polymer and have no effect on another? Most published works to date indeed attribute this often observed weaker reinforcement effect in thermosetting polymers (such as epoxy) to poor nanotube dispersion and weak interfacial adhesion, leading to tube aggregation and easier tube pull out under an external force [4, 23]. We postulate here that another possibly determining cause of the lack of reinforcing effect in thermosets has its roots in a modification of the macromolecular network structure of epoxy resulting from the addition of

5 Strength of MWNT nanocomposites 289 Figure 1. Typical stress strain curves for (a) pure epoxy and epoxy-based nanocomposites; (b) PMMA and PMMA-based nanocomposites. MWNT into the matrix. Recent work with SWNT and nanofibers [24 26] reveals that the presence of nanotubes may alter the curing process, and a link between this effect and the mechanical behavior of SWNT-based epoxy composites has been seen [26] Differential scanning calorimetry We investigate possible changes in epoxy structure due to the introduction of f - MWNT by means of differential scanning calorimetry (DSC) in the dynamic and isothermal modes, and study the curing reaction of epoxy matrix with and without nanotubes. In the following, the discussion is based on the assumption that the heat evolved during the curing reaction is proportional to the degree of reaction.

6 290 L.-Q. Liu and H. D. Wagner Figure 2. Comparison of mechanical properties of epoxy-based and PMMA-based nanocomposites: (a) Tensile modulus (b) tensile strength. Dynamic DSC scans of pure Epon 828/T-403 at four different heating rates are presented in Fig. 3. The overall heat evolved in the reaction is taken as the average heat of reaction calculated from each thermogram shown in the figure. As seen, the peak exothermal temperature (T p ) shifts to higher temperatures as the heating rate is increased. Similar thermograms were obtained for all epoxy-based nanocomposites. From the data presented in Table 1, the heat of polymerization ( H 0 ) does not vary much with the heating rate and the coefficients of variation of the average heats of polymerization are below 9% in all cases. This justifies the validity of this method to measure the heat of polymerization [27]. As seen, the total heat of reaction H 0 strongly decreases with an increase in the nanotube content, an indication that the reaction mechanism has possibly been

7 Strength of MWNT nanocomposites 291 Figure 3. Dynamic DSC scans of Epon 828/T-403 at four heating rates. Table 1. Heat of polymerization for pure epoxy and epoxy-based nanocomposites, under different heating rates q ( Cmin 1 ) Heat of polymerization H 0 (J g 1 ) Pure epoxy 0.1 wt% f -MWNT 0.5 wt% f -MWNT 1.0 wt% f -MWNT nanocomposites nanocomposites nanocomposites Average value 298 ± ± ± ± 20.3 affected by the presence of the nanotubes. A similar decrease was also reported for poly(etherimide) (PEI) mixed with epoxy [27], for carbon fiber in epoxy resin [28], as well as for in-situ polymerization PMMA in the presence of carbon nanotube [29]. In the latter study, the average molecular weight and the glass transition temperature of the nanocomposite were found to be lower in comparison with pure PMMA prepared under the same condition. As a further step, dynamic mode DSC analysis was used to measure the activation energy of the reaction. Two kinetic models the Kissinger and the Flynn Wall Ozawa methods [30 33] were used to analyze the relation between the activation energy (E a ), the heating rate (q), and the peak exotherm (T p ). According to the Kissinger method, the activation energy is obtained from the maximum reaction rate, where d(dα/dt)/dt is zero at a constant-heating rate. The parameter α is the degree of cure, and is proportional to the heat generated during reaction. The key

8 292 L.-Q. Liu and H. D. Wagner expression resulting from the Kissinger method is as follows: d[ln(q/tp 2)] = E a d(1/t p ) R, (1) where R is the ideal gas constant. A plot of ln(q/tp 2) versus 1/T p provides the activation energy E a. The Flynn Wall Ozawa model provides the following equation: [ ] AEa log(q) = log E a, (2) g(a)r RT p where g(a) is a function of the degree of cure α. Applying these two models results in the linear plots presented in Fig. 4. The slopes of the lines provide the activation energies presented in Table 2. Compared to pure epoxy, the activation energy increases significantly with the weight content of nanotube using both models (the Flynn Wall Ozawa method provides slightly higher values than the Kissinger method). It may be concluded that the reaction between the epoxy and the curing agent is probably hindered by the presence of the nanotubes. This conclusion is further supported by the isothermal DSC analysis presented below. The degree of cure, α, up to time t in the isothermal reaction process can be obtained from isothermal DSC kinetics [25 27, 34]: α = H t /(H + H S ),where H t denotes the evolved heat up to time t,h is the total heat under the isothermal curve, and H s is the residual heat. As shown in Fig. 5, the degree of cure decreases noticeably as a result of the introduction of nanotubes into epoxy. This decrease in the degree of cure is consistent with the data of Table 2: indeed, the reported increase in activation energy does not favor crosslinking of the epoxy network. Furthermore, the glass transition temperature was measured to add to the above argument: the value of T g for pure epoxy is 81 C, and the addition of 0.1 wt% f -MWNT decreases the value down to 74 C. The likely conclusion, as above, is that the crosslink density of epoxy is reduced compared to that of pure epoxy, due to the presence of nanotubes. However, at higher nanotube content, the nanotubes increasingly prevent the free movement of polymer chains [35], an effect that is opposite to the lower crosslinking effect: this is reflected in the slight increase of T g to 77 C for 0.5 wt% nanotube content and to 78 C at 1 wt% nanotube content, respectively. Since a direct correlation exists between the Young s modulus of epoxy and the degree of crosslinking [36], the above result is reflected in the behavior of the modulus (and strength) observed in Fig. 2. The fracture surfaces of the nanocomposites were investigated by SEM. Fig. 6(a) provides evidence for good nanotube dispersion within matrices and strong interfacial adhesion between the nanotubes and epoxy, and the same is observed in the PMMA-based nanocomposites, see Fig. 6(b). In view of this, we therefore conclude that the lower mechanical properties observed in the nanotube-epoxy system has its origin in the lower degree of molecular crosslinking.

9 Strength of MWNT nanocomposites 293 Figure 4. Activation energies obtained by the Kissinger and Flynn Wall Ozawa methods: (a) pure epoxy system; (b) 0.1 wt%; (c) 0.5 wt%; (d) 1.0 wt% f -MWNT epoxy-based nanocomposites. 4. CONCLUSIONS Small amounts of functionalized multi-wall carbon nanotubes were well dispersed in an epoxy resin and a PMMA matrix, respectively. Contrasting properties were observed in these systems: (i) no significant improvement in modulus or strength was obtained for epoxy-based nanocomposites; (ii) a systematic improvement of modulus and strength was observed in the PMMA-based nanocomposites, as a function of the amount of nanotubes, up to 32% increase in tensile modulus and 28%

10 294 L.-Q. Liu and H. D. Wagner Figure 4. (Continued.) Table 2. Activation energies for epoxy and epoxy-based nanocomposites Kissinger (kj/mol) Epoxy 44.9 ± ± wt% f -MWNT nanocomposites 48.7 ± ± wt% f -MWNT nanocomposites 52.4 ± ± wt% f -MWNT nanocomposites 57.2 ± ± 2.5 Flynn Wall Ozawa (kj/mol)

11 Strength of MWNT nanocomposites 295 Figure 5. Degree of cure of the reaction, α, against time at 100 C isotherm for pure epoxy and 1.0 wt% f -MWNT epoxy-based nanocomposites. Figure 6. SEM fracture surfaces of (a) 1.0 wt% f -MWNT epoxy-based nanocomposites, and (b) 1.0 wt% f -MWNT PMMA-based composites. increase in strength, with 1.0 wt% nanotube content. The presence of nanotubes promotes an increase in the activation energy of the curing reaction in epoxy, and a decrease of the degree of curing. As a result, the crosslinking density of the

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