Material Ireland Polymer Research Centre, Dept. of Physics, Trinity College .... H. Doshi, M. Srinivasarao, J. O. Park and D. A. Schiraldi, Polymer, 43, 1701.
Mechanical and Thermal Properties of CNT and CNF Reinforced Polymer Composites M. Cadek1, B. Le Foulgoc1, J. N. Coleman1, V. Barren1, J. Sandier2, M. S. P. Shaffer2, A. Fonseca3, M. van Es4, K. Schulte5 and W.J. Blau1 Material Ireland Polymer Research Centre, Dept. of Physics, Trinity College Dublin, Ireland. Department of Materials Science & Metallurgy,University of Cambridge, Cambridge CBQZ,U.K. 3 Lab. de Resonance Magnetique Nucleaire., Fac. Univ. N.D. de la Paix, rue de Bruxelles 61,5000 Namur, Belgium. 4 DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands. 5 Polymer Composite Group, Technical University of Hamburg Harburg, 21073 Hamburg, Germany. 2
Abstract. In this research study carbon nanotubes and carbon nanofibres were investigated as possible reinforcements to improve the mechanical and thermal properties of several polymer matrix systems. A range of polymer matrices were examined and include polyvinyl alcohol, poly(9-vinyl carbazole) and polyamide. To compare production methods, polymer composite films and fibres were produced. It was found by adding various mass fractions of nanofillers, that both the Young's modulus and hardness increased dramatically for both films and fibres. In addition, the thermal behaviour was seen to be strongly dependent on the nanofillers added to the polymer matrices.
INTRODUCTION Over the last 50 years the use of polymer composites has been exploited in many applications from biomedical devices to aerospace applications because of their excellent physical properties such as high strength to weight ratio. Today nanotube based polymer composites are at the forefront of technological developments and present an exciting challenge because of their unique mechanical and electrical properties. Their high Young's modulus of 1-2 TPa[l] makes them promising candidates for reinforcement applications, but there are still many problems that must be overcome. In this research, mechanical properties such as Young's modulus and hardness are investigated for a number of different carbon nanofilled polymer composites.
EXPERIMENTAL The multi walled carbon nanotubes used in this study were produced by the arc discharge method[2, 3] in our laboratory, and subsequently dispersed in matrices of polyvinyl alcohol (PVA) and polyvinyl(9-carbazole) (PVK) to form composite
CP633, Structural and Electronic Properties of Molecular Nanostructures, edited by H. Kuzmany et al. © 2002 American Institute of Physics 0-7354-0088-l/02/$19.00
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materials. De-ionized water was used as the solvent for PVA and dichloromethane was used in the case of PVK. The concentration of polymer in solution was 30 g/L in each case. The polymer was dissolved in solvent using a high power sonic tip for 1 minute. Untreated 'Kraetschmer'-generated soot [2, 4] was added to the polymer solutions in various ratios and sonicated for 1 minute using a high power sonic tip. This was followed by a gentle sonication for 2 hours in a low power (60 W) sonic bath to ensure good dispersion of the nanotubes and homogeneity of the solutions. Afterwards, the solutions were left for 24 hours to allow any impurities present to form a sediment on the base of the sample container. For each sample the resulting suspension was separated from the sediment by decantation[5, 6]. True nanotube mass fractions were measured by thermogravimetric analysis and found to range between 1% and 10%. In order to produce films for mechanical testing Poly(m-phenylenevinylene-co-2,5dioctyl-/?-phenylenevinylene) (PmPV) and PVA solutions were drop cast onto a glass substrate to allow film formation by evaporation of the solvent in a 60°C heated oven. PVK samples were spin coated using a spin speed of 1500 min"1 and a spinning time of 1 minute. Several layers had to be spun to obtain a film thickness of >lum. In between spinning the layers, the samples were stored in an 80°C heated oven for 30 minutes to allow reorientation of the polymer chains and better interfacial bonding between the layers. The film thickness was measured using white light interferometry. In addition several polymer fibres reinforced with different nanofillers were examined. Arc-discharge grown multi walled carbon nanotubes (AGMNT) produced in our laboratory at Trinity College Dublin, aligned (acgMNT) (produced at the University of Cambridge) and entangled (produced at the University of Namur) catalytic grown multi walled carbon nanotubes (ecgMNT)[7] were used. For comparison commercially available carbon nanofibres (CNF) were also studied. Different mass fractions of nanofillers ranging from 1.5% to 15% by mass were mixed with the polymer matrices. Poly amide 12 (PA 12) was chosen as the polymer host. The fibre production process involved the dispersion of the nanofillers in the polymer matrices and re-melting of the polymer composite. The dispersion of the nanofillers in the polymer host was made in a double screw micro extruder[8, 9]. The temperature of the mould was 220°C while the residence time in the mould was 5 minutes at a screw speed of 80 revolutions per minute. In the case of the PA 12, the granules were dried for 12 hours in an 80°C heated vacuum oven at 50 mbar. After the mixing process the composite strand was extruded and cooled down to room temperature. The spinning of thin fibres was carried out using a melt flow index tester (MFT) and a combined take-off and wind-up unit. Five grams of the nano-filled polymer composite was re-melted in a classical MFI with a plunger 10 mm in diameter and a speed of advance of lOmm/min. Extrusion of the polymer melt followed through a 0.1 mm hole where high orientation of the nanotubes occurred. A stripper roller (50 mm in diameter) was used to convey the extruded strand off. The haul off speed was300rpmforPA12. The mechanical properties of the polymer fibres were measured by mounting them in a polymer support matrix. After curing of the support matrix, the samples were ground and polished. Values for hardness and Young's modulus were obtained by nano-hardness testing(HNT) [10]. Ten indentations were made and averaged for each
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sample. The maximum indentation force was 0.5 mN with a loading/unloading rate of ImN/min for the polymer films and 100 mN indentation force with a loading/unloading rate of 200mN/min for the polymer fibres. The thermal properties were examined using thermo gravimetric analysis (TGA). lOmg of either polymer film or fibres were used for these measurements. The heating rate was 10 K/min for a temperature range of 25-1000°C. The PA12 samples were held at 100°C for 1 hour prior to thermal analysis to remove any water present.
RESULTS & DISCUSSION Significant increases in both hardness and Young's modulus were observed for both nanotube reinforced polymer samples. For the PVA based composite, an increase from 7 GPa to 12 GPa and 300 MPa to 470 MPa was recorded for Young's modulus and hardness values respectively with a 5wt% nanotube loading level. Similar results, as shown in Figure 1, were observed for the PVK based composites. In this case the hardness increased linearly from a value of 300 MPa for pure PVK to 600 MPa for the 5wt% sample before levelling off. Similarly, Young's modulus increased from 2 GPa for the pure polymer to 6 GPa for the 8wt% sample. Hardness of PVK / AGMWNT
I
Young's modulus of PVK / AGMNT
, ^
I 500
1
^
I
i
400 I
E
0
2
4
6
8
10
2
nanotube content [%]
a)
4
6
8
10
nanotube content [%]
b)
FIGURE 1. a) hardness increase of 100 % for Polyvinyl-(9-)carbazole by adding 8 weight-% of carbon nanotubes b) Young's modulus increase of 200% for Polyvinyl-(9-carbazole) by adding 8 weight-% of carbon nanotubes.
This relatively large improvement in the mechanical properties of the above polymers on the introduction of nanotubes suggests a relatively good dispersion of nanotubes in the polymer matrices. In addition these results are suggestive of good interfacial bonding between polymer chain and nanotube. At present, further work is underway to understand the nature of the polymer nanotube interaction. A similar trend was also observed for the acgMNT reinforced polyamide fibres presented in Figure 2. The Young's modulus increased from 1.08 GPa for the pure polymer to 1.7 GPa for the 5wt% sample. In addition the hardness increased from 100 MPa for the pure polymer to 150 MPa for the 5wt% sample over the same loading range.
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Hardness of Polyamid 12 / acgMNT
2 |
Young's modulus of Polyamide 127 acgMNT
2, w
130
1,6
120
•§
110
X
100
1,2
O
90 1
2
3
1,0
4
1
a)
2
3
4
massfraction[%]
mass fraction [%]
b)
FIGURE 2. a) hardness increase of 66% for polyamide by adding 100 weight-% of carbon nanotube containing soot b) Young's modulus increase of 70% for polyamide by adding 100 weight-% of carbon nanotube containing soot.
CONCLUSIONS The mechanical properties of nanotube reinforced polymer films have been studied by nano-indention. In all cases significant increases in both Young's modulus and hardness were observed. In the case of poly(9-vinylcarbazole) based composites the Young's modulus and hardness increased by 200% and 100% respectively. As far as the authors are aware is this is the highest increase in mechanical properties observed when using carbon nanotubes as a reinforcement material.
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