Results and Discussion
4.1 Mechanical properties of unirradiated UHMWPE/MWCNTs composites and UHMWPE/ T blends
4.1.1 Mechanical properties of unirradiated UHMWPE/MWCNTs composites
4.1.1.1 Mechanisms involved for the enhancement of mechanical properties
The reasons for the enhancement of mechanical properties of the composites up to 2 wt. % of MWCNTs can be explained as follows: (a) MWCNTs are known to have extraordinarily high mechanical properties, Treacy et al. [1996], thus the addition of them would obviously enhance the overall mechanical properties of the composites; (b) functionalization of MWCNTs attaches certain chemical groups on their surface, which bond with the polymer and thus aids in stress transfer from UHMWPE to MWCNTs; (c) homogenous distribution of MWCNTs in UHMWPE;
and (d) the addition of MWNCTs increased the crystallinity of the polymer resulting the enhanced mechanical properties, as confirmed by Perepechko [1981]. In order to confirm the above, micro-Raman spectroscopy, rheological percolation and DSC studies were done on the test sample.
Raman spectroscopy has become a first hand and standard tool for the characterization of nanostructures and confirms the stress transfer from the matrix to the filler, Samrutishika [2010].
The Raman spectra of 2 wt. % composites before and after the tensile testing have been studied and shown in Figure 4.3. It is observed that the G- band peak of the test sample before tensile testing was obtained at 1541 cm-1, and it was shifted to 1550 cm-1 for the same sample after tensile testing. The G-band observed between 1500 to 1600 cm-1 denotes the in Plane vibration of sp2 carbon-carbon bonds. When MWCNTs are subjected to either tensile or compressive stresses, the C-C bond vibrations change due to the induced strain, which is reflected in the Raman spectra, Cronin et al. [2004]. It leads to a change in either intensity or position of the G - band. Thus, the observed shift in G-band, shown in Figure 4.3, can be used to qualitatively confirm the effective stress transfer from UHMWPE to MWCNTs. According to Minfang et al.
[2009], the MWCNTs surfaces provided nucleation sites for the crystallization that subsequently
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Raman intensity (arb. unit)
Raman shift (cm-1) a
b
a- Before tensile testing b- After tensile testing
0.01 0.1 1 10 100
10000 100000 1000000 1E7
wt. % of MWCNTs in UHMWPE
0 % 0.2 % 0.4 % 0.5 % 1.0 % 1.5 % 2.0 % 2.5 % 5.0 %
Storage modulus (Pa)
Frequency (rad/s)
enhance the MWCNT–polymer interaction leading to effective stress transfer from matrix to reinforcement. Since a shift in G- band confirms the stress transfer. Thus, the enhancement of mechanical properties of the UHMWPE/MWCNTs composites was achieved.
Figure 4.3. Raman spectra of 2 wt. % UHMWPE/MWCNTs composite: before and after the tensile test
Figure 4.4. Assessment of homogeneous dispersion of MWCNTs in UHMWPE using
The homogeneous dispersion of the MWCNTs is confirmed by estimating the rheological percolation of the composites. The term percolation is defined as the concentration of filler material, where a long range connectivity is exhibited by the filler network in the matrix. In rheological experiments, the property that is typically used for estimating the percolation threshold is the storage modulus, Grady [2011]. Figure 4.4 shows the storage modulus of the composites against angular frequency. In order to obtain more accurate threshold value, two additional composites having 0.2 and 0.4 wt. % MWCNTs were prepared and tested. It is observed that no significant change in the storage modulus was observed upto 0.4 wt. % of MWCNTs. The storage modulus of virgin UHMWPE was found to be 26.7 kPa at 0.015 rad/s, which was slightly increased to 33.7 kPa at 0.4 wt. % of MWCNTs. When the concentration of reinforcement was increased to 0.5 wt. %, a very large change in storage modulus was observed and it was increased to 233.2 kPa which is one order higher. However, the change in storage modulus of the composites was not so significant beyond 0.5 wt. %. In other words, the percolation threshold was found to be at 0.4 wt. % above which a significant change in storage modulus was observed. A lower percolation threshold implies better debundling of reinforcement in the matrix. If it is less than 1 wt. %, then the dispersion at nanoscale is considered to be very good, Du et al. [2004]. The rheological studies carried out by Zhang et al. [2006] obtained a threshold limit at 0.6 wt. % of SWCNTs in UHMWPE, whereas the same was found to be 0.4 wt. % in our case. The variation of threshold value may be due to the fact that it varied significantly with type, size, shape and aspect ratio of the filler.
In order to confirm the increase of crystallinity of the composites, DSC studies were performed on the test sample. Figures 4.5a and 4.5b show the DSC thermograms and the crystallinity of the composites, respectively. It is observed from Figure 4.5a that the melting
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-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0
Weak shoulder peak
DSC (mW/mg)
Temperature (oC) Pure
0.5 % 1.0 % 1.5 % 2.0 % 2.5 % 5.0 %
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 52
56 60 64 68 72 76
% Crystallinity
MWCNTs wt.%
y=4.52x +53.82
temperature of the composites was not affected by the presence of MWCNTs. A weak shoulder peak was also observed indicating the presence of small crystals. It is observed from Figure 4.5b that the crystallinity of virgin UHMWPE was found to be 52.5 %, which was increased to 64.7 % upon addition of 2 wt. % MWCNTs corresponding to an enhancement of 23 %. An increase of crystallinity can be attributed to enhance the mechanical properties of composites. It is interesting to note that although optimum mechanical properties were obtained at 2 wt. % composites, the crystallinity was continued to increase to 67 and 72.3 % at 2.5 and 5 wt. % composite, respectively. The reason for the enhanced crystallinity with an increase of MWCNTs concentration is that the MWCNTs acted as sites for nucleation of crystallization of polymer and thus it enhanced the crystallinity of the composite, Sreekanth et al. [2012]. Although the crystallinity of the composites was increased beyond 2 wt. % of MWCNTs, the mechanical properties were found to be reduced. The reasons for the reduction of mechanical properties of the composites beyond 2 wt. % MWCNTs are discussed in the next section
Figure 4.5. a) DSC thermograms and b) Crystallinity of the UHMWPE/MWCNTs composites
(a) (b)
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20 40 60 80 100 120 140 160 180 200 220
Complex viscosity [kPa.s]
Temperature [oC]
Pure 0.5 % 1.0 % 1.5 % 2.0 % 2.5 % 5.0 %
4.1.1.2 Reasons for the reduction of mechanical properties of composites beyond 2 wt. %