Results and Discussion
4.3 Studies on network parameters and radical scavenging capability of composites and T
4.3.4 ESR studies on composites and UHMWPE/ T blends
The ESR studies were used to quantify the free radicals present in the composites and T
blends. Although MWCNTs and T reacted with the free radicals in the polymer, there will be good amount of unbounded free radicals left over in the polymer. The free radicals that are trapped in the crystalline phases of the UHMWPE slowly diffuse towards amorphous regions and they react with atmospheric Oxygen leading to the formation of peroxy radicals. They abstract a hydrogen atom from the polyethylene chain leading to the generation of further new radicals, Oral et al. [2006], and finally it leads to the shelf ageing induced degradation of the mechanical properties. Figures 4.35a, 4.35b and 4.35c show the ESR spectra of 100 kGy irradiated composites within 10 days, 60 and 120 days after irradiation, respectively. It is observed from the spectra of irradiated samples obtained at different ageing periods that the intensity of the ESR signal is reduced with an increase of MWCNTs concentration. It indicates that the number of free radicals present in the polymer causing the degradation of its
Figure 4.35. ESR spectra of 100 kGy irradiated composites at different periods of ageing a) Within 10 days b) 60 days c) 120 days after irradiation
(a) (b) (c)
0.0 0.5 1.0 1.5 2.0 0
20 40 60 80 100 120 140
10 Days 60 Days 120 Days
Relative intensity
MWCNTs wt.%
335 336 337 338 339 340 341 342
2 wt.%
pure
Field (mT)
mechanical properties reduced with an increase in MWCNTs concentration in UHMWPE.
An asymmetric broad singlet at room temperature serves as an indicator of the presence of peroxy radicals, Jahan et al. [2001]. The number of free radicals present in the test samples at any given time is proportional to the intensity of the spectrum. The spectral splitting factor (g) is evaluated from the equation 3.5 in order to confirm the formation of peroxy radicals because it is a characteristic value for a certain species of free radicals. In the present study, the obtained g- value varied between 2.003 to 2.005 (when H is app. 338.43 mT), where the usual range of ‘g’
for a peroxy radical is 2.001 to 2.005, Pedro et al. [2008].
The experimentally recorded first derivative of ESR spectrum, shown in Figure 4.35, is integrated to obtain the absorption curve. The absorption curve of 100 kGy irradiated composites after 120 days of irradiation is shown in Figure 4.36a. The area under the absorption curve is proportional to the radical concentration present in the test sample. It is observed from Figure 4.36a that the intensity of the absorption curve is reduced with an increase of MWCNTs
Figure 4.36. a) Absorption spectra of the ESR signal obtained from 100 kGy irradiated composites after 120 days of irradiation b) Relative intensity of the irradiated composites at
different periods of ageing
(a) (b)
concentration. The relative intensity of composites at different ageing periods at 100 kGy dose was obtained by normalizing the area under the absorption curve of each composite with that of an unirradiated sample, Suarez et al. [2003] and it is shown in Figure 4.36b. It is observed from Figure 4.36b that the reduction of relative intensity of free radicals present in the composites followed a linear trend upto 1 wt. %, beyond which the reduction rate was negligible. The relative radical intensity of pure UHMWPE after 10 days of irradiation was found to be 12.1, which was reduced to 4.9 at 2 wt. % MWCNTs corresponding to a reduction of 59 %. In case of irradiated pure UHMWPE after 120 days of irradiation, the relative radical concentration was calculated to be 137, and it was reduced to 79 for 2 wt. % composite corresponding to the reduction of 42.3 %. The presence of MWCNTs has effectively restricted the concentration of radicals present in the composites against ageing compared to that of virgin UHMWPE. It can be inferred from the above studies that the presence of MWCNTs reduced the number of free radicals in UHMWPE and thus it helps to restrict the deteriorating effects of oxidation.
The ESR spectra of 100 kGy irradiated UHMWPE/ T blends within 10 days, 60 and 120 days after irradiation are shown in Figures 4.37a, 4.37b and 4.37c, respectively. It is observed from the spectra that the intensity was reduced with an increase of T concentration at any given shelf ageing period. Being an antioxidant material, T reacts with free radicals present in the polymer and thus reduces its radical concentration. The absorption curve of 100 kGy irradiated UHMWPE/ T blends after 120 days of irradiation is shown in Figure 4.38a. Figure 4.38b shows the relative intensity of blends after different periods of irradiation. The relative intensity of pure polymer was found to be 137, which was reduced to 63 and 45 for GI100 T0.1 and GI100 T0.5 after 120 days of irradiation. The relative intensity was reduced exponentially from 0 to T 0.5
334 335 336 337 338 339 340 341 342 343 0.5 % αT
0.3 % αT 0.1 % αT
Field (mT)
334 335 336 337 338 339 340 341 342 343
0.5 % αT 0.3 % αT 0.1 % αT
FIeld (mT)
333 334 335 336 337 338 339 340 341 342
0.5 % αT 0.3 % αT 0.1 % αT
FIeld (mT)
335 336 337 338 339 340 341 342
0.3 % αT
0.5 % αT
Field (mT)
0.1 % αT
0.0 0.1 0.2 0.3 0.4 0.5
0 20 40 60 80 100 120 140
Relative intensity
αT wt. % 10 Days 60 Days 120 Days
wt. % T for 60 and 120 days aged samples. However, there was no significant influence of T
concentration on the relative intensity of the sample within 10 days after irradiation.
A comparison of the relative intensity of radical concentration of 100 kGy irradiated composites and UHMWPE/ T blends after 120 days of irradiation is shown in Figure 4.37. It is observed that the radical intensity of virgin UHMWPE was found to be 137, which was reduced to 79 and 63 for NC2.0 and T0.1 blend, respectively. The relative intensity of NC2.0 and T0.1 was found to be reduced by 43 and 54 %, respectively compared to that of virgin UHMWPE.
Figure 4.37 ESR spectra of 100 kGy irradiated UHMWPE/ T blends at different periods of ageing a) Within 10 days b) 60 days c) 120 days after irradiation
Figure 4.38. a) Absorption spectra of the ESR signal of 100 kGy irradiated UHMWPE/ T blends after 120 days of irradiation b) Relative intensity of the UHMWPE/ T blends at different periods
after irradiation
(a) (b) (c)
(a) (b)
0.0 0.5 1.0 1.5 2.0 0
20 40 60 80 100 120 140
Relative intensity
MWCNTs/ αΤ(wt. %)