Effect of absorbed dose

21 The radiolysis of acetone and dimethylformamide were not as widely studied compared to methanol, but it has been established that the amount of radicals created by irradiation of methanol is greater than the amount of radicals from acetone and dimethylformamide radiolysis [10]. This partly explains the higher Dg values calculated for WHF and MCC grafted in methanol based solvents, as shown in Figure 1.3. Based on available literature, the major primary reactive intermediates produced in the gamma irradiation of acetone are solvated electrons and methyl radical [43].

Products of the γ-radiolysis of dimethylformamide suggest that fission of the carbonyl-nitrogen and nitrogen-methyl bonds are the most probable reactions [44].

Solvated electrons react slowly, if at all, with common carbohydrates and the hydrated electron had been found to have low reactivity toward carbohydrates [3, 10].

Water is known to be a good swelling agent for cellulose [35]. Adding water to methanol and acetone solvents affect the balance between the dissolution of GMA by methanol/acetone and the swelling ability of water for cellulose. Methanol and acetone dissolves and facilitate the diffusion of GMA to the surface of MCC. Water on the other hand swells MCC to increase the number of available sites for reactions.

However, the high number of radicals produced in water-based solvents contributed significantly to the formation of large amounts of homopolymers in the grafting mixture during irradiation. The difficulty of removing the homopolymers from the grafted MCC rendered the use of water/methanol solvent impractical; therefore methanol was used as the solvent for the succeeding experiments.

22 kGy. The Dg value could have leveled off at higher absorbed dose, but no attempt was made to irradiate the MCC in the monomer solution beyond 20 kGy because of too much homopolymer formation in the solution. Therefore, subsequent preparation of WHF and MCC were performed at 10 kGy absorbed dose.

Figure 1.4 Effect of absorbed dose on degree of grafting in the radiation-induced graft polymerization of GMA from WHF. Grafting conditions: 5% GMA, 8 kGy hour

-1 dose rate, 1:3 water/methanol, 3 trials.

Figure 1.5 Effect of absorbed dose on degree of grafting in the radiation-induced graft polymerization of GMA from MCC. Grafting conditions: 7% GMA, 8 kGy hour

-1 dose rate, methanol, 2 trials.

0 20 40 60 80 100

0 5 10 15 20 25 30 35

Degree of Grafting (%)

Absorbed Dose (kGy)

0 5 10 15 20 25

0 5 10 15 20

Degree of Grafting (%)

Absorbed Dose (kGy)

23 The amount of free radicals formed by interaction with radiation increases in a linear fashion with absorbed dose and then reaches a certain limiting value at higher absorbed doses. At higher absorbed doses, the degree of grafting tended to level off because of the recombination of some free radicals before they can initiate graft polymerization. The leveling off could be attributed to the fact that at higher degree of grafting values, the polymerization becomes a diffusion-controlled process [45]. The almost constant grafting extent at high absorbed doses may also be attributed either to monomer exhaustion or to the increased viscosity of the bulk of grafting mixture which restricts the glycidyl methacrylate diffusion to the grafter chains.

The number of free radicals created in the grafting mixture increased at higher absorbed dose. The radicals produced upon interaction of radiation with the sample were generated from GMA, trunk polymer (i.e. WHF and MCC) and solvent molecules because the grafting method used in the above discussions was simultaneous grafting. The free radicals from monomer and solvent can react with the trunk polymers to generate additional active sites on WHF and MCC, which lead to the higher Dg. Therefore, grafting of GMA on WHF and MCC, was enhanced at higher absorbed dose. However, larger amounts of homopolymers were also formed when higher absorbed doses were utilized because the monomer molecules were also exposed to γ-radiation.

The dose rate dependence of the degree of grafting was observed in several studies [10, 46]. Table 1.1 shows that decreasing the dose rate from 8 kGy hour^-1 to 1.4 kGy hour^-1 resulted in a 6% increase in Dg. High dose rates produce higher free radical density, which may favor recombination of radicals generated in close vicinity or faster generation of the homopolymer in bulk and this effect may lessen the available monomer available for grafting. However, the decrease in dose rate translated to six additional hours of irradiation time. The small increment in Dg did not compensate the added irradiation time so 8 kGy hour^-1 was the preferred dose rate.

24 Table 1.1 Effect of dose rate on degree of grafting for the radiation-induced graft polymerization of GMA from WHF. Grafting conditions: 5% GMA, 1:3 water/methanol, 10 kGy absorbed dose, nitrogen atmosphere.

Dose Rate Degree of grafting

8 kGy hour^-1 58%

1.4 kGy hour^-1 64%

The achieved Dg values in the grafting of WHF are comparable with the values obtained by other researches that utilized grafted lignocellulosic and cellulosic materials. Sokker et al. (2009), used cotton fabric wastes as polymer backbone in the radiation-induced graft polymerization of GMA. After optimizing the grafting conditions, they were able to achieve around 80% Dg, but the monomer concentration used in their study is four times the optimum concentration in radiation grafting of WHF [18]. In the study of Khan (2005), he achieved 53% Dg in the pre-irradiation grafting of emulsified methyl methacrylate from γ-irradiated jute fibers [47]. Abdel-Aal et al. (2005), performed radiation-induced graft copolymerization of maize starch/acrylic acid, but they did not report the degree of grafting values [48].

Although the Dg values calculated from the grafted MCC were less than the values from grafted WHF and other lignocellulosic materials discussed here, it is already sufficient to change the surface property of MCC, as shown in the wettability test.

Figure 1.6 Effect of absorbed dose on degree of grafting at different reaction times for graft polymerization of GMA from electron beam pre-irradiated APNWF.

Grafting conditions: 5% GMA, 0.5% Tween 20, 200 kGy (^◆), 100 kGy (■) and 50 kGy (●) absorbed doses, 5 trials.

0 100 200 300 400 500 600

0 1 2 3 4

Degree of Grafting (%)

Time (hours)

25 The Dg of grafted polymers increased with increasing absorbed dose in the pre-irradiation grafting of APNWF with GMA monomer (Figure 1.6), similar to the trend observed from the previous discussion of results from simultaneous grafting of GMA onto WHF. The trunk polymer contains abaca fibers, which are inherently tough even in its processed form. The interaction of electron beams with APNWF resulted in the production of free radicals. Higher absorbed dose yielded higher free radical concentration that translated to higher number of initiation sites for graft polymerization. Figure 1.6 shows the relationship between Dg and grafting time at different absorbed doses. It can be observed from Figure 1.6 that at a fixed reaction time, higher absorbed dose gave APNWF-g-PGMA with higher Dg. APNWF exposed to a pre-irradiation dose of 200 kGy gave a 303% Dg after an hour of reaction with 5% GMA emulsion and this increased to 550% after 4 hours of reaction time.

However, the APNWF-g-PGMA obtained at these conditions was brittle because of the very high amount of PGMA graft chains. The brittleness made the handling of the grafted APNWF difficult for the subsequent processes. With an absorbed dose of 100 kGy, the Dg increased from 207% to 252% when the reaction time was increased from 1 to 4 hours. At both absorbed doses, prolonged contact of the irradiated APNWF with the GMA emulsion resulted in higher Dg. This can be attributed to the fact that an increase in reaction time allows more GMA molecules to react with the active sites (i.e. free radicals initiation sites) on the surface of irradiated APNWF.

Prolonged contact with the GMA emulsion also enhanced the propagation of the PGMA chains, resulting in higher Dg.

According to Sekine et al. (2010), a Dg greater than 100% is preferable for synthesis of a metal ion adsorbent via chemical modification [36]. APNWF exposed to 50 kGy irradiation dose gave 93% Dg after an hour of contact with the GMA emulsion. Similar to the results observed before, the Dg increased with increasing grafting time. An almost constant Dg value of 180% was achieved after 3 hours of reaction time and this is sufficient for preparation of metal ion adsorbent. A 180% Dg corresponds to an epoxide group density of 4.5 mmol/gram-adsorbent. The grafted APNWF obtained at these conditions was more flexible compared to those exposed at higher irradiation dose. Attaining a sufficient Dg after irradiation at a low dose of 50 kGy is valuable for development of a metal ion adsorbent. It was found that irradiation at higher doses results in degradation of the molecular chain of the

26 polymeric material that leads to easy fiber-breaking [28]. Irradiation at lower absorbed dose also gives economic advantage for the process.