The trend in the activation energy variation noted for the degradation of the material CBMN1 is very similar to that of CBM but the activation energy at the initial stage is lower than the CBM but after α = 0.40, the activation energy value is 30-40 kJ mol-1 higher than CBM. At the initial stage the nanosilica particles may act as medium for thermal transport for the degrading material, thus requiring lower activation energy for degradation. The increase in activation energy value after α = 0.40 may be attributed to the shielding effect of nanoparticle in the evolution of formed gases from polymer matrix, during its thermal decomposition [41].
CBMN2 also shows similar trend as that of CBMN1, but its activation energy values are lower than CBM and CBMN1 up to α = 0.60, but afterwards the value reaches a higher region compared to both the systems. At the initial stage the catalytic activity exhibited by NSOM particle makes the degradation of CBMN2 as an easier one; as a result the Ea values are lower. Further, the degradation of the long alkyl chains present in the NSOM particles may also add to this factor. But after 0.60 reaction extent value the Ea value get increased this may due to the restriction of mobility of the segmental movement of degrading chains exhibited by the enhanced interaction between the silica and the polymer matrix.
The trend shown by CBMN3 is completely different from the other three systems. The activation energy values are present in the lower region. In the chemically modified nanosilica particles incorporated BMIM system, the maleimide units chemically bound with the silica units is having the possibility to polymerize with the maleimide double bonds present in the BMIM monomer. The chemically modified silica particles act as spacers and this bulky spacer invariably reduces the crosslinking density in the cured BMIM (CBMN3). Due to this aspect, during the degradation there will be enhanced motion of the chain segments favoring easy degradation [42].
Conclusions
BMIM was prepared by the classic amic acid approach. Nanosilica particles were obtained in a very economical way from the largest available agricultural waste rice husk, and organic and chemical modifications were done in the nanosilica particles. The interaction existing between the nanosilica particles and BMIM was confirmed by FTIR studies. The curing studies of BMIM and its nanosilica blends were done by DSC analysis. Nanosilica and modified nanosilica particles decrease the melting point, curing temperatures and enthalpy of curing of BMIM. This decrease in curing parameters may be due to the catalytic activity exhibited by the NS and modified NS particles, which can accelerate the curing process. The degradation temperatures of BMIM/nanosilica nanocomposites determined by TGA were lower than that of pure polyBMIM matrix. The thermal degradation products produced from the modified NS particles induced catalytic activity on the thermal degradation of polyBMIM matrix. This leads to decrease in the degradation temperature. The curing kinetics of BMIM and its nanosilica blends and the degradation kinetics of polyBMIM and its nanosilica nanocomposites were calculated using three model free kinetic methods KAS, FWO and FRD methods. The Ea values for the curing of NS reinforced BMIM increased as the extent of polymerization increased, but NSOM and NSCM reinforced BMIM show higher Ea values at the initial stages of curing but as the curing proceeds the Ea values decreased because of the catalytic activity exhibited by NSOM and NSCM particles. The trend in the variation of Ea values for the degradation of BMIM/nanosilica nanocomposites was more or less similar up to the reaction extent value 0.45.
Afterwards, NS, NSOM particles restrict the molecular segmental movement of polyBMIM, leading the increase in Ea values. For NSCM reinforced system, the Ea values decreased because of physical and chemical changes took place in the chemical moieties present it. The SEM images of modified nanosilica particles reinforced BMIM matrix show broken surfaces and rough fractured strips indicating the improvement in the toughness of the nanocomposites.
Acknowledgements
The authors wish to express sincere thanks to the management and principal of the respective colleges for providing all facilities to do the work. The authors would like to thank the Directorate of Extramural Research & Intellectual Property Rights (ER&IPR), Defence Research & Development Organization, Ministry of Defence, Government of India, New Delhi-110 105 for financially supporting this work under the grant ERIP/ER/0704359/M/01/1101 dated 12-12-2008.
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