Study On The Evaluation Of Carbon Nanotube Dispersion State Based On Fractal Theory

The dispersion state of carbon nanotubes(CNTs) is one of key factors influencing the performance of CNT composites. By far, various methods have been applied to characterize CNTs dispersion state. However, due to difference in working mechanism and measurement limitation of characterization methods, there is no unified view on the evaluation of CNT dispersion state. which hinders the study of reinforcement mechanism thereby restricting the improvement of macroscopic properties of CNT composites. Therefore, working out the evaluation method of CNT dispersion state and establishing the relationship between CNT dispersed state and macroscopic properties of composites have become the most important problem in the fabrication and application of CNT composites. To address the above issues,a large number of samples with different CNT dispersion state were obtained by adjusting disperse methods, disperse parameters and CNT concentration. CNT dispersion state was quantitatively characterized by calculating fractal dimension of CNT dispersion observed at different scales. With fractal dimension as a “bridge”, the relationship between CNT dispersion state and macroscopic properties of CNT composite was explored. Following works were carried out:(1)CNT dispersion with the mass fraction of 0.1wt%, 0.5wt% were prepared by sonication, CNT dispersion state were systematically characterized by SEM, TEM, UV-VIS and rheological property. By testing the electrical, mechanical, thermal properties of CNT/PVA films, we discussed the applicability of different methods used for characterization of CNT dispersion state.(2)For the present difficulties encountered in characterization of CNT dispersion state, a new characterization method based on fractal theory has been proposed. By adjusting CNT concentration(0.1wt-5.0wt%), dispersion methods(sonication, ball milling) and disperse time(30min, 60 min, 90 min, 120min), CNT dispersion with different dispersion state were obtained, we calculated the fractal dimension of CNT dispersion at different scales by box counting method and found that the fractal dimension can quantitatively describe CNT dispersed state. When fractal dimension is calculated by box counting method, the minimum mesh size of digital image should be smaller than the diameter of CNTs and larger than thedistance between CNTs. The minimum mesh size of optical images is out of the defined scale, so the fractal dimension cannot reflect the real CNT dispersion state. While the minimum mesh size of SEM and TEM images are in the defined scale, the fractal dimension can reflect the real CNT dispersion state.(3)When the minimum mesh size of digital images are beyond the defined scale, there is no significant relationship between fractal dimension and properties of CNT/PVA composites. When the minimum mesh size of digital images are in the defined scale, fractal dimension has exponent relation with the electrical conductivity and has linear relation with the tensile strength of CNT/PVA composites respectively. With the increase of fractal dimension, electrical conductivity will show percolation phenomenon, after that, electrical conductivity of CNT/PVA composites increase gradually. Fractal dimension can be used as a "bridge" to make an effective connection between CNT dispersed state and properties of CNT composites.