Quantitative analysis of grafted CNT dispersion and of their stiffening of polyurethane (PU)

Electroactive devices are developed for energy conversion purposes. In particular, polyurethanes (PU) are lightweight and flexible materials, which have demonstrated their ability to convert electrical energy into mechanical energy (actuation by electrostriction) and vice-versa (energy harvesting). It has been shown that energy conversion efficiency can be increased by incorporating carbon nanotubes (CNTs) into a PU matrix. The counterpart of this improvement is the stiffness increase, which in turn limits the electrostriction efficiency. On the other hand, it is well known that CNTs are hardly dispersed in a polymeric matrix, and that the interfacial adhesion strength is generally poor. One solution to improve both dispersion and adhesion consists in grafting polymeric chains onto the CNT surfaces. As most of the works dedicated to improve material electroactivity are mainly empirical, this work aims to (i) better characterize these material microstructures by electron tomography, through the measurement of the CNT tortuosity, the CNT-CNT minimum distance and the number of their contacts, and (ii) and to predict their mechanical stiffness from these microstructural data. From electron microscopy observations of the studied materials, CNTs can be assumed to be composed of successive stiff rods of measured length and orientation, linked together by flexible kinks. Their mechanical stiffening effect in PU is, simply and in an original way, evaluated using the classical analytical equations derived by Halpin and Kardos, accounting for the microstructural parameters determined by electron tomography. It appears clearly that, due to their tortuosity and despite their ultra-high longitudinal stiffness, CNTs only poorly stiffen soft matrices. Fully stretching 10 μm long nanotubes increases the composite modulus by almost 10 for a fraction of only 2 vol%.