Crystallization of poly(ε-caprolactone)/MWCNT composites: A combined SAXS/WAXS, electrical and thermal conductivity study

Abstract

In situ crystallization of poly(ε-caprolactone) (PCL) filled with different contents (0.2–5 wt%) of multiwalled carbon nanotubes (MWCNTs) was investigated in X-ray (SAXS/WAXS) synchrotron experiments simultaneously with thermal and electric conductivity measurements. The combined study provides information on nucleation ability of MWCNT, crystallization and melting kinetics, degree of crystallinity as well as the evolution of thermal diffusivity and electrical conductivity of PCL/MWCNT composites during isothermal and non-isothermal crystallization.In both, non-isothermal and isothermal experiments, MWCNTs act as strong nucleation agents. Two separate melting peaks in calorimetric experiments indicate (i) nucleation by the nanotubes and (ii) heterogeneous nucleation by the polymer itself. WAXS shows identical local crystal structure (unit cell) for both and a similar degree of the total crystallinity independent of thermal treatment and relative fraction. The crystallization kinetics of crystals nucleated by MWCNT is much faster and the crystallization and melting temperatures are higher than those for the neat PCL. By isothermal crystallization at higher temperatures (here: T = 327 K) the heterogeneous nucleation of the polymer itself can be suppressed and in the length scale of SAXS experiments no indication of periodic lamella stacks was detected. Electrical conductivity measurements performed simultaneously with SAXS/WAXS during isothermal crystallization showed for MWCNT contents above 0.5 wt% an increase in electrical conductivity by more than one order of magnitude, which can be related to the formation of a conductive nanotube network. At the same time, thermal conductivity of the composite is increasing only slightly, which is rather related to the increase of the crystalline phase than to heat transfer in a percolated structure. The dependence of thermal conductivity on MWCNT content can be related to the addition of filler with high thermal conductance and does not show a simple correlation to the electrical conductivity.