Nucleation and antinucleation effects of functionalized carbon nanotubes on cyclic and linear Poly(e-caprolactones)

Abstract

Novel nanocomposites were prepared by blending linear or cyclic poly(e-caprolactones) with two types of chemically modified carbon nanotubes (CNTs). The low-polydispersity cyclic PCL samples (C-PCLs) were synthesized by click chemistry with a number-average molecular weight (M n) of 22 kg/mol. Linear analogues (L-PCLs) with the same M n value were also prepared. Two types of CNTs were employed (with 1% w/w content): single wall CNTs functionalized with octadecylamine (SWNT-ODA) and multiwall carbon nanotubes grafted with linear PCL chains (i.e., MWNT-g-PCL prepared by ring-opening polymerization on previously functionalized MWNTs with a composition of 10% MWNT and 90% L-PCL). The nanocomposites were characterized by transmission electron microscopy (TEM), polarized light optical microscopy (PLOM), and differential scanning calorimetry (DSC). A nucleating effect was detected in both PCLs when SWNT-ODAs were employed. However, in the case of MWNT-g-PCL, the nanofiller nucleated L-PCL but caused an unexpected antinucleation effect on C-PCL. Another interesting behavior displayed by this novel C-PCL/MWNT-g-PCL nanocomposite (composed of 90% C-PCL, 9% L-PCL, and 1% MWNTs) was not only a reduction in nucleation density and in Tc temperatures during cooling from the melt, as expected for an antinucleating agent, but also a decrease in spherulitic growth rate and in overall isothermal crystallization kinetics as compared to C-PCL. The results were explained by realizing that new topological effects were created upon mixing the grafted L-PCL chains within MWNT-g-PCL with C-PCL molecules. When these linear chains come into contact with cyclic PCL chains, a threading effect is produced that dramatically affects chain dynamics by forming a transient entanglement network. As a consequence, cyclic molecules relax and diffuse more slowly than anticipated, decreasing both nucleation and growth kinetics. Results on linear and cyclic PCL blends are also presented here, and they support our explanation of the unexpected antinucleation effect reported for C-PCL/MWNT-g-PCL nanocomposites.