Nucleation and Antinucleation Effects of Functionalized Carbon Nanotubes on Cyclic and Linear Poly(ε-caprolactones)

Abstract

Novel nanocomposites were prepared by blending linear or cyclic poly(ε-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 T c 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.

Review on PCL, PBS, and PCL/PBS blends containing carbon nanotubes

“…In most of the kinetic studies [50,66,69] on the crystallization of PCL/CNTs nanocomposites there were three general observations: (i) The crystallization temperature range for neat PCL was lower than those for the nanocomposites. This was an indication that the supercooling needed for the development of isothermal crystallization in the nanocomposites is much lower than that for neat PCL in view of the nucleation effect that they cause on the PCL matrix; (ii) The overall crystallization rate increased with MWCNTs loading for the systems explored, which was probably due to the nucleation effect that accelerated the primary nucleation and contributed to increasing the overall crystallization kinetics; (iii) The temperature dependence of the overall crystallization rate was not the same [46,50,66,69]. However, in study [46] the crystallization temperature range for C-PCL was higher than that of the C-PCL/MWNT-g-PCL nanocomposite, and the nanocomposite exhibited a decrease in overall crystallization rate compared to C-PCL.…”

“…A characteristic of this behaviour is a large increase in T c . Some authors did, however, encounter antinucleation effects when the interaction between the CNTs' surfaces and the polymer was very poor [46]. Figure 3 summarizes the shifts in non-isothermal crystallization temperatures from that of neat PCL after introduction of CNTs, and the reasons for the differences observed in this figure are discussed in the next paragraphs.…”

“…In previous research [46,60,64,[67][68][69], different functional groups such as N-methylpyrrolidine, carboxyl and hydroxyl, urethane, phenylmethanol, primary amine groups and 2-hydroxyethylbenzocyclobutene were used to functionalize MWCNTs. In some cases, the MWCNTs were not modified [40,50,66,69].…”

“…Not all the literature reported increases in T c for the PCL/CNT nanocomposites, because the efficiency of a nucleating agent depends on, among other factors, the interaction between the polymer and the CNT surface, and the quality of the dispersion and distribution of the particles. Pérez and co-workers [46] reported that MWCNTs, functionalized with 2-hydroxyethyl benzocyclobutene (BCB-EO) through a Diels-Alder cycloaddition reaction, nucleated linear PCL (L-PCL), but showed an antinucleation effect in cyclic PCL (C-PCL). This was due to the reduced diffusion of C-PCL induced by the threading effect induced by the L-PCL molecules grafted to the MWCNTs, which opposed the nucleating effect of the MWCNTs.…”

“…The ultimate properties of the nanocomposites are dependent on the processing methods and processing conditions. Most CNTs/polymer nanocomposites were processed using the following methods: melt blending, solution mixing and in situ polymerization [43,46,47]. Melt blending is one of the most economical and environmentally friendly methods of fabricating composites.…”