Competition between supernucleation and plasticization in the crystallization and rheological behavior of PCL/CNT-based nanocomposites and nanohybrids

Abstract

PCL was blended with pristine multiwalled carbon nanotubes (MWCNT) and with a nanohybrid obtained from the same MWCNT but grafted with low molecular weight PCL, employing concentrations of 0.25 to 5 wt % of MWCNT and MWCNT-g-PCL. Excellent CNT dispersion was found in all samples leading to supernucleation of both nanofiller types. Nanohybrids with 1 wt % or less MWCNTs crystallize faster than nanocomposites (due to supernucleation), while the trend eventually reverses at higher nanotubes content (because of plasticization). Rheological results show that yield-like behavior develops in both nanocomposites, even for the minimum content of carbon nanotubes. In addition, the MWCNT-g-PCL family, when compared with the neat polymer, exhibits lower values of viscosity and modulus in oscillatory shear, and higher compliance in creep. These rheological differences are discussed in terms of the plasticization effect caused by the existence of low molecular weight free and grafted PCL chains in the nanohybrids.

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

“…This result indicated that the CNTs supernucleation could be dominated by (i) the strong interaction between the polymer and the MWCNTs and (ii) the dispersion quality of the blend. A similar or better supernucleation effect was reported when PCL was blended with PCL grafted MWCNTs (MWCNTs-g-PCL) [69]. The supernucleation effect of the MWCNTs was the result of an excellent dispersion of the MWCNTs in the polymer matrix.…”

“…Generally, neat PCL revealed Avrami index values of approximately n = 3. This indicates that (i) the crystal geometry is spherulitic and follows an athermal nucleation which leads to spherulites of roughly the same size during isothermal crystallization, and that (ii) the nucleation is very fast (instantaneous), and the nucleation starts almost immediately after the isothermal crystallization temperature was reached [47,66,69]. Trujillo et al [66] reported lower Avrami index values (approximately n = 2) for PCL/CNT nanocomposite systems compared to the n = 3 for neat PCL, corresponding to the formation of axialites.…”

“…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]. In order to evaluate the efficiency of the MWCNTs as nucleating agents, it is necessary to compare their effect with that of the PCL self-nuclei or self-seeds.…”

“…The efficiency of the MWCNTs as nucleating agents for PCL was calculated according to Equation (11), which was proposed by Fillon et al [71]: (11) where T c,NA is the peak T c value determined from a non-isothermal DSC cooling run for a sample of the polymer with the nucleating agent (NA), T c,PCL is the peak T c value for neat PCL after its crystalline history has been erased (also determined from a non-isothermal DSC cooling scan), and T c,max is the maximum peak crystallization temperature determined after neat PCL has been self-nucleated at the ideal selfnucleation temperature or T s,ideal (i.e., the self-nucleation temperature that produces maximum self-nucleation without any annealing) [71,72]. Müller and coworkers [66,69] first described supernucleation effects for PCL/CNT nanocomposites. Supernucleation refers to the nucleation action that is better than that provided by the polymer's own crystals.…”

“…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.…”