Thermal Fractionation and Isothermal Crystallization of Polyethylene Nanocomposites Prepared by in Situ Polymerization

Abstract

Nanocomposites of high-density polyethylene (HDPE) and carbon nanotubes (CNT) of different geometries (single wall, double wall, and multiwall; SWNT, DWNT, and MWNT) were prepared by in situ polymerization of ethylene on CNT whose surface had been previously treated with a metallocene catalytic system. In this work, we have studied the effects of applying the successive self-nucleation and annealing thermal fractionation technique (SSA) to the nanocomposites and have also determined the influence of composition and type of CNT on the isothermal crystallization behavior of the HDPE. SSA results indicate that all types of CNT induce the formation of a population of thicker lamellar crystals that melt at higher temperatures as compared to the crystals formed in neat HDPE prepared under the same catalytic and polymerization conditions and subjected to the same SSA treatment. Furthermore, the peculiar morphology induced by the CNT on the HDPE matrix allows the resolution of thermal fractionation to be much better. The isothermal crystallization results indicated that the strong nucleation effect caused by CNT reduced the supercooling needed for crystallization. The interaction between the HDPE chains and the surface of the CNT is probably very strong as judged by the results obtained, even though it is only physical in nature. When the total crystallinity achieved during isothermal crystallization is considered as a function of CNT content, it was found that a competition between nucleation and topological confinement could account for the results. At low CNT content the crystallinity increases (because of the nucleating effect of CNT on HDPE), however, at higher CNT content there is a dramatic reduction in crystallinity reflecting the increased confinement experienced by the HDPE chains at the interfaces which are extremely large in these nanocomposites. Another consequence of these strong interactions is the remarkable decrease in Avrami index as CNT content increases. When the Avrami index reduces to 1 or lower, nucleation dominates the overall kinetics as a consequence of confinement effects. Wide-angle X-ray experiments were performed at a high-energy synchrotron source and demonstrated that no change in the orthorhombic unit cell of HDPE occurred during crystallization with or without CNT.

Preparation and characterization of nanocomposites based on COOH functionalized multi-walled carbon nanotubes and on poly(trimethylene terephthalate)

“…The decrease of the dielectric strength (reduction of the relaxing material) is caused by crystal formation due to the progressive transfer of the amorphous material to the growing crystalline phase [34,[56][57][58]. For neat PTT and PTT/MWCNTs nanocomposites it is observed a decrease in the intensity of the !-relaxation process, characterized by the value of the dielectric loss at the maximum ($# max ) and a positional shift of the maximum of the !-relaxation towards lower frequencies with increasing crystallization time in agreement with previous results [55][56][57][58]. [58].…”

“…The topological confinement of macromolecular chains at their interfaces and strong interactions between them can increase of supercooling. Recent study on crystallization behaviour of nanocomposites based on high density polyethylene and CNTs performed by Müller and coworkers [54,55] have shown that in nanocomposites a competition between super-nucleation and confinement occur. At low contents the nucleation effect dominates.…”

“…Regarding the analysis of the dynamics of the amorphous phase during isothermal crystallization, the segmental motions of the amorphous polymer chains are confined between the crystalline regions thus modifying the main !-relaxation. It was established for many polymers, that the occurrence of crystallinity in a polymer affects the segmental dynamics, as characterized by DS, in the following way: i) a decrease of the dielectric strength, ii) a shift of the relaxation rate to lower values and iii) a considerable broadening of the relation function [34,[55][56][57][58]. The dielectric strength of a relaxation process is, in a first approach, proportional to the total amount of relaxing units participating in the process.…”

Effects of carbon nanotubes on polymer physics

“…129 However, another study, also with metallocene-grafted HDPE, found a significant increase in the crystallization rate while also finding a substantial decrease in the Avrami exponent n. This study also found significantly thicker crystals as measured by a successive self-nucleation and annealing thermal fractionation technique performed in a DSC. 260 Both studies found that the fractional crystallinity initially increased to $10% nanotube content, then decreased as the nanotube content increased above 10%. Studies on covalently bonded polycaprolactone and polyethylene oxide also found a significant increase in overall crystallization rate with essentially no decrease in n in the former case and a decrease in the latter case.…”

Effect of Multi Wall Carbon Nanotube Content on The Electrical and Rheological Properties of Polypropylene-based Nanocomposites

“…Some techniques were using compatibilizer such as PP-g-MA [6], in-situ polymerization of polyolefin with CNT [7,8], and by modification of the CNT's surface [9].…”