Role of matrix crystallinity in carbon nanotube dispersion and electrical conductivity of iPP‐based nanocomposites

Abstract

A homopolymer iPP and a series of propylene‐ethylene random copolymers with a content of ethylene from 7 to 21 mol % were used as matrices to prepare single‐walled carbon nanotube (SWCNT) nanocomposites in a range of SWCNT concentration from 0.15 to 1 wt %. The solution blending and melt‐ compression molding procedures were kept identical for all nanocomposites. The poly(propylenes) have crystallinities ranging from 70 to 10%, and serve to test the role of SWCNTs acting as nucleants to preserve in the nanocomposites the uniform dispersion of SWCNTs after sonication. The major role of polymer crystallinity is to mediate toward a more open and more connected SWCNT network structure. Fast nucleation and growth of high crystalline matrices on multiple sites along the surface of the nanotubes prevents SWCNT clustering, and entraps the SWCNT network between the semicrystalline structure reducing the driving force of nanotubes to curl and twist. Depletion of crystallites in the less crystalline matrices (<35% crystallinity) leads to curled and poorly connected nanotubes. A consequence of the gradual loss of SWCNT connectivity is a decreased electrical conductivity; however, the change with crystallinity is not linear. Conductivity decreases sharply with decreasing crystallinity for SWCNT contents near the percolation region, while for contents approaching the plateau region the electrical conductivity is less sensitive to matrix crystallinity. The percolation threshold decreases rapidly for polymers with <∼30% crystallinity and slowly levels off at crystallinities >∼40%. At SWCNT concentrations of 0.15 wt %, SEM images of nanocomposites with the highest crystallinity matrix indicate debundled and interconnected nanotubes, whereas more disconnected and curled SWCNTs remain in the lowest crystallinity nanocomposites. Electrical conductivity in the former is relatively high, whereas the latter are insulators. Also discussed is the nucleating effect of nanotubes and restrictions of the filler to polymer chain diffusion in the crystallization of the polymers. SEM images and Raman spectra in the radial breathing modes region (100–400 cm−1) are complementary tools to extract the quality and details of the SWCNT dispersion in the nanocomposites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2084–2096, 2010

Effects of carbon nanotubes on polymer physics

“…DSC experiments were used to justify an increase in crystal thickness for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) 225 as well as in polypropylene and ethylene-propylene copolymers. 123 Peaks in small-angle scattering experiments generally shift towards lower angles indicative of thicker crystals with the introduction of nanotubes. 222,226 However, a very detailed analysis of small-angle scattering patterns for polylactide found a decrease in crystal thickness with the introduction of nanotubes 202 while another study also found a decrease in thickness for polybutylene succinate.…”

“…244 In a case where random ethylene-propylene copolymers were used with matched molecular weights and synthesized in the same manner, the increase in crystallization temperature in nonisothermal studies was much smaller in a sample that had a more disordered repeat unit structure; the crystallization temperature of the unfilled copolymer was almost 100 C less than the unfilled homopolymer. 123 In a very interesting study where the only variable changed was polypropylene molecular weight, the crystallization temperature in nonisothermal experiments varied widely depending on the molecular weight. There was no obvious dependence on molecular weight, and in fact the nanotube-nucleated samples all had pretty similar crystallization temperatures; it was the pure PP that had differing temperatures.…”

Thermal Conductivity and Interconnectivity of Hexamethylene Diisocyanate Contained Polyurethane Grafted Multiwall Carbon Nanotube/Polyurethane Nanocomposite Film

“…Brooks et al reported that matrix crystallinity affected the dispersibility of CNTs. 9) They indicated that MWNTs are poorly dispersed in low crystallizable polymers. As they explained, the crystallinity of the PU matrix provided the dispersion conditions of CNTs.…”

“…Also, the growth of a semicrystalline structure from the CNTs acts as a barrier and prevents CNT clustering during the casting process. 9) The thermal properties of the composite films were measured by the laser flash method to determine the thermal diffusivity and thermal conductivity. The thermal conductivity was calculated using the following equation:…”

Visualization of the conductive paths in injection moulded MWNT/polycarbonate nanocomposites by conductive AFM

“…Formerly Jeon et al accessed the dispersion of SWCNTs in crystalline iPP matrix with increasing SWCNT content by CCI SEM as well [22]. Even though CCI method is a very powerful tool for imaging which nanotubes contributes to the electrical percolation, the charge transport induced by accelerated electrons differs from the processes which occur during a usual I-V resistivity measurement.…”