Special morphology and its role in mechanical enhancement of linear low‐density polyethylene/multiwalled carbon nanotubes composites

In this work, multiwalled carbon nanotubes (MWCNTs), as reinforcing agent, were blended with linear low‐density polyethylene (LLDPE), then molded by hot compression molding to prepare LLDPE/MWCNTs composites. Tensile tests indicate that the strength, Young's modulus, and toughness are all improved for LLDPE/MWCNTs composites containing 1 and 3 wt % MWCNTs. Compared with LLDPE, the Young's modulus of LLDPE/MWCNTs composites rises from 144.8 to 270.8 MPa at 1 wt % MWCNTs content. At the same time, increases of 18.5% in tensile strength and 16.6% in yield strength are achieved. Additionally, its toughness is enhanced by 26.7% than that of LLDPE. Microstructure characterizations, including differential scanning calorimetry, X‐ray diffraction, and scanning electron microscopy were performed to investigate the variations of microstructure and further to establish the relationship between microstructure and mechanical properties. Homogeneous dispersion of MWCNTs, network formation, and development of an oriented nanohybrid shish‐kebab structure contribute to the enhanced strength and toughness. The increased crystallinity is beneficial to the reinforcement and increased modulus. Additionally, the thermal stability of the LLDPE/MWCNTs composites is enhanced as well. This work suggests a promising routine to optimize polymer/MWCNTs composites by tailoring the structural development. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45525.     

Mechanical,rheological, and electrical properties of multiwalled carbon nanotube reinforced ASA/Na‐ionomer blend

Varying amounts of multiwalled carbon nanotubes (MWCNTs) was melt‐extruded with the acrylonitrile‐styrene‐acrylate (ASA)/Na‐ionomer blend, and mechanical, rheological, and electrical properties were studied Optical micrographs show good dispersion level at low MWCNT content and network formation at higher nanotubes percentage. DC conductivity model data shows percolation threshold reached at 1% MWCNT content and after percolation, two‐dimensional network structure was formed. The “peak and valley” type surface topology of matrix may be responsible for low percolation threshold limit. The polymer/nanotubes interactions at low MWCNT content increased the mechanical strengths, which were reduced by the network structure and agglomerates of nanotubes at higher nanotubes content. The MWCNTs interacted differently with the architecturally complex polymer chains and controlled chain dynamics accordingly. The Carreau‐Yasuda model was found fit to viscosity data and the model parameters data suggest the zero shear viscosity is function of MWCNTs content but the infinite shear viscosity is independent of nanoparticles content. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42516.     

The influence of matrix viscosity on MWCNT dispersion and electrical properties in different thermoplastic nanocomposites

Composites of MWCNTs having each three different levels of matrix viscosity with five different polymers (polyamide 12, polybutylene terephthalate, polycarbonate, polyetheretherketone and low density polyethylene) were melt mixed to identify the general influence of matrix viscosity on the electrical properties and the state of MWCNT dispersion. Huge differences in the electrical percolation thresholds were found using the same polymer matrix with different viscosity grades. The lowest percolation thresholds were always found in the composites based on the low viscosity matrix. The state of primary MWCNT agglomerate dispersion increased with increasing matrix viscosity due to the higher input of mixing energy. TEM investigations showed nanoagglomerated structures in the low viscosity samples which are obviously needed to achieve low resistivity values. The effect of nanotube shortening was quantified using two different viscosity grades of polycarbonate. Due to the higher mixing energy input the nanotube shortening was more pronounced in the high viscosity matrix which partially explains the higher percolation threshold.     

Compatibilized polypropylene nanocomposites containing expanded graphite and graphene nanoplatelets

We present a non-covalent compatibilization approach to prepare polypropylene (PP) composites containing expanded graphite (EG) and graphene nanoplatelets (GNPs) by melt compounding. This method involves PP matrix functionalization with pyridine (Py) moieties, which are capable of engaging in π-π interactions with the surface of the EG and GNPs. The addition of 10 wt% of PP grafted with amino-pyridine (PP-g-Py) to neat PP facilitated the break-up of EG particles, by intercalating between their layers and facilitating their separation into smaller tactoids. GNPs were prepared starting from EG through a thermomechanical exfoliation method. Addition of GNPs to PP resulted in well-dispersed platelets having aspect ratios as high as 40, whereas in the presence of the PP-g-Py compatibilizer the matrix contained sub-micron scale platelets. The electrical percolation thresholds were in the vicinity of 6 and 10 vol% in the compatibilized PP-EG and PP-GNP composites, respectively, and the maximum value of the electrical conductivity achieved was 10⁻¹ S/m for the compatibilized GNP composites. Addition of GNPs resulted in increases in the flexural moduli by as much as 95% compared to the unfilled PP, whereas the impact strength remained unaffected up to 10 wt% GNP content.     

Effects of multi‐walled carbon nanotubes and conductive carbon black on electrical,dielectric, and mechanical properties of epoxidized natural rubber composites

The influence of multi‐walled carbon nanotubes (MWCNTs) and conductive carbon black (CCB) on cure, electrical, dielectric, and mechanical properties of epoxidized natural rubber (ENR) composites was investigated. It was found that short MWCNTs (S‐MWCNTs) with low loading significantly affected the cure characteristics in a way similar to high loading of CCB. Moreover, the ENR/S‐MWCNTs composites exhibited high AC conductivity, dielectric constant, and dielectric loss tangent (tan δ ) compared to the ENR/CCB and ENR/L‐MWCNTs (long MWCNTs) composites. In addition, the S‐MWCNTs composites showed the lowest percolation threshold concentration, defined as the lowest loading to form conductive paths in the insulating ENR matrix. This might be attributed to the comparatively high interfacial polarization, with good dispersion and distribution, of the S‐MWCNTs in ENR matrix. These characteristics were confirmed by TEM imaging and by a high bound rubber content, corroborating strong filler–rubber interactions in the ENR/S‐MWCNTs composites. However, the L‐MWCNTs composites showed the lowest electrical and other related properties, despite the highest aspect ratio and specific surface area of this filler. This might be because of the flocculation of nanotubes by mutual entanglement, leading to a poor uneven distribution in the ENR matrix. POLYM. COMPOS., 38:1031–1042, 2017. © 2015 Society of Plastics Engineers     

Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique

A new method was developed to disperse carbon nanotubes (CNTs) in a matrix polymer and then to prepare composites by melt processing technique. Due to high surface energy and strong adsorptive states of nano-materials, single-walled carbon nanotubes (SWNTs) were adsorbed onto the surface of polymer powders by spraying SWNT aqueous suspected solution onto fine high density polyethylene (HDPE) powders. The dried SWNTs/powders were blended in a twin-screw mixture, and the resulting composites exhibited a uniformly dispersion of SWNTs in the matrix polymer. The electrical conductivity and the rheological behavior of these composites were investigated. At low frequencies, complex viscosities become almost independent of the frequency as nanotubes loading being more than 1.5 wt%, suggesting an onset of solid-like behavior and hence a rheological percolation threshold at the loading level. However, the electrical percolation threshold is ∼4 wt% of nanotube loading. This difference in the percolation thresholds is understood in terms of the smaller nanotube-nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. The measurements of mechanical properties indicate that this processing method can obviously improve the tensile strength and the modulus of the composites.     

强剪切流动下聚丙烯基复合材料介电性能和结构演变的研究

研究了多壁碳纳米管(MWCNT)、线型低密度聚乙烯(LLDPE)和强剪切流动场对聚丙烯(PP)/MWCNT/LLDPE复合材料的导电性能、结晶性能、力学性能和介电性能的研究。结果表明,PP/MWCNT复合材料随着MWCNT含量的提高,导电性能逐步改善,导电逾渗值约为1.8%,而第二分散相LLDPE的引入使复合材料的导电性能表现出了先降低后升高的现象,并且随着LLDPE含量的增加,MWCNT逐步向LLDPE内部迁移,构成了双连续结构。复合材料经过固态口模拉伸的强剪切流动场之后,随着拉伸速率从10 mm/min增加到300 mm/min的过程中,复合材料的融化温度逐渐向高温方向偏移,表现出了片层厚度增加的现象。复合材料内部分子链取向和纤维结构的形成,能够显著提高复合材料的拉伸强度和介电性能,拉伸强度提高从约50 MPa提高到了约240 MPa,提高了约380%。MWCNT和强剪切流动场的引入也使复合材料的介电常数大幅提升。     

Reinforcing and Flame-Retardant Effects of Halloysite Nanotubes on LLDPE

Halloysite nanotubes (HNTs) were used to compound with linear low density polyethylene (LLDPE) to prepare composites with better mechanical properties and higher flame retardancy. The PE graft was used as interfacial modifier in the LLDPE/HNTs composites. HNTs were showed to be a promising reinforcing and flame retardant nano-filler for LLDPE. The mechanical properties and flame retardancy as well as thermal stability of the composites can be further enhanced by the addition of the graft copolymer. Morphological observation revealed that the graft copolymer could facilitate the dispersion of HNTs in LLDPE matrix and enhance the interfacial bonding.     

Dynamic rheological,morphology and mechanical properties of compatibilized LLDPE/EMA blends

Blends of linear low density polyethylene (LLDPE) and ethylene-co-methyl acrylate (EMA) having 60/40 composition was studied with and without compatibilizing agent. The compatibilizing agent used was maleic anhydride grafted linear low density polyethylene (LLDPE-g-MA). The LLDPE backbones of the compatibilizer are compatible with LLDPE blend component, whereas the maleic anhydride is affinated with carbonyl groups of EMA. The effectiveness of the compatibilizing agent was evaluated using different techniques like mechanical, thermal, scanning electron microscopy and rheological studies. Best compatibilization effect was found in the blend at a loading of 3 wt% of compatibilizer since at this level of compatibilizer complex viscosity, tensile strength, modulus, elongation at break, impact strength was found to be higher. The increase in the melt viscosity, storage modulus and thermal stability of the compatibilized blends indicated enhanced interactions between the discrete LLDPE and EMA phases induced by the functional compatibilizer.     

Supramolecular bionanocomposites 3: Effects of surface functionality on electrical and mechanical percolation

Percolation phenomena in bionanocomposites composed of the bioplastic polylactide and surface decorated supramolecular multiwalled carbon nanotubes (MWCNTs) are investigated. MWCNTs with three distinct surface chemistries—a native surface, a hydroxyl functionalized surface, and a short hydrocarbon functionalized surface—are prepared and studied. Two experimental methods are used to determine percolation thresholds; melt rheology provides a measure of the mechanical percolation threshold and electrical impedance spectroscopy provides values of the electrical percolation threshold. The MWCNT‐loading level required to obtain mechanical percolation is systematically found to be lower than the loading level needed to achieve electrical percolation. Hydroxylated MWCNTs have the highest percolation thresholds of 1.8 (mechanical) and 6.7 wt % (electrical), which is attributed to aggregation caused by hydrogen bonding. Alkane‐grafted MWCNTs have lower percolation thresholds of 0.76 (mechanical) and 2.8 wt % (electrical). However, untreated MWCNTs have the lowest thresholds of 0.16 (mechanical) and 1.5 wt % (electrical). The reduced percolation threshold observed upon conversion of the hydroxylated surface to the alkylated surface is attributed to morphological differences—functionalization disrupts hydrogen bonding and leads to better nanotube dispersion. The combination of higher inherent conductivity and more favorable nanotube‐polylactide interactions implies that unmodified MWCNTs are preferred to produce electrically conductive bionanocomposites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Noncovalent compatibilization of polypropylene/MWCNT composites using an amino‐pyridine grafted polypropylene matrix

Polypropylene (PP)/multiwalled carbon nanotube (MWCNT) composites are prepared by implementing noncovalent compatibilization. The compatibilization method involves PP matrix functionalization with pyridine (Py) aromatic moieties, which are capable of π–π interaction with MWCNT sidewalls. Imaging revealed that the addition of 25 wt% of PP‐g‐Py to neat PP is capable of drastically reducing nanotube aggregate size and amount, compared to a matrix containing the equivalent amount of a maleated PP (PP‐g‐MA). Raman spectroscopy confirms improved polymer/nanotube interaction with the PP‐g‐Py matrix. The electrical percolation threshold appears at a MWCNT loading of approximately 1.2 wt%, and the maximum value of the electrical conductivity achieved is 10⁻² S/m, irrespective of the functionalization procedure. The modulus of the composites is improved with the addition of MWCNTs. Furthermore, composites functionalized with Py display significant improvements in composite ductility compared with their maleated counterparts because of the improved filler dispersion. POLYM. COMPOS., 37:2794–2802, 2016. © 2015 Society of Plastics Engineers     

Dispersion of carbon nanotubes through amphiphilic block copolymers: Rheological and dielectrical characterizations of poly(ethylene oxide) composites

In this article, we report on some properties of polymer nanocomposites prepared from dispersions of multiwall carbon nanotubes (CNT) in aqueous solution prepared using amphiphilic block copolymers. These nanocomposites are made of polyethylene oxide as matrix and CNT wrapped with copolymers as fillers. We investigated the rheological and electrical behavior of such composites with the objectives of underlined the effect of wrapping. Two rheological and only one electrical percolation thresholds have been observed and related to polymer–CNT and CNT–CNT networks. The low values of these percolation thresholds agree with a homogeneous dispersion of CNT in the matrix. We also demonstrated that specific wrapping may induce an increase of electrical conductivity without affecting too much the viscosity of the melt. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Effects of Surface Modifiers on Mechanical and Rheological Properties of Halogen-Free Flame Retarded Polyethylene Composites

Halogen-free flame-retarded linear low density polyethylene (LLDPE) composites were prepared in a melt process using magnesium hydroxide (MH) as a flame retardant. The effects of surface modifiers on mechanical properties and rheological behavior of the filled LLDPE have been studied. The results show that the use of surface modifiers with low molecular weight (MW) such as stearic acid or its salts, and silane coupling agents can improve greatly the elongation at break of the composites, and decrease the tensile strength in most cases. However, PE- g -DBM (LLDPE grafted with dibutyl maleate) as a polymeric compatibilizer can make the strength and elongation at break of the composites increase simultaneously. It was found from the rheological measurements that the composites containing low MW surface modifiers exhibited a lower viscosity compared with that of the composites without them, whereas the use of PE- g -DBM compatibilizer increased the viscosity of the composites. In addition, their effects on the morphological structure of the flame-retarded LLDPE composites containing surface modifiers have been investigated using scanning electron microscope (SEM).     

Control of carbon nanotubes at the interface of a co-continuous immiscible polymer blend to fabricate conductive composites with ultralow percolation thresholds

The concept of “double percolation”, i.e., conductive fillers are selectively located in one phase of a co-continuous polymer blend to form a percolated network in the selected phase, is widely used to reduce the percolation thresholds of conductive polymer composites to a fraction of their original values. However, it is expected that the percolation threshold can be significantly reduced further if the conductive fillers are only selectively distributed at the continuous interface of the co-continuous polymer blend, where only a very small amount of fillers are needed to build up the conductive percolated network. Multiwalled carbon nanotubes (MWCNTs) with very high aspect ratio (ca. 1000) are selectively distributed at a continuous interface of a co-continuous immiscible poly(lactic acid)/poly(ε-caprolactone) (PLA/PCL) blend at a weight ratio of 50/50 by controlling the migration process of MWCNTs from the unfavorable PLA to the favorable PCL phase. Compared to the PLA/PCL/MWCNTs composites by the traditional double percolation method (percolation threshold is ca. 0.97 wt%), the percolation threshold of PLA/MWCNTs/PCL composites (ca. 0.025 wt%) drops 2 orders of magnitude due to controlling the MWCNTs at the continuous interface between the PLA and PCL phases.     

Investigation of electrical,mechanical, and thermal properties of functionalized multiwalled carbon nanotubes‐reduced graphene Oxide/PMMA hybrid nanocomposites

Electrical, mechanical, and thermal properties of the poly(methyl methacrylate) (PMMA) composites containing functionalized multiwalled carbon nanotubes (f‐MWCNTs) and reduced graphene oxide (rGO) hybrid nanofillers have been investigated. The observed electrical percolation threshold of FHC is 0.8 wt% with maximum conductivity of 1.21 × 10^?3 S/cm at 4 wt% of f‐MWCNTs. The electrical transport mechanism and magneto resistance studied of hybrid composites have also been investigated. Progressive addition of f‐MWCNTs in rGO/PMMA composite results increase in mechanical (tensile strength and Young's modulus) and thermal (thermal stability) properties of f‐MWCNTs‐rGO/PMMA hybrid nanocomposites (FHC). The increased mechanical properties are due to the efficient load transfer from PMMA matrix to f‐MWCNTs and rGO through better chemical interaction. The strong interaction between PMMA and f‐MWCNTs‐rGO in FHC is the main cause for improved thermal stability. POLYM. ENG. SCI., 59:1075–1083, 2019. © 2019 Society of Plastics Engineers     

Improved mechanical properties of carbon nanotube/polymer composites through the use of carboxyl-epoxide functional group linkages

Single-walled and multi-walled carbon nanotubes (CNTs) were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group. We have then observed experimentally that mutual chemical reaction between the functional groups on the CNTs with the polymer epoxide group can enhance, two-fold, both the tensile strength and elastic modulus, E, of single walled CNT/polymer composites. A simple model was formulated to understand the variation of E with CNT volume fraction, considering agglomeration effects as well. An increase in the work of fracture, obtained from the experimental stress-strain curves, was seen at low nanotube filling fractions and is presumably due to crack bridging of the polymer matrix by CNTs. The influence of CNT length and geometry on mechanical properties, along with the influences of electrical and mechanical percolation thresholds was considered.     

Mechanical and electrical properties of multi walled carbon nanotube/ABC block terpolymer composites

Composites of multi-walled carbon nanotubes (MWCNTs) in ABC block terpolymer matrices of different compositions are studied. The composites were obtained by dispersion of MWCNTs in poly(styrene-block-butadiene-block-methyl methacrylate) (SBM) in a selective solvent for the M block, followed by solvent evaporation and compression molding. The structures of the MWCNT/SBM composites are investigated by transmission electron microscopy. The processing conditions, i.e. solvent cast or compression molding, induce different non-equilibrium microstructures and the MWCNTs modify the SBM organization only locally. We show that by fixing the processing procedure we are able to obtain samples with reproducible microstructure and properties. The electrical conductivity thresholds of these composites are lower than 1 wt.%. The reinforcing effect of the MWCNTs measured by dynamical mechanical analysis is mainly related to the SBM microstructures of the matrix and to the MWCNT dispersion quality.     

Low percolation threshold and high conductivity in carbon black filled polyethylene and polypropylene composites

High electrically conductive composites have been manufactured using twin and single screw extruders from carbon black with polyolefin. High density, low density polyethylene, polypropylene, polyethylene/polypropylene copolymer, and maleic anhydrite grafted polypropylene have been compounded with three carbon blacks (CBs), i.e., Black Pearl, Printex, and Ketjen, respectively. The lowest percolation threshold (0.8 vol %) for conductive composite was obtained using Ketjen CB blended with high density polyethylene (HD3690, MFI = 36 g/10 min). Polypropylene composites also achieved low percolation thresholds of 1.5 vol % when compounded with Printex or Ketjen CB. Decreasing melt viscosity of polymer matrix resulted in decreasing resistivity of composites. Ketjen CB showed the best conductive behavior for both polyethylene and polypropylene composites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Graphene/polyethylene nanocomposites: Effect of polyethylene functionalization and blending methods

Since its recent successful isolation, graphene has attracted an enormous amount of scientific interest due to its exceptional physical properties. Graphene incorporation can improve electrical and mechanical properties of polymers including polyethylene (PE). However, the hydrophobic nature and low polarity of PE have made effective dispersion of nano-fillers difficult without compatibilization. Graphene was derived from graphite oxide (GO) via rapid thermal exfoliation and reduction. This thermally reduced graphene oxide (TRG) was blended via melt and solvent blending with linear low density PE (LLDPE) and its functionalized analogs (amine, nitrile and isocyanate) produced using a ring-opening metathesis polymerization (ROMP) strategy. TRG was well exfoliated in functionalized LLDPE while phase separated morphology was observed in the un-modified LLDPE. Transmission electron micrographs showed that solvent based blending more effectively dispersed these exfoliated carbon sheets than did melt compounding. Tensile modulus was higher for composites with functionalized polyethylenes when solvent blending was used. However, at less than 3 wt.% of TRG, electrical conductivity of the un-modified LLDPE was higher than that of the functionalized ones. This may be due to phase segregation between graphene and PE, and electrical percolation within the continuous filler-rich phase.     

Electrical,mechanical, and crystallization properties of ethylene‐tetrafluoroethylene copolymer/multiwalled carbon nanotube composites

Multiwalled carbon nanotubes (MWCNTs) were melt‐mixed in a conical twin‐screw extruder with a random copolymer of ethylene and tetrafluoroethylene. Surprisingly, the electrical percolation threshold of the resultant composites was quite low; ～0.9 wt %. In fact, this value is as low or lower than the value for most MWCNT/semicrystalline polymer composites made with roughly equivalent aspect ratio tubes mixed in a similar manner, for example, melt mixing. This low percolation threshold, suggestive of good dispersion, occurred even though the polymer surface energy is quite low which should make tubes more difficult to disperse. Dynamic mechanical measurements confirmed the rather low percolation threshold. The effect of nanotubes on crystallization kinetics was quite small; suggesting perhaps that a lack of nucleation which in turn reduces/eliminates an insulating crystalline polymer layer around the nanotubes might explain the low percolation threshold. Finally, the modulus increased with the addition of nanotubes and the strain at break decreased. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41052.