Marianna Kontopoulou

Influence of clay exfoliation on the physical properties of montmorillonite/polyethylene composites

Abstract Melt compounding was used to prepare conventional composites of montmorillonite clay and polyethylene (PE) as well as nanocomposites of exfoliated montmorillonite platelets dispersed in a maleated polyethylene (PE- g -MAn) matrix. The extent of clay platelet exfoliation in the PE- g -MAn nanocomposites was confirmed by X-ray diffraction and resulted in a significant reduction of the degree of crystallinity and increased polymer crystallization rates. Studies of non-isothermal crystallization kinetics suggested that the exfoliated clay promotes heterogeneous nucleation and two-dimensional crystallite growth. PE/clay composites behaved in a similar manner as conventional macrocomposites, exhibiting modest increases in their rheological properties and Youngs modulus. Conversely, the nanoscale dimensions of the dispersed clay platelets in the nanocomposites led to significantly increased viscous and elastic properties and improved stiffness. This was attributed to the high surface area between the polymer matrix and the exfoliated clay, which resulted in enhanced phase adhesion.

A noncovalent compatibilization approach to improve the filler dispersion and properties of polyethylene/graphene composites.

Graphene was prepared by low temperature vacuum-assisted thermal exfoliation of graphite oxide. The resulting thermally reduced graphene oxide (TRGO) had a specific surface area of 586 m(2)/g and consisted of a mixture of single-layered and multilayered graphene. The TRGO was added to maleated linear low-density polyethylene LLDPE and to its derivatives with pyridine aromatic groups by melt compounding. The LLDPE/TRGO composites exhibited very low electrical percolation thresholds, between 0.5 and 0.9 vol %, depending on the matrix viscosity and the type of functional groups. The dispersion of the TRGO in the compatibilized composites was improved significantly, due to enhanced noncovalent interactions between the aromatic moieties grafted onto the polymer matrix and the filler. Better dispersion resulted in a slight increase in the rheological and electrical percolation thresholds, and to significant improvements in mechanical properties and thermal conductivity, compared to the noncompatibilized composites. The presence of high surface area nanoplatelets within the polymer also resulted in a substantially improved thermal stability. Compared to their counterparts containing multiwalled carbon nanotubes, LLDPE/TRGO composites had lower percolation thresholds. Therefore, lower amounts of TRGO were sufficient to impart electrical conductivity and modulus improvements, without compromising the ductility of the composites.

Chain-topology-controlled hyperbranched polyethylene as effective polymer processing aid (PPA) for extrusion of a metallocene linear-low-density polyethylene (mLLDPE)

Hyperbranched polymers have recently received extensive interest for applications as polymer processing aids (PPAs). In this work, we present the first study on the performance of novel hyperbranched polyethylene as a processing aid for a metallocene linear-low-density polyethylene (mLLDPE). Moreover, we investigated for the first time the unique role that chain topology plays in determining the efficiency of the polymers as PPA. Two polymers of drastically different chain topologies, a hyperbranched polyethylene (HBPE) and a linear branched polyethylene (LBPE) having a linear chain topology, were prepared by ethylene polymerization using chain walking Pd-diimine catalysis. The performances of these two polymers of different chain topologies in improving the processability of the mLLDPE were investigated and compared by using two-step capillary extrusion experiments, including constant-shear testing at 200s−1 and shear-rate sweep testing. For comparison purposes, a commercial fluoropolymer-based PPA, Vito...

Tuning structural parameters of polyethylene brushes on silica nanoparticles in surface-initiated ethylene “living” polymerization and effects on silica dispersion in a polyolefin matrix

Surface-initiated ethylene “living” polymerization with covalently tethered Pd–diimine catalysts represents a novel technique for covalent surface functionalization of silica nanoparticles with polyethylene (PE) brushes. In this paper, we report on the successful tuning of various structural parameters of PE brushes in this surface-initiated polymerization technique, including brush length, density, and topology. To control/reduce the brush density, the density of the surface-tethered acryloyl groups for catalyst immobilization is adjusted by using mixed silane agents comprised of effective 3-acryloxypropyltrichlorosilane and inert ethyltrichlorosilane at different compositions in the surface functionalization step, which in turn adjusts the density of immobilized catalysts for rendering PE brushes. This approach gives rise to low-polydispersity PE brushes of controllable densities at the polymerization condition of 27 atm/5 °C: 0.022–0.055 chains per nm2 on a precipitated silica (Silica-I) and 0.07–0.17 chains per nm2 on a fumed silica (Silica-II). The length of PE brushes is controlled by adjusting the polymerization time, with the highest brush length of about 45 kg mol−1 achieved at 6 h of polymerization at 27 atm/5 °C. Unlike the linear brushes with short branch structures obtained at 27 atm/5 °C, hyperbranched PE brushes with compact topology are obtained at 1 atm/25 °C, benefiting from the chain walking mechanism of the Pd–diimine catalyst. The PE-grafted silicas of varying brush density and length are subsequently used as nanofillers to construct polymer nanocomposites with an ethylene–α-olefin copolymer as the matrix polymer. The effects of brush length and density on the nanofiller dispersion and physical properties of the composites are examined. This represents the first study on polyolefin composites containing silica nanoparticles grafted with polyolefin brushes as nanofillers.

Correlation between the length reduction of carbon nanotubes and the electrical percolation threshold of melt compounded polyolefin composites.

The objectives of this work are to quantify the degree of multiwalled carbon nanotube (MWCNT) length reduction upon melt compounding and to demonstrate unambiguously that the length reduction is mainly responsible for the increase in electrical percolation threshold of the resulting composites. Polyolefin matrices of varying viscosities and different functional groups are melt compounded with MWCNTs. A simple method is developed to solubilize the polymer matrix and isolate the MWCNTs, enabling detailed imaging analysis. In spite of the perceived strength of the MWCNTs, the results demonstrate that the shear forces developed during melt mixing are sufficient to cause significant nanotube breakage and length reduction. Breakage is promoted when higher MWCNT contents are used, due to increased probability of particle collisions. Furthermore, the higher shear forces transmitted to the nanotubes in the presence of higher matrix viscosities and functional groups that promote interfacial interactions, shift the nanotube distribution toward smaller sizes. The length reduction of the MWCNTs causes significant increases in the percolation threshold, due to the loss of interconnectivity, which results in fewer conductive pathways. These findings are validated by comparing the experimental percolation threshold values with those predicted by the improved interparticle distance theoretical model.

Linear viscoelastic properties of ethylene–octene copolymer/nanosilica composites investigated over a broad range of frequencies

The interrelations between microstructure and rheology of melt compounded poly(ethylene-co-octene) (EOC) based nanocomposites containing low amounts of silica (SiO2) nanoparticles were studied. In the presence of a maleated EOC compatibilizer, hydrogen bonds are established between the silanol groups located on the surface of the particles and the succinic anhydride functionality of the compatibilizer, resulting in improved filler dispersion during compounding and the formation of a bound layer of polymer surrounding the particles. This layer alters the interface between the bulk polymeric matrix and the particles, influencing the linear viscoelastic (LVE) response. The compatibilized composites exhibited a more stable response in time sweeps and a higher critical strain for the onset of nonlinearity, compared to their noncompatibilized counterparts. Superposition of small angle oscillatory shear and creep/creep recovery experiments revealed enhancements in the LVE functions in the absence of a compatibil...

Optimization of Dispersion of Nanosilica Particles in a PP Matrix and Their Effect on Foaming

Abstract Nanocomposites based on isotactic polypropylene (PP) and nanosilica (SiO2) were prepared using a co-rotating twin-screw extruder (TSE). The effect of operating variables, such as screw speed and screw configuration on the dispersion of nanosilica in the polymer matrix has been studied, using TEM imaging. High shear stress, sufficient residence time, and high fill ratio in the melting section of the screw were the most important factors in achieving good nanosilica dispersion. Furthermore, the effects of filler loading and amount of a maleated polypropylene (PP-g-MA) compatibilizer on the degree of SiO2 dispersion were investigated. The foaming performance of the composites was evaluated using a batch foaming simulation system, and an extrusion foaming setup that employed respectively N2 and CO2 blowing agents. Well-dispersed surface-modified hydrophobic SiO2 particles acted as effective nucleating agents for foaming, when used at loadings below 1 phr.

Foaming of reactively modified polypropylene: Effects of rheology and coagent type

A series of linear controlled rheology polypropylenes were produced through reaction with dicumyl peroxide in the melt state, to investigate the effect of molecular weight and viscosity on foaming. Foaming experiments by compression molding, using a chemical blowing agent (azodicarbonamide), and by batch foaming using nitrogen-blowing agent, revealed that lower viscosity promoted higher expansion ratios and larger cells, due to reduced resistance to cell growth. Linear polypropylene was further modified using trimethylolpropane trimethacrylate and triallyl trimesate coagents. Coagent modification resulted in pronounced changes in the shear thinning and elasticity of the modified polypropylenes. However, evidence of long-chain branching was only present in polypropylene modified by triallyl trimesate. Foams based on coagent-modified polypropylenes had higher expansion ratios than their degraded counterparts. This was ascribed in part to the lower viscosities, and to a nucleating effect, arising from the presence of a finely dispersed phase of coagent-rich nanoparticles. Strain hardening in polypropylenes modified by triallyl trimesate further resulted in a finer cell structure, due to suppressed coalescence.

Laser Transmission Welding of Glass Reinforced Nylon 6

The effect of laser welding parameters such as laser power, laser speed, working distance and weld pressure on the weld strength, microstructure and meltdown of modified T-joints were studied using a diode laser and a contour welding technique. Specimens made of 30% glass reinforced nylon 6 were used in this study. A regression model was fitted to the data based on a central composite experimental design. The model showed that low levels of laser power at lower laser speed gave the maximum weld strength. It was observed that increases in weld pressure had a negative effect on weld strength. Meltdown was found to increase proportionally to the line energy and weld pressure.

Coagent Modified Polypropylene Prepared by Reactive Extrusion: A New Look into the Structure-Property Relations of Injection Molded Parts

Abstract Peroxide-mediated reactive extrusion of linear isotactic polypropylene (L-PP) was conducted in the presence of trimethylolpropane trimethacrylate (TMPTMA) and triallyl trimesate (TAM) coagents, using a twin screw extruder. The resulting coagent-modified polypropylenes (CM-PP) had higher viscosities and elasticities, as well as increased crystallization temperature compared to PP reacted only with peroxide (DCP-PP). Additionally, deviations from terminal flow, and strain hardening were observed in PP modified with TAM, signifying the presence of long chain branching (LCB). The CM-PP formulations retained the modulus and tensile strength of the parent L-PP, in spite of their lower molar mass and viscosities, whereas their elongation at break and the impact strength were better. This was attributed to the finer spherulitic structure of these materials, and to the disappearance of the skin-core layer in the injection molded specimens.