E. Roumeli

Factors controlling the enhanced mechanical and thermal properties of nanodiamond-reinforced cross-linked high density polyethylene.

A systematic investigation of the factors influencing the notable enhancement of the mechanical and thermal properties of nanodiamonds (NDs)-reinforced cross-linked high density polyethylene (PEX) is presented in this work. The effects of crystal structure and molecular conformation as well as filler dispersion and adhesion with the matrix were found to govern the mechanical properties of the final composites. A considerable increase in the strength, toughness, and elastic modulus of the materials was found for the composites with filler content below 1 wt %. For higher NDs concentrations, the properties degraded. When filler concentration does not exceed 1 wt %, enhanced adhesion with the matrix is achieved, allowing a more successful load transfer between the filler and the matrix, thus enabling an effective reinforcement of the composites. The higher degree of crystallinity along with larger crystal size are also positively influencing the mechanical properties of PEX. Higher filler concentrations, on the other hand, lead to the formation of larger aggregates, which lead to lower adhesion with the matrix, while they also constitute stress concentrators and therefore reduce the positive reinforcement of the matrix. The thermal conductivity of the composites was also found to be significantly increased for low-filler concentrations. This enhancement was less significant for higher NDs concentrations. It is concluded that this reinforcement is due to the heat capacity increase that NDs incorporation causes in PEX. Additionally, a thermal stability enhancement was found for the composite with minimum filler content.

Amino-Functionalized Multiwalled Carbon Nanotubes Lead to Successful Ring-Opening Polymerization of Poly(ε-caprolactone): Enhanced Interfacial Bonding and Optimized Mechanical Properties.

In this work, the synthesis, structural characteristics, interfacial bonding, and mechanical properties of poly(ε-caprolactone) (PCL) nanocomposites with small amounts (0.5, 1.0, and 2.5 wt %) of amino-functionalized multiwalled carbon nanotubes (f-MWCNTs) prepared by ring-opening polymerization (ROP) are reported. This method allows the creation of a covalent-bonding zone on the surface of nanotubes, which leads to efficient debundling and therefore satisfactory dispersion and effective load transfer in the nanocomposites. The high covalent grafting extent combined with the higher crystallinity provide the basis for a significant enhancement of the mechanical properties, which was detected in the composites with up to 1 wt % f-MWCNTs. Increasing filler concentration encourages intrinsic aggregation forces, which allow only minor grafting efficiency and poorer dispersion and hence inferior mechanical performance. f-MWCNTs also cause a significant improvement on the polymerization reaction of PCL. Indeed, the in situ polymerization kinetics studies reveal a significant decrease in the reaction temperature, by a factor of 30-40 °C, combined with accelerated the reaction kinetics during initiation and propagation and a drastically reduced effective activation energy.

Thermal degradation kinetics and decomposition mechanism of PBSu nanocomposites with silica-nanotubes and strontium hydroxyapatite nanorods

Novel poly(butylene succinate) (PBSu) nanocomposites containing 5 and 20 wt% mesoporous strontium hydroxyapatite nanorods (SrHNRs) and silica nanotubes (SiNTs) were prepared by melt-mixing. A systematic investigation of the thermal stability and decomposition kinetics of PBSu was performed using pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS) and thermogravimetry (TG). Thorough studies of evolving decomposition compounds along with the isoconversional and model-fitting analysis of mass loss data led to the proposal of a decomposition mechanism for PBSu. Moreover, the effects of SrHNRs and SiNTs on the thermal stability and decomposition kinetics of PBSu were also examined in detail. The complementary use of these techniques revealed that the incorporation of SiNTs in PBSu does not induce significant effects neither on its thermal stability nor on its decomposition mechanism. In contrast, the addition of SrHNRs resulted in the catalysis of the initial decomposition steps of PBSu and also in modified decomposition mechanisms and activation energies. The evolving gaseous products of PBSu and their evolution pattern in the SiNT nanocomposites were the same as in neat PBSu, while they were slightly modified for the SrHNR nanocomposites, confirming the findings from thermogravimetric analysis.

Understanding the mechanical and thermal property reinforcement of crosslinked polyethylene by nanodiamonds and carbon nanotubes

A comprehensive investigation of the mechanical and thermal property reinforcement of silane-crosslinked polyethylene nanocomposites, containing small amounts of multi-walled carbon nanotubes (MWCNTs) and nanodiamonds (NDs), is presented in this work. Lower filler concentrations allowed a satisfactory dispersion, enabling the successful reinforcement of the matrix in every aspect. As the nanofiller content increased, the formed aggregates enlarged and the performance of the composites became more brittle. The measured stiffness enhancement of all the composites was found to be mainly influenced by the crystalline characteristics of the matrix and filler–matrix adhesion. Moreover, it was concluded that filler dispersion and filler–matrix interactions govern the ultimate strength and toughness behavior of these composites, which were found to slightly increase in minimum filler concentrations. Fractography was employed to study the embrittleness of the composites with higher filler loadings, and the observations revealed that a ductile to brittle transition is caused by a micro-deformation mechanism change in these composites. Furthermore, the prepared composites had a significantly improved thermal conductivity, which was mainly related to their superior specific heat capacity, while a great thermal stability enhancement was also revealed.

Effect of high energy ball milling on the structure and mechanical properties of cross-linked high density polyethylene

The effects of high energy ball milling (HEBM) on the structure and some key-properties of crosslinked high density polyethylene (PEX) have been thoroughly examined with a combination of X-ray diffraction analysis, IR and Raman spectroscopy, differential scanning calorimetry, gel content measurements, and tensile properties tests. A structure–property relationship, which provides a reasonable explanation for the studied case has been developed based on the experimental results and their analysis. It is proposed that the HEBM provides some of the silane-grafted macromolecular chains, which have a specific orientation, with sufficient energy in order to crosslink and form small crystalline-like areas. The arrangement of chains in the “reformed” domains leads to a total increase of the overall crystallinity, but also a decrease of the crystalline size. The proposed model can also support the fact that by increasing the milling time, the overall crystallinity of PEX and some important mechanical properties are found to increase.

Identifying pigments in the temple of Seti I in Abydos (Egypt)

The temple of Seti I in Abydos, a sacred city noted as the most venerated place in Egypt, was built by the 19th dynasty (ca.1294-1279 BCE). The temple is famous for its remarkably unique design; it is in the shape of an “L” and its wall paintings are decorated with the most complete series of Kings and Gods in Egypt, which virtually helped to decode Egyptian history. Our first results concern to samples from these wall paintings which are examined by SEM-EDS and FTIR microscopy in order to identify the used pigments. The dimensions of the samples were about 3×6mm, with blue, green, yellow and red colors on the surfaces.