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All issues / Volume 14 (2020) / Issue 12 (December)

Panacea: A Greek ancient myth ‘living’ in materials science?
G. C. Psarras
Vol. 14., No.12., Pages 1105-1105, 2020
DOI: 10.3144/expresspolymlett.2020.89
This is an editorial article. It has no abstract.
Graphene oxide modified carbon fiber prepregs: A mechanical comparison of the effects of oxidation methods
V. Acar, S. Erden, M. Sarikanat, Y. Seki, H. Akbulut, M. O. Seydibeyoglu
Vol. 14., No.12., Pages 1106-1115, 2020
DOI: 10.3144/expresspolymlett.2020.90
The objective of this study is to understand the significance of graphene oxide type for the improvement of mechanical properties of carbon fiber reinforced epoxy composites. Modified epoxy systems were prepared by adding two different graphene oxide nanoparticles prepared by the modified Hummers method (GOH) and Laachachi method (GOL). In the second stage, carbon fiber reinforced epoxy composites were produced using prepreg manufacturing technique. X-ray Photoelectron Spectroscopy characterizations of the pure graphene and modified graphene nanoparticles (GOH and GOL) were performed to confirm the oxidation of the nanoparticles. The morphology of the nanocomposite epoxy resin was investigated with scanning electron microscopy. Mechanical tests were conducted with composites to determine the properties of the final materials. It was observed that the addition of the GOH improved the longitudinal tensile and flexural strength values by 41 and 33%, respectively. Interlaminar shear strength (ILSS) values of epoxy/carbon fiber significantly increased up to 58% with GOH addition. Moreover, the results show that nano-enhanced resins could be a key component for many composites, and it can be a suitable material for long term usage and resistance of sudden failure of composites.
Triple-shape memory effect in 3D-printed polymers
L. Dai, J. Song, S. Qu, R. Xiao
Vol. 14., No.12., Pages 1116-1126, 2020
DOI: 10.3144/expresspolymlett.2020.91
A triple-shape memory effect represents that polymers can memorize two temporary shapes and show a subsequent recovery from the second temporary shape to the first temporary shape and further to the permanent shape with heating, which provides more design freedom for the applications of shape-memory polymers. In this work, we investigate the shapememory performance of three polymers produced by a commercial 3D printer. Two polymers with a pure component exhibit distinct glass transition regions spanning 30°C for each material. The copolymer has a broader glass transition region from –4 to 59 °C. The dual-shape memory tests show that the deformation temperature has a limited influence on the two pure polymers, while a clear temperature memory effect can be observed in the copolymer. For the copolymer, the recovery region has only a small overlap when deformed at 60 and 20 °C, which allows programming two temporary shapes at these two temperatures. Both uniaxial tension tests and three-dimensional demonstration show that the copolymer exhibits a good triple-shape memory effect.
Mechanical behaviour of poly(lactic acid)/cellulose nanocrystal nanocomposites: A comparative study between conventional tensile test and small punch test
S. Singh, O. Santana-Perez, C. Rodriguez, K. Oksman, M. Ll. Maspoch
Vol. 14., No.12., Pages 1127-1136, 2020
DOI: 10.3144/expresspolymlett.2020.92
The use of nanocomposites is increasingly frequent as a way to improve the mechanical behaviour of polymers. In the specific case of poly(lactic acid) (PLA), the use of cellulose nanocrystals (CNC) as a reinforcing material is an interesting option, once the tendency of CNCs to agglomerate has been solved. One of the possible solutions to this problem is a superficial modification of CNC’s nanocrystals through a ring-opening polymerization (ROP) process. This work analyzes the use of CNC nanocrystals modified using ROP (mCNC) as a reinforcement of PLA. The mechanical properties of PLA/CNC nanocomposites are evaluated using tensile tests and small punch tests (SPT) on films prepared by extrusion calendering and post processed by compression molding. The addition of non-modified CNC promotes multiple crazing in PLA, increasing its ductility. mCNC leads to a more dispersed nanocomposite, a slight increase in the elastic modulus and a drastic decrease of crazing in tensile tests. The same tendency has been observed with SPT, and the applicability of this test in the prediction of the tensile modulus (E) of polymeric nanocomposites has been demonstrated. However, more work is needed to find the ideal SPT parameter to estimate the yield point.
Natural rubber nanocomposites based on hybrid filler of zinc nanoparticles and carbon nanotubes: Electrical conductivity and other related properties
N. Thongkong, S. Wisunthorn, S. Pichaiyut, C. Nakason, S. Kiatkamjornwong
Vol. 14., No.12., Pages 1137-1154, 2020
DOI: 10.3144/expresspolymlett.2020.93
Hybrid filler of carbon nanotubes (CNT) and zinc oxide (ZnO) nanoparticles were prepared and then mixed with natural rubber (NR). The ZnO nanoparticles were first synthesized by the sol–gel method. The NR-CNT/ZnO nanocomposites were prepared by latex mixing. Various proportions of ZnO in the CNT/ZnO hybrid filler were tested by mixing with NR to eventually form NR-CNT/ZnO nanocomposites. It was found that the optimum ZnO content in CNT/ZnO hybrid filler was about 3 phr. These ZnO contents provided the NR hybrid composite with superior mechanical properties in terms of tensile strength and modulus, together with good electrical conductivity and dielectric properties. In addition, stress relaxation tests revealed stronger filler network formation after incorporation of CNT-ZnO hybrid filler into the NR matrix. It might be due to electrostatic interactions between CNT and ZnO, also contributing to high electrical conductivity and dielectric properties of the NR-CNT/ZnO composites, when compared to a solely CNT filled NR composite. Furthermore, the Payne effect and morphological properties were qualitatively analyzed, indicating that CNT dispersion was finer in the NR-CNT/ZnO composites having about 1 to 3 phr ZnO content in CNT/ZnO. Higher contents of ZnO in CNT/ZnO hybrid filler caused large filler aggregates degrading mechanical and electrical properties.
Significant modification of the surface morphology of polylactide (PLA) and PLA-halloysite nanocomposites in the presence of N,N’-ethylenebis(stearamide) upon thermal treatment
M. Pluta, J. Bojda, T. Makowski, E. Piorkowska, M. Murariu, L. Bonnaud, P. Dubois
Vol. 14., No.12., Pages 1155-1168, 2020
DOI: 10.3144/expresspolymlett.2020.94
Surface morphology and bulk structure of polylactide (PLA) modified with halloysite nanotubes (HNT) treated with N,N’-ethylenebis(stearamide) (EBS) and untreated, as well as reference materials – PLA, EBS, and a blend of PLA with EBS, were examined. HNT is known to improve PLA properties, including mechanical and barrier properties. The materials were cold crystallized and tested using DSC, TGA, PLM, SALS, SEM, AFM. The additives affected nucleation and crystallization of PLA, but did not influence crystallinity, which was of approx. 37–40 %. In bulk, PLA crystallized in the form of spherulites in all materials. However, on surfaces of EBS-containing materials, a thin EBS layer with domain morphology was found. The EBS layer induced epitaxial crystallization of PLA in the form of parallel stacks of edge-on lamellae, the phenomenon reported for this polymer pair for the first time. It was observed in PLA with EBS modified HNT and in the PLA blend with EBS. Moreover, the EBS layers imparted hydrophobicity to the EBS-containing materials.
Amino multi-walled carbon nanotubes further improve the thermal conductivity of boron nitride/liquid crystal epoxy resin composites
T. F. Qin, H. Wang, J. He, Q. Q. Qu, Y. S. Da, X. Y. Tian
Vol. 14., No.12., Pages 1169-1179, 2020
DOI: 10.3144/expresspolymlett.2020.95
In this work, we introduced highly thermally conductive and fibrous amino multi-walled carbon nanotubes (MCNT-NH2) into hexagonal boron nitride/liquid crystal epoxy resin (h-BN/LCER) composites to improve the thermal conductivity of the composites. First, we prepared hexagonal boron nitride@amino multi-walled carbon nanotubes (h-BN@MCNT-NH2) hybrid fillers. Then, the amino group in the hybrid filler participated in the curing process of the epoxy resin to prepare hexagonal boron nitride@amino multi-walled carbon nanotubes/liquid crystal epoxy resin (h-BN@MCNTNH2/LCER) composites. Subsequently, its thermal conductivity was tested and analyzed using the Agari’s model and microstructure of the composites, and we can come to conclude that the thermal conductivity of h-BN@MCNT-NH2/LCER composites is higher than that of h-BN/LCER at the same filler content. The main reason is that the addition of MCNT-NH2 plays a role in increasing the thermal conduction path of h-BN/LCER composites and decreasing the large interface thermal resistance of fillers and resin matrix. Finally, the usability and thermal conductivity of h-BN@MCNT-NH2/LECR composites were verified by light-emitting diode (LED) lamps. The temperature of LED lamp using 50% h-BN@MCNT-NH2/LCER composites was eventually stabilized at 27.7 °C, it is expected that 50% h-BN@MCNT-NH2/LCER composites will be used in LED electronic products.
Insights into the curing kinetics of epoxy/PLA: Implications of the networking structure
I. D. S. Silva, J. J. P. Barros, A. Albuquerque, N. G. Jaques, M. V. L. Fook, R. M. R. Wellen
Vol. 14., No.12., Pages 1180-1196, 2020
DOI: 10.3144/expresspolymlett.2020.96
The curing kinetics of thermoset-thermoplastic compounds based on diglycidyl ether of bisphenol A (DGEBA), methyl tetrahydrophthalic anhydride (MTHPA) as the hardener, 2,4,6-tris(dimethylaminomethyl)phenol (DEH 35) as a catalyzer, and poly(lactic acid) (PLA) as workable (i.e., with repairable bonds) phase was investigated using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Hydrogen bonding between PLA carbonyl and epoxy hydroxyl and oxirane groups is the main influent interaction taking place, which acts as changing providers (i.e. hydrogen bonded groups able to move) of curing rate and degree of curing as evaluated from the vibration bands and exothermal released heat. With increasing PLA content, the real crosslink density decreases as also the curing rate is delayed. Partially miscibility between epoxy and PLA is proposed, whereas after reaching epoxy’s solubility limit PLA precipitates, hence the microstructure is suggested to be composed by the epoxy network, interacted Epoxy/PLA and precipitated PLA. Reported data offer reliable tools to reach the aimed degree of crosslinking, controlling of epoxy microstructure overcoming the brittle fracture providing a wider window of processing and applications.
Published by:

Budapest University of Technology and Economics,
Faculty of Mechanical Engineering, Department of Polymer Engineering