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All issues / Volume 14 (2020) / Issue 5 (May)

The Polypropylene Pope would be 70
T. Barany
Vol. 14., No.5., Pages 400-400, 2020
DOI: 10.3144/expresspolymlett.2020.33
This is an editorial article. It has no abstract.
Fast curing versus conventional resins – degradation due to hygrothermal and UV exposure
N. M. Barkoula, M. Karabela, N. E. Zafeiropoulos, P. Tsotra
Vol. 14., No.5., Pages 401-415, 2020
DOI: 10.3144/expresspolymlett.2020.34
In the current paper, three resin systems are investigated, the first is a high temperature fast curing system, the second is a high-temperature system that requires several hours to cure and the third is a slow room temperature curing system. All systems are exposed to hygrothermal and combined moisture/temperature/UV protocols, and their durability is assessed using Fourier transform infrared (FTIR) spectroscopy, mechanical and thermomechanical characterization. The conventional slow curing system presents a wide distribution of polymer chains that leads in excess free volume/increased susceptibility to water. The medium curing system is more homogenous and shows lower absorption profiles. The fast curing system shows two network domains, one that is more crosslinked and contributes towards higher free volume/water absorption and the other that is partially cured and supports chain scission/leaching processes during exposure. Continuation of crosslinking prevails in conventional resins upon hygrothermal and combined UV exposure. At the same time antagonistic effects (i.e. continuation of the crosslinking process versus plasticization, chain scission, and leaching) are observed in medium curing and fast curing systems upon exposure.
Smart polymer nanocomposites: A review
W. S. Chow, Z. A. Mohd Ishak
Vol. 14., No.5., Pages 416-435, 2020
DOI: 10.3144/expresspolymlett.2020.35
The development of smart polymer nanocomposites (SPN) has been an area of high scientific and industrial interest in recent years, due to fantastic improvements achieved in these materials. SPN found potential applications in shape memory, self-healing, self-sensing, self-heating, self-cleaning, and energy harvesting. This mini-review highlights the current research and development on SPN, specifically on shape memory polymer (SMP) and self-healing polymer (SHP) nanocomposites. The processing techniques for SPN nanocomposites, for example, polymerization, melt-compounding, solution mixing, electrospinning, and thermoset-curing are discussed. Some of the potential strategies to modify the properties of SMP and SHP nanocomposites are highlighted. The future perspective and recommended works for the SPN nanocomposites are shared in this mini-review.
Polymer nanocomposites: Why their mechanical performance does not justify the expectation and a possible solution to the problem?
S. Fakirov
Vol. 14., No.5., Pages 436-466, 2020
DOI: 10.3144/expresspolymlett.2020.36
The expected order-of-magnitude enhancement in modulus (at loadings less than 1%) of polymer nanocomposites has proven elusive – the observed improvements are 10–35% only. The failure of the concept has been attributed to poor dispersion, poor interfacial load transfer, process-related deficiencies, and others. Due to the inherent property of the nano-size materials, their extremely high specific surface, they tend to agglomerate, and their further dispersion in the matrix component is practically impossible. For this reason, the composites prepared via blending a polymer with a nano-size material are microcomposites instead of nanocomposites, as demonstrated by light scattering studies. So long as reliable tools and/or techniques for proper dispersion of nanomaterials are missing, we must use methods free of the dispersion step in the manufacturing process. The relatively new ‘concept of converting instead of adding’ offers two such techniques – instead to take the matrix and the reinforcement in their final form and blend them, one takes one component only and during the processing creates the missing second component. Both approaches result in true nanocomposites with superior mechanical performance – the improvements of the tensile strength for nanofibrillar polymer-polymer and single polymer composites are up to 200 and 440%, respectively, (or even up to 650% if trans-reaction catalyst is used).
Effects of cellulose nanofiber on the thermal, mechanical, and optical properties of triacetate cellulose nanocomposites
C. M. Wu, K. S. Danh, A. N. Nakagaito
Vol. 14., No.5., Pages 467-476, 2020
DOI: 10.3144/expresspolymlett.2020.37
In this study, well-dispersed nanocomposite films with triacetate cellulose (TAC) as a matrix and reinforced with lemon peel-cellulose nanofibers (lemon peel-CNF) were developed. The nanofibers in aqueous solution were solvent-exchanged to methanol by a series of centrifuging and re-dispersing steps. Afterward, using the solution casting method, the mixture of recycled TAC (rTAC) film and nanofibers prepared by stirring combined with ball milling technique to achieve full dispersed solution was coated onto glass to obtain a thin film. The dispersion effect of CNF in TAC was observed by transmission electron microscopy. The optical, mechanical, and thermal properties of the nanocomposite films were characterized experimentally. The results showed that with varying CNF content (1–7 wt%), the haze was slightly increased while the transmittance was not affected compared with that of rTAC film (92.7%). The addition of CNF increased the tensile strength by 60%, tensile strain by 150%, and yield strength by 50%. Creep compliance improved for all nanocomposites compared with rTAC film. CNF led to a significant reduction in the thermal expansion properties of rTAC film.
A comparative study on the electrical properties of different forms of carbon allotropes – epoxy nanocomposites
S. G. Stavropoulos, A. Sanida, G. C. Psarras
Vol. 14., No.5., Pages 477-490, 2020
DOI: 10.3144/expresspolymlett.2020.38
In this study, series of nanocomposites consisting of an epoxy matrix and different carbon nanoinclusions (Carbon Black, Multiwall Carbon Nanotubes (MWCNT), Graphene nanoplatelets (GnP) and nanodiamonds) were developed, and their electrical response was examined in wide frequency and temperature ranges. Depending on the filler type and concentration, nanocomposites exhibited either insulator to conductor transitions or dielectric relaxation phenomena. Recorded relaxations were attributed to interfacial polarization, glass to rubber transition and motion of polar side groups. Nanocomposites integrating carbon black or MWCNTs exhibit an abrupt increase in permittivity and conductivity at a critical concentration (or percolation threshold). The insulator to conductor transition is described by means of percolation theory and critical concentration and exponent are determined. Conductance mechanisms are investigated in all sets of nanocomposites, by accounting the influence of temperature on conductivity and by applying the Variable Range Hopping model. Further, analysis reveals hopping conductivity as the main charge migration process below critical concentration, while hopping and metallic-like conduction coexist above it.
β-Nucleation of isotactic polypropylene: Chain structure effects on the effectiveness of two different nucleating agents
J. Wang, M. Gahleitner, D. Gloger, K. Bernreitner
Vol. 14., No.5., Pages 491-502, 2020
DOI: 10.3144/expresspolymlett.2020.39
Of all crystal modifications of isotactic polypropylene (iPP), the β-phase has received particular attention in both academia and industry because of the toughness increase resulting from its formation. Several novel nucleating agents have been presented and studied in recent years, most prominently the ‘lanthanum complex’ WBG. The majority of these studies are based only on one type of iPP, mostly an iPP homopolymer, despite the fact that chain irregularities are known to affect β-phase formation negatively. In the present study, several different iPP homo- and copolymers based on Ziegler-Natta and single-site catalysts differing mostly in chain defect concentration were used to evaluate the sensitivity of a ‘reference’ nucleating agent, quinacridonequinone, as well as WBG to chain structure effects. The results in terms of crystal structure, as well as mechanics, indicate a much greater sensitivity of WBG, resulting in a massively reduced β-phase formation for SSCiPP hompolymers and ZN-iPP random copolymers with ethylene. An increase of γ-phase content in nucleation of the polymers with least regular chain structure hints at some duality for WBG as also observed for other β-nucleating agents before.
Static and dynamic mechanical characterization of cross-linked polyethylene foams: The effect of density
A. Kmetty, M. Tomin, T. Barany, T. Czigany
Vol. 14., No.5., Pages 503-509, 2020
DOI: 10.3144/expresspolymlett.2020.40
We performed the complex static and dynamic mechanical characterization of cross-linked polyethylene foams; we investigated the effect of foam density on various mechanical properties and the relationship between cell structure and mechanical behavior. The impact damping and energy absorption properties of the foams were determined by falling weight impact tests, while static mechanical properties were determined by compression and compression set tests. The experimental results validated our hypothesis that the relationship between the above-mentioned mechanical properties and density can be described with the power law with good approximation.
Published by:

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