Fotios Christakopoulos


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Christakopoulos

First Name

Fotios

ORCID

0000-0001-7383-5875

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2022 - 20244
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Publications 1 - 4 of 4
  • Gas-phase polymerization of ultra-high molecular weight polyethylene with decreased entanglement density
    Item type: Journal Article
    Lopes do Rosario, Roberta; Christakopoulos, Fotios; Tervoort, Theo A.; et al. (2023)
    Journal of Polymer Science
    It is well known that ultra-high molecular weight polyethylene (UHMWPE) is a polymer with long chains and very high molecular weight that poses difficulties in terms of processability due to the presence of chain entanglements. In many cases is thus necessary to treat the material in different ways after the polymerization to minimize the amount of entanglements and improve the processability. Based on observations that the use of inert condensing agents (ICA) had a noticeable impact on molecular weight and crystallinity, it was decided to develop a gas-phase polymerization process with addition of ICA for UHMWPE with a high fraction of disentangled chains. For the optimization of this process, the comparison with slurry is important for the understanding the improvement. Thus, a clear difference between slurry and gas phase is observed in terms of crystallinity and the lamellar thickness of the crystals, molecular weight and entanglements. Characterization techniques are developed to measure the properties of the reactor powder and understand the impact of the alkanes in situ. Using solid-state drawability, the entanglement degree of the reactor powder is analyzed. From the small-angle x-ray scattering and wide-angle x-ray scattering techniques, it is possible to find a correlation of entanglements and lamellar thickness. Moreover, crystallization kinetics measurements of the polymer in presence of ICA constitutes a powerful method to explain the phenomena of entanglement and crystal formation.
  • Additive Manufacturing of Polyolefins
    Item type: Review Article
    Christakopoulos, Fotios; van Heugten, Paul M.H.; Tervoort, Theo A. (2022)
    Polymers
    Polyolefins are semi-crystalline thermoplastic polymers known for their good mechanical properties, low production cost, and chemical resistance. They are amongst the most commonly used plastics, and many polyolefin grades are regarded as engineering polymers. The two main additive manufacturing techniques that can be used to fabricate 3D-printed parts are fused filament fabrication and selective laser sintering. Polyolefins, like polypropylene and polyethylene, can, in principle, be processed with both these techniques. However, the semi-crystalline nature of polyolefins adds complexity to the use of additive manufacturing methods compared to amorphous polymers. First, the crystallization process results in severe shrinkage upon cooling, while the processing temperature and cooling rate affect the mechanical properties and mesoscopic structure of the fabricated parts. In addition, for ultra-high-molecular weight polyolefins, limited chain diffusion is a major obstacle to achieving proper adhesion between adjunct layers. Finally, polyolefins are typically apolar polymers, which reduces the adhesion of the 3D-printed part to the substrate. Notwithstanding these difficulties, it is clear that the successful processing of polyolefins via additive manufacturing techniques would enable the fabrication of high-end engineering products with enormous design flexibility. In addition, additive manufacturing could be utilized for the increased recycling of plastics. This manuscript reviews the work that has been conducted in developing experimental protocols for the additive manufacturing of polyolefins, presenting a comparison between the different approaches with a focus on the use of polyethylene and polypropylene grades. This review is concluded with an outlook for future research to overcome the current challenges that impede the addition of polyolefins to the standard palette of materials processed through additive manufacturing.
  • Disentangled Melt of Ultrahigh-Molecular-Weight Polyethylene: Fictitious or Real?
    Item type: Journal Article
    Litvinov, Victor; Christakopoulos, Fotios; Lemstra, Pieter Jan (2024)
    Macromolecules
    There are two opposing views on the role of the melting of low-entangled polyethylenes on the obtained entanglement density in the molten state and the time required for its equilibration, namely, instantaneous recovery to the equilibrium melt state upon melting (chain explosion) versus slow recovery (melt memory effect). A series of rheological studies have shown that slow heating of low-entangled nascent ultrahigh-molecular-weight polyethylene (UHMWPE) powders to temperatures above the equilibrium melting temperature causes the formation of “the disentangled nonequilibrium melt” due to consecutive detachment of chain stems from the edges of crystals keeping the largely intact low-entangled middle part of chains in the melt. Contrary to the rheology findings, studies of several mechanical properties of recrystallized UHMWPE have found that the equilibrium entanglement density is reached instantly upon the melting of low-entangled UHMWPE, suggesting a chain explosion mechanism. To obtain additional information that can help in understanding the melt memory phenomenon better, several UHMWPE samples are studied by ¹H NMR T₂ relaxometry. The NMR experiments are performed for melts prepared from UHMWPE powders with different entanglement densities that were molten using fast (∼10 K/min) and slow (∼0.2 K/min) heating rates. In all cases, the existence of “the disentangled nonequilibrium melt” was not observed. The results are explained by the chain explosion mechanism that leads to the equilibrium volume-average entanglement density, already at the final stage of melting. Cautious rheological experiments also do not detect “the disentangled nonequilibrium melt”. Possible artifacts of previous rheological studies of disentangled UHMWPE melts are discussed. The conclusion of the present study is supported by a large number of previous investigations that are briefly reviewed. Is the disentangled melt state fictitious or real? The answer is yes and no. Chain explosion causes instantaneous equilibration of the volume-average entanglement density upon melting by the formation of local entanglements that play a major role in several volume-average properties, i.e., modulus, drawability, adhesion, segmental mobility, and some other properties. However, the uniform distribution of topological knots between chains is a slow process that is largely governed by chain reptation. The heterogeneity of the entanglement network as well as the impurities in UHMWPE can influence (1) the local nucleation phenomenon at crystallization that does not characterize the entanglement network and (2) deformation properties at ultimate strains. Therefore, the definition of melt memory and chain explosion should be specified to properties that are used for the characterization of low-entangled and equilibrium melt states.
  • Solid-state extrusion of nascent disentangled ultra-high molecular weight polyethylene
    Item type: Journal Article
    Christakopoulos, Fotios; Busato, Stephan P.; Kong, Xiang; et al. (2024)
    Polymer Engineering & Science
    Ultra-high molecular weight polyethylene (UHMWPE) stands out as the preferred material across a wide spectrum of demanding applications, thanks to its remarkable mechanical, chemical and physical attributes. Nevertheless, processing UHMWPE can be an arduous and time-consuming endeavor. Traditional melt-processing techniques, commonly employed for polymers, prove impractical due to the exceedingly high viscosity of the melt that follows from the highly entangled macromolecular chains. Several methods are known to lower the entanglement density of UHMWPE, such as crystallization from a semidilute solution or direct polymerization, resulting in a less interconnected polymer structure. Unfortunately, even with low-entangled UHMWPE, melt processing remains unviable because of the rapid re-entanglement of polymer chains upon melting. However, these low-entangled UHMWPE materials can still be effectively processed in the solid state, a technique employed in the production of high-performance UHMWPE fibers and tapes, currently the only two feasible geometries. This research reports the development of a solid-state extrusion method for nascent low-entangled powder, centering on manipulating the surface energy of the interior wall materials within the extruder and die. Ordinary steel dies proved ineffective for solid-state extrusion, as they generated excessive friction between the die and UHMWPE, resulting in a rapidly escalating extrusion pressure. However, utilizing dies constructed from temperature-resistant polymers allowed for extrusion at a consistent pressure without the need for lubricating agents. We demonstrate the practicality of the developed solid-state extrusion method by fabricating UHMWPE monofilaments and tubes, showcasing its potential in expanding the range of producible UHMWPE geometries. Additionally, a first evaluation of the influence of processing conditions and the entanglement density of the reactor powder used on extrudability, and the properties of the resulting components is presented.Highlights A process for the solid-state extrusion of UHMWPE is developed. UHMWPE processing with higher design flexibility than current industrial standards. UHMWPE processing with higher throughput than current industrial standards. Contrary to previous techniques no lubricating agent or co-extrudate is used.
Publications 1 - 4 of 4