Melting Kinetics of Nascent Poly(tetrafluoroethylene) Powder

The melting behavior of nascent poly(tetrafluoroethylene) (PTFE) was investigated by way of differential scanning calorimetry (DSC). It is well known that the melting temperature of nascent PTFE is about 344∘ C, but reduces to 327∘ C for once molten material. In this study, the melting temperature of nascent PTFE crystals was found to strongly depend on heating rate, decreasing considerably for slow heating rates. In addition, during isothermal experiments in the temperature range of 327∘C<T<344∘ C, delayed melting of PTFE was observed, with complete melting only occurring after up to several hours. The melting kinetics of nascent PTFE were analyzed by means of the isoconversional methodology, and an apparent activation energy of melting, dependent on the conversion, was determined. The compensation effect was utilized in order to derive the pre-exponential factor of the kinetic model. The numerical reconstruction of the kinetic model was compared with literature models and an Avrami-Erofeev model was identified as best fit of the experimental data. The predictions of the kinetic model were in good agreement with the observed time-dependent melting of nascent PTFE during isothermal and constant heating-rate experiments.ISSN:2073-436

Additive Manufacturing of Polyolefins

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?

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

Additive Manufacturing of Polyolefins

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

Melting kinetics, ultra-drawability and microstructure of nascent ultra-high molecular weight polyethylene powder

ISSN:0032-3861ISSN:1873-229

Melting-Induced Evolution of Morphology, Entanglement Density, and Ultradrawability of Solution-Crystallized Ultrahigh-Molecular-Weight Polyethylene

The melting-induced change in the density of physical network junctions, which are formed by chain entanglements and network junctions due to anchoring of chain segments to crystals, is studied by 1H NMR T2 relaxometry for solution- and melt-crystallized ultrahigh-molecular-weight polyethylene (UHMWPE), sc-UH, and mc-UH, respectively. The NMR results are complemented by real-time synchrotron wide- and small-angle X-ray scattering (WAXS and SAXS) analyses to extract the sizes of the crystalline lamellae and intercrystalline domains. Below the melting temperature, the network of physical junctions is denser in the amorphous phase of mc-UH than the one in sc-UH owing to a lower entanglement density and a smaller number of physical junctions from polymer crystals in sc-UH. However, the difference in the total density of physical junctions between mc-UH and sc-UH films decreases with decreasing crystallinity during melting. At the end of the melting trajectory, at vanishing crystallinity, the volume-average entanglement density, as characterized by the NMR method, is approximately the same in sc- and mc-UH. This indicates that the entanglement density in sc-UH films increases during melting owing to the fast buildup of local chain entanglements. These entanglements are formed by segments of the same chain, neighboring chains, or both due to a displacement of chain fragments upon lamellar thickening and due to the so-called “chain explosion” that occurs locally in the amorphous domains. The increase in the entanglement density in sc-UH is additionally confirmed by the solid-state drawability of sc-UH films that were annealed in the melting region but below the end of melting. The maximum draw ratio decreases and the drawing stress increases with the increasing annealing temperature