Semi-crystalline polymers containing amorphous and crystalline regions usually have intimately mixed chains. The resulting topological constraints (entanglements) in the amorphous regions limit the drawability in the solid state. By controlled synthesis the number of entanglements can be reduced. Ultimately, crystals composed of single chains are feasible, where the chains are fully separated from each other. If such separation can be maintained in the melt a new melt state can be formed. Here we show that through slow and carefully controlled melting such polymer crystals form a heterogeneous melt with more entangled regions, where the chains are mixed, and less entangled ones, composed of individually separated chains. Chain reptation, required for the homogenization of the entanglement distribution, is found to be considerably hindered. The long-lived heterogeneous melt shows decreased melt viscosity and provides enhanced drawability on crystallization. This novel route to create heterogeneous melt should be applicable to polymers in general.
The ease in processability of synthetic polymers has led to their fast growth as commodity as well as engineering plastic, whereas the latter are used in case of demanding applications. Among several, the most well-studied polymer is probably polyethylene. This polymer, based on a simple À(CH 2 ÀCH 2)À repeating unit, can be used for day-today applications as well as for more challenging duties such as prostheses, soft and hard ballistics, light weighted ropes, etc. The wide range of physical and mechanical properties is accessible due to fine control of the molecular architecture, coming from the better understanding of polymer synthesis. The development of a material with the desired properties requires the combination of several disciplines in science, including chemistry, physics, rheology, and processing. It is recognized that, for linear polyethylene, virtually free of chain branching, the physical properties such as wear, abrasion resistance, and impact strength enhance with increasing molar mass. However, the material becomes more difficult to process because the zero shear viscosity follows the well-known power law η 0 µ M 3.4 , thus making the polymer having weight-average molar mass greater than 1 million g/mol nearly impossible to process via the conventional processing route. 1,2 The cause for the increase in viscosity, when increasing the molar mass of the polymer, is related to the increasing number of physical entanglement (friction points due to contact of chain segments) as the molar mass between the entanglement, M e , is considered to be a constant for a given polymer. For example, in the case of linear polyethylene, the molar mass between entanglement is ∼1200 g/mol. 3 The number of entanglement per chain can be effectively suppressed by dissolution of the polymer in a suitable solvent. In dilute solutions, below the so-called overlap concentration Φ*, the number of entanglement per chain can be reduced considerably. 4 In the case of crystallizable polymers, such as linear polyethylene, the reduced entanglement density in the solution can be made permanent since the long chain molecules form folded-chain crystals on cooling where entanglement reside in the amorphous phase, a well-studied phenomenon in polymer physics. 5,6 The reduction of entanglement in the amorphous region of the semicrystalline polymer favor the postdrawing process. 7 Following these concepts, the existing commercial route has been adopted to develop high-modulus high-strength 50 fibers, where dissolution of less than 6 wt % of the ultrahigh 51 molecular weight polymer in a high boiling point solvent such as 52 decalin 8 is required. Prior to removal of the 94 wt % of the 53 solvent, the solution is cooled for crystallization. The disen-54 tangled solid state thus achieved is used for its ease in drawability 55 to make high-modulus high-strength fibers. However, on heating 56 the solution crystallized polymer, the disentangled chains tend to 57 re-entangle, erasing the disentangled state. 9,10 58 A more elegant and also te...
The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order-disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.
A simple and scalable electrospinning process followed by thermal treatment was used to fabricate carbon nanofibers (CFs). The asprepared CFs were investigated as anode materials for sodium ion batteries (SIBs). Remarkably, due to their weakly ordered turbostratic structure and a large interlayer spacing between graphene sheets, the CFs exhibit a dominant adsorption/insertion sodium storage mechanism that shows high reversibility. As a result, the CFs show excellent electrochemical performance, especially cycle stability (97.7% capacity retention ratio over 200 cycles). Reversible capacities of 233 and 82 mA h g À1 are obtained for the CFs at a current density of 0.05 A g À1 and even a high current density of 2 A g À1 , respectively. The excellent cycle performance, high capacity and good rate capability make the CFs promising candidates for practical SIBs.
Recently, the influence of reduced graphene oxide nanoplatelets (rGON) on the rheological response of polymers has been a subject of interest. In the case of disentangled UHMWPE, it has been shown that the chain-filler interaction in the UHMWPE/rGON composite results into an everlasting non-equilibrium melt state having heterogeneous distribution in entanglement density. In this study, a thermal analysis protocol is used to follow the influence of the non-equilibrium polymer melt on the crystallization kinetics of disentangled UHMWPE with, and without, rGON. The analysis is carried out by means of differential scanning calorimetry (DSC) and the results are supported by rheology. When the disentangled UHMWPE sample, without the filler rGON, is left to crystallize under isothermal condition after melting, two endothermic peaks are observed: the high temperature peak (close to the equilibrium melting point, 141.5 °C) is related to the melting of crystals obtained on crystallization from the disentangled domains of the heterogeneous (nonequilibrium) polymer melt, whereas the low melting temperature peak is related to the melting of crystals formed from entangled domains of the melt. On increasing the annealing time in melt (160 °C), the enthalpy of the lower melting temperature peak increases at the expense of the high melting temperature peak, confirming a transformation of the nonequilibrium polymer melt to a fully entangled equilibrium melt state. However, independent of the equilibrium or non-equilibrium melt state, the recurrence of the high melting temperature peak is observed when the samples synthesized using the single-site catalytic system are left to isothermal crystallization at a specific temperature. The recurrence of the high melting temperature, close to the equilibrium melting point of the polymer, questions the differences in entanglements formed before and after polymerization in these high molar masses. The differences in the topological constraints are likely to influence the difference in melting temperatures of the isothermally crystallized samples. In the presence of rGON, the melting response of disentangled UHMWPE crystallized from its heterogeneous melt state changes; at a specific filler concentration, it is observed that the high endothermic peak remains independent of the annealing time in melt. This observation strengthens the concept that in the presence of the filler, chain dynamics is arrested to an extent that the nonequilibrium melt state having lower entanglement density is retained, facilitating the formation of crystals having high melting temperature. IntroductionThe topology of methylene segments in the non-crystalline region of the semi-crystalline polymer Ultrahigh Molecular Weight Polyethylene (UHMWPE), has a profound influence on the mechanical deformation either uniaxially or biaxially [1]. The topology can be tailored by controlling the crystallization kinetics either by dissolution and crystallization or controlled polymerization [2,3,4,5]. The influence of mo...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.