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...