Optimal Entanglement of Polymers Promotes the Formation of Highly Oriented Fibers

Polymer fibers consist of macromolecules oriented along the fiber axis. Better alignment of chains leads to an increased strength of the fiber. It is believed that the key factor preventing formation of a perfectly oriented fiber is the entanglement of polymers. We performed large-scale computer simulations of uniaxial stretching of semicrystalline ultrahigh molecular weight polyethylene (UHMWPE). We discovered that there is an optimal number of entanglements per macromolecule necessary to maximize chain orientation in a fiber. Polymers that are entangled too strongly form less-oriented fibers. On the other hand, when polymers have too few entanglements per chain, they disentangle during stretching, and a strong fiber is not formed. We constructed a microscopic analytical theory describing both the fiber formation and disentanglement processes. Our work presents a novel view of the role of entanglements during fiber production and predicts the existence of an optimal number of entanglements per chain maximizing the fiber quality that scales with chain length as ≈ N2/5. A total of 102 entanglements per chain is the optimum for UHMWPE having typical degree of polymerization of N ≈ 105.