Chapter 1: High-performance polyethylene fibers

In 1960, Treloar [1] published a seminal paper showing that the stiffness (E-Modulus) of an extended viz. fully stretched polyethylene and polyamide chain is close to 200 GPa, which is appr. the same stiffness as steel. In the 1970s, Zhurkov [2] and Boudreaux [3] estimated that the strength of an extended PE chain could be as high as 25 GPa. These theoretical calculations/predictions rose the interest of many scientists and engineers to pursue chain-extension which in practice implies that fibers are made in which a maximum degree of chain-extension, viz. chain alignment, in the fiber direction is attempted. These earlier publications were in fact not very realistic because even if polymer chains could be perfectly aligned in the fiber direction, then the maximum length of a polymer chain is limited to some 10 μm. In measuring fiber properties macroscopic dimensions are encountered with gauge length's in the order of several centimeters in the tensile tester. Consequently, in a tensile tester an ensemble of a larger number of parallel chains is tested and the stress-transfer and chain overlap between individual chains determine the tensile properties like strength and stiffness/E-Modulus. Consequently, the strength of a polyethylene fiber is determined by the rather weak intermolecular van der Waals interactions with important consequences for chain alignment and chain length.

In the case of the non-polar polyethylene with only weak van der Waals intermolecular interactions one would expect no impressive tensile fiber properties whereas in the case of more polar polymers, such as the polyamides, one would expect quite some interesting properties when the hydrogen-bonds between the individual chains could be aligned along the fiber axis. The reality turned out to be quite the opposite as will be discussed in this chapter for polyethylene and in the chapter on ‘high-performance fibers based on flexible polar molecules’ for the polyamides (nylons).