Development of High-Modulus and High-Strength Poly(tetrafluoroethylene) Fibers by Elongation at a Liquid Crystalline State

Raw filaments of ultrahigh molecular weight poly(tetrafluoroethylene) (PTFE) (8.4 × 10 6 ) were prepared by the paste extrusion of fine powder. The raw filaments with a sag ratio of 25% were annealed at 360 °C, higher than the apparent melting point of 330 °C estimated by differential scanning calorimetry (DSC), at a heating rate of 10 °C/min. After annealing for 30 min, they were cooled to room temperature at desired rates. The resultant monofilaments were annealed for 30 min at 388 °C and were elongated up to 100 times at the same temperature. Young's modulus and the tensile strength were sensitive to the sag ratio of raw filaments under heating and cooling processes. The maximum values of Young's modulus and the tensile strength of drawn fibers reached 57.6 and 2.31 GPa, respectively, at 25−26 °C associated with the crystal transition at room temperature, when the sag ratio was 25% corresponding to the intrinsic shrinkage of the raw filaments. This indicates that a suitable level of the entanglement mesh to assure the maximum values of the tensile strength and Young's modulus was formed by the drastic shrinkage (25%) of the raw filaments, leading to molecular motion without constraints. This phenomenon is discussed in terms of the morphology of the monofilaments and drawn fibers as studied by optical microscopy (crossed-polarized), differential scanning calorimetry, and X-ray diffraction techniques. The production of high-strength PTFE fibers is attributed to the appearance of thermotropic liquid crystals at temperatures higher than the apparent melting point, reflecting the chain rigidity of PTFE.