THE ULTIMATE STRENGTH AND STIFFNESS OF POLYMERS

Ultraoriented polymer fibers have elastic modulus E as large as 350 GP a and tensile strength sigma(b) as large as 7 GPa in materials with a density rho approximate to 1200 Kg/m(3). Keys to achieving these prope rties are near perfect orientation of polymer chains along the fiber a xis and reduction of the number of chain ends. The two materials that have been most thoroughly studied are polyethylene (PE) and poly(p-phe nylene terephthalamide) (PPTA). Various schemes for calculating the el astic modulus are reviewed, together with estimates of effects of impe rfect chain orientation and the presence of chain ends. Fracture of fi bers is treated in terms of covalent bond scission and/or chain slip o riginating at chain ends. Under laboratory conditions the experimental modulus E can be >90% of the theoretical modulus. It appears that fra cture is more sensitive to chain end defects, limiting practical stren gth to less than 25% of the ultimate strength predicted from bond scis sion models.