4 Conclusions - POLYTECH - ETH Zürich

2 From Fiber to Fabric and Film and FoilAs mentioned above, the origin of the outstanding mechanical properties of highperformancepolymer fibers is the covalently bonded nature of their constituentmacromolecules and their predominantly uniaxial arrangement. Unfortunately, theresponse of such highly anisotropic structures, not surprisingly, is strongly dependenton the direction of applied load or strain, as is quantitatively reflected in their stiffnessmatrices, shown in Figure 2A for ideal single crystals of PPTA 41 and PE 42 . Figure 2Bdisplays the extraordinary pronounced dependence of the Young’s modulus on the testangle of such structures, calculated according to Bastiaansen et al. . 43 Mostillustratively, at an inclination of only 10 °, the modulus of PE reduces to only aboutone-tenth of the axial (macromolecular) stiffness. The somewhat less sensitive “offaxisperformance” of PPTA is, of course, due to the stronger hydrogen bonds betweenthe polymer chains in comparison to the weak van der Waals interactions between PEmacromolecules.ACPPTACPE⎛40 23 13 0.1 1.2 2.4⎞⎜⎟⎜31 41 1.5 3.2 1.9⎟⎜360 0.3 5 11 ⎟= ⎜ ⎟GPa⎜5.5 3.7 2.7⎟⎜22 3.2⎟⎜7.5⎟⎝⎠⎛8.0 3.3 1.1 0 0 0⎞⎜⎟⎜9 2.1 0 0 0⎟⎜316 0 0 0 ⎟= ⎜ ⎟GPa⎜3.2 0 0⎟⎜1.6 0⎟⎜3.6⎟⎝⎠BE (GPa)3503002502001501005000 30 60 90θ (°)FIGURE 2 A. Stiffness matrices of PPTA 41 (top) and PE 42 (bottom). B. Theoretical test-angle, θ,dependence of the Young’s modulus, E, of an ideal, extended-chain single crystal of PPTA (dashedcurve) and PE (solid curve), calculated according to ref. 43.The above strong test-angle dependence of the mechanical properties of highperformancepolymer fibers, naturally, has major consequences for the design and-15-