4 Conclusions - POLYTECH - ETH Zürich

iaxially oriented polymer films. Such films have been produced by biaxialdeformation in the solid state or, for instance in the case of PPTA, using a mandrelguidedprecipitation process. 49-51 Another technique, specifically designed to processthermotropic liquid-crystalline polymers, such as Vectra ® , employs a die with threerotating cylinders to form films. 52, 53 An alternative - most elegant - process is the insitugrowth of planar oriented bacterial cellulose films. 54 Table 2 displays relevantmechanical properties of a number of such “high-performance” polymer films. Evencursory examination reveals that their characteristics are far inferior when compared tothose of the corresponding uniaxially oriented fibers (Table 1).TABLE 2 Mechanical properties of selected high-performance polymer films. *Value provided bythe supplier.material Young’s modulus E Strength σ Strain at break ε Density ρ ref.GPa MPa % g/cm 3Bacterial Cellulose 18 230 2 0.99 54PPTA Aramica ® 15 400 20 1.44 55HBA/HNA Vectra ® 10 240 3* 1.40 56UHMW PE 6 600 50 0.98 57PET Mylar ® 5 250 120 1.39 58Calculations of Bastiaansen et al. 43 indicate that, owing to their structure, the stiffnessof such films, in fact, can be expected to be relatively low. Biaxially oriented filmstypically resemble a planar aggregate of small crystals and not a sheet-like laminate ofextended-chain single crystals (see Figures 4A and 4B). Assuming, for instance, thatthe constituent entities are ideal single crystals of polyethylene with the stiffnessmatrix presented in Figure 2A, the maximum Young’s modulus of such an aggregatewas shown to be only 12 GPa, while that of the hypothetical laminate of ideal singlecrystals was calculated to be 111 GPa. As the latter structure is of key interest in thepresent work, their mechanical properties will be elaborated upon in some more detailin the next section.-17-