Morphology and orientation of crystallites of simultaneous biaxially stretching ultra-high molecular weight polyethylene films prepared by gelation/crystallization from solutions

Simultaneous biaxially stretching was carried out using ultra-high molecular weight polyethylene dry gel films which were prepared by crystallization from solutions. The concentrations were 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 g/100 mt. The maximum draw ratio was dependent upon the concentration of the solutions. The greatest significant drawability could be realized at 0.9 g/100 mt which is much higher than the optimum concentration of 0.45 g/100 mt assuring the draw ratio > 300-fold for uniaxially stretching. Young's modulus of the biaxially stretched film was much lower than that of uniaxial films with the same draw ratio (70-fold), although the Young's modulus of the film with the maximum draw ratio was much higher than that of commercial films. To address this problem, the orientation function of crystallites was determined from the orientation of the reciprocal lattice vectors of the crystal planes. As the result, it turned out that the c- and a-axes are oriented predominantly to the film surface but the orientational degree of the c-axis is not remarkable in spite of high draw ratio of 8.7 x 8.7. The second order orientation factor was estimated from birefringence by subtracting the crystalline contribution from the total birefringence. These results indicated that the preferential orientation of the c-axis to the stretching direction is mainly due to the rotation of crystallites around their b-axis leading to straining of tie molecules. Furthermore, the ultimate value of Young's modulus was estimated by assuming an ideal simultaneous biaxially stretching film with 100% crystallinity and the perfect orientation of the c-axis parallel to the film surface. Even so, the predicted value was less than 10 GPa, when the elastic compliance of a crystal unit by Odajima and Zehnder were employed. This indicates the difficulty in producing high modulus and high strength polyethylene sheets in terms of theoretical aspects.