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 molecula r 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 o f 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 uniax ial 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 commer cial films. To address this problem, the orientation function of crystallit es 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 ar e oriented predominantly to the film surface but the orientational degree o f the c-axis is not remarkable in spite of high draw ratio of 8.7 x 8.7. Th e second order orientation factor was estimated from birefringence by subtr acting the crystalline contribution from the total birefringence. These res ults indicated that the preferential orientation of the c-axis to the stret ching direction is mainly due to the rotation of crystallites around their b-axis leading to straining of tie molecules. Furthermore, the ultimate val ue of Young's modulus was estimated by assuming an ideal simultaneous biaxi ally 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 aspec ts.