A novel orientation technique for semi-rigid polymers. 1. Preparation of cross-linked cellulose acetate and hydroxypropylcellulose films having permanent anisotropy in the swollen state

It has been predicted that unusually good mechanical properties can be obtained by drying swollen networks of semi-rigid chains while they are in the deformed state, as described in several theoretical investigations Macromolecules,23: 5335, 5341 (1990),24: 901 (1991)]. The present investigation involves the preparation of networks of this type from cellulose acetate (CA) and hydroxypropylcellulose (HPC), in order to test these concepts. The cross-linking required to maintain anisotropy during the drying process was obtained using formaldehyde, while the polymers were in either the anisotropic or isotropic state. Control of the cross-linking was obtained by studying the effects of the concentration of formaldehyde, temperature, and reaction time.The liquid-crystalline phase separations in CA and HPC, and in their networks, were studied with cross-polarized optical microscopy. CA and HPC showed anisotropic phases in trifluoroethanol and in methanol, respectively, and under shear the HPC systems exhibited the band textures associated with macroscopic orientation. In the case of the uncross-linked polymers, this band texture disappeared shortly after shearing was discontinued. The networks prepared by cross-linking the HPC in either liquid-crystalline solutions or in isotropic solutions also showed band textures, but these textures now persisted long after removal of the shearing stress. As shown in the following paper, the extensibility required in the proposed processing technique was highest for the networks prepared in the isotropic state, suggesting that these materials should have the greatest potential for dramatic improvements in mechanical properties.