MORPHOLOGICAL-CHARACTERISTICS OF THE LYOTROPIC AND GEL PHASES IN THE CELLULOSE NH3/NH4SCN SYSTEM/

Solutions of cellulose in the ammonia/ammonium thiocyanate (24.5/75.5 w/w) solvent form several stable phases. Of particular interest in thi s work are the temperature-dependent liquid crystalline and gel phases which are stable at cellulose concentrations above 6% w/v. While the temperature-composition conditions yielding these phases are reasonabl y well established, very little is currently known about the morpholog ical characteristics of lyotropic and gelled cellulose. Polarized ligh t microscopy is employed here to demonstrate that solutions at tempera tures above the gel melting point are birefringent, composed of liquid crystals. Field-emission scanning electron microscopy has been used t o (i) examine the three-dimensional network in cellulose gels, and (ii ) correlate network morphology with cellulose molecular weight and sol ution concentration. Results obtained from two complementary sample pr eparation techniques (i.e., critical point drying and freeze drying) a re compared to identify and minimize artifacts, and reveal that gel fo rmation occurs as the solutions phase-separate into polymer-rich aniso tropic and solvent-rich isotropic phases. The polymer-rich phase is hi ghly interconnected and forms a fibrillar network, with fibrils measur ing 20-70 nm in diameter. (C) 1996 John Wiley & Sons, Inc.