COMPOSITIONAL CHANGES DURING THE WET SPINNING OF ACRYLIC FIBERS FROM AQUEOUS SODIUM THIOCYANATE SOLVENT

The compositional changes taking place during the wet spinning of acry lic fibers from an aqueous sodium thiocyanate solvent were investigate d. The composition of the fibers diverted from the precipitation bath after various immersion times was determined gravimetrically, while fi ber diameters were imaged to ascertain volumetric changes with time. T he kinetics of phase separation were approximated using light transmis sion and video techniques applied to acrylic films. For coagulation in to water at 20 and 40 degrees C and into 15% aqueous NaSCN at 20 degre es C, a greater influx of the nonsolvent to outflow of the solvent was recorded at short timescales. Unexpectedly, both the outflow of the s olvent and nonsolvent against the concentration gradient was noted at longer timescales, suggested by the light transmission data to be afte r the primary phase separation. The consequent reduction in filament d iameter, hence, the volume, is discussed in terms of a coarsening mech anism, whereby the mobile polymer lean phase has a route away from the filament into the bath during polymer coarsening. Finally, the compos itional changes are plotted on a phase diagram for the system as traje ctories into the two-phase region. The polymer-coarsening effect rende rs the interpretation at longer timescales uncertain. (C) 1998 John Wi ley & Sons, Inc.