Mass transport and radial composition evolution during coagulation in dry-wet spinning of cellulose/TBAP/DMSO solutions

Component exchange and radial composition changes during coagulation were investigated for cellulose filaments spun from TBAP/DMSO solutions. Here, the exchange of water, dimethyl sulfoxide (DMSO), and tetrabutylammonium hydrogen phosphate (TBAP) in cellulose/TBAP/DMSO filaments was quantified by gravimetry and ¹H NMR. Fick’s second law in cylindrical coordinates was used to estimate apparent diffusion coefficients and calculate radial composition paths. Experimentally determined pseudo-ternary cloud-point diagrams were then used to assess the onset of phase separation along these paths. Water exhibited the highest apparent diffusivity at low temperatures, while DMSO became the fastest at high temperatures and TBAP remained the slowest throughout. Increasing bath temperature shortened the period of net liquid uptake. At equal local water content, the maximum skin-core composition differences increased from 1.8 to 5.6 wt% for DMSO and from 2.3 to 5.8 wt% for TBAP as temperature increased from 10 to 70°C. Adding DMSO to the bath generally reduced apparent diffusion and radial composition differences. Cellulose concentration had a limited effect on apparent diffusivity. This work links component transport to local composition changes during phase separation and provides a quantitative basis for selecting coagulation conditions to reduce radial composition heterogeneity.