Interaction Measure
Thermodynamic compatibility between a polymer and a solvent is represented by a dimensionless value that determines solubility and phase separation. The flory-huggins parameter quantifies this interaction, with lower values indicating a strong affinity that promotes a stable, homogeneous spinning dope. When the value rises above a critical point, the polymer starts to precipitate out of the solution, which initiates the solid fiber formation.
Phase Behavior
Spinning dopes must be stable at the extrusion temperature but ready to solidify when they enter the coagulation bath. This transition is controlled by how the flory-huggins parameter shifts during the process.
Coagulation Rate
Water entering the spin bath acts as a non-solvent, which increases the local value of the thermodynamic interaction variable. This change forces the flory-huggins parameter to exceed its critical limit, resulting in the rapid phase separation that forms the microporous core of the spandex fiber. Adjusting the water concentration allows the mill to control the speed of this phase separation.
Solvent Choice
Industrial fiber extrusion relies on solvents that maintain a low interaction value to prevent premature clogging of the spinneret holes. If the flory-huggins parameter is too high, the polymer chains curl up and aggregate, causing blockages and spinning breaks. Choosing the correct solvent blend ensures a smooth, continuous extrusion process with highly uniform polymer distribution.
