  Gel spinning is the industrial method of choice for combining hydrophilic
ultra-high molecular weight (UHMW) polymer resins with a hydrophobic support
polymer to produce composite filaments for cytapheresis. Cytapheresis is a
medical technique for removal of leukocytes from blood. Gel spinning is used to
avoid high melt viscosity and thermal sensitivity of UHMW resins and the high
melt temperature of the substrate resin but requires the recovery of toxic
solvents. The UHMW resin is used because it forms a stable gel phase in the
presence of water; a lower molecular weight resin (LMW) simply dissolves. UHMW
and LMW resins were both poly(ethylene oxide) (PEO) and the substrate was
polyarylsulfone (PAS). The literature indicated PEO undergoes non-oxidative
thermal degradation above 200 {\deg}C and PAS is processed up to 350 {\deg}C.
Dynamic oscillatory shear rheometry was used to study 0, 25, 40, 50, 60, and 75
wt. % UHMW PEO in LMW PEO to take advantage of the sensitivity of viscosity to
changes in molecular weight and material configuration, indicating degradation.
Samples were exposed to 220 {\deg}C, 230 {\deg}C, 240 {\deg}C, 250 {\deg}C, 275
{\deg}C, and 300 {\deg}C temperatures for 5 min to explore conditions that
could result in sample degradation. The viscosity decreased less with
increasing UHMW PEO content for samples exposed to the same temperature and the
viscosity decreased more with increasing exposure temperature for samples with
the same UHMW PEO content. Parameters were regressed from observed data to
predict the change in molecular weight via empiricisms relating the viscosity
to molecular weight, shear rate, temperature, and time.
