THE EFFECT OF SUPPRESSION OF OFFGASSING ON THE RHEOMETRY OF THERMOTROPIC LIQUID-CRYSTALLINE POLYMERS

Thermotropic liquid crystalline polymers commonly develop a bubble tex ture at time scales necessary to perform low-rate rheological experime nts. We have fabricated a capillary rheometer capable of high-sensitiv ity (relatively low-rate) measurements under a hydrostatic pressure ad equate to suppress formation of visible bubbles. Vectra A (a copolyest er of 73% HBA and 27% HNA) exhibited a transient reduction in viscosit y at low rates that is independent of strain history and dependent onl y on time in the melt; no change in inherent viscosity was observed de spite a threefold change in viscosity. The transient, which was not ob served at high rates on the time scale of the experiment, may reflect changes in the defect structure. The flow curve for a given thermal hi story (time in the melt) exhibited a flattening at low to intermediate rates, suggestive of the flow curves reported for lyotropic systems. This result is qualitatively in agreement with rotational rheometer me asurements known to be contaminated by offgassing, which exhibit highe r apparent viscosities but give the same flow curve shape. Rotational rheometer transients scaled with strain in a manner similar to that re ported for lyotropic systems. Transient reduction in viscosity and flo w curve flattening with decreasing rate was also observed in pressuriz ed rheometer measurements of the fully nematic copolyester of 80% HBA and 20% PET; a decrease in inherent viscosity was observed for this po lymer, but it appears to be too small to account for the observed decr ease in viscosity with time.