Melt rheological behavior of a triblock copolymer based on aramide end-segments

Melt rheological behavior of a ABA triblock polymer made of poly(tetramethylene oxide) (PTMO) (Mn = 2,900 g mol⁻¹) soft segment and aramide hard segment was studied. The aramide end-segments (A) were short and mono-disperse in length. The mid-segment (B) consisted of PTMO₂₉₀₀ extended with terephthalate units to a molecular weight of 9000 g mol⁻¹. The molecular weight of the triblock was 9700 g mol⁻¹. Rheological behavior of this material was studied by parallel-plate and capillary method. The ABA triblock copolymer was compared with a B polymer (PTMO-terephthalate) of a similar molecular weight. The low molecular weight B polymer had a Newtonian behavior. The low molecular weight triblock copolymer had at high frequencies a low complex viscosity. However, at low frequencies the triblock copolymer had a very high complex viscosity. Also the G"/G' ratio decreased with decreasing frequency to values less then one and the G' seemed to have at low frequencies a plateau value. The activation energy of the process increased in value with decreasing shear rate. All these results indicate that the triblock copolymer at low frequencies had a gel-like behavior and this probably due to the clustering of the aramide segments. The aramide clusters are thought to be the (weak) network points of the gel. This network was also found to have a time dependant rheological response and thus a thixotropic behavior.