Rapid End-Block Pullout in ABA Triblock Polymer Gels

The end-block pullout time of the ABA triblock polymer poly[styrene-b-(ethylene-alt-propylene)-b-styrene] (SEPS) has been studied via rheology. Gels of SEPS in squalane of varying concentrations were produced via a cosolvent method and then rheometry was performed at various temperatures. Master curves were produced by time-temperature superposition, and longest relaxation times were extracted. These relaxation times were then compared with the results of previous time-resolved small-angle neutron scattering (TR-SANS) experiments that focused on chain exchange kinetics in SEPS/squalane micelles. It was found that the relaxation time of the gels (associated with the end-block pullout time) was up to four orders of magnitude smaller than would be expected based on the TR-SANS experiments of equivalent diblock polymers. A model is proposed that accurately reproduces the triblock TR-SANS results by incorporating two additional factors: 1) a reduction in the energy barrier to end-block pullout due to the triblock architecture, and 2) a bias towards shorter chain lengths in the relaxation of the gels. The first factor was quantified by a recalculation of the energy barrier to end-block extraction using the gel relaxation time. The second was accounted for by the creation of an effective chain length distribution that assumes the shorter of two end-blocks will pull out before the longer block. These two effects are then incorporated into a model that accurately reproduces the triblock TR-SANS results and consistently explains the unexpectedly rapid pullout in ABA triblock rheology and TR-SANS experiments.