STRUCTURAL-CHANGES UPON ANNEALING IN A DEFORMED STYRENE BUTADIENE STYRENE TRIBLOCK COPOLYMER AS REVEALED BY INFRARED DICHROISM

Structural and morphological changes in a deformed styrene-butadiene-styrene (SBS) tri-block copolymer were usually studied by means of small-angle X-ray scattering techniques through analysis of scattering patterns which are related to rigid polystyrene (PS) microdomains. In the present study, a different approach was demonstrated. Infrared dichroism was used to elucidate these changes induced by annealing through measurements of molecular orientation of rubbery polybutadiene (PB) chains. This was made possible by the fact that (1) structural rearrangements of PS microdomains are inevitably associated with changes of the PB orientation and (2) strain-induced orientation of the PB chains depends on the arrangement or orientation of the PS microdomains which act as cross-links. Infrared dichroism analysis showed that the relaxation of rigid PS microdomains, which occurs even at annealing temperatures (T(g)) below the glass transition temperature (T(g)) of PS, can be studied by observing the relaxation of the PB orientation and that the remaining PB orientation under constant strain at long annealing time (> 30 min) characterizes the orientation of rubbery chains required for activating PS plastic flow. Furthermore, it was found that the loss of PB orientation under strain with annealing time is not equally associated with the lost recovery of the sample length after removal of the extensional stress, suggesting the existence of different mechanisms of PB orientation relaxation. Finally, an anisotropic morphology in annealed SBS polymer was revealed by a strain-induced PB orientation which depends on the redrawing direction with respect to the initial stretching direction (SD). These measurements indicate that, at T(a) less than or-similar-to T(g) of PS, the PS rearrangement involves essentially rotation of cylindrical microdomains, with the interdomain vector connecting the centers of the cylinders remaining preferentially parallel to the SD; while, at T(a) > T(g) of PS, the rearrangement process involves more displacement of the PS cylinders, with the interdomain vector oriented perpendicularly with respect to the SD.