NMR INVESTIGATION OF THE ORDER-DISORDER AND RHEOLOGICAL TRANSITIONS IN STYRENE-ISOPRENE DIBLOCK COPOLYMERS

C-13 NMR spectroscopy was used to investigate the physical basis of th e rheological thermal transition (T(rheo)) in symmetric poly(styrene-b -isoprene) (SI) diblock copolymers. Among the studied samples, those w ith M(n) = (3 - 32) x 10(3), which all exhibited two glass transition features (T(g,I) and T(g,S)) in their DSC curves, had T(rheo) values r anging from -10-degrees-C to 205-degrees-C. The NMR line width data re lated to the styrene units in the bulk materials show that a strong se gregation behavior persists over a wide range of temperature above T(r heo). For M(n) > 1.2 X 10(4), the rheological transition coincides wit h the last trace of ordering (extinction of the lamellar grain diffrac tion), but in the disordered state the materials contain polystyrene ( PS) block heterogeneities large enough to exhibit NMR features compara ble to those of homo-PS samples having the same T(g) as T(g,S). Over t his range, T(rheo) increases in the same ratio as M(n)0.35. An analysi s of this relationship suggests an order-disorder transition governed by an energy barrier related to the interface tension in the ordered s tate. For Mn < 1.2 x 10(4), T(rheo) no longer corresponds to an order- disorder transition because vitrification of the PS block heterogeneit ies occurs before a lamellar ordering can take place in the materials.