Motional perturbation at the microphase boundary in thermoplastic elastomers

exp 13 C NMR spin-lattice relaxation times, T sub 1 , and nuclear Overhauser enhancement factors, NOEF, have been measured at two Larmor frequencies (20.1 and 100.6 MHz) and over a range of temperatures (40-100 deg C) for the polyisoprene (PI) microphase in poly(styrene-b-iosprene) (SI) diblock polymers having a lamellar morphology. The study was carried out over a range of molecular weights (or lamella thicknesses) in order to scrutinize the effect of the microdomain boundary upon the dynamics of the PI chains. It is shown that the perturbation of the high-frequency segmental motion probed by T sub 1 and NOEF is limited to a zone of finite thickness (3.6 nm) near the microphase interface. This perturbation is characterized by a twofold increase in correlation time for segmental motion together with a significant increase in motional dispersion of cooperativity. Contrary to exp 13 C NMR line widths and glass transition temperature (T sub g,I ) of the PI microphase, which both increase with decreasing lamellar thickness in the range < 3.6 nm, T sub 1 and NOEF exhibit an asymptotic behavior in this range. T sub 1 and NOEF are also independent of the PS microphase rigidity. These features provide evidence that the changes in the PI chain dynamics probed by these parameters are mainly due to chain end immobilization rather than to change in free volume near the microphase boundary. Graphs. 34 ref.--AA