Stretch and orientational mode decoupling in relaxation of highly stretched polymer melts

We study the relaxation of linear polymer molecules following fast uniaxial extension. Polystyrene melts of M-w = 80 kg/mol are elongated at a Rouse-Weissenberg number W-iR = 1.5 to Hencky strain epsilon = 3, where steady state is approached using a filament stretch rheometer. Samples were quenched for different duration of stress relaxation at constant Hencky strain to preserve molecular conformation by rapid cooling below the glass transition temperature. Expansion of ex situ small-angle neutron-scattering data in spherical harmonics shows experimental evidence for the decoupling of local and global relaxation phenomena from highly stretched states. This work demonstrates both that the decoupling can be seen in the spherical harmonics expansion and that it can be seen even for short chains provided the initial state is highly oriented.