X-ray photon correlation spectroscopy (XPCS) studies show that the viscosities of thin film linear chain polymers at temperatures sufficiently high above the bulk glass transition temperature Tg bulk are equal to the bulk zero shear viscosities. The free surface dynamics of thin films of macromolecules of branched topologies have been shown to be slower than the bulk at temperatures above Tg bulk, in some situations. To understand this phenomenon, a study of 8-arm star-shaped polystyrenes (SPS) of varying arm lengths, were investigated using a combination of XPCS, oscillatory shear rheology, dielectric spectroscopy, and molecular dynamics simulations and. The surface dynamics of SPS molecules with short arms are slower than the bulk, whereas for longer entangled arms the surface relaxation dynamics were comparable to the bulk. The mechanism of surface relaxations of the short-arm SPS molecule is also different -cage-like from the long armed SPS molecule. Molecular dynamics simulations suggest that the slower surface dynamics are associated with spatial ordering of the short-arm molecules at the free surface.
Citation: Physical Review Letters
Pub Type: Journals
star polymers, dynamics, thin films