Relaxation Dynamics in Bimodal Polystyrene Melts: A Fourier-Transform Infrared Dichroism and Small-Angle Neutron Scattering Study

Abstract

Polymer chain orientational relaxation has been studied in bimodal polystyrene melts composed of long hydrogenated chains and 20 wt % short deuterated chains. Thin polymer films were uniaxially stretched above the glass transition temperature, relaxed, and then quenched after different relaxation times. The chain orientation in the deformed samples was analyzed on different length scales using two different techniquesFourier-transform infrared dichroism (FTIRD) and small-angle neutron scattering (SANS). The local orientational relaxation of the long matrix chains, as well as that of the short deuterated chains, is measured using FTIRD. It is shown that the relaxation of the long chains (Mw ∼2 × 106) is not affected by the presence of the short chains, for the particular weight fraction of short chains (20%), and for the two different short chain masses used: Mw ∼27 000 and Mw ∼188 000. The SANS isointensity patterns are elliptical in shape for short relaxation times but take the form of lozenges when the terminal/Rouse time of the short chains is approached. For relaxation times of the order of (or longer) than the terminal/Rouse time of the short chains, that is, in the relaxation time range where a residual short chain orientation is measured, isointensity curves in the form of butterflies are observed. For the sample containing small chains of Mw ∼27 000, a new isointensity curve is observed at relaxation times of the order of the Rouse time. This isointensity curve has the form of a “4-leaved shamrock”. A qualitative interpretation is given in terms of the stochastic process governing chain relaxation.