RELAXATION DYNAMICS IN BIMODAL POLYSTYRENE MELTS - A FOURIER-TRANSFORM INFRARED DICHROISM AND SMALL-ANGLE NEUTRON-SCATTERING STUDY

Polymer chain orientational relaxation has been studied in bimodal pol ystyrene 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 dif ferent relaxation times. The chain orientation in the deformed samples was analyzed on different length scales using two different technique s-Fourier-transform infrared dichroism (FTIRD) and small-angle neutron scattering (SANS). The local orientational relaxation of the long mat rix chains, as well as that of the short deuterated chains, is measure d using FTIRD. It is shown that the relaxation of the long chains (M(w ) similar to 2 x 10(6)) is not affected by the presence of the short c hains, for the particular weight fraction of short chains (20%), and f or the two different short chain masses used: M(w) similar to 27 000 a nd M(w) similar to 188 000. The SANS isointensity patterns are ellipti cal in shape for short relaxation times but take the form of lozenges when the terminal/Rouse time of the short chains is approached. For re laxation times of the order of (or longer) than the terminal/Rouse tim e of the short chains, that is, in the relaxation time range where a r esidual short chain orientation is measured, isointensity curves in th e form of butterflies are observed. For the sample containing small ch ains of M(w) similar to 27 000, a new isointensity curve is observed a t relaxation times of the order of the Rouse time. This isointensity c urve has the form of a ''4-leaved shamrock''. A qualitative interpreta tion is given in terms of the stochastic process governing chain relax ation.