Small-angle neutron scattering from deuterated polystyrene in dioctyl phthalate solution under shear

Abstract

Small-angle neutron scattering measurements were taken from high molecular weight (1.95 X lo6) deuterated polystyrene in dioctyl phthalate solution at low concentration (3 % weight fraction) under steady shear with rates ranging from the quiescent condition (0 s-l) to 1100 s-l at room temperature (22 OC). The scattered intensity increased drastically beyond a characteristic shear rate (+c = 57 9-1) similar to the increase following a temperature drop from the one-phase region (ambient temperature) to the two-phase region in this upper critical solution temperature system. The cloud point is estimated to be a few degrees lower than 14 OC which is the cloud point for protonated polystyrene in dioctyl phthalate at this molecular weight and concentration. On the basis of this analogy, the random-phase approximation (RPA) approach (the Zimm inverse scattering formula) was used along with a swollen radius of gyration in the Debye function to analyze the data and extract a statistical segment length b and a polymer-solvent interaction parameter xp. The segment length was seen to increase slightly, indicating a slight increase in chain volume beyond +c, while the interaction parameter showed a dramatic increase up to the 'spinodal value +* = 280 8-1 where the RPA breaks down, therefore giving nonreliable values. This data treatment method has no justification on physical grounds: it is merely a way to quantify our observations since specific models that could explain our data are not available at this time.