Shear-induced phase separation and crystallization in semidilute solution of ultrahigh molecular weight polyethylene: Phase diagram in the parameter space of temperature and shear rate

In the previous papers, we elucidated enhancement of concentration fluctuations, phase separation, and crystallization induced by steady state or step-up shear flow, as observed by shear small-angle light scattering, optical microscopy, and birefringence, for a semidilute solution of ultrahigh molecular weight polyethylene in paraffin as an athermal solvent. However the studies were done only at a given temperature of 124 °C, which is higher than the nominal melting temperature of the quiescent solution Tnm (115-119 °C). It is crucial to extend the studies over a wider temperature range in order to generalize shear-induced phase behavior of the solution. Thus in this work we constructed a kind of phase diagram in the parameter space of temperature (T) and shear rate (γ˙). The temperature range covered was higher than Tnm, so that the phase diagram is strictly concerned with shear-induced phase behavior (i.e., without shear the solution is homogeneous and in a single-phase state). The diagram identified Regimes I-III in the T-γ˙ space as will be detailed in the text. In constructing the phase diagram we found the following new points also. (i) The critical shear rate γ˙cx which defines the boundary between Regimes I and II was independent of T. (ii) Regime III identified previously through the γ˙ dependence of the integrated scattered intensity only at a particular temperature T = 124 °C was further separated into two regimes of IIIa and IIIb below and above a critical temperature (147 °C), respectively, through the observation of the γ˙ dependence as a function of T: In Regime IIIa, the sheared solution developed the optically anisotropic fibrous structures, indicative of the shear-induced crystallization triggered by the shear-induced concentration fluctuations in Regime II; In Regime IIIb, the solution is so stable that it did not show a trend of the shear-induced crystallization even at the highest shear rates accessible in this experiment, but it only showed the shear-induced phase separation. (iii) The critical shear rates γ˙c,streak and γ˙cz, which define respectively the boundary between Regimes II and IIIa and that between Regimes II and IIIb, are sensitive to temperature.