Nonlinear rheology of immiscible polymer blends: Step strain experiments

Relaxation experiments after simple shear flow were performed on (50/50) PB /PDMS poly(1-butene)/polydimethylsiloxane immiscible model blends and the r esults were compared to the predictions of the Doi-Ohta and Lee-Park models . Three situations of flow were examined: (i) first the variation of stress relaxation was followed in time at various step strain amplitudes, (ii) va riation of stress relaxation as a function of the amplitude of preshear rat e at a fixed strain, and (iii) at a fixed strain and preshear rate, the rel axation of the stress was studied as a function of the time elapsed between the end of the preshear and the step strain. After application of step str ains of various magnitudes, the stress relaxation modulus G(t,gamma) at sho rt times was found to obey the Wagner time-strain separability [Wagner (197 6)]. It was possible to separate linear effects from the nonlinear ones via a damping function h( gamma) of sigmoidal form. After cessation of steady shear flow of different magnitudes, the linear stress relaxation modulus at long time scale was found to be very sensitive to the shear flow condition s and to the elapsed time between the end of the preshear and the step stra in. The morphology evolution characterized by the droplet radius extracted from emulsion models as a function of the steady shear rate was found to be fairly described by the empirical partially mobile interface coalescence m odel and did not obey the inverse proportionality to shear rare as predicte d by the Doi-Ohta theory. After cessation of steady shear flow, the blend m orphology continues to evolve until a steady state was reached. Two kinetic s seem to govern the establishment of a stable morphology: a rapid retracti on process of elongated droplets leading to an increase of terminal relaxat ion time followed by breakup via Rayleigh instabilities and end-pinching me chanisms as was confirmed by in situ morphological observations carried out between two sliding plates at equivalent deformation. (C) 2000 The Society of Rheology. [S0148-6055(00)00306-0].