The effect of silica nanoparticles on the morphology of(10/90 wt%) PDMS/PBD blends during the shear induced coalescence of droplets of the minor phase at low shear rate was investigated systematically in situ by using an optical shear technique.Two blending procedures were used:silica nanoparticles were introduced to the blends by pre-blending silica particles first in PDMS dispersed phase(procedure 1) or in PBD matrix phase(procedure 2).Bimodal or unimodal droplet size distributions were observed for the filled blends during coalescence,which depend not so much on the surface characteristics of silica but mainly on blending procedure.For pure(10/90 wt%) PDMS/PBD blend,the droplet size distribution exhibits bimodality during the early coalescence.When silica nanoparticles(hydrophobic and hydrophilic) were added to the blends with procedure 1,bimodal droplet size distributions disappear and unimodal droplet size distributions can be maintained during coalescence;the shape of the different peaks is invariably Gaussian.Simultaneously,coalescence of the PDMS droplets was suppressed efficiently by the silica nanoparticles.It was proposed that with this blending procedure the nanoparticles should be mainly kinetically trapped at the interface or in the PDMS dispersed phase,which provides an efficient steric barrier against coalescence of the PDMS dispersed phase.However,bimodal droplet size distributions in the early stage of coalescence still occur when incorporating silica nanoparticles into the blends with procedure 2,and then coalescence of the PDMS droplets cannot be suppressed efficiently by the silica nanoparticles.It was proposed that with this blending protocol the nanoparticles should be mainly located in the PBD matrix phase,which leads to an inefficient steric barrier against coalescence of the PDMS dispersed phase;thus the morphology evolution in these filled blends is similar to that in pure blend and bimodal droplet size distributions can be observed during the early coalescence.These results imply that
We propose a simple and effective boundary model in a nonequilibrium molecular dynamics(NEMD) simulation to study the out-of-equilibrium dynamics of polymer fluids. The present boundary model can effectively weaken the depletion effect and the slip effect near the boundary, and remove the unwanted heat instantly. The validity of the boundary model is checked by investigating the flow behavior of dilute polymer solution driven by an external force. Reasonable density distributions of both polymer and solvent particles, velocity profiles of the solvent and temperature profiles of the system are obtained. Furthermore, the studied polymer chain shows a cross-streaming migration towards center of the tube,which is consistent with that predicted in previous literatures. These numerical results give powerful evidences for the validity of the present boundary model. Besides, the boundary model can also be used in other flows in addition to the Poiseuille flow.
