Shear induced mixing

Another example of shear-induced mixing comes from the mbber field [Izumitani and Hashimoto, 1985]. A solution cast film of SBR/BR blend with two-phase stmcture was folded and pressed by hand. After the folding and pressing process was repeated several times, a homogeneous film was obtained. Note that this process is similar... 

If so, then PE chains had to exist in the PP phase to start with, i.e., under high shear rate in injection molding machine PP and PE could be locally miscible. Upon removal of the shear field the PE chains crystallized in PP-rich matrix. Also note that in the PE phase, there are the highly disordered regions in which normal PE lamellar growth may be prevented by the presence of PP chains as impurities. This implies that shear-induced mixing took place in the PE-rich phase as well. 

This can be explained as follows. It is known that PP and EPDM are immiscible materials and they exhibit a lower critical temperature (LCST) phase diagram (19). During mixing, especially at high shear rates, the LCST curve elevates with temperature and shear-induced mixing takes place. Thus, in the process of dynamic vulcanization, PP and EPDM can be considered as miscible materials under high shear rates. As a result, after the cross-linking reaction, the unmixed EPDM component forms the dispersed domain, while the matrix consists of mixed PP (dominant) and EPDM (minor) components connected by chemical cross-links (20). 

Shear-induced mixing was reported for polystyrene/polyisobutylene, PS/PIB, blends (Wu et al. 1992). Optical measurements indicated that shearing within the... 

The relation predicts that, initially, as y increases the system undergoes homogenization, then phase separation, followed by another homogenization and demixing. These predictions were confirmed by experiments on blends of ethylene-co-vinyl acetate (EVAc) with chlorinated polyethylene, CPE, and PS/PVME [240-244]. The shear-induced mixing was also observed in PS blends with polyisobutylene, PIB [245]. 

SCH Schmidt, J., Burchard, W., and Richtering, W., Shear-induced mixing and demixing in aqueous methyl hydroxypropyl cellulose solutions. Biomacromolecules, 4, 453, 2003. 

If d F/dcf) < 0 the mixed state of the blend is unstable, so if the contribution that the stored energy makes to the spinodal is such that the curvature is negative than shear-induced de-mixing will result, whereas if the curvature is positive then shear-induced mixing occurs. 

As a consequence, whether shear-induced mixing or de-mixing occurs in the strong shear limit depends only on G and a. For G = 1, Axc( z)> which is always positive at low shear rates, becomes zero in the strong shear limit. For G 1, the result of Eq. 97 can be combined with the predictions of the previous section. A range of complex shear dependent behaviours were predicted. 

Figure 5.7, the CP first decreases, indicating demrxing and, above a critical shear rate, it increases, suggesting shear-induced mixing, a behavior that was later confirmed for other blend systems. 

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