Ductile interphase

Despite the emphasis on favorable interactions between the matrix and reinforcement and compound formation between them, it may be beneficial in certain circumstances for the interaction between the two primary constituents to be relatively weak. This is especially true in ceramic-ceramic composites, where both constituents are brittle, and the only way to impart some ductility on the composite is for the interphase to fail gracefully —that is, for the fibers to actually pull out of the matrix in a controlled manner. Optimization of the interphase properties in advanced composites is currently the focus of much research. 

An additional elegant route to obtain disentangled UHMW-PE, and thus to control the interphase, is by direct polymerization in the reactor. In order to make UHMW-PE, a relatively low polymerization temperature is needed and a situation is easily encountered where the polymerization temperature is lower than the crystallization temperature of UHMW-PE in the surrounding medium in which the catalyst is suspended. In this situation, the growing chains on the catalyst surface tend to crystallize during the polymerization process. These UHMW-PE reactor powders, often referred to as nascent or virgin UHMW-PE, can be remarkably ductile. It was shown by Smith et al. [26] that reactor powders, in the same manner as solution-cast UHMW-PE, could be drawn easily into high-modulus structures. 

Figure 20.15 shows b of PE/PP/PS blends as a function of catalyst concentration. The TPB reveals a fragile behavior, with only 4% of b. To assess the effect of PP in these TPBs, and taking into account their relative component amounts, this ternary blend can be considered as a binary 80/20 PE/PS blend with a small amount of PP. As was shown before (Fig. 20.7), the PE/PS physical blend exhibits Cb of about 120%, much greater than the value for the ternary blend. It is clear that the low ductility of PE/PP/PS blend must be related to the PE-PP interphase. 

The PE/PP/PS reactive blends show b up to three times higher than that of TPB (Fig. 20.15). Taking into account that F-C reaction is not possible between PE and PP, and PP-PS interphase could not be detected (Fig. 20.13), the increase in ductility can be attributed to the compatibilization of the PE-PS interphase. Moreover, such behavior was also observed for the binary PE/PS blend (Fig. 20.7). 

In the case of PO blends, compatibilization most frequently aims for improved ductility and/or transparency. The Z-N-LLDPE obtained using multi-sited catalyst constitutes a specific case - the homopolymer may have phase-separated morphology that requires compatibilization. It has been known that addition of 5-20 wt% LDPE needs to be used for improved performance. However, explanation for this is rather recent (Robledo et al. 2009). The relaxation spectrum of the blend may be decomposed into three components (1) Z-N-LLDPE matrix, (2) LDPE dispersed drops, and (3) a thick interphase with its own viscoelastic properties, obtained by interaction between the high-MW linear fraction of the LLDPE and the low-MW linear LDPE macromolecules. 

Miscibility is identified as an existence of a single phase thus, the term refers to liquid systems solutions and melts (some authors treat co-ciystallization as a solid-state miscibility). Most polymer blends available oti the market are immiscible, but with adequate interactions across the interphase. For example, by 1980 ca. 42 % film producers used immiscible LLDPE/LDPE blends, where LLDPE improved modulus and strength and LDPE enhanced processability and ductility. Properties of LLDPE/LDPE blends have been described in several publications (Utracki and Schlund 1986,1987 Schlund and Utracki 1987 Zahavich and Vlachopoulos 1997). Blends of LLDPE with PP were also studied (Dumouhn et al. 1987, 1988, 1991 Dumoulin and Utracki 1990). Similarly, blends of PE with PC were described (Utracki and Sammut 1989, 1990b). 

Abstract. C- and BN-interphases on SiC fibers for unidirectional SiCf/SiC composites were formed by EPD process, and their microstructure and mechanical properties were investigated. Whereas the C-SiCf/SiC composites showed a pseudo-ductile fracture behavior with large amount of fiber pullout, the BN-SiCf/SiC composites fractured in a brittle maimer without fiber pullout in spite of sufficient thickness of BN interphase. It is inferred from the results of EDS that sintering additives would react with h-BN-interphase, and the interphase did not act effectively for toughening the SiCf/SiC composites. 

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