Effect of Nanoparticles on Droplet-Matrix Morphology

Substantial reductions in the size of the domains of the dispersed phase, as well as narrowing of the particle size distribution, have been reported extensively, in various immiscible blends containing nanoclay or nanosilica fillers [9,11-14,26,27,45,51]. As an example in the functionalized PP/ethylene-octene copolymer (80/20 wt%) system prepared by Bailly and Kontopoulou, the average particle size of the dispersed phase was reduced by about 

Effect of nanosilica addition on the morphology of a 80/20 PP/ethylene-octene copolymer blend (a) unfilled blend (b) composite containing 7 wt% surface-modified silica. The PP matrix has been grafted with a silane. The holes correspond to the etched dispersed phase. (Reprinted from Bailly, M. and Kontopoulou, M., Polymer, 50,2472,2009.) 

Theories that describe the reduction of the size of the dispersed phase in the presence of nanoparticles vary, depending on whether the filler is located in the continuous phase, in the dispersed phase, or at the interphase between the two blend components. Compatibilizing effects due to polymer adsorption on the filler surface, as well as reduction in the interfacial tension between the two phases in the presence of the filler, are the generally accepted mechanisms when the fillers are located at the interface [11,13,26]. Ray et al. [11] showed that upon addition of only 0.5 wt% of organically modified clay, the interfacial tension decreased from 5.1 to 3.4 mN/m for a PS/PP blend and from 4.8 to 1.1 mN/m for PS/PP-g-MA, suggesting a possible interfacial activity of the clay that is localized at the interface in similar fashion to classical compatibilizers. 

However, these mechanisms are obviously not dominant when the fillers reside in the matrix, as shown through estimations of the interfacial tension using the Palierne model [44]. First of all, it should be kept in mind that when the filler partitions in one phase, the reduction in dispersed particle size may be attributed to a compositional effect. In the presence of the nanofiller, the ratio of compositions is altered. For example, 5 wt% of a filler localized in the matrix will correspond to a higher ratio of matrix over the dispersed phase, which in turn may affect the morphology. This effect should be more pronounced for relatively high filler concentrations that are not commonly encountered. 

Effect of organoclay content on dispersed phase particle size in a maleated EPR/PP 70/30 blend. 

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