**Acknowledgements**

**3.4. Relevant aspects to consider in the dispersion mechanisms of NPs by** 

causes of the dispersion of nanoparticles in the formation of nanocomposites.

A number of researches are still necessary to understand the effects of the different parameters (power, amplitude, and working frequencies) of the ultrasound waves in the preparation of nanocomposites, as well as the mechanism of action. To understand the nature of a

Ultrasound-assisted extrusion process turns out to be a very promising technology and that in the last 10 years has shown great advances in its application to the elaboration of polymeric nanocomposites. However, the mechanism to achieve the deagglomeration of nanoparticles and their dispersion in the polymer remains unknown. The phenomenon of acoustic cavitation is mainly proposed for the explanation of bubble dynamics, but it is possible to find in the literature and as mentioned above that the strict notion of cavitation is an isothermal transition of the liquid–vapor phase limit in a fluid of a single component, due to a decrease in pressure [34, 35, 73]. In other words, the cohesion between the fluid particles is overcomed by an externally applied stress, which causes the homogeneous nucleation of the vapor. Based on this argument and answering the initial question of the text on the phenomenon by which the dispersion of nanoparticles in polymeric systems results, the phenomenon of acoustic cavitation is questionable, since it is not enough to be able to explain the dispersion of nanoparticles during the ultrasound-assisted extrusion process to produce nanocomposites, since there is no phase change and also due to the viscoelastic characteristics of the polymer matrices that imply higher cutting efforts, which would hinder the formation of bubbles. However, it has been proven that in polymeric solutions, there is poor bubble formation due to cavitation effects [74]. On the other hand, there is also the idea that ultrasound causes vibrational effects on the polymer related to relaxation times at the chain level [75], which could help explain that the effects of friction in the polymer are the main

**Figure 13.** Phenomenon of internal acoustic cavitation (a) and external (b) in polymeric nanocomposites according to

**ultrasound-assisted extrusion**

Zhong et al. [71].

180 Nanocomposites - Recent Evolutions

The authors acknowledge the financial support of CIQA through grant 6438 (2018), and of CONACyT through grants 294030 (LANIAUTO) and 296395 is greatly appreciated.
