The filler concentration depends on filler size, shape, and type and on the polymer matrix structure. If polymer is branching you can add more concentration than the linear polymer.Also the molecular weight distribution of polymer chain is impolrtant.If it is narrow MWD you can add more than broad MWD.You can check the optimum filler concentration by viscosity,density and surface tension tests or by AFM, SEM, and TEM tests.

My best wishes

The problem of maximum filler content is taken into account in several theories related to filled polymer systems, especially in the Lewis-Nielsen theory (see e.g. www.papers.itc.pw.edu.pl/index.php/JPT/article/download/463/637). There are estimates of maximum filler concentration for monodisperse sphese or even bimodal and more complicated spherical particles, and there are experimental estimates for random distribution using e.g. metal spheres. if you deviate form the ideal spherical shape the attainable maximum concentration decreases. The same is true for agglomerated particles. See also: https://books.google.hu/books?isbn=0442260245, http://www.kfm.p.lodz.pl/projekty/tok-lectures/Yevgen%20Mamunya%20-%20lecture1.pdf

http://cdn.intechopen.com/pdfs/35438.pdf

I hope it helps

The filler concentration depends on filler size, shape, and type and on the polymer matrix structure. If polymer is branching you can add more concentration than the linear polymer.Also the molecular weight distribution of polymer chain is impolrtant.If it is narrow MWD you can add more than broad MWD.You can check the optimum filler concentration by viscosity,density and surface tension tests or by AFM, SEM, and TEM tests.

My best wishes

 Hi

I think that the problem is related to the critical volume fraction of the reinforcement. You can find its math. model in (mechanical behavior of eng.. material) Meyer & Chawla...............................  

This depends on the filler and polymer structures