Try some surfactants... we can discuss it in detail as few scientists at my place are expert for uniform particle dispersion using surfactants.. can write me at [email protected]

I guess you are speaking about silica like Aerosil. It should be no problem when you start from an aqueous dispersion of silica prepared by sol-gel method. You may try commercial ones like Ludox. With dispersions of Aerosil in ethanol or isopropanol you should achieve considerable deagglomeration using intense sonication with a tip not ultrasonic bath.

 If you added silica to the Pebax solution (I assume you used mixture of water/ethanol), then you might have some difficulty for diffusion and wetting of the solution into the silica agglomerates due to the viscosity of the solution. you either can prepare less concentrated solution or you can first disperse silica in water/ethanol (with sonication) and then add Pebax to this mixture.

Also, consider the possible reaction of amino end groups of Amide block in Pebax with Si-OH on the surface of silica.

Thank you so much for your answers. Dear Ebrahim Jalali Dil, the solvent of the pebax is butanol.

With an hydrophilic polymer, why did you not consider using water as solvent? 

Anyway,  important parameters to take into account are the volume fraction of nanoparticles and the polymer concentration. In principle, polymer may interact with NPs and it may adsorb onto NP surface (whatever the type of interaction). Agglomeration could be due to an inefficient stabilization of NP in the solvent used, but also it may be induced by depletion forces set by polymer excess. 

If you cannot change polymer concentration, you should at least try with different NP volume fraction. I do not think using surfactant can be a solution. Surfactant can interact  both with polymer and NP, with an overall complication of the system. 

If you need some advices about depletion in NP suspensions you could take a look at these attached publications.

Article Phase Behavior of DNA-Based Dispersions containing Carbon Na...

Article Attempts to control depletion in the surfactant-assisted sta...

Article Effects of Single-Walled Carbon Nanotubes on Lysozyme Gelation

Dear Navid,

you did not gave information about concrete nature of the silica (Aerosil?) and whether the nature of silica nanoparticles can be different (e.g. Ludox)?

disperse with non-aqueous solvent or in oil phase with proper surfactant like tween or, span

Dear Titus

I hope you are fine. It is aerosil

Aerosil is composed of aggregated and agglomerated nanoparticles. Starting from dry the aggregates are interconnected by interparticle bonds between silanol groups. To disperse them to the nano level you need high shear (at least rotor stator system better ultrasonication or high pressure homogenization) and an appropriate dispersion media. In my experience you may reach good dispersion, e.g. in cyclohexane with low content of octanol or somewhat higher content of shorter chain alkanols (https://drive.google.com/file/d/0B-eBFrtlEwlhZGQ2MWZhZWItZDQ1Zi00ODQxLTg5ZjctNzBkNGZlNDVmNzFm/view?usp=sharing Characterization of nanostructuring effects at solid/liquid interfaces by analytical centrifugation)

The other side of the problem is likely caused by higher affinity of butanol to the polymer compared to silica, i.e. this should then lead to depletion flocculation. This maybe avoided by mixing the above mentioned stabilized aerosil dispersion with the Pebax solution.

Would be very interested in your observations with the real system compared to my speculations.