To create the material for my model, I've constructed a node tree. This node tree is interactive (based on the dimension of the model it changes its values). When I export the final model as an obj file, the corresponding MTL file is generated. The problem is that when I import again the model in Blender the node tree is not present and the color of the model is white. I can' t create a group and simply load it, as the node tree is interactive and not unique, I've tried to check the box "copy" when I export the model and I've also checked that the files have the same name and are in the same directory. I don't understand if the problem is during the export or import operation. Do you have any suggestion?
The only equivalency between blender's material and .mtl is the principled BSDF node.
Blender's materials can be infinitely complex but the material format associated with .obj is simpler and limited to fixed arguments. anything other than a single 'Principled BSDF' node will get lost in translation.
I'd suggest looking into blender's Link and Append features