Hello , everyone, about periodic boundary conditions  in DPD simulations ,I was confused. It is that means " repulsive wall at cell boundary" or "Shear rate"?

For me , i had always made "Wall " off , and when i chose different cell size, like 12X12X12, 15X15X15 , 20X20X20, with other simulation conditions kept same,

the morphology of the same  polymer has changed .

so i want to know if it is the problem of periodic boundary conditions?

Thanks to all the guy considering my question.