What causes the other: Stress cause strain or Strain cause stress ( Conjugated Polymers)

 I am struggling in understanding a theorem related to a conjugated polymer which is based on the electro-chemo-mechanical working principle.

The basic idea of the actuator is constructed from three layers, the up and bottom are conductive and the medium layer is dielectric and when the voltage applied the ions migrates to fill the holes.

It is mentioned that the expansion is happening because of the migration of the ions and then the stress occurs.

I don't agree with it as what makes the ions migrate is the electrostatic force and then the strain occurs.

I don't know whether the stress can be a result of a strain.