A well system includes a drop plug element and an internal plug seat in an internal flow path of a well tubing. At least one of the plug seat or the drop plug element includes a polymer that is deformable, having a first stiffness when subjected to a first strain rate, to allow the drop plug element to pass through the plug seat. The polymer resists deformation, having a second, higher stiffness when subjected to a second, higher strain rate, to resist allowing the plug element to pass through the plug seat and to seal the plug element and plug seat.