How can I change boundary conditions for the plate equation in PDE toolbox ?

Dear all,

I am trying to use the PDE toolbox to solve the classical plate equation with different boundary conditions. My starting point is the example given in PDE toolbox documentation: "Clamped, Square Isotropic Plate with Uniform Pressure Load" (https://fr.mathworks.com/help/pde/examples/clamped-square-isotropic-plate-with-a-uniform-pressure-load.html)

It works perfectly for clamped edges. But I could not implement "free edges" boundary condition: If I do not specify any boundary conditions to an edge, the edge can move as expected but the tangent is forced to be horizontal. It looks like w' (derivative of displacement) is still prescribed to be zero.

Does anyone knows how to remove any constraint on w and w' based on this example ?

Julien