The behavior is due to additional chemical bonds between polymer
chains that prevent them from shearing, which can be derived by thermodynamics and rubbery
elasticity, and even used as a measurement of crosslink density.[113] Another important feature
of the UV-DE is the stiffening after a critical stretch ratio. As the crosslink density increases, a
smaller stretch is needed in UV-DE to drive the elastomer into its non-Gaussian region, wherein
the non-crystalizable elastomer starts to have stress redistribution and causes substantial
stiffening.[114–116] Such critical stretch ratio shifts to a smaller value at a higher crosslink density, which makes the mechanical properties of the elastomer to shift from being VHB-like
to IPN-like in nature.