When an alternating stack of molten polymer sheets is pulled in uniaxial tension parallel to the layers, interfacial area per unit volume increases, amplifying interfacial effects. Multilayers of several polymer pairs were prepared by lamination (up to 100 layers). Pairs with low and high interfacial tension, Γ, and pairs with functional groups which could react were selected. These were stretched at 220 °C at various constant extension rates in a rotating clamp extensional rheometer. At low rates the extra measured stress could be related to Γ and the number of interfaces. At higher extension rates extra stress exceeded Γ. This method for measuring interfacial tension can be applied to opaque samples and does not require knowing sample density or viscosity. For the reactive pairs there was pronounced strain hardening caused by coupled and even cross linked chains at each interface. The contribution of the cross linked interface was shown to follow rubber elasticity theory. As a result of the reaction the measured stress was two orders of magnitude higher than for the nonreactive case. Extra stress due to the grafting reaction was also measured. It increased with extension rate, presumably due to entanglements of the grafts across the interface. No evidence of graft reaction reducing Γ was found.