There is rigidity in the Ethene molecule due to the double-bonded carbons. In Ethane there are two carbons that share a single bond, this allows the two Methyl groups to rotate with respect to each other. These different conformations result in higher and lower energy forms of Ethane. In Ethene there is no free rotation about the carbon-carbon sigma bond. There is no rotation because there is also a (pi) bond along with the sigma bond between the two carbons. A (pi) bond is only formed when there is adequate overlap between both top and bottom p-orbitals. In order for there to be free rotation the p-orbitals would have to go through a phase where they are 90° from each other, which would break the (pi) bond because there would be no overlap. Since the (pi) bond is essential to the structure of Ethene it must not break, so there can be not free rotation about the carbon-carbon sigma bond.