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A sigma bond remains unchanged upon rotation and so can rotate freely, but to rotate a pi bond you must break the chemical bond, rotate and reform. Bond breaking requires energy, and at room temperature the molecules do not have sufficient thermal energy to overcome the energy barrier. It is possible that at very high temperature (such that the thermal energy kT is larger than the bond-breaking energy) or by using photons to break the pi bond, the isomerism would be lost as the excited molecule would be able to rotate freely.
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