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Abstract: With three polymers (poly(1-butene), isotactic polypropylene and syndiotactic polypropylene) as model systems, the effect of extensional strain on the nucleation rate was studied using a home-made miniature extensional rheometer and in-situ X-ray scattering. A non-monotonic correlation between nucleation rate and strain is observed around rheological yield point. Increasing strain leads to an increase in nucleation rate as expected when strain is smaller than yield strain, however, it results in a decrease in nucleation rate when strain slightly exceeds yield. We proposed that disentanglement occurs at yield point and thus decelerates flow-induced nucleation. Disentanglement provides freedom for chains to retract and leads to a dissipation of the energy initially imposed by extensional flow, which consequently weakens the effect of flow field on the enhancement of nucleation. [Copyright &y& Elsevier]