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Poly (vinylidene fluoride) (PVDF) is a crystalline polymer with three main crystal forms, i.e.α-, β-and γ-form.Of them, the β-form is most interesting because of its superior piezoelectric properties.It makes PVDF widely used as actuator, energy harvesting materials, and so on.However, under normal conditions crystallization of PVDF usually yields the α-form that is electrically inactive.With aid of mechanical deformation the α-form can be transformed into the β-form.To date, α→β transformation induced by stretching is mostly concentrated in PVDF and is less concerned in the blends containing PVDF.This study reports phase transformation of poly (vinylidene fluoride) (PVDF) in its miscible blends with poly (3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) upon stretching.The original α-form of the PVDF component in the blends is converted into the β-form by stretching,which is decreased with the increasing of the P34HB content.Moreover, α→β transformation of the PVDF component in the blends exhibits a sharp transition at the P34HB content of about 15 wt%.It is correlated with composition dependent dissolution and crystallization of the P34HB component in the amorphous phase of the PVDF component.