Purpose/Aim
Biaxially oriented polypropylene (BOPP) films used for capacitors need to be stretched in longitudinal and transverse directions. Stretching causes orientation and rearrangement of polymer molecular chains, resulting in variation of crystallinity. In addition, the crystal size and β-crystal content change while stretching. Crystallization morphology affects dielectric properties, which will influence the practical application of capacitors. 
Experimental/Modeling methods
After pretreatment and melting, the polypropylene resin was cooled and shaped by the casting system to produce 320μm thick cast film. The thickness of the MD-stretched film was 74~77μm after MD(Machine Direction) stretching, and then 7.3μm BOPP film was obtained after TD(Transverse Direction) stretching. In this paper, DSC(Differential Scanning Calorimetry) method was used to measure the heat flow curves of three kinds of films, from which the crystallinity and β-crystal content were obtained. The relative dielectric constant ԑ_r and the dielectric dissipation factor tanδ were obtained by FDS(Frequency-Domain Spectroscopy) method, from which the polarization and dielectric loss characteristics of the films during stretching were analyzed. 
Results/discussion
The results show that the crystallinity increases 3.8% and the β-crystal content decreases 14.6% after MD stretching, and TD stretching does not change crystallinity but increases β-crystal content. In addition, the results of FDS show that ԑ_r and tanδ of cast film are the largest and most sensitive to frequency, ԑ_r and tanδ of BOPP film are the stablest, while tanδ of MD-stretched film shows an obvious relaxation peak.
Conclusions
In this paper, the variation of crystallization and dielectric properties of polypropylene film during sequential biaxial stretching were studied, which to some extent provided theoretical guidance for the stretching process of polypropylene film for capacitors.
 

sequential biaxial stretching,polypropylene film,Crystallization,dielectric properties