Double-step moulding: An effective method to induce the formation of β-phase in PVDF

Nowadays, the investigation of smart methods promoting PVDF beta phase has become an important research topic in many energy related fields. In this work, compression moulding was proposed as an effective, simple and easys-calable technique to tune PVDF polymorphism and obtain the desired crystalline form, exploiting a procedure in two steps. After a preliminary principal component analysis of the infrared spectra, the different moulding conditions tested were correlated with the electroactive phase amount, obtaining a non-monotonous trend. Finally, dielectric and dynamic-mechanical measurements were carried out on selected samples. Experimental evidence indicates that the double-step moulding induces the beta phase formation due to the orienting effect generated by the shear stresses in the not completely molten polymer. Moreover, this method determines an increase in permittivity, a reduction of the dielectric losses and a change of PVDF molecular relaxations with respect to the reference film moulded through a simple single-step moulding.