Effects of halloysite nanotubes addition and annealing on crystal polymorphism of PVDF molded via microinjection-compression molding

Poly(vinylidene fluoride)/halloysite nanotubes (PVDF/HNTs) nanocomposites were prepared via melt compounding and then molded into thin-walled plates via microinjection-compression molding (μ-ICM). It was found that the HNTs could not induce the electroactive β- and γ-phases of the PVDF during μ-ICM, although the HNTs showed β- and γ-nucleating activity on the PVDF during quiescent melt crystallization. High melt temperature and subsequent high cooling rate occurring during μ-ICM were responsible for the depression of β- and γ-nucleating effects of the HNTs, because long trans conformations on PVDF molecular chains could not be induced at such a thermal environment. However, electroactive phases of the PVDF could form by simply annealing the μ-ICM thin-walled plates (even the neat PVDF plate) at 180 °C for 30 min. This was ascribed to oriented PVDF molecular chains existing in the thin-walled plates due to strong shear and elongation occurring during μ-ICM, which facilitated the formation of long trans conformations of the PVDF.