Strain and temperature induced phase changes inspin‐coated PVDFthin films

Abstract

Poly (vinylidene fluoride) (PVDF) is widely utilized for its unique pyro and piezoelectric properties related to its electro-active β-PVDF. In the past few decades, methods of producing β-PVDF by mechanical stretching of non-polar α-PVDF have been extensively documented. The aim of the present study is to understand the correlation between phase and mechanical behavior of β-PVDF obtained directly from spin-coating, and its stretch-induced transformation from α-PVDF. The effect of thermal annealing and in-plane anisotropy on mechanical properties of spin-coated PVDF freestanding thin films is investigated, under in-situ tensile loading, in conjunction with digital image correlation, to measure the full-field deformation. Stress-strain behavior of spin-coated β-PVDF and α-PVDF are correlated with mechanical stretchinduced phase transformation in α-PVDF. The mechanical strength and failure strain exhibited by spin-coated and annealed α-PVDF

Disclosing Solid-Phase-Transition Mechanism from Nonpolar to Polar Poly(vinylidene fluoride) via In Situ Real-Space Visual Methods

“…The manipulation of polymorphic phases of polymers has consequently received widespread concerns. − Poly­(vinylidene fluoride) (PVDF) is a typical polymorphic semicrystalline polymer with at least three common crystal forms: α, β, and γ. − The α form is thermodynamically most stable and accessible. , It is, however, not wanted owing to its nonpolar nature. Therefore, either avoidance of its formation or conversion of it into the polar phase for piezo-, pyro-, or ferroelectric application has to be made. − Among the various polar phases, the β phase exhibits the highest polarization, resulting in excellent electrical properties. − Despite the superior polarization performance of the β phase, its practical application and development are often limited because of the harsh conditions required for its formation, including the use of nucleating agents, , high electric fields, , rapid quenching, or strong external force fields. , By contrast, the γ phase also displays higher polarity and can be obtained by a simple thermal treatment, such as direct crystallization from the melt at a high temperature, , or annealing the α phase at proper temperature through the solid phase transition. − Even though the γ crystals obtained via solid phase transition exhibit exactly the same crystal structure, they possess yet a much higher melting temperature (around 185–190 °C depending on the chemical structure) than those directly obtained from melt crystallization and thus been referred to as the γ′ phase. ,,− Notably, the Curie temperature of PVDF is expected to be higher than the melting temperature; thus, the polarity of the γ′ phase is expected to be kept above its melting temperature, which is significantly higher than that of the widely used piezoelectric poly­(vinylidene fluoride- co -trifluoroethylene) (P­(VDF-TrFE)) copolymers in the range of 55–128 °C. − Hence, it enables the development of heat-proof piezoelectric …”

“…16−21 Despite the superior polarization performance of the β phase, its practical application and development are often limited because of the harsh conditions required for its formation, including the use of nucleating agents, 22,23 high electric fields, 24,25 rapid quenching, 26 or strong external force fields. 27,28 By contrast, the γ phase also displays higher polarity and can be obtained by a simple thermal treatment, such as direct crystallization from the melt at a high temperature, 29,30 or annealing the α phase at proper temperature through the solid phase transition. 31−38 via solid phase transition exhibit exactly the same crystal structure, they possess yet a much higher melting temperature (around 185−190 °C depending on the chemical structure) than those directly obtained from melt crystallization and thus been referred to as the γ′ phase.…”