Processing and characterization of a novel nonporous poly(vinilidene fluoride) films in the β phase

Abstract

Poly(vinylidene fluoride) (PVDF) has remarkable properties leading to electro-optics, electro-mechanical and biomedical applications. In particular, its piezo-and pyroelectric properties provide possibilities for many technological applications. The semicrystalline nature of PVDF, combined with the occurrence of at least four crystalline phases implies a complicated physical microstructure. The most frequently described and important phase is the b phase. The piezo-and pyroelectric properties mainly depend on this phase, so increasing the b phase content has always been a great concern. It is possible to obtain films in the b phase by solution but this material presents a high porosity leading to an opaque appearance and a decrease of the mechanical and electrical properties. In this work, porous films in the b phase were obtained directly from the solution at 60°C. After applying pressure perpendicular to the surface of the film at elevated temperature, the pores in the original sample are eliminated. The changes on the morphology and crystallinity associated to the pressure treatment were also studied.

The Role of Solvent Evaporation in the Microstructure of Electroactive β-Poly(Vinylidene Fluoride) Membranes Obtained by Isothermal Crystallization

“…Characteristic absorption bands from the -phase have been identified at 840, 745, 510, and 445 cm −1 . One can also clearly observe the absence of -phase (408, 532, 614, 764, 796, 855 and 976 cm -1 ) or -PVDF (431, 776, 812 and 833 cm -1 ) characteristic bands [3,6]. These results were observed for all samples, showing that if the crystallization temperature is lower than 70 ºC, the polymer crystallizes only in the electroactive -phase [3,5,6].…”

“…In this way, the reaction order n can be obtained ("general rate expression", The determination of the kinetic parameters over a series of weight losses has more accuracy than the method of the "general rate expression". Flynn [23,24] developed a method to calculate the kinetic parameters at different conversion levels using isothermal techniques: (6) In equation 6, t is time, g() is constant at  intervals, A is a constant, R is the ideal gas constant, T is the temperature of isothermal measurement and E act is the activation energy.…”

“…Un-oriented films exclusively in the  phase can also be obtained from the crystallization of PVDF from solution with N,N-dimethylformamide (DMF) or dimethylacetamide (DMA) at temperatures below 70 ºC [4,5]. These films showed a high degree of porosity, which makes them opaque and fragile [6]. This fact opens, on the other hand, the possibility of preparing porous electroactive membranes, with tailored porosity.…”

Effect of filler content on morphology and physical–chemical characteristics of poly(vinylidene fluoride)/NaY zeolite-filled membranes

“…At temperatures above 120 ºC polymer crystallization occurs in the -phase. Within this temperature range a co-existence of  and -phases is observed [7].…”

Caracterização de filmes de PVDF-β obtidos por diferentes técnicas

“…Entretanto, nessas aplicações a manta de nanofibra deve ser previamente polarizada e a elevada porosidade apresentada por essa manta, extremamente conveniente em outras aplicações, pode dificultar esse processo, como ocorre com os filmes obtidos a partir solução com baixa taxa de evaporação. Para este úl-timo caso, a solução encontrada foi prensagem a 140 °C, que reduz a porosidade e torna os filmes transparentes e com melhores propriedades mecânicas [37] . Para mantas eletrofiadas o mesmo procedimento poderá ser adotado.…”