Fluoropolymer microstructure and dynamics: Influence of molecular orientation induced by uniaxial drawing

Abstract

Fluorinated semi-crystalline polymers are attractive for dielectric film applications due to their chemical inertness and high thermal stability. In the present investigation we explore the influence of orientation induced by uniaxial drawing on the crystalline microstructure and relaxation processes of poly(ethylene-tetrafluoroethylene) (ETFE), in order to ascertain how morphological control can benefit polymer dielectric design. When drawn below or near the T g , the crystallinity of the drawn films is unchanged, and oriented amorphous structures and crystalline microfibrils form at high draw ratios. This orientation slows segmental relaxation, reflected by an increase in the dynamic T g. When drawing above T g , the films undergo straininduced crystallization at high draw ratios. For these films, an increase in dynamic T g is also observed, in addition to a second segmental relaxation process, appearing as a shoulder on the primary process. We propose that this represents a contribution from a rigid amorphous fraction, having slowed chain dynamics.

Crystalline microstructure and dielectric properties of oriented poly(ethylene-co-tetrafluoroethylene)

“…frequencies. As noted in our earlier research on ETFE and by others, 10,11 ETFE possesses quite low dielectric loss at 25 ˚C (tan δ ~ 10 -3 at 10 Hz) but this increases somewhat at higher frequencies due to contribution from the very broad γ relaxation. When drawn at 50 ºC to 3λ (50C-3λ), the shape parameter (β) is lower than that of the undeformed film, indicating a wider probability distribution (Figure 8a).…”

“…After drawing, the scattering intensity becomes concentrated into diffraction arcs arising from the oriented lamellar morphology. As noted previously 11,26 , a prominent feature in the 2D patterns of the drawn films is the strong equatorial arcing of the After drawing to 4λ (50C-4λ and 140C-4λ), the diffraction peak at ~19.5º becomes broader compared to the unoriented film: the full width at half maximum (FWHM) increases from 1.3˚ for the unoriented film to 2.8˚ and 2.5˚ for 50C-4λ and 140C-4λ, respectively.…”

“…This treatment was shown to effectively erase the history imparted by extrusion. 11 Samples for drawing were then cut from the unoriented film and marked with horizontal and vertical gage lines using a square grid template, each line separated by 0.5 cm.…”

“…Although their dielectric constants are in the same range as BOPP, they can be melt extruded into thin films, have high dielectric stability at elevated temperatures, and exhibit very low dielectric loss. 10 Building on our previous work, 11,12 we focus in this paper on the role of uniaxial orientation and subsequent thermal annealing on the semi-crystalline microstructure and low electric field dynamics of ETFE, as well as high field dielectric breakdown measurements on selected drawn and annealed ETFE films. Experimental Weibull dielectric breakdown strengths are found to increase with uniaxial orientation (draw ratio = 3) to values as high as 870 MV/cm, over 10% larger than commercial BOPP films measured under similar conditions.…”

Microstructure evolution of poly[tetrafluoroethylene-co-(perfluoropropylvinylether)] films under uniaxial deformation

“…For the as-received PFA film, an isotropic SAXS pattern is observed, indicating a randomly oriented lamellar texture. At draw ratios of 1.4 and 2.0 (i.e., in Region II of the stress-strain curve,Figure 1), the dark inner ring begins to split into two symmetric arcs located along the equatorial direction, similar to our recent findings on semi-crystalline poly(ethylene-tetrafluoroethylene)[20]. This scattering arises from crystal lamellae having their normals directed transverse to the draw direction.…”