Water absorption and dielectric changes in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymer films

Abstract

Crystalline Langmuir–Blodgett copolymer films of vinylidene fluoride with trifluoroethylene (70%:30% and 80%:20%) absorb water. Water absorption is accompanied by film swelling, as indicated by an increase in lattice spacing, sometimes by as much as 5%. This water absorption, between 0 and 40 °C, is a result of intercalation or occupation of interstitial sites between the layers of the film, not just water molecules filling voids and defect sites alone. An increase in the film capacitance is observed, although the polymer chains retain all trans configuration of the ferroelectric phase.

Adsorbate/absorbate interactions with organic ferroelectric polymers

“…A capacitance model for the estimation of water uptake in polymer PVDF coatings and thin films was presented [15] that is consistent with our own studies of the dielectric properties of P(VDFTrFE 70:30) [19,21]. Such studies implicate a dipole interaction between water and the ferroelectric polymer dipoles [27].…”

“…Poly(vinylidene fluoride) [PVDF, -(CH 2 -CF 2 ) n -] copolymers with trifluoroethylene [TrFE, -(CHF-CF 2 )-] can form highly ordered crystalline ferroelectric polymer ultrathin films as has been demonstrated by X-ray and neutron scattering [19,20,22,35,37,38,40], scanning tunneling microscopy [27, 33-37, 39, 40, 42, 45, 46], low energy electron diffraction [36,37] and band mapping [27,36,37,46]. Although not always evident in scanning tunneling microscopy (Figure 1), the band structure shows a characteristic super-periodicity dominated by -(CH 2 -CF 2 ) 2 -or -(CH 2 -CF 2 )-(CHF-CF 2 )-"dimer" pairs [36,37,46] in the ferroelectric phase.…”

“…Surface dipoles can affect the binding site of water species adsorbed at the surface and sterically hinder or enhance desorption of adsorbed and absorbed water. Perturbations of local surface dipoles can affect desorption of absorbed water [19,25,27]. As we are dealing with polymers, the absorption of water is persistent, even with largely hydrophobic polymers.…”

The elimination of the influence of ambient environmental effects on the structure of `inert' polymers

“…This water absorption, between 273 and 313 K, is a result of intercalation or occupation of interstitial sites between the layers of the ®lm, not just water molecules ®lling voids and defect sites alone. An increase in the ®lm capacitance is observed, although the polymer chains retain the all-trans con®guration of the ferroelectric phase (Jacobson et al, 2004). While it has been known that small amounts of water can be absorbed (Castela & Simoes, 2003;Castela & Simoes, 2002), reperforming structural studies in vacuo demonstrates that even hydrophobic polymers can absorb signi®cant amounts of water and, presumably, other contaminants.…”

“…Water is not a standard solvent for poly(vinylidene¯uoride), P(VDF), nor tri¯uoroethylene, but well ordered P(VDF-TrFE) copolymer ®lms swell when immersed in water (Jacobson et al, 2004). By comparing the ±2 X-ray diffraction lines from crystalline Langmuir Blodgett copolymer ®lms of vinylidene¯uoride with tri¯uoroethylene (70%:30% and 80%:20%) exposed to substantial amounts of water with those same ®lms vacuum annealed and characterized in vacuum, we ®nd that vinylidene¯uoride with tri¯uor-oethylene (70%:30% and 80%:20%) does absorb water.…”