Comprehensive Characterization of Chain End Groups of Vinylidene Fluoride Based Polymers

Abstract

Summary:In this work an investigation of the chain end groups produced in the free radical copolymerization of vinilydene fluoride (VDF) and hexafluoropropylene (HFP) is performed. Type and amount of chain end groups are evaluated by a meticulous analytical characterization of VDF/HFP copolymer. At first pulsed gradient spin-echo nuclear magnetic resonance (spin-echo NMR) is used to identify all the chain end groups also at very low concentration (equal to 0.1 mmol Á Kg À1). The instrument sensitivity is increased of an order of magnitude in comparison with the traditional NMR. Moreover potentiometric titration and ion chromatography (IC) are also used to study the chain end groups and, as a consequence, the nature and the amount of the acidity showed by the polymer chains. In details two intensity of acidity are detected by potentiometric titration, namely strong and weak. The strong acidity is associated to the presence of residual surfactant and can be removed washing the polymer, while the weak acidity is due to free molecules of fluoride acid (HF). The standard ion chromatography facility is properly modified to quantify the fluoride in the polymer matrix without any pre extraction in water. Thanks to this the HF concentration in the polymer is evaluated with high accuracy. A detailed kinetic scheme for the VDF/HFP polymerization is also proposed taking into account all the findings obtained studying the chain end groups.

NMR Study of the Chain End and Branching Units in Poly(vinylidene fluoride-co-tetrafluoroethylene)

“…This is consistent with recently reported studies of copolymers from VDF and hexafluoropropylene (HFP), where −COOH end groups were not detected. In the present work, their resonances are below the detection limit …”

“…4 Since such polymers contain both proton and fluorine atoms, the couplings between these nuclei made the 1D-NMR spectra very complicated when observing one nucleus without decoupling the other. In Wormald's work, 1 H{ 19 F} and 19 F{ 1 H} 1D-NMR and some 2D-NMR experiments were conducted on VDF telomers. The superior dispersion obtained in their spectra made it possible to obtain correct assessment of chain end and short-chain branching units.…”

“…6 In this study, a variety of complementary 1D-and 2D-NMR techniques were used to obtain unequivocal resonance assignments for chain end and branching units in poly(VDFco-TFE). The ability to apply 19 F decoupling during 1 H detected experiments (and vice versa) provides considerable spectral simplification. More importantly, the signal enhancement arising from collapse of multiplets is especially useful for detecting signals from these low-occurrence units with 2D-NMR.…”

“…More importantly, the signal enhancement arising from collapse of multiplets is especially useful for detecting signals from these low-occurrence units with 2D-NMR. First, 19 F T 1 and DOSY (diffusion ordered spectroscopy) experiments were carried out to identify the possible chainand branch-end resonances. Then, data from 19 F{ 1 H} gHETCOR (heteronuclear correlation) and 19 F− 19 F selective COSY 9,10 in the structures of the end units.…”

“…First, 19 F T 1 and DOSY (diffusion ordered spectroscopy) experiments were carried out to identify the possible chainand branch-end resonances. Then, data from 19 F{ 1 H} gHETCOR (heteronuclear correlation) and 19 F− 19 F selective COSY 9,10 in the structures of the end units. In 19 F{ 1 H} gHETCOR experiments, three-bond correlations between 1 H and 19 F can be observed, and four-bond correlations between 1 H and 19 F are too weak to be detected.…”