Peter Holstein

Semicrystallinity and polymorphism in PVDF : a solid-state 19F N.M.R. investigation

Abstract Solid-state 19 F nuclear magnetic resonance spectroscopy has been employed to investigate the semicrystallinity and polymorphism of poly(vinylidene fluoride) (PVDF). Fast MAS and high-power proton decoupling were applied to separate the spinning sidebands from the relevant spectral range and to remove the effects of dipolar coupling between the protons and the fluorine nuclei. Amorphous and crystalline regions provide distinct chemically-shifted signals. The application of relaxation filters can suppress the signal either of the amorphous or of the crystalline part. The polymorphism, which characterizes the PVDF crystallites, is reflected in spectra of the crystalline part. Some of the variety of possible conversions between the modifications have been exemplified. Variable-temperature experiments demonstrate both crystallization effects and changes in the modifications.

Triple-Channel Solid-State NMR Investigation of Poly(vinylidene fluoride) Polymorphs

Triple‐channel solid‐state NMR investigations of two different poly(vinylidene fluoride) (PVDF) materials are described. Carbon‐13 cross‐polarization magic angle spinning NMR spectra were recorded using simultaneous high‐power decoupling on both the proton and fluorine channels. Both 1H to 13C and 19F to 13C cross‐polarization experiments were applied, giving identical results apart from intensity variations due to the cross‐polarization efficiency. An attempt was made to characterize the polymorphism of PVDF by means of solid‐state triple‐resonance spectroscopy. Two principal signals, for the CF2 and the CH2 groups, were observed, for both the non‐polar α‐phase and the polar β‐phase. There was a small difference (ca. 4 ppm) in the chemical shift positions for these distinct crystalline modifications.

19F NMR OF PROTON-CONTAINING SOLIDS

Using three fluorinated steroids as examples, it is demonstrated that for simple multi‐fluorinated organic molecules single‐pulse and cross‐polarization magic‐angle spinning experiments with proton decoupling are both feasible and informative. Spinning sideband manifolds are used to obtain shielding tensor components, and shielding tensor information is also obtained from static lineshape analysis. Dipolar dephasing and rotational resonance effects are illustrated, and fluorine spin–lattice relaxation times have been measured. Spin‐diffusion effects are demonstrated for one compound via 2‐D spin‐exchange experiments. In addition, one compound is shown to exhibit polymorphism. Fluorine‐19 solution‐state NMR spectral information is also presented, for comparison with the accrued solid‐state NMR data.

Solid-state 19F NMR investigation of poly(vinylidene fluoride) with high-power proton decoupling.

The use of high-power proton decoupling has enabled highly-resolved spectra of fluorine polymers to be recorded, as is exemplified herein for semicrystalline poly(vinylidene fluoride) (PVDF). By means of high MAS speeds (up to 17 kHz), the spinning sidebands are removed from the whole of the relevant chemical shift range. For spectra of the crystalline regions of the polymer, the high-power decoupling is necessary, though its effect is not large. Various relaxation techniques have been used to examine the semicrystallinity and the polymorphism of PVDF, with special pulse sequences used to discriminate between the various domains. Different chemical shifts have been observed for the signals of the amorphous and crystalline phases. Those of the more immobile parts cover a substantial range.

Electrically induced dynamic processes in nematic liquid crystals: 1H nuclear magnetic resonance investigations

The director reorientation of some nematic liquid crystals in the presence of both magnetic and electric field is described by a modified from of the Leslie equation. Proton nuclear magnetic resonance experiments observing static director orientations in different angles between the director and the magnetic field are described together with reorientation experiments driven by various electric fields. A new technique to measure Δχ/Δe is presented which gives direct access to the anisotropies without any influence of elastic properties. Using electric fields of different strength and in different angles with respect to the magnetic field the dynamic processes in liquid crystals can be investigated in a very flexible way. A number of experiments investigating the homogeneous director reorientation in the electric field is presented. As one result the rotational viscosity was determined. As a surprising result we found a homogeneous reorientation in the electric field and an inhomogeneous reorientation back ...

Proton spin-diffusion in PVDF: a 1H–19F CP/MAS NMR study

Proton spin-diffusion experiments were carried out in order to investigate the domain structure of semicrystalline poly(vinylidene fluoride) by means of different approaches. Besides static proton NMR experiments of the Golden–Shen type the spin-diffusion phenomenon has been studied by means of 1H–19F NMR dual-channel cross-polarization in conjunction with magic-angle spinning. The fluorine NMR detection of proton spin-diffusion gives a much higher discrimination of the effect due to the large chemical shift differences between the signals of the crystalline and amorphous regions. Different mobility filters were applied in order to create a magnetization profile which allowed the measurement of the transfer from the crystalline regions to the mobile parts and viceversa. The data for the magnetization transfer of the spin-diffusion process support the two-component model of the polymer.

Nematic reorientation in electric and magnetic fields

Homogeneous reorientation processes of two nematic liquid crystals in electric and magnetic fields have been observed using proton nuclear magnetic resonance spectroscopy (NMR). Using a recently developed experimental set-up, it is possible to study reorientation processes in liquid crystals by means of NMR experiments in a very flexible way. The time constant τ describing these processes has been determined as a function of the applied electric field. It emerges that the electric field cannot only be used to increase the reorientation time but also to slow the director reorientation by approximately one order of magnitude. Experimental data for 5CB and a fluorinated liquid crystal (BCH-5 FFF) are presented. The reorientation time measured as a function of the electric field can be used to calculate the rotational viscosity γ 1. By repeating these experiments at different temperatures it was possible to investigate the temperature behaviour of γ 1.

Study of fast switching processes due to electric and magnetic fields--an NMR approach.

Solid state NMR techniques have been developed to investigate dynamic molecular effects (e.g., molecular reorientations) due to simultaneously applied external electric fields on electrically sensitive materials such as liquid crystals (LC), liquid crystalline polymers (LCP) and polymeric electrets. Such effects can be observed only on relatively thin systems (10-200 microm). That means that many scans are necessary to achieve a sufficiently high signal-to-noise-ratio in the spectra (500-1000 scans). If the material is also magnetically sensitive, the electric field can be used to orient molecules in a starting orientational state and by switching-off the voltage to access fast reorientation processes in the magnetic field B0. Until now, the behaviour of orientable molecular systems under the influence of electric fields has been investigated by means of a more or less quasistatic approach (LCP: 100 V, electrets: 1 kV) in equilibrium states. The achievable time resolution depends on the desired signal-to-noise-ratio. For the case of proton NMR this means a time resolution of about 10 min. However, very often switching processes occur on a much shorter time scale. Using conventional techniques it is impossible to observe fast (ca. 100 micros) electrically or magnetically induced reorientation processes. In this work, we present a concept to overcome the problems outlined above and to extend the area of our current in situ NMR investigations on thin electrically-switched or poled polymeric layers. The basic idea is to include synchronized electric pulses during the NMR experiment using the preparation and/or mixing periods of a 1D or 2D pulse sequence for the application of an orienting field (electric or magnetic) and to use the reversibility of the molecular switching phenomenon to achieve a sufficient signal-to-noise-ratio. The techniques extend the range of possible investigations from about 100 micros to approximately T1 for correlated spectra (and to longer times of applied fields for uncorrelated spectra). Results are shown for a nematic LC and a nematic polymer having a similar side chain.

1H MULTIPLE-PULSE SOLID-STATE NMR INVESTIGATIONS OF ELECTRICALLY ORIENTED LIQUID CRYSTALS

For the discussion of orientional states in liquid crystals, quadrupolar (2H NMR) and dipolar (1H NMR) interactions have mainly been exploited. There have also been attempts to describe the individual (regarding different positions in a molecule) orientation and order by means of chemical shift information (13C NMR). Owing to the large chemical shift range of carbons, 13C NMR spectroscopy is suitable for providing resolved spectra of highly ordered systems such as low molecular mass liquid crystals. In the case of 1H NMR, the chemical shift range is much smaller than the dominating dipolar interactions. An attempt to use multiple‐pulse techniques to extract chemical shift information for static samples is presented. The tensor character of the chemical shift is reflected in the orientational dependence of the line position. The orientation of the liquid crystals can be manipulated by the in situ application of electric fields.

Chapter 6.6 – 19F NMR

19 F has been recognized as one of the most important nuclei and been widely used for solution-state studies. It is present in 100% natural abundance, is second only to the proton in its resonance frequency, and has a spin quantum number of ½. These properties convey very favorable sensitivity in the nuclear magnetic resonance (NMR) experiment—the receptivity is 83.4% of that for 1 H and 4.73 x 10 3 of that for 13 C. Moreover, the chemical shift range for 19 F is comparable to that of 13 C and well over an order of magnitude greater than 1 H. To obtain high-resolution 19 F spectra, special techniques are necessary. The most widely used is magic-angle spinning. In the case of perfluorinated polymers, homonuclear dipolar coupling will cause substantial broadening of the fluorine spectrum, and MAS will not be very effective in achieving high resolution because of spin diffusion. The resonance band will behave homogeneously, so that linewidths under MAS will vary inversely as ν r , where ν r is the sample rotation frequency, requiring spinning at rates greater than or equal to 10 times the static bandwidth to be fully effective.