Selective NMR Pulse Sequences for the Study of Solid Hydrogen‐Containing Fluoropolymers

Abstract

Summary: Fluorine‐19 NMR spectra of solids have some special features, which are discussed in this article. In particular, they generally contain two abundant spin baths (protons and fluorine nuclei). This situation throws up some special operational requirements, as does the study of heterogeneous samples. The relaxation characteristics of heterogeneous systems, which are briefly described herein, frequently permit the use of specific pulse sequences to obtain subspectra for individual components. Various possible selective sequences for use in fluorinated heterogeneous organic solids are listed and their actions rationalized on the basis of molecular mobility. Semicrystalline hydrogen‐containing fluoropolymers form especially suitable systems for such operations, and in order to understand their domain structures it is essential to obtain subspectra of the amorphous and crystalline domains. Examples are given of the use of selective pulse sequences for studying fluoropolymers, especially for poly(vinylidene fluoride) (PVDF) and the copolymer P(VDF75/TrFE25) (TrFE = trifluoroethylene).

Fluorine-19 solid state NMR study of vinylidenefluoride polymers using selective relaxation filters

“…Alternatively, recording the FID immediately following the 12 minipulses will give the 19 F spectrum mainly for the mobile domain. This may also be obtained by use of larger minipulse angles leading to, say, a net nutation angle of 3601 for M(mobile), by which time M(rigid) may have been lost, so that a final 901 pulse will yield a spectrum of the mobile region only [21]. The direct polarisation with delayed acquisition (DPDA) sequence is not shown; this is a simple one-pulse experiment with a delay time prior to acquisition during which decoupling can be gated off [22].…”

“…Its implementation and operation are described in Ref. [21]. Although it has some similarity to the dipolar filter sequence (Fig.…”

“…The minipulse plus interval steps are then looped n times (usually 12). Varying the parameters (y and/or t) allows selection of a specific component of the free induction decay (FID), giving spectra containing only either the rigid or the amorphous component of semicrystalline polymers [21]. A simplified explanation of discrimination by DIVAM is as follows.…”

Efficient Strategy for Determining the Atomic-Resolution Structure of Micro- and Nanocrystalline Solids within Polymeric Microbeads: Domain-Edited NMR Crystallography

“…Fortunately, solid-state NMR spectroscopy offers a range of experimental techniques enabling selective suppression or enhancement of the required signals. For polymeric composites, the T 1 - and T 2 -filtered experiments, which are sensitive to differences in segmental dynamics, are particularly convenient techniques. − This is given by the fact that local motions in even highly crystalline polymers efficiently reduce 1 H spin–lattice relaxation times T 1 ( 1 H), which then tend to be several seconds only. In contrast, low-molecular-weight crystalline compounds often exhibit much longer relaxation times reaching up to several hundreds of seconds.…”

“…In contrast, low-molecular-weight crystalline compounds often exhibit much longer relaxation times reaching up to several hundreds of seconds. As a variety of different relaxation filters and editing techniques can be applied, − the appropriate experimental conditions enabling efficient suppression of the unwanted signals can often be readily attained.…”