Authors: Jeffrey H. Simpson
Source: Scholarworks (University of Massachusetts Amherst) (article, 2021-10-26)
Field: Chemistry — Spectroscopy

Use of solid-state NMR to probe the order, dynamics and conformations of poly(p-phenylene vinylene) and poly(2,5-dimethoxy-p-phenylene vinylene)/ Solid-state $\\sp{13}$C cross-polarization magic-angle-spinning (CPMAS) nuclear magnetic resonance (NMR) spectra have been obtained for films of poly (p-phenylene vinylene) (PPV) and PPV-$d\\sb4,$ poly (p-2,3,5,6-tetradeutero-phenylene vinylene). All four phenylene and vinylene resonance have been assigned. The $\\sp{13}$C CPMAS NMR spectra show the presence of phenylene ring 180$\\sp\\circ$ rotation jumps. Solid-state $\\sp2$H quadrupole-echo nuclear magnetic resonance spectra of PPV-$d\\sb4$ films show that all the phenylene rings of PPV undergo 180$\\sp\\circ$ rotational jumps about the 1,4 ring axis with a median activation energy, E$\\sb{\\rm a}$ = 15 kcal/mol and a distribution of activation energies of less than $\\pm$2 kcal/mol. The effects of concentrated sulfurice acid doping of PPV-$d\\sb4$ have been examined with $\\sp2$H quadrupole echo NMR spectroscopy. The activation energy for ring flips is observed to increase upon doping, as does the width of the jump rate distribution. Stretched PPV films have been prepared in which the vinylene protons have been substituted with deuterium (PPV-$d\\sb2).$ $\\sp2$H spectra of aligned samples at high temperature show molecular motion. The observed chain motion is consistent with a 180$\\sp\\circ$ rotational jump of the PPV chain about the crystallographic c axis. Inversion recovery quadrupole echo spectra also support a chain jump model and indicate the jump rate distribution is similar to that of the phenylene rings. $\\sp2$H quadrupole echo NMR spectra of oriented PPV-$d\\sb4$ aligned in the NMR magnetic field have been simulated and thereby have yielded the chain orientation distribution in these films. $\\sp2$H spectra obtained at $-58\\sp\\circ$C are well simulated by an orientation distribution consisting of two Gaussian components. In addition, the average tilt of the phenylene ring relative to the chain axis has been found to be 7.7$\\sp\\circ,$ not the 9.2$\\sp\\circ$ predicted for a trans-stilbene-like structure. PPV has been doped with sodium metal. The chemistry of this process has been examined with elemental analysis. ESR spectroscopy has followed the evolution of unpaired spins as the doping reaction proceeds. The $\\sp2$H NMR line shape of sodium-doped PPV-$d\\sb4$ is relatively insensitive to changes in temperature and consists of two distinct components. The origin of these components is considered. Two-dimensional $\\sp{13}$C-$\\sp1$H HETCOR CP-MAS NMR spectra of p-dimethoxybenzene (DMB) and poly(2,5-dimethoxy-p-phenylene vinylene) (PDMPV) with variable $\\sp1$H dipolar mixing times have been analyzed to determine the rates of $\\sp1$H magnetization transfer between chemically distinct protons. Internuclear distances have been obtained from these rates and the conformation of the PDMPV repeat unit has been calculated and discussed. Phenylene ; Solid-state ; Poly(p-phenylene) ; Polymer chemistry ; Materials science ; Order (exchange) ; Chemistry ; Polymer ; Physical chemistry ; Composite material Advanced NMR Techniques and Applications ; Chemical Synthesis and Characterization ; Conducting polymers and applications