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13C NMR solid-state spectra have been obtained over the temperature range 150-350 K for four cured epoxy polymers. Each is the diglycidyl ether of bisphenol A (DGEBA) reacted respectively with (i) piperidine, (ii) m-phenylenediamine, (iii) hexahydrophthalic anhydride, and (iv) nadie methyl anhydride. These spectra are compared to the spectra of the unreacted DGEBA monomer, in both crystalline and amorphous forms. The polycrystalline DGEBA 13C spectrum suggests that there is more than one monomer conformation or configuration within the unit cell. This is consistent with X-ray structural assignment, which finds that only one stereoisomer is present but that one end of the monomer is slightly disordered so that either of two possible conformations is possible for the epoxide ring. The origins of some of the observed chemical shift splittings are tentatively assigned. The temperature dependence of the spectra of the piperidine-cured epoxy is analyzed. The rotation by 180°of the phenylene rings accounts for the observed coalescence of certain spectral lines. Analysis of the motion by a single relaxation time model suggests a non-Arrhenius process; this behavior is, however, an artifact of the assumption of a single relaxation time. The full temperature dependence is better described by invoking a distribution of correlation times or, equivalently (vide infra), a nonexponential autocorrelation function. The autocorrelation function ( ) = exp[-(f/rp)"] (0 < a < 1 and tp given by an Arrhenius relation) is used both for these NMR results and for existing dynamical mechanical results for the piperidine-cured epoxy. Analysis with this correlation function is particularly simple in the a = 0 limit, as shown. The NMR line shape is critically sensitive to whether the distribution is inhomogeneous, arising from a true (spatially varying) distribution of single-exponential processes, or homogeneous, with a nonexponential autocorrelation function which describes all common molecular processes and is independent of position. The NMR results are consistent with the activation energy (E = 63 kJ/mol, 15 keal/mol) and width parameter (a = 0.28 ± 0.02) found by mechanical spectroscopy, but the 180°f lipping of the phenylene ring detected by NMR is either approximately 3000 times slower (inhomogeneous distribution) or 20 times slower (homogeneous distribution) than that for the motion that accounts for mechanical loss. However, the NMR and mechanical relaxation results can be reconciled by presuming that the phenylene rings reorient by small diffusive steps. A reorientation of the phenylene ring by about 3°( inhomogeneous distribution) or 40°( homogeneous distribution) has the same correlation time as the mechanical relaxation process and suggests that small-angle phenylene reorientation occurs with or may be identical with the mechanical relaxation process.

Radio-frequency pulse methods have been used to measure the N14 spin-lattice relaxation time in 25 molecular liquids, with the N14 in several types of groups. For eight of the compounds, microwave or pure quadrupole resonance measurements of the N14 quadrupole coupling constant were combined with our T1 results to give values for the correlation time τq describing the molecular reorientations which govern T1. These values for τq at 25°C are about an order of magnitude shorter than reorientational correlation times calculated from the viscosity and molecular radius using the Debye—BPP approach. For the other compounds, τq was estimated and used with the observed T1 for N14 to predict values of the quadrupole coupling constant. The temperature dependence of T1 was observed for nine compounds, leading to activation energies for molecular reorientation of 1.4 to 3.2 kcal/mole and inverse frequency factors τq0 of 2×10—14 to 9×10—14 sec. The temperature dependence of the proton T1 was observed in CH3CN and it is compared with that of N14. It appears that the relaxation of some quadrupolar nuclei affords considerable promise for studying molecular reorientations in liquids and for separating diffusional and rotational processes. These possibilities, as well as that of estimating quadrupole coupling constants not otherwise readily accessible, are discussed.

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An analysis of liquid state cross polarization in AXN spin systems is presented. A two-level geometrical formalism derived from symmetry considerations provides a closed form description of the quantum dynamics for arbitrary N. A parallel discussion using a classical vector model explains the significance of the quantum mechanical solutions and illustrates the nature of the spin–spin correlations accompanying magnetization transfer. General expressions for the J cross polarization A signal with and without X decoupling are given. A discussion of RJCP and PCJCP pulse sequences introduced previously shows how manipulation of Hartmann–Hahn mismatch can advantageously modify cross-polarization dynamics. The quantum formalism demonstrates that efficient polarization transfer is equivalent to population inversion of an ensemble of fictitious spins subjected to different rf field intensities, and that pulse sequences for cross polarization can be constructed by analogy with conventional spin inversion techniques.