Nuclear magnetic resonance in solid ethylene/α-olefin copolymers: Relaxation,spin-diffusion and lamellar widths

¹H and ¹³C-n.m.r. spectra and spin-lattice relaxation behaviour (laboratory frame, T₁ (¹³C and ¹H), and rotating frame, T_1ρ (¹H)) are reported for a range of solid ethylene copolymers with α-olefins having different types and concentrations of branches. The spin-diffusion model for relaxation in semicrystalline polymers is summarised and some new theoretical results given. The ¹³C high-resolution n.m.r. spectra obtained using both cross-polarisation (c.p.) and single pulse excitation (s.p.e.) methods associate side chain resonances mainly with the mobile, short T_1ρ protons and, hence, the more disordered region of the solids, consistent with the short ¹³C T₁ components. ¹H laboratory-frame spin-lattice relaxation is single component whilst those for ¹³C and the ¹H on-resonance rotating-frame relaxation are both multiexponential processes, requiring a minimum of three components to describe them. Annealing of samples, which is known to increase the lamellar thicknesses of the crystalline region, causes large increases in the longer relaxation time components. The ¹H and ¹³C spin-lattice relaxation data for a set of annealed and quenched ethyl-branched materials having different branch contents are compared in detail with the predictions of the spin-diffusion relaxation model. The results are internally and semi-quantitatively consistent with this theory and it is concluded that for the ¹H spin-lattice relaxation the model is clearly appropriate and for ¹³C it is consistent with observations. Questions concerning the relevance of spin-diffusion for the magnetically dilute ¹³C nucleus at natural abundance are mentioned and possible alternative explanations for the relationships observed are referred to briefly.