Changes of proton-enhanced, magic-angle-spinning 13C-NMR dynamic parameters during cure of tetra(ethylene glycol)dimethacrylate

Dynamic NMR and mechanical properties T_SL, T_1p(C) at 60 kHz, T_g at ≈1 Hz and its half width, log decrement (Δ_max) and compression modulus (E) were measured for poly[tetra(ethylene glycol)dimethacrylate] (PTEGDMA) at different extents of cure. The β, γ and water-induced transitions were also noted. The spin-lock, cross-polarization time-constant, T_SL, declined to its limiting value before the vitrification point. T_1p(C), relaxation time in the rotating ¹³C field, increased sharply at or just after vitrification. The change is greatest at quaternary C, decreasing through CH₂, CH₂O and CH₃, being barely significant in the latter. The observed changes affirm that T_1p(C) is sufficiently influenced by spin-lattice relaxations to provide a monitor of the damping of mid kHz components during cure. The small change observed at CH₃ shows that this group is not the origin of changes in relaxation rates at other groups. Correlations with E and T_g show the importance of mid kHz components of group motion to the cooperative motions determining the response to macroscopic strain at lower frequencies. MAS ¹³C-NMR showed that PTEGDMA contains residual unsaturation at limiting cure and that two types, constrained and unconstrained, can be distinguished.