Solid state nuclear magnetic resonance study of highly oriented poly(glycolic acid)

The structure of isotropic and highly drawn poly(glycolic acid) (PGA) fibres is investigated using solid-state 1H NMR spectroscopy. Relaxation times in the rotating frame ( T 1 ρ ) using a spin-lock method at 100 °C indicate that isotropic PGA can be adequately described by two phases, a rigid crystalline phase and an amorphous mobile phase with relaxation times of 16.6 and 0.9 ms, respectively. A crystallinity of 40% is obtained by this method and agrees well with previous calorimetric studies. In contrast, T 1 ρ measurements indicate that oriented PGA is 50% crystalline and can be adequately described by a three phase model consisting of: rigid crystalline material with long relaxation time, semi-rigid non-crystalline material with intermediate mobility and relaxation time and highly mobile non-crystalline material with the shortest relaxation time. It was found that the crystalline phase has a T 1 ρ = 53 and T 1 ρ = 39 ms when the fibre direction is at 0 and 90° relative to the external magnetic field, respectively. This difference in relaxation time is associated with higher spin interactions at 0°, reducing the effect on the mobility of the chains. The WAXS orientation averages P2=0.99 and P4=0.96 obtained from the azimuthal scans of the (020) and (002) reflections indicate a highly oriented crystalline structure. These results are used to contrast the structural information obtained from measurements and theoretical calculations of the rigid-lattice anisotropy of the second moment (M2), from which the orientation averages P2=0.96 and P4=0.94 were obtained on the basis of a published crystal structure. The discrepancies found are associated with small differences between the published crystal structure and that required to explain the spin interactions among adjacent molecules.