Nuclear Magnetic Resonance of Polymer Fibers

Abstract

Nuclear magnetic relaxation times T2, T1, and T1ρ are reported for a tetrafluoroethylene–hexafluoropropylene copolymer (FEP), both in bulk and in drawn fiber form. The measurements cover a temperature range from − 200 to + 250°C. Relaxation times T2 and T1ρ exhibit marked anisotropy with fiber orientation in the magnetic field. The effect is less pronounced in the T1 case. Theoretical expressions for T2, T1, and T1ρ have been derived for the fiber case, and the results are presented. Lattice sums have been computed for rotation and rotation–translation modes of molecular activity and the values used to predict theoretical relaxation times. Quantitative agreement between theory and experiment is excellent for T2 and satisfactory for T1 and T1ρ, in support of the proposed molecular models. Relaxation effects previously labeled γ or Glass II are assigned to molecular reorientation about the helix axis, with the motion occurring in both the amorphous and crystalline regions of the polymer. The α or Glass I relaxation is assigned to longitudinal molecular translation. The possibility of a helix interconversion cannot be ruled out. The NMR data show that the perfluoromethyl–CF3 groups are locked in and do not rotate. Finally, no indication of a preferred orientation of chains in the amorphous regions is in evidence.