Study of stretched polypropylene fibres by 1H pulsed and CW NMR spectroscopy

A set of stretched isotactic polypropylene fibres prepared with the draw ratio λ=4 at four different stretching temperatures was investigated by H1 pulsed relaxation NMR methods and CW NMR spectroscopy. We have studied the influence of the stretching temperature and draw ratio upon the changes of structure and molecular mobility. Some information on the influence of these conditions was obtained from CW NMR measurements by means of the temperature dependences of second moment M2 and decomposition of NMR spectra into elementary components corresponding to the chains with different mobility. H1 CW NMR spectra were measured at two (14.1 and 10.5 MHz) Larmor frequencies in the temperature range 200-420 K. An analysis of the experimental data shows that the stretching of the fibres at different temperature results in a change of molecular mobility. Spin-lattice relaxation times in laboratory (T1) and rotating (T1ρ) frames were also measured on the set of the fibres in the temperature range 239-423 K at 30 MHz Larmor frequency employing a home made pulse spectrometer. In the rotating frame spin-lattice relaxation time measurements in the temperature range above 278 K three relaxation times T1ρ have been observed. The minima of the temperature dependences of the observed relaxation times reflect an α-relaxation process in crystalline regions and β-relaxation process related to a double glass transition in the non-crystalline regions of the studied fibres.