Solid and melt-state 1H NMR studies of relaxation processes in isotactic polypropylenes

Abstract

Relaxation processes in metallocene and Ziegler–Natta isotactic polypropylenes were studied using 1H MAS NMR spectroscopy. 1H MAS NMR spectra and laboratory-frame spin-lattice relaxation times T1(1H) were measured within the temperature range 30–190 °C, which covers the glass transition relaxation and melting processes of both polymers. Splitting of the peak related to protons in amorphous regions of the studied samples into three sharp peaks at elevated temperatures made it possible to determine the spin-lattice relaxation times T1(1H) for particular iPP proton groups. The melt-state NMR spectra of ZN-iPP display three sharp peaks with three additional weak peaks positioned on the less shielded side. Entanglements of ZN-iPP chains are suggested as a possible source of these additional peaks. The spectra of m-iPP indicate substantially fewer entanglements due to its lower molecular weight compared with that of ZN-iPP. The temperature dependences of the relaxation times T1(1H) relating to specific groups of ZN-iPP were shown to reach minima associated with the motions of amorphous chain segments (glass transition relaxation), which are very close to the melting temperature and minima associated with the melting process. Each of the T1(1H) temperature dependences for m-iPP shows only one minimum associated with the melting process. When the particular relaxation times T1, min relating to the minima that occur above the melting temperatures were considered, and these relaxation times were compared for the same proton groups in different samples, significant differences in the relaxation times were observed between samples. Polymer chain motion was more restricted in melted ZN-iPP than in m-iPP, as inferred from the T1, min values.