NMR RELAXATION STUDY OF SEGMENTAL MOTIONS IN POLYMER-NORMAL-ALKANES

Abstract

C-13 spin-lattice relaxation times were measured for n-alkanes of moderate chain length, ranging from n-octane to n-dodecane, under the condition of proton broad-band decoupling within the temperature range of 248-318 K in order to gain some insight into basic features of segmental motions occurring in long chain ploymeric molecules. The NOE data showed that except for methyl carbon-13 dipole-dipole interactions between C-13 and directly bonded H-1 provide the major relaxation pathway, and we have analyzed the observed T1 data on the basis of the internal rotational diffusion theory by Wallach and the conformational jump theory by London and Avitabile. The results show that the internal rotational diffusion constants about C-C bonds in the alkane backbone are all within the range of 10(9)-10(10) sec-1 in magnitude while the mean lifetimes for rotational isomers are all of the order of 10(-11)-10(-10) sec. Analysis by the L-A theory predicts that activation energies for conformational interconversion between gauche and trans form gradually increase as we move from the chain end toward the central C-C bond and they are within the range of 24 kcal/mol for all the compounds investigated.