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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 13, Number 3
BKCSDE 13(3)
March 20, 1992 

NMR Relaxation Study of Segmental Motions in Polymer-n-Alkanes
Jeong Yong Chung, Jo Woong Lee*, Hyungsuk Pak, Taihyun Chang
13C 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 13C and directly bonded 1H 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 109-1010 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 2-4 kcal/mol for all the compounds investigated.
296 - 306
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