Segmental mobility spin-lattice relaxation times

In order to study the segmental mobility of the copolymer, the 13C spin-lattice relaxation times (Tx) were measured on the copolymer containing... 

Differing 7j values for CH3, CH2, and CH carbon nuclei within a molecule can arise not only by methyl rotation or anisotropic molecular motion, but also from the segmental mobility of partial structures, even when the dipolar mechanism predominates. Thus the spin-lattice relaxation times of methylene carbon atoms in long alkane chains pass through a minimum at the middle of the chain. In the presence of heavy nonassociating... 

Carbon-13 spin-lattice relaxation times TL (Section 3.3) are relatively insensitive to the chain length of polymers [531]. The influence of local segmental motions predominates, as shown for low-density polyethylenes in which Tx values are one to two seconds for the main chain but up to seven seconds for peripheral side-chain carbon nuclei at 120 C [532] due to segmental mobility (Section 3.3.3.4). To conclude, quantitative evaluation of polymer carbon-13 spectra as necessary for side-chain determination requires the knowledge of spin-lattice relaxation times. 

There is considerable evidence from a variety of techniques for a dendrimer model composed of a relatively soft or spongy interior surrounded by a considerably harder outer molecular surface, the so-called dendritic box. C-NMR measurements of spin lattice relaxation times of specifically tagged PAMAM dendrimers have shown considerably reduced mobility in the outer surface groups relative to the interior segments. Meltzer et performed C-... 

Proton spin resonance measurements on carbon black filled rubbers confirm the relatively small effect of the black on local segmental mobility. Waldrop and Kraus (107) were unable to find evidence for two spin-lattice relaxation times (one for surface rubber and one for bulk rubber) and found very little effect of carbon blacks on the position of the minimum in the spin-lattice relaxation time (7j) vs. temperature curve. The shape of the curve was also substantially unaffected (107). Extraction of free rubber from an uncross-linked SBR-HAF black mix did not accentuate the effect of the carbon black. More recently Kaufmann, Slichter and Davis (108) reported the observation of two spin-spin relaxation times (T2) in the bound rubber phases of polybutadiene and ethylene-propylene rubber, each reinforced with 50 phr of an SAF black (155 m2/g surface area). The amount of fully immobilized polymer was only 4% of the total, but the remainder of the bound rubber displayed... 

Unlike small molecules, whose spin-lattice relaxation times are typically controlled by their tumbling motions, the relaxation times of polymers are governed by the local segmental motions of groups of atoms. The atoms of the chain ends and the branch ends, typically those attached to the last few carbons, are more mobile and exhibit longer relaxation times. In comparison, the motions of the atoms in the polymer backbone are more restricted and therefore have shorter Tj s. In this way, the spin-lattice relaxation experiment provides a way to distinguish the resonances of chain ends/branch ends from those of the backbone. 

High-resolution C spectra of solid polymers can principally be obtained by two ways from normal Bloch decays (SPE single-pulse excitation) of the carbon magnetisation, just as in 1-NMR, or from cross-polarisation. These techniques are complementary. Discriminating experiments may consist of comparing CP/MAS and SPE spectra (the latter obtained without cross-polarisation). Whereas the former depends on proton relaxation, the latter is affected only by carbon relaxation. Because of the great segmental mobility in elastomers, these systems have shorter spin-lattice relaxation times (in the order of seconds), which makes SPE feasible. 

Copolymerization of the it- and st-macromonomers in toluene at 50-80 C, in chloroform at 60 C and in dimethylsulfoxide at 60°C also confirmed higher reactivity of the i -niacromonomer[19]. Higher reactivity of i -PMMA macromonomer may be due to the higher segmental mobility of i -PMMA chain than that of st-PMMA chain, which was evidenced by the measurement of C NMR spin-lattice relaxation times in solution [20]. 

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