Spin segmental mobility

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... 

In ribonuclease A 13C spin-lattice relaxation of the carbonyl and a and / carbon atoms is slower in the denaturated protein than in the native sample [177]. Apparently, the skeleton of this macromolecule becomes more flexible on denaturation, probably owing to conformational changes. However, the s carbons of lysine in the native protein exhibit relatively large T, values which change only insignificantly on denaturation [177]. This behavior is attributed to a considerable segmental mobility of the lysine side chain (Table 3.17 [177]). 

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. 

The influence of complex formation on segment mobility of polymer chains was studied on the example of PMAA complexes with DTAB, TTAB and CTAB [19], and also PEVP complexes with SDD [26], The formula of spin-marked PMAA (PMAA ) is presented in Scheme 1 formula of spin-marked PEVP (PEVP ) is presented in Scheme 4. Content of spin marks in the case of PMAA was mark per 30 units, in the case of PEVP 1 mark was per 100 macromolecules. 

Fig. 18a, b Spectra of the CAVl 1/DMF crystals a CP/MAS spectrum recorded with 4 ms contact time. Asterisks denote spinning sidebands b dipolar-dephased CP/MAS spectrum. Peaks C2 and C5 are absent from the spectrum while C7 and CH2 peaks are still visible, probably due to segmental mobility of the aliphatic chains. (Adopted from [55] with permission)... 

Observable NMR spectral parameters such as chemical shift, spin-spin coupling, and peak intensities in simple ID spectra allow one to obtain polymer composition, tacticity, sequence distribution, and mechanism of polymerization. However, the poor mobility of polymers often leads to high viscosity solutions and rapid Tj relaxation, resulting in the poor spectral resolution. The viscosity can be decreased by dilution and high measurement temperature. Fortunately, many polymers possess segmental mobility, which facilitates the observation of high-resolution spectra with the line widths in the range of 1-10 Hz. 

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... 

Nmr relaxation measurements allow the molecular motions of the segments of a polymer molecule to be investigated. The spectral line width, which is proportional to I/T2 (i.e. the reciprocal of the spin-spin relaxation time of proton nmr) is a direct measure of segment mobility. It would be anticipated that the anchor segments in the trains attached to the surface of the adsorbent particle would display low mobility. On the other hand, those segments in loops and tails that project into the continuous phase should possess much higher mobility. Two distinct linewidths would therefore be expected for the two different types of segments. 

Klepac extensively studied the effect of uniaxial deformation on the molecular chain segmental mobility of linear low density polyethylene (LLDPE) by ESR spin probe method. The simulated ESR spectra of undeformed and deformed (parallel and perpendicular) LLDPE samples at 0 °C are shown as dotted line in Figure 25.31. The results obtained by simulations indicate that the amount of slow components and corresponding Tr of LLDPE films deformed in both parallel and perpendicular directions are significantly larger than those of the undeformed LLDPE films. However, the amount of fast components decreased in the deformed sample. It was concluded from the results that uniaxial deformation reduces the molecular chain segmental mobility in the amorphous region and increased the amount of crystalline phase of the LLDPE films. 

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