Polymers spin-lattice relaxation times

It appears that purification of commercially available solvents is sometimes required for the complete elimination of impurity resonances. Occasionally, these impurities may be turned into advantage, as in the case of C2D2CI4 where the (known) C2DHCI4 content may be used as an internal standard for quantitation. Thus, removal of every impurity peak is not always essential for identification and quantitative analysis of stabilisers in PE. Determination of the concentration of additives in a polymer sample can also be accomplished by incorporation of an internal NMR standard to the dissolution prepared for analysis. The internal standard (preferably aromatic) should be stable at the temperature of the NMR experiment, and could be any high-boiling compound which does not generate conflicting NMR resonances, and for which the proton spin-lattice relaxation times are known. 1,3,5-Trichlorobenzene meets the requirements for an internal NMR standard [48]. The concentration should be comparable to that of the analytes to be determined. 

Approximately 1 g polymer and 0aQ6 M Cr(acac). were dissolved in CDCl. to prepare solutions for ySi and JC NMR spectroscopy. NMR spectra were run on a Varian XL-200 FT-NMR instrument. To aid in obtaining quantitative data, the solution was doped with 0.06 M chromium acetylacetonate [Cr(acac) )] to remove possible signal artifacts resulting from long spin-lattice relaxation times (T s) and tt> nucleay Overhauser effect, well-known features associated with 3Si and JC NMR spectroscopy. This permits quantitative signal acquisition. From the literature (16) and additional work done in this laboratory, it was expected that Cr(acac) would be an inert species. A solution of HMDZ (2.04 g, 12.67 mmole),... 

NMR 13C spin-lattice relaxation times are sensitive to the reorientational dynamics of 13C-1H vectors. The motion of the attached proton(s) causes fluctuations in the magnetic field at the 13C nuclei, which results in decay of their magnetization. Although the time scale for the experimentally measured decay of the magnetization of a 13C nucleus in a polymer melt is typically on the order of seconds, the corresponding decay of the 13C-1H vector autocorrelation function is on the order of nanoseconds, and, hence, is amenable to simulation. 

The second difficulty is not encountered in proton spectroscopy, where proportionality between peak area and concentration of the respective sequence is virtually guaranteed, but is present in caibon spectroscopy where one works under heteronuclear broad-band decoupling conditions. Under such conditions, both the nuclear Oveihauser effect (NOE) and the differences in spin-lattice relaxation time T, can alter the intensity. In this connection, however, Schaefer showed that for the different C nuclei inside the polymer chain, because of the restricted molecular movement, there are no large differences in NOE (121). 

Polymer Dynamics. 13C spin-lattice relaxation times (Ti) were determined with either an inversion-recovery sequence (16) (for carbons observed by direct polarization) or with a modified cross-polarization experiment (17). 13C rotating-frame relaxation times (Tip(C)) were derived from measurements of the carbon signal that remained after a Tjp(C) hold time of... 

Now it will be necessary to elucidate the location of the butyl isopropenyl ketone unit in the polymer chain. The spin-lattice relaxation time, Tj., of the protons in the polymer and oligomer was measured. The Ti of methylene protons adjacent to the carbonyl group was nearly the same level as the T of methyl protons in the terminal butyl group or terminal methine proton (Table ) but much longer than the T of the protons in interior sequences of polymer (13). These indicate that the butyl carbonyl group in the polymer or oligomer locates at or near to the forefront or the end of the chain. 

The NMR spectra were taken on a JEOL JNM-MH-100 (CW) spectrometer using tetramethylsilane as an internal standard. 13C spin-lattice relaxation time of the polymer was measured by the inversion-recovery Fourier transform method on a JNM-FX100 FT NMR spectrometer operating at 25 MHz. 

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. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

MHz, a it/2 pulse was 2.2 ysec and the data were acquired with the carrier frequency below resonance (the rhs of the spectrum in each figure). These spectra may be used to determine polymer crystallinity and to determine various kinds of macromole-cular motion. Also spin-lattice relaxation times in the rf interaction frame (Ti ) addition to conventional T- and T p relaxation times have been measured to help elucidate the various mechanisms responsible for the observed chemical shift line shapes. 

Relaxation parameters of interest for the study of polymers include (1,2,5,40) the spin-lattice relaxation time in Ho,... 

Spin-lattice relaxation measurements (7j) are also common,78 83 though these of course monitor more rapid motions. Spin lattice relaxation times have nevertheless proved a simple but effective means of distinguishing different structural regions within polymer samples, i.e. crystalline and amorphous regions and interfacial regions between crystalline and amorphous parts. 

If the motion of an acrylic polymer radical about the Cp bond is hindered, changing the temperature should lead to changes in the TREPR spectrum. This is indeed observed for all acrylic polymers we have examined to date. Simulation of the complete temperature dependence of TREPR spectra of acrylic polymer main-chain radicals should allow information regarding the conformational motion of the polymer in solution to be extracted, such as rotational correlation times, spin-lattice relaxation times (Ti), and activation energies for conformational transitions. 

Molecular motions not only affect the NMR line shape, but also determine the spin-lattice relaxation time T. Measurement of the T relaxation provides information about fast motions with frequencies near the NMR fiequency of deuterons, i.e., 46 MHz [25]. Moreover, Ti relaxation experiments are very useful for detecting motional heterogeneity in polymers [25]. Motional effects in both relaxation and line shape studies are completely dominated by reorientations of C-D bonds. Therefore, in motionally heterogeneous polymers, different T] relaxation times can be related to chain units with different mobility as reflected in different line shapes. 

The Pti samples (182) were prepared as colloids, protected by a PVP polymer film. Layer statistics according to the NMR layer model (Eqs. 28-30) for samples with x = 0,0.2, and 0.8 are shown in Fig. 63. The metal/ polymer films were loaded into glass tubes and closed with simple stoppers. The NMR spectrum and spin lattice relaxation times of the pure platinum polymer-protected particles are practically the same as those in clean-surface oxide-supported catalysts of similar dispersion. This comparison implies that the interaction of the polymer with the surface platinums is weak and/or restricted to a small number of sites. The spectrum predicted by using the layer distribution from Fig. 63 and the Gaussians from Fig. 48 show s qualitative agreement w ith the observed spectrum for x = 0 (Fig. 64a). 

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