Broad component

Another problem in many NMR spectrometers is that the start of the FID is corrupted due to various instrumental deadtimes that lead to intensity problems in the spectrum. The spectrometer deadtime is made up of a number of sources that can be apportioned to either the probe or the electronics. The loss of the initial part of the FID is manifest in a spectrum as a rolling baseline and the preferential loss of broad components of... 

Solid-state C NMR techniques have been applied to the characterization of the different phases of several polybibenzoates [25,30], including P7MB, PDTMB and PTEB. The last two polymers offer the advantage of the stability of the mesophase at room temperature. The spectra corresponding to the pure mesophase of these samples only exhibited a broad component, while the spectra of the annealed samples were separated into two components crystal and noncrystal. The shapes of the mesophase and the noncrystal components are very similar, and only modest variations in the relaxation times were observed between these two components. The degree of crystallinity of these samples was determined... 

Fig. 4.2. The structure of the doublet (a) before collapse and (b) after (the broken line is the broad component of the collapsed doublet, that has negative intensity).

The nature of the clustered phase is not well understood. One possible interpretation is that a cluster is composed of a relaxed divacancy whose inner surface is dressed with hydrogens however there is no direct NMR data which supports this identification (Reimer and Petrich, 1988). Alternatively, it has been suggested that the broad component of the NMR spectrum arises from hydrogen atoms lined up alone microtubular structural defects (Chenevas-Paule and Bourret, 1983). 

At lower temperatures, the spectrum of the protons changes. Between —130 and —154° this peak separates into two equal but broad components at 5 3 05 and 6-59. The methylene spectrum at T86 p.p.m. also separates and exhibits a broad high-field shoulder at 1-70. These shifts are all measured relative to the He band at 2-82 p.p.m., which is assumed to remain constant. 

The ESR spectra of polyisobutylene after irradiation with ultraviolet light (6) are different from those obtained after irradiation with ionizing radiation. The spectra consists mainly of two components one, a sharp quartet which has a half life of 1% hours at liquid nitrogen temperature, has been attributed to free methyl radicals (XI), in analogy with ultraviolet-irradiated polypropylene (51). The broad component is composed of many superimposed lines and was interpreted as caused by three different radicals, all stable at liquid nitrogen temperature. One of these radicals (XV) is the counterpart to the methyl radical (XI) while the others are the two radicals (XIII and XVI) which can both be formed by hydrogen abstraction. 

The ESR spectrum of radicals in poly(4-methyl-l-pentene) induced by ultraviolet light 4,5) is composed of a sharp quartet with the hyper-fine splitting constant of 22.5 gauss and a broad quartet. The sharp component has been attributed to methyl radicals (XI) while the broad component could be caused by polymer radicals of structures XXIII and/or XXIV. 

The area below the absorption curve is proportional to the number of protons in the sample which permits the decrease of free plasticizer to be calculated from the change of this area upon cooling. For each sample the area below the curve is constant, regardless of whether the absorption is narrow or broad. Below the glass temperature the narrow component of the absorption curve can be attributed to the plasticizer, whereas the broad component represents the glassy polymer and that part of the plasticizer which is already immobilized. 

Irradiation of butadiene rubbers gives rise to a broad component, with the intensity increasing with the irradiation time. It is found that rubbers crosslinked with irradiation have more hindred motion than rubbers crosslinked with sulfur this is explained by the higher potential barrier of rotation about C—C bonds, as compared to C—S bonds ... 

Figure 8 shows the relationship between the mass fraction wb of the broad component and the crystallinity (1 —X)[Pg.151]

Fig. 7. Three-component analysis for bulk-crystals. Dashed, dotted, and broken lines indicate narrow, medium, and broad components, respectively. Thick dotted and solid lines indicate the composite curve for the three components and the experimental spectrum, respectively.

Fig. 8. Mass fraction of broad component vs. crystallinity obtained from density measurements66. Solid circles indicate data of Mandelkern ef ai.68, 69 Straight line indicates the relation, (l-k)

The spectrum for samples with a very low molecular weight, ie., lower than about 1000, is fairly independent of the mode of crystallization, whether from the melt or from dilute solution. The spectrum is characterized by a very large broad component (wb 0.95) and a medium component with a large second moment, but no narrow component. In such samples the extended molecular chain length will be comparable to or slightly larger than the lamella thickness. The conformation of molecular chains to form the lamellar crystallites will be similar to that depicted schematically in Fig. 10 (B), independent of the crystallization mode. 

Similar deviation was also recognized by Berg inarm14 for solution-grown samples. This will be caused by the elementary spectrum used for the broad component. Detailed discussion is outside the scope of this review. 

---