Crystalline and noncrystalline components

Spin-Lattice and Spin-Spin Relaxation. In order to examine the content of these crystalline and noncrystalline components, we examined the spin relaxation of each resonance line. Firstly it was found that the line due to the orthorhombic crystalline component at 33 ppm involves plural Tic s for all samples, as summarized in Table 4. In relation to the T1C for each sample, very long Tic values are recognized for three higher molecular weight samples. These values are expected... 

Since Holmes observation of the X-ray diffraction of nylon [1], many fruitful studies have been presented using X-ray diffraction, infrared absorption and other techniques. It can be expected that solid-state NMR provides useful information about the structure and dynamics of the crystalline and noncrystalline components of polyamides [2, 3]. Actually, solid-state H, and NMR have been successfully used to clarify various crystalline and amorphous components. 

In these connections, it is useful to consider the behavior of the chemical shift of the CH2 carbons of paraffins and polyethylene in the crystalline and noncrystalline components. It is known that the CH2 carbons in paraffinic chains appear at lower frequency by 4-6 ppm if a carbon atom three bonds away is in a gaMc/ie-conformation rather than in a frans-confor-mation (y-effect) [7]. In fact, cyclic paraffins which crystallize in a conformations are characterized by two parallel all-fran5-planar zigzag strands connected by two GGTGG loops, it is found that the CH2 carbons with a y-effect resonates at a lower frequency by about 6.5 ppm as compared with those with no y-effect [8, 9]. Furthermore, it is found that the CH2 carbons of cyclic paraffins, and n-paraffins in the noncrystalline state, appear at lower frequency by 2-3 ppm more than those in the crystalline state. In the crystalline state, the CH2 carbons assume the all-trans-zigzag conformation, which is fixed because motion is frozen, but in the noncrystalline state a rapid transition between the trans- and gawc/ie-conformations occurs [11]. Weeding et al. [12] obtained the same results on the noncrystalline state. 

The Tic values for the crystalline component are more than 5 times larger than the values for the noncrystalline component. Using this difference in Tic, CP/MAS NMR spectra of the two components are recorded separately by selective measurements of the crystalline component by the CPTl pulse sequence and the following spectral subtraction method. Figure 19.5 shows the CH resonance lines of the crystalline and noncrystalline components of A-PVA films thus obtained [33]. In this figure, the results of the computer lineshape analyses for these resonance lines are also shown. Here,... 

Our recent CP/MAS 3c NMR work(l, 2) on native and regenerated celluloses has shown that the resonance lines can be analyzed in terms of the contribution from the crystalline and noncrystalline components. The chemical shifts of Cl, C4, and C6 carbons thus ascribed to the respective components could be correlated to the torsion angles i(> and Tp, which define the conformation about the 3-1,4-glycosidic linkage, and x about the exo-cyclic C5-C6 bond,... 

On the other hand, the resonance line of the Cl carbon is also assumed to contain crystalline and noncrystalline components. In order to examine this problem, we measured spin-lattice relaxation times using a pulse sequence as developed by Torchia(16). As a result, it has been found that each resonance line of ramie contains two components with values of 4.6-16s and 65-130s which correspond to the noncrystalline and crystalline components, respectively(1 ). ... 

On the basis of these different Tj values, we were able to record the spectra of the crystalline and noncrystalline components of native cellulose separately by appropriate pulse techniques. 

Figure 4(A) shows the spectrum of the crystalline component of ramie which was obtained tmder the MAS condition by a pulse sequence for measurements developed by Torchia(16). The time between the two ir/2 pulses was about 30s. As is clearly seen, the broad upfleld components of the C4 and C6 resonance lines disappear and another fine splitting appears in the Cl resonance line which originates only from the crystalline component. Figure 4(B) shows the spectrum of the noncrystalline component of ramie which was obtained by a conventional ir/2 single pulse sequence with a short waiting time of 4s employing the DD/MAS technique. As the difference in between the crystalline and noncrystalline components is not very large, a small amount of the downfleld crystalline lines also appears for the C4 and C6 carbons. Although a small amount of the crystalline component is also involved in the Cl resonance line, the characteristics of the noncrystalline component of this line can be well observed the chemical shift of this component is about 106 ppm from IMS and the line width is about 90 Hz.

Figure A Partially relaxed 25 MHz C NMR spectra of the crystalline and noncrystalline components of ramie which were separately measured by a pulse sequence for CP measurements developed by Torchla(16) and a single ir/2 pulse sequence, respectively.

The chemical shifts of Cl, C4, and C6 carbons of the crystalline and noncrystalline components of native and regenerated celluloses are tabulated in Tables I and II, respectively. Table II shows also the line width Av of each resonance line. Before we discuss these results, let us consider the meanings of the solid-state chemical shifts in the next section. 

Chain Conformation of the Crystalline and Noncrystalline Components of Cellulose. 

The contributions of the crystalline and noncrystalline components to the resonance lines of Cl, CA and C6 carbons of different native and regenerated cellulose samples can be analyzed by using the respective line shapes. The chemical shifts of both components thus obtained were precisely determined. 

The application of a properly timed rf pulse disturbs the net equilibrium magnetization aligned parallel to the magnetic field Hq. The return of nuclear magnetization to equilibrium for solid polymers depends on the chain mobility and is monitored by the FID. The crystalline phase content can be extracted by deconvoluting the decay curve into portions corresponding to crystalline and noncrystalline components. Crystallinity determination of various polyethylene samples by FID NMR based on two- and three-component approaches, as well as... 

In conclusion, it must be emphasised that although it is convenient for the purpose of constructing models to assume a composite that comprises distinct crystalline and noncrystalline components, on the molecular level a gradual transition must occur, extending over a number of monomer units, between the well-orientated and ordered crystallites and the bulk of the remaining material. 

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