Chemical shift of polyethylene

In Table 7.3 is shown the chemical shift of polyethylene as a function of temperature [20]. The chemical shifts for the crystalline phase are independent of temperature, but those for peak A are dependent on temperature. At -120°C, the chemieal shifts for the crystalline and amorphous phases are about 33 ppm and 32 ppm, respectively. The peak for the crystalline phase arises from the erystalline part of the orthorhombic form. The chemical shift of the amorphous peak is about 32 ppm which is in the range of the tran -conformation. The methylene carbons seem to take a trani-conform-ation in the nonerystalline phase in this temperature range because of the... 

Table 7.3. NMR chemical shift of polyethylene as a function of temjjerature [20]...

Fig. 9.1. C chemical shift of polyethylene and n-paraffins. ( ) C chemical shift in solution (O) chemical shift in the solid state [3, 5, 16].

Addendum Subsequent to the presentation of this work, a report by D.L. VanderHart (J. Chem. Phys. (1986) 84, 1196) estcUsllshed a field dependency for c NMR chemical shifts of polyethylene in the solid state. At the 50.3 MHz frequency used in the present work, the correct value is 32.9 ppm not 33.6 and hence all chemical shifts should be decreased by 0.7 ppm from the values reported above. 

The estimation of chemical shifts by examining the spectra of model compounds is not always feasible, and the prediction models fail to distinguish between two or more stereosequences as they cannot always be distinguished on the basis of intensity alone. To overcome these limitations, large numbers of organic compounds have been analyzed by NMR and their chemical shifts have been used to determine a set of empirical correlations that are used to determine the structure based on the polymer s NMR spectrum. The carbon chemical shifts of hydrocarbon-based polymers such as polyethylenes can be predicted by empirical additivity rules such as ... 

The chemical shift is determined by the relatively local electronic structure. One of the most important parameters which affect chemical shift is conformation. As mentioned in the section about crystalline and amorphous phases, a typical example for the conformational effect on the chemical shift is the chemical shift difference between the crystalline and amorphous phases of polyethylene. In the crystalline phase, polyethylene takes the all trans-zigzag conformation, while, in the amorphous phase, a rapid transition between the trans and gauche conformations takes place. As a result, the chemical shift of the amorphous phase is the average of the trans and gauche conformations. 

Structure [3, 4]. As described in Chapter 7, the chemical shifts of CH2 for paraffins and polyethylene in orthorhombic form are shifted by about 1 ppm when compared with those in the monoclinic and triclinic forms. Quantum chemical calculations reveal that the chemical shift difference is caused by a local difference of mutual orientation for the trans-zigzag plane in inter-molecular interactions in the orthorhombic form and the triclinic and monoclinic forms [5]. 

Table 9.1. C chemical shifts of cyclic paraffins, n-paraffins and polyethylenes in the solid state.

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. 

Amorphous polymers or regions of polymers can be regarded as micro-porous materials, and can be studied, therefore, by monitoring the Xe chemical shifts of adsorbed xenon gas. Mansfeld et al. [94] used this method to distinguish between incompatible blends (of polypropylene with a poly-propylene/polyethylene copolymer) and compatible blends (PMMA and PVDF). In the former case, two Xe signals were observed, whereas only... 

Chemical Shift of A1 2p Binding Energy in the Aluminum-Polyethylene Interface. 

Assuming no conformation dependent chemical shift effects to occur and using the chemical shift of orthorhombic polyethylene (33 ppm) [6] we can now calculate the chemical shifts of the methine carbon atoms in the three triads of the solid crystalline E-VOH copolymer, respectively 000 (67 ppm), CX)E (70.4 ppm) and EOE (73.8 ppm) where 000, OOE, EOE are abbreviations for (VOH, VOH, VOH), (VOH, VOH, E) and (E, VOH, E) triads. The chemical shift values presented above are only meant to yield useful assignments of the several methine carbon NMR signals of E-VOH copolymers. These assignments are necessary because Ovenall [5] did not report dependable estimates for all three types of methines sustained by experimental results. We are aware of chemical shift differences between liquids and solids. Moreover, the choice of orthorhombic polyethylene as a basis for the shift calculations is rather arbitrary but this will only cause the same uncertainty in each of the three shifts. Of more importance is the known sensitivity of substituent-induced shifts towards different conformational equilibria. From results obtained by Cantow [7] for different poly (1,2-dimethylbutane) polymers it can be estimated that the uncertainties in our estimations amount to ca. 2 ppm. It is, however, improbable that the order of the three methine carbon signals will be misjudged. 

The chemical shifts of the functionalized polyethylene and the two model compounds were so close that isolated CH(CCX)C2H5)(CH2COOC2H5) units along the macromolecular chains appear to account very well for the functionalities present in the examined polyethylene sample. 

The experimental evidence for this theory has come from nuclear magnetic resonance studies of a numba of 2-hydro3gd)eiizophenones (110). The n ative chemical shift of the hydro g proton, characteristic of the hydr< en bonding formation, is closely correlated with their efficiency as stabilizers for polyethylene. 

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

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