Methylene sequence

Figure 3.4 Packing of amides in unit cell (a) a-parallel structure of even-even PA (b) /3-antiparallel structure of even-even PAs with equal methylene sequence length in amine and acid unit, as in PA-4,620 (c) antiparallel PA-6-type polymer.

PTT has three dynamic mechanical viscoelastic relaxations [61, 62], a, (j and Y (Figure 11.9). The 70°C a-relaxation is the glass transition. In a study on the effect of methylene sequence length on aromatic polyester viscoelastic properties, Farrow et al. [63] reported a PTT a-relaxation as high as 95 °C. They also found that Tg of this series of aromatic polyesters did not show any odd-even effects, which was later confirmed by Smith et al. [64],... 

The reaction probability model produces a complete calculated C nmr spectrum as a best fit to the observed experimental spectrum. The deduced probabilities provide the following derived quantities (1) "rira", a measure of monomer sequence randomness, (2) the distribution of methylene sequence lengths,... 

We considered the polymer chain could be formed by either primary or secondary insertion. Primary insertion is when the monomer forms a bond with the methylene group to the catalyst metal and secondary insertion is where the metal and monomer form a bond with the propylene methine group. Ethylene must always add by primary insertion, but propylene can add either way. And, in fact, the presence of the ag secondary carbon shows the presence of a two carbon methylene sequence which can occur only if both types of insertion are in force. 

Table II gives our description for each occurring methylene sequence in terms of conditional probabilities. We take a measured nmr spectrum of an ethylene propylene elastomer and derive a set of six reaction probabilities that fit it best. For possible sets of probability values, a spectrum is calculated and a sum of weighted squares of errors is found. The set of probability values having the smallest sum of weighted squares of error is used as a starting point to minimize the error using the...

Table VI yields the methylene sequence distribution of these two EPDM rubbers. A measure of the fraction of long methylene runs is important to the understanding of physical properties of these and similar materials. Polyethylene blends with EPDM rubbers, having appropriate long runs of ethylene, have been shown ( 1 ,1 ) to have unusually high tensile strengths.

An interesting phenomenon of macromolecular isomorphism was detected in copolymers of a-butene with a-M-hexene, a-M-octene, a-n-decene, a-w-dodecene, and a- -octadecene (<5). In these copolymers isomorphous replacement occurs between the butene units and the methylene sequences of the side groups in both the known modifications I and II of poly-a-butene. 

As discussed in the section B of this chapter, the sample with a very low molecular weight is predominantly composed of a lamellar crystalline region, with a minor amount of interfacial region, and no liquidlike interzonal region at room temperature, as can be schematically depicted in either Fig. 10 (B) or (C). The interfacial region comprises relatively short methylene sequences with very limited mobility that are excluded from the crystalline region. This characteristic feature of the phase structure is also reflected in the temperature dependence of the NMR spectrum. 

At low temperatures, the y transition is centred around - 150 °C. It has been assigned to motions of the central methylene groups of the amine moiety. Indeed, it appears with diamines containing at least four methylene units and increases in intensity with longer methylene sequences. 

The half-width of the crystalline component line was estimated to be 18 Hz. This value reflects the very stable orthorhombic crystalline phase of this sample. The component line shape centered at 31.0 ppm represents the contribution from the amorphous phase in which the molecular conformations are changed rapidly over all permitted conformations. The relatively narrow line width estimated as 38 Hz is caused by the rapid molecular motion. The line centered at 31.3 ppm represents the noncrystalline phase in which the local molecular motion can occur in the same manner as in the amorphous phase (in Tic time frame), but a long-range molecular motion accompanying a conformational change related to a 10-20 methylene sequence is severely restricted. The wide line width as 85 Hz... 

Molecular Weight Dependence of Phase Structure. Similar line shape analysis was performed for samples with molecular weight over a very wide range that had been crystallized from the melt. In some samples, an additional crystalline line appears at 34.4 ppm which can be assigned to trans-trans methylene sequences in a monoclinic crystal form. Therefore the spectrum was analyzed in terms of four Lorentzian functions with different peak positions and line widths i.e. for two crystalline and two noncrystalline lines. Reasonable curve fitting was also obtained in these cases. The results are plotted by solid circles on the data of the broad-line H NMR in Fig. 3. The mass fractions of the crystalline, amorphous phases and the crystalline-amorphous interphase are in good accord with those of the broad, narrow, and intermediate components from the broad-line NMR analysis. 

For this cycloalkane, the Tic s of the methylene carbons in the folded sequence are estimated to be 35-39 s at room temperature and become shorter with increasing temperature [66]. Hence, the Tic s for the regularly folded methylene sequence in polyethylene, if it exists, will be appreciably shorter than 35 s at room temperature, and much shorter than ca. 220 s of the inner methylene carbons of polyethylene (cf. Table 1). Hence, if we measure the DD/CP 13C NMR spectrum of the solution-grown polyethylene sample by a single pulse sequence with a rep-... 

An EPDM rubber is produced by the terpolymerisation of ethylene and propylene with a small amount (typically of the order of 5%) of an unconjugated di-olefin. The di-olefins used, include dicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene and methyl tetrahydroindene, 1,5 cyclo octadiene. A number of other dienes [74,75] have been tried. Infrared spectroscopy [35] is used to find out the ter monomer content. The characteristic peaks for the ter monomer are shown in Table 3.4. In view of the relatively low concentrations, it is probable that ter monomer base units are present largely as isolated units in EPDM but the distribution of propylene and methylene sequences is of considerable interest. 

A band of moderate intensity has been observed at 1080 cm-1 in the Raman spectrum of once-folded solution-crystallized n-alkane C168H338.72 This spectral region is associated with C—C stretching vibrations of methylene sequences containing gauche bonds. Extended-chain shorter alkanes (/v-CixHox, n-C72H146) did not show this band. The 1080 cm-1 band is also observed in semicrystalline polyethylene.73 Low-temperature spectra were not reported. 

In crystalline polymers, the principal relaxation process is associated with melting. In polyethylene, a (1-. and 7-transit ions have been identified and, particularly in higb-tlensity polyethylene, the a-transition has been sub-divided into a and a. In ethylene-based polymers, the y-transitions at —120°C is generally associated with the amorphous phase, in particular, with crankshaft motion of methylene sequences. " However, based upon studies of solution grown lamellae, it has also been suggested that this may llien be associated with... 

Figures 3.9 and 3.10 show the temperature dependencies of Ti and NOE of the CH2 (rrr) of the same PMMA solution and the results (solid and broken curves) simulated by the second-order model-free treatment with p = 3 [17]. Here, the Arrhenius equation was assumed for the respective correlation times tj = tio exp(AEi/RT) and ta/ = ta,o exp(AEA,/RT). In this case the simulated results with p = 3 are also in good accord with the experimental results, indicating the validity of the model-free treatment. Similar analyses of the temperature dependencies of the Tj were successfully performed for the rubbery components of the solid polyesters with different methylene sequences [20, 21]. These results are also well analyzed by the second-order model-free treatment with p = 3. There are a large number of the publications of the temperature dependencies of Ti and NOE analyzed by different models of molecular motions for polymers in the dis-...

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