Polyethylene methyl groups

Friedrich et al. also used XPS to investigate the mechanisms responsible for adhesion between evaporated metal films and polymer substrates [28]. They suggested that the products formed at the metal/polymer interface were determined by redox reactions occurring between the metal and polymer. In particular, it was shown that carbonyl groups in polymers could react with chromium. Thus, a layer of chromium that was 0.4 nm in thickness decreased the carbonyl content on the surface of polyethylene terephthalate (PET) or polymethylmethacrylate (PMMA) by about 8% but decreased the carbonyl content on the surface of polycarbonate (PC) by 77%. The C(ls) and 0(ls) spectra of PC before and after evaporation of chromium onto the surface are shown in Fig. 22. Before evaporation of chromium, the C(ls) spectra consisted of two components near 284.6 eV that were assigned to carbon atoms in the benzene rings and in the methyl groups. Two additional... 

This reaction leads to drastic decrease in the number of methyl groups in polyethylene and to an increase in the number of double bonds. 

IR analysis can also be used quantitatively to determine the EO-PO ratio [12]. Using mixtures of polyethylene glycol and polypropyene glycol as calibration standards, the ratio of two absorbances, one due to the methyl group of the PO unit (e.g., the C-H stretch band at 2975 cm ) and one due to the methylene group (e.g., the C-H stretch band at 2870 cm ), are plotted against percent of PO content. The ratio of the same two absorbances taken from the IR spectrum of a poloxamer may then be used to determine its percent of PO content by interpolation. 

Figure 8.13 DSC data for model polyethylene with a methyl group on every 21st carbon. [Reproduced with permission from Macromolecules, 33, 3781-3794, 2000. Copyright 2000, American Chemical Society.]...

Since the reactivity ratios of ethylene-polar monomer pairs are quite different, the preparation of copolymers with precisely the same comonomer composition can be a challenging endeavor. Earlier in this chapter, we described the synthesis and characterization of precisely placed methyl groups on a polyethylene... 

Polypropylene was not developed until the 1950s when Ziegler and Natta invented coordination catalysts. The structural difference between polyethylene and polypropylene is the methyl group in the propylene unit. Its presence makes a difference because it makes possible three different polymer structures Isotactic, with all methyl groups in the same plane makes the best plastic syndiotactic, in which the methyl groups alternate in the same plane and atactic, with the methyl groups randomly in and out of the plane is soft and rubbery. Polypropylene is used as film and in many structural forms. It is also used as fibers for carpet manufacture and for thermal clothing. 

Figures 59 and 60 show the ATR-FTIR spectra recorded from areas C and D, respectively, from the cross-section of catheter sample 1. Both spectra show features that indicate that each is essentially a polyethylene. The shoulder seen at 2,960 cm-1 (due to terminal methyl groups in the branching) in Figure 59 indicates that the polymer is probably a low-density polyethylene (LDPE). Other weak absorption bands are also evident in this spectrum between 1,800-1,600 cm-1 and 1,300-1,000 cm-1, which are not attributable to polyethylene. These may well indicate it is a copolymer or contains an additive. The sharp well-resolved doublet at 730/720cm 1 evident in the infrared spectum of Figure 60 and lack of shoulder at 2,960 cm"1 indicate that this layer is probably...

We have synthesized such a material, which is called perfectly imperfect polyethylene, where each branch is a methyl group and its frequency along the backbone is controlled by the nature of the symmetrical diene used in the ADMET polycondensation reaction [37]. Equation 12 illustrates the chemistry used to produce polyethylene by a step polycondensation route rather than a chain propagation mechanism. 

Due to the symmetry of 19a, which possesses a methyl group on each and every ninth carbon, the carbon NMR spectrum displays only six resonances. This conclusively demonstrates that these techniques are able to prepare polymers whose regularity can be controlled with ease. The polymers themselves are sufficiently high in molecular weight to mimic crystallization behavior of polyethylene in its substituted forms. 

Some special problems arise in explaining PP, and they breed a set of new terms that are used throughout discussions on any polymer more complicated than polyethylene. The problem lies in the extra group that propylene carries along. Except for that methyl group, -CH3, propylene (CH3-CH=CH2) would be ethylene. 

More importantly, understanding the chemistry of PP requires you to know the critical difference between PP and the polyethylenes—the asymmetry of the PP molecules. backbone. In polyethylene, every carbon looks like every other carbon in the chain. In PP, the polymer linkage is between succeeding double-bonded carbons, like polyethylene. But, the methyl group survives as a branch on every second carbon in the PP backbone chain. See Figure 23—7.) Furthermore, the orientation of that branch is crucial to the properties of the polymer. See Figure 23-8.)... 

The intramolecular mobility, which yields many other conformations between these two extremes, is the basis for viscoelasticity in polymers. Pendant groups, such as the methyl group in isobutane, and lower temperatures decrease the mobility of model compounds such as ethane as well as the polymer chains of polyethylene. 

This covers methods that depend upon the fact that branching introduces groupings with different chemical structure from that of the repeat units of linear chain, namely branch-points and end-groups. These can sometimes be detected and estimated by physical or chemical methods. However, short branches as well as long ones introduce these groups, and it may not be justifiable to attribute them, or all of them, to long branches. Methyl groups in polyethylene are a case in point. 

Nowadays, the leading two polyolefin companies are producing more than 20 million metric tons of polyethylene (PE) and polypropylene (PP) annually. This is about 35% of the world production of all POs. The demand for high impact polypropylene HIPP, and linear low density polyethylene, LLDPE, especially (see Fig. 5.4-2) is permanently growing at about 7% per year. "Distance holders", such as the methyl groups and the butene branch shown in Fig. 5.4-2, help to control the degree of crystallinity, and consequently the processability. 

If groups A and B are methyl groups (butane), the steric hindrance between A and B leads to a rotational barrier of 25 kj ( 6 kcal) per mole. The consequence of this simple fact is that in fatty acids and related substances and in polyethylene the chains of CH2 groups tend to assume fully extended zigzag conformations. 

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