The CH 2 chains in these molecules can orient parallel to each other forming areas of long range order (that is, they crystallize as labeled b in Figure 3). What HDPE and petroleum jelly have in common is that they contain long chains of methylene groups, and that they are solids at room temperature. The methyl group umbrella mode is at 1377 as expected, but note that similar to HDPE, the CH 2 rocking peak in Figure 2 is split. As expected, we have three peaks between 30, confirming we have CH 3 and CH 2 groups present. This sample consists of a mixture of straight chain alkanes with at least 18 carbons total and, at minimum, 16 methylenes in a row. The spectrum of petroleum jelly will serve as a good example, and its spectrum is seen in Figure 2. To understand this splitting, we need to first examine the spectrum of a mixture of long chain but not polymeric alkanes. What was not discussed in the previous column (1) was why the CH 2 rocking peak in HDPE is split, with peaks at 730 and 720 rather having one expected peak at 720☑0. Figure 1 shows the details of the HDPE spectra. However, I left you hanging regarding the details of the HDPE spectra. In a previous column (1), we examined the spectra of LDPE and HDPE,both alkane-based polymers, and saw how the existence of alkyl side chains in LDPE gave it different properties and spectra than HDPE. High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Crystalline Splitting Table III shows the peak positions for two useful alkane bending vibrations, the CH 3 umbrella mode and the CH 2 rock. Table I lists the group wavenumber peak position ranges for CH 3 and CH 2 stretching vibrations. The spectra of alkanes are dominated by the C-H stretching and bending peaks from the methyl and methylene groups. In fact, the extended hydrocarbon family is a large one. However, this does not mean that alkanes and alkane polymers comprise a small family of molecules. The simplest hydrocarbons are alkanes, which contain CH3 (methyl) and CH 2 (methylene units) (2). It is important to recall from an early installment in this series that hydrocarbons are molecules that contain hydrogen and carbon only (2), thus hydrocarbon polymers are large molecules that contain hydrogen and carbon only. In the last installment (1), I showed that the spectra of functional groups in polymers are similar to those of functional groups in small molecules, which means that our analysis of polymer spectra provides us a great opportunity to review the spectra of the functional groups we have covered over the last several years. ![]() We explore how different PE syntheses produce materials with different physical and spectroscopic properties, which introduces us to the concept of crystalline splitting and the rest of the story on CH 2 rocking peaks. ![]() In our second installment of the infrared (IR) spectra of polymers, we finish the discussion of the spectrum of polyethylene (PE) started last time.
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