INFRARED EXAMINATION OF POLYMER

Taken From: J. Haslam, H.A. Willis, D.C. Squirrel, Identification and Analysis of Plastics, Iliffe, London, 1972. p. 368, 369, 385,386

INFRARED EXAMINATION OF POLYMER

Polythene

When examined as thin films (0.001 to 0.002 in or 0.025 to 0.05 mm thickness) all kinds of polythene appear very similar in their spectra. The strong absorption bands at 3.4 μm (2941 cm^-1), 3.8 μm (2632 cm^-1), 7.3 μm (1370 cm^-1) and 13.8 μm (725 cm^-1) all arise from --CH₂-- chain. When the infrared spectrum of a resin shows these features, and no others of significance , it may be presumed to be polythene, but there is naturally a strong similarity between the spectrum of polythene and the spectrum of high molecular weight linear aliphatic hydrocarbons, including paraffin wax.

There is usually no difficulty in deciding, from its physical properties whether a substance is polythene or paraffin wax, since the latter is soluble in ether polythene is substantially insoluble. This test is useful when dealing form example with wax-coated paper: both polythene and paraffin wax are removed by boiling toluene, but only the latter is ether soluble.

Polypropylene

The spectrum of isotatic polypropylene is somewhat surprising, since, in addition to the bands normally attributable to --CH₂-- and –CH₃ groups at 3.4 μm y near 7 μm, a number of sharp bands of medium intensity occur in the 8 μm to 12 μm region.

In the spectrum of molten isotactic polypropylene, which is presumably amorphous, and so-called atactic polypropylene which is amorphous at room temperature all the prominent structure between 8 μm and 12μm disappears, except for bands at 8.7 μm and 10.3 μm. The only important difference in the spectra of the molten polymer and atactic polymer is the band in the latter at 11.3 μm. This is thought to be due to the chain-terminating group R·C·(CH3)=CH₂ and its prominence in this spectrum of the atactic resin is presumably due to the relatively low molecular weight of the sample examined.

It is reasonable to presume therefore that the additional bands which appear in the spectrum of the isotactic resin are not due directly to the chemical structure of the resin. Nor are they due to the interaction of molecules when packed together in the crystal, since different crystalline forms of polypropylene in which the molecules are packed in different ways, have virtually identical spectra. It is therefore concluded that the features of the spectrum of isotactic polypropylene are due to the helical arrangement of the chain of the individual polymer molecule.

Unsaturation in polypropylene

Examination of polypropylene as comparative thick specimens (≈ 0.015 in or 0.4 mm thickness) reveals a shoulder (which varies in intensity in different specimens) at approx. 11.25 μm (889 cm^-1) on the side of the polypropylene band at 11.1 μm (901 cm^-1). This shoulder, which is characteristic of pendant methylene unsaturation, arises from the end group on the polypropylene chain: