ASTM D2238 – Standard Test Methods for Absorbance of Polyethylene Due to Methyl Groups at 1378 cm−1
Description:
Significance and Use
4.1 When interpreted with the aid of appropriate calibration data, either test method is acceptable for use to compare the total methyl contents of polyethylenes made by similar processes. Data on infrared absorption at certain other wavelengths is potentially useful for information on certain other wavelengths (7).
Note 4: The bias of determination of the concentration of total alkyl groups depends on knowing the concentrations of methyl and ethyl branches present, since these branches have anomalously high absorptivities per group at 1378 cm−1 (7.25 μm).
4.2 Knowledge of total methyl groups in polyethylene, when combined with data on molecular weight and on reactive end groups such as vinyl, is potentially useful so as to lead to assignment of end-group structures and to shed light upon polymerization mechanisms.
4.3 Qualitative correlations are possible between data on total methyl groups in polyethylene with certain polymer properties such as melting point, density, stiffness, and other mechanical properties that are closely dependent on the degree of crystallinity of the polymer.
4.4 These test methods are especially suitable for research. They have not been tested for use in manufacturing control.
Scope
1.1 These test methods cover measurement by infrared absorption spectrophotometry of the 1378 cm−1 (7.25 μm) band in polyethylene due to methyl groups. (1, 2, 3-6)2 Two test methods are covered:
1.1.1 Test Method A uses compensation with a standard sample film of known methyl content.
1.1.2 Test Method B uses compensation with a wedge of polymethylene or a polyethylene of known low methyl content.
1.2 These test methods are applicable to polyethylenes of Types I (density 0.910 to 0.925 g/cm3), II (density 0.926 to 0.940 g/cm3), and III (density 0.941 to 0.965 g/cm3).
Note 1: For determination of density, see Specifications D1505.
Note 2: In cases of Type III polyethylene with densities greater than 0.950 g/cm3, different results are obtained with the two test methods.
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
ASTM D2236