The existence of isotactic, syndiotactic, and atactic polypropylene varieties is an important consideration regarding the polymer's structure and morphology.
Initially, the stereoregularity of the chains was studied primarily using X-rays and IR spectroscopy. Many of the bands observed at that time were attributed to the molecule’s helical structure.
Subsequent studies sought to correlate the intensity of some of these bands with the content of isotactic polymer, based on the differences found in the spectra of crystalline (solid), molten, and atactic PP. The most notable advances in understanding the possible crystalline structures of PP and their relationship to the observed stereoregularity have been achieved through C13-NMR.
The preparation of samples for analysis is based on the insolubility of the isotactic forms in boiling n-heptane, as mentioned in section I.1.b. The spherulites exist in different morphological configurations, depending on the crystallization conditions. The sign of birefringence (direction of deviation) depends on the relative amounts of radial and tangential platelets. In commercial products, the monoclinic crystalline form is the most commonly found.
Molecular weight is determined using standardized size-exclusion chromatography techniques. The viscosity-average molecular weight (VAMW), derived from intrinsic viscosity, is calculated using the Houwink equation:
Molecular weight distribution is analyzed using two methods. The first involves fractionation, typically performed on columns containing an inert support, followed by the characterization of each fraction using conventional techniques. The second is based on size-exclusion chromatography (SEC). The polymer in solution is separated according to molecular size, as larger molecules are retained for shorter times than smaller ones. The technique requires very precise calibration using samples with extremely narrow distributions.