Structure of mesomorphic form of isotactic polypropylene

Abstract

The structure of quenched isotactic polypropylene (iPP) films, including samples etched with fuming nitric acid (FNA), has been studied by infrared (IR) spectra, wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) measurements. The changes of IR, SAXS, DSC, and WAXD results induced by annealing for etched samples have been compared with those for unetched ones. The IR absorbance spectrum of the quenched iPP etched by FNA did not change. In addition, the SAXS intensity did not increase when these samples were annealed, indicating that the total (IR) crystallinity (i.e., the content of chain segments in the helical conformations) of the etched samples does not increase. However, WAXD patterns of the samples changed in a manner similar to those of the unetched one, from the original two blurred diffraction peaks to the sharp crystal patterns of alpha-form iPP, suggesting that the mesomorphic (or liquid crystal-like) phase has reorganized to alpha-form crystals. It is concluded that the change of WAXD of quenched iPP films during annealing results mainly from transformation of order in the mesomorphic phase, rather than only from an increase of crystal size. In other words, mesomorphic-form iPP is not constituted by any known crystals (such as alpha or beta crystals) in small sizes; its WAXD pattern reflects truly the degree of order in the mesomorphic phase.