Crystalline forms of isotactic polypropylene

TLDR

X‑ray diffraction was used to investigate the production conditions and melting behavior of the β‑ and γ‑forms of isotactic polypropylene, and to derive their unit cells and speculate on helix packing. The study identified β‑ and γ‑crystalline forms of isotactic polypropylene, obtained them free of the α‑form, characterized their unit cells (β pseudohexagonal with 6–12 helical chains, γ triclinic), confirmed a ternary helical chain configuration, and found that drawing converted both forms to α‑crystal, preventing oriented fiber formation.

Abstract

Abstract The X‐ray diffraction patterns of two crystalline forms of isotactic polypropylene, the β‐ and γ‐forms distinct from the α‐form described by NATTA 1 , have been recorded. The β‐form, previously observed only as single spherulites 6 has now been obtained in a well crystalline polymer, practically free of α‐form. The γ‐form is obtained, free of α‐form, in stereoblock fractions taken over the range 35–70°C. from normal commercial polypropylenes; the γ‐form is formed after cooling from the melt. The conditions for the production of the two forms and their melting behaviour, have been investigated by X‐rays. Oriented fibres were not obtained in either form as conversion to α‐form took place on drawing. Both forms have a ternary helical chain configuration. The derivation of the unit cells of the β‐ and γ‐forms from X‐ray photographs of unoriented specimens has been attempted and speculations have been made as to the packing of the helices in both forms. There are at least six and possibly twelve helical chains in the pseudohexagonal unit cell of the β‐form; the unit cell of the γ‐form is triclinic.