Gamma-phase of isotactic polypropylene

Abstract

There are basically three crystal modifications known for isotactic polypropylene (iPP) which forms in the crystalline state exclusively a 31-helix caused by the energy minimum at this conformation [1], The α-modification is the common form of iPP and well characterized already by Natta and coworkers. The β-modification occurs in a certain interval of crystallization temperatures and is usually induced by nucleation agents. Turner-Jones et al. assigned a triclinic unit cell to the γ-modification of iPP [2]. Later studies applying a Rietveld analysis showed that the γ-modification of iPP forms an orthorhombic unit cell. The original triclinic unit cell can be considered as a part of the orthorhombic unit cell. The main difference of the new model is the nonparallel chain packing in the crystal of the γ-modification which is unique in the field of synthetic polymers and was introduced by Meille et al. [3]. The orthorhombic unit cell of the γ-modification is formed by bilayers composed of two parallel helices. The direction of the chain-axis in adjacent bilayers is tilted with an angle of 80°. This is a unique packing arrangement for polymers but has been known e.g. for fatty acids. The dimensions of the unit cell are a = 0.854 nm, b = 0.993 nm, c = 4.241 nm. Lotz et al. have made a remarkable work to support this nonparallel chain packing model [1]. They pointed out that the electron diffraction experiments on a flat-on γ-phase single crystal resulted in the same patterns after clockwise 40° rotation and anticlockwise 40° rotation, which corresponds to an angle of 80° between the double layers of helices arranged in the unit cell. Figure 1 shows four unit cells of the β-modification formed by 31-helices of iPP.