The methods of preparation and formation of supermolecular structures in quiescent and sheared melts and the properties of the β-modification of isotactic polypropylene (β-iPP) are reviewed. The introduction of selective β-nucleants is the most reliable method for preparation of samples rich in β-modification or of pure β-iPP. The advantages and drawbacks of the known β-nucleating agents are summarized. It is emphasized that pure β-iPP can be prepared under laboratory and processing conditions in the presence of highly active and selective β-nucleants. Nevertheless, there are no literature data—apart from that of the author's groups—which evidenced unambiguously the formation of pure β-iPP. It hints at the insufficient selectivity of β-nucleants used or at the inappropriate crystallization or melting conditions applied by other scientists. The structure formation during the high-temperature hedritic crystallization is discussed comprehensively and illustrated by polarized light microscopy and scanning ele...

β-MODIFICATION OF ISOTACTIC POLYPROPYLENE: PREPARATION, STRUCTURE, PROCESSING, PROPERTIES, AND APPLICATION

The crystalline structure, spherulitic crystallization and melting behaviour of isotactic polypropylene (iPP) is comprehensively reviewed, illustrated and discussed. It is demonstrated and summarized how the nucleation, development and growth of the different spherulites depend on the crystallization and melting conditions both in quiescent and sheared melt. Based on the results the microtexture development during real processing conditions is elucidated for selected examples. Several subjects of intense debate on the above topics are clarified using evidence from polarized optical micrography and differential scanning calorimetry.

Supermolecular structure of isotactic polypropylene

Wide-angle X-ray diffraction (WAXD) was used to investigate the effects of shear on the crystallization behavior of polypropylene (PP) with β-nucleating agent. The melt was subjected to shear at the shear rate from 0.5 to 60 s-1 for 5 s with a CSS450 shear stage. For the PP with low content of the additive, the formation mechanism of the β crystals is almost the same as that of pure isotactic polypropylene (iPP), viz., shear induces. Otherwise, for the samples with high content of the additive, the formation mechanism of the β form are nucleating agent induces. The results clearly show that shear restrains the formation of high β phase for the melt with additive.

Influence of Shear on Crystallization Behavior of the β Phase in Isotactic Polypropylene with β-Nucleating Agent

Calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca-Sub proved to be the most effective β-nucleating agent of iPP. The Ca-Sub nucleating agent widens the upper crystallization temperature range of pure β-iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β-iPP was markedly higher than that of α-iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α-iPP. The morphology of the β-iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β-spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999

Highly active thermally stable β-nucleating agents for isotactic polypropylene

The metastable phase of isotactic polypropylene (βiPP) is crystallized epitaxially on two specific nucleating agents:  γ-quinacridone and dicyclohexylterephthalamide (DCHT). The resulting thin film...

Epitaxial Crystallization and AFM Investigation of a Frustrated Polymer Structure: Isotactic Poly(propylene), β Phase

The conditions for the formation of the β-form of isotactic polypropylene in the presence of a β-nucleator and the β-α transition during heating were studied by wide angle X-ray diffraction. A conditional crystallinity of the β-form, X′β, and a structural disorder parameter, S, deduced from WAXD data were defined for the characterization of the β-form in addition to the K value. The results show that X′β and S obviously depend on the content of β-nucleator, and that the β-α transition during heating essentially consists in the melting of the β-form and the successive recrystallization to the α-form.

Studies on the β‐form of isotactic polypropylene, 1. Characterization of the β‐form and study of the β‐α transition during heating by wide angle X‐ray diffraction

Effects of crystallization conditions, heating rate, and isothermal annealing on the melting behavior of isotactic polypropylene samples with predominantly β-form were studied by differential scanning calorimetry. The results show that the β-form of these samples has a higher thermal stability and a lower recrystallization ability to the α-form after melting than the β-form of other samples.

Zhi‐qun He

Papers

Studies on the β‐form of isotactic polypropylene, 1. Characterization of the β‐form and study of the β‐α transition during heating by wide angle X‐ray diffraction

Studies on the β‐form of isotactic polypropylene, 2. The melting behavior of predominantly β‐form samples