Structural and Kinetic Factors Governing the Formation of the γ Polymorph of Isotactic Polypropylene

Abstract

The molecular, thermodynamic, and kinetic factors that govern the formation and concentration of the α and γ polymorphs in metallocene-catalyzed isotactic poly(propylenes) have been studied with a set of polymers that have a wide range in molecular weight and defect contents. With these polymers it was possible to investigate the influence of molecular weight on γ formation at a fixed defect concentration, as well as the role of the defect concentration at constant molecular weight. The major experimental techniques used were wide-angle X-ray scattering and differential scanning calorimetry complemented by microscopy. From these studies the role of chain microstructure, the crystallization temperature, and the thermodynamic and kinetic requirement for the formation of the γ form could be established in more quantitative detail than heretofore. A particular important finding was the fact that at fixed defect concentration the fractional content of the γ polymorph goes through a maximum with crystallization temperature. The results that were obtained establish a quantitative framework within which the underlying bases that lead to formation of the γ form, and its unique crystalline structure, are discussed.

Crystallization of α and γ Phases in Isotactic Polypropylene with Low Ethylene Content: Isothermal Crystallization and Secondary Crystallization

“…At the isothermal crystallization temperature of T c =117ºC, both phases formed from the early stage of the crystallization. This result is consistent with the result in previous work (27).…”

“…The lamellar thickness of the phase which generates during the isothermal process is considered to be thinner than that of the phase. The melting behavior of the IPP material containing the and crystals was investigated by XRD also in the previous work (27). Although the material used in the previous work was isothermally crystallized, the sample was first cooled to room temperature and then heated again.…”

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

“…Only the α-parents morphology, possessing chain axes in the direction of flow, has an increased growth rate during a certain time scale, as stated in Eq. (10). The growth of all the other morphology is assumed to be proportional to the quiescent growth rate of each phase.…”

“…The β modification is usually formed using specific nucleating agents [2][3][4] or from oriented α-crystals created by moderately strong flow [5]. Elevated pressure and high crystallization temperatures induces the γ-phase [6,7], which is found also in iPPs with stereo-and/or regio-errors [8][9][10] or small amount of α-olefines comonomer [11]. Elevated cooling rates (>100°C/s) inhibit the stable monoclinic modification in favour of a mesomorphic form [1,12].…”

Modeling Flow-Induced Crystallization

“…Most dominantly present is typically the α-phase [6], formed at atmospheric pressure and low to moderate cooling rate. Crystallization under the influence of moderately strong flow conditions or specific nucleating agents can induce the β-phase [7], whereas crystallization of the γ-phase can be achieved in several ways, such as inclusion of small amounts of 1-olefin co-units [8], introduction of stereo-and regio-irregularities controlled by a metallocene catalyst [9,10], use of materials of very low molecular weight [11], or crystallization under elevated pressure and high temperature [12,13]. The latter is especially relevant in the context of the present work.…”