Thermal analysis and X‐ray scattering study of metallocene isotactic polypropylene prepared by partial melting

Abstract

In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m-iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic ␣ and ␥ phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self-nucleation temperatures (T n 's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160°C. The relative amounts of ␣ and ␥ crystals are determined from the area under the unique wide-angle X-ray reflections. The lower and upper endotherms are caused by the melting of ␥ and ␣ crystals, respectively. Four distinct regions of T n were identified on the basis of the thermal and structural parameters of m-iPP. In region I, T n is below the peak melting temperature of the ␥ phase. Here, ␥ crystals are annealed and ␣ crystals are barely affected by T n . In region II, T n is above the peak of the lower endotherm but below the peak of the upper endotherm. ␥ crystals melt, and ␣ crystals anneal. In both regions I and II, the portion of the sample melted at T n recrystallizes epitaxially with existing parent ␣ lamellae as the substrates, and the amount of ␣ always exceeds the amount of ␥. In region III, T n is above the peak of the upper endotherm, and all ␥ crystals and some or all ␣ crystals are melted at T n . The number of ␣-crystal nuclei steadily decreases as T n increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, T n exceeds the upper endotherm, and only small self-nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of ␣ and ␥ is observed during cooling from T n . Because of the effective nucleating ability of ␣ toward ␥, as the temperature drops, the amount of ␥ increases and then exceeds the amount of ␣. With subsequent reheating, the reverse crossover occurs because of the lower melting point of ␥.

Self-Nucleation of Crystalline Phases Within Homopolymers, Polymer Blends, Copolymers, and Nanocomposites

“…Dai et al [132] studied the effect of SN treatment on the thermal properties and structure of PP. The PP employed was synthesized using a metallocene catalyst.…”

Overall crystallization kinetics of polymorphic propylene–ethylene random copolymers: A two-stage parallel model of Avrami kinetics

“…Both polymorphs are observed at the earliest detection of crystallinity. This feature is associated with the likelihood of the a phase to act as effective seeds to nucleate and grow g branches as documented in other studies [32,33]. The implication in the isothermal evolution of both types of crystals is that once an a surface is generated, a subsequent epitaxial growth of g crystals from this surface follows.…”

“…These a crystals are oriented along the radius of the spherulite [11,12]. In addition, self-seeding crystallization studies also indicate that the (010) plane of the monoclinic phase serves as a very efficient seed to nucleate the g phase [33].…”

Crystallization of Polypropylene: Application of Differential Scanning Calorimetry Part I. Isothermal and Non-Isothermal Crystallization

“…Then, the relative crystallinity as a function of temperature ) (T X can be formulated as: (11) where o T denotes the initial crystallization temperature, c T and ∞ T the crystallization temperature at time, t and after the completion of the crystallization process, respectively.…”

“…Even if the occurrence of the set of transverse crystallites for the metallocenecatalyzed PP is known, it still remains a difficult matter to analyze the crystallization and melting processes for a given sample [8]. In this sense, crystallization and melting characteristics of the metallocene-catalyzed isotactic PP (iPP) have recently attracted interest from both theoretical and practical point of view [7,[9][10][11][12][13].…”