Synergies among the self-assembled β-nucleating agent and the sheared isotactic polypropylene matrix
Abstract The impacts of both shear temperature and shear rate on both the self-assembled β-nucleating agent (β-NA) in the hosting isotactic polypropylene (iPP) matrix and the subsequent crystallization of the hosting iPP matrix were investigated by polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) measurements, respectively. Under static condition, β-NA could self-assemble into snowflake-like aggregate. Once shear was applied, three kinds of self-assembled aggregates could be developed prior to the crystallization of matrix depending on the applied shear temperatures, that is, snowflake-like self-assembled aggregate at 200 °C, deformed snowflake-like one with smaller branches at 180 °C, tiny and dense needle-like one if shear temperature was further decreased to 160 °C. Since β-NA with various self-assembled aggregates can act as nucleating templates for the subsequent nucleation of iPP matrix, it is conceivable that the crystallization behavior of the iPP matrix can be altered. As expected, β-phase crystallinity, long period and lamellar thickness are steadily dependent on the shear temperature and shear rate. That is, when shear was applied at 200 °C, β-phase crystallinity, long period and lamellar thickness keeps almost constant regardless of the shear rate. If melt was sheared at 180 °C, β-phase crystallinity was decreased while long period and lamellar thickness remained unchanged with elevating shear rate. At lower shear temperature (i.e., 160 °C), β-phase crystallinity, long period and lamellar thickness were further decreased with increasing the shear rate. These results are of practical significance to understand the long-term ignored issue: apart from the loading of β-NA, different self-assembled aggregates formed in flow field are another crucial factor in determining the microstructure development of matrix.
Material properties Mechanical enhancement of melt-stretched b-nucleated isotactic polypropylene : The role of lamellar branching of b-crystal
Melt-stretched isotactic polypropylene (iPP) filled with different weight fractions (0, 0.1, 0.2 and 0.4 wt %) of b-nucleating agent (b-NA) was extruded via a single screw extruder with a slit die and immediately melt-stretched at the die exit. The microstructure and mechanical properties (i.e., monotonic tensile, step-cycle and stress relaxation behavior) of these samples were analyzed. Unexpectedly, the lamellar branching of b-crystal, which is similar to lamellar branching of a-crystal, is observed in melt-stretched b-nucleated iPP. More interestingly, it is found that, compared with other samples, iPP containing 0.2 wt % of b-NA has the best mechanical properties. Through thorough analysis of various structural characterizations, the improved tensile strength obtained from step-cycle tests and viscous force obtained from stress relaxation measurements for iPP with 0.2 wt% of b-NA is ascribed to lamellar branching of bcrystal. The study is an example of preparing high performance polymers via practical polymer pro-
Beta-modification of isotactic polypropylene
Commercial grades of isotactic polypropylene (iPP) crystallize essentially into α-modification (α-iPP) with sporadical occurrence of the β-phase (β-iPP). Crystallization in temperature gradient or in sheared melt encourages the development of the β-phase in commercial, non-nucleated iPP [1]. For preparation of samples rich in β-modification or of pure β-iPP, the introduction of selective β-nucleants is the most reliable method [1]. The known β-nucleating agents are collected in Tables 1 and 2, indicating their advantages and drawbacks. The most widespread high active β-nucleating agent is a γ-quinacridone red pigment. Some two-component compounds obtained by the reactions of certain organic acids with CaCO3, also possess a very high β-nucleating activity. Different calcium and zinc salts of aliphatic and aromatic dicarboxylic acids having high thermal stability, belong to the selective β-nucleants, as well [1]. The β-content of iPP samples — and so the efficiency of β-nucleants and the influence of thermal and mechanical conditions of the crystallization on the polymorphic composition - can be characterized by the k-value determined from wide angle X-ray scattering (WAXS), by differential scanning calorimetry (DSC) on non-recooled samples or by polarized light microscopy (PLM) using thin sections of the samples [1]. It should be pointed out that the melting curves of iPP samples cooled to room temperature do not give correct information about the β-content. This statement is due to the βα-recrystallization which takes place during heating of the sample and overlaps with the melting process of the β-phase [1, 2]. Unfortunately, the efficiency of β-nucleants was characterized by DSC on cooled samples in many studies. Therefore, the quantitative conclusions of these studies should be treated with caution. Studies on β-nucleated iPP revealed that the formation of pure β-iPP has an upper (T(βα) = 140°C) and a lower limit temperature (T(aβ) = 100-110°C) [1]. The crystal structure, morphology, melting and recrystallization behavior of β-iPP are discussed in detail in a later chapter’ spherulitic crystallization and structure’. This section deals with the preparation, properties and application of β-iPP.