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.
Correlation between the fracture toughness and β‐crystal fraction in a β‐nucleated propylene‐based propylene–ethylene random copolymer
Propylene-based propylene-ethylene random copolymer (PPR) has been widely used in the production of hot-water pipes. To further improve its toughness and thermal resistance, -nucleating agents (-NAs) are frequently incorporated. In this study, PPR containing 5.6 mol % ethylene units was modified by two kinds of -NAs, that is, calcium pimelate and N,N-dicyclohexylterephthalamide. The notched Izod impact strength of PPR increased with the addition of the -NAs. Drastically different toughening effects were found between the two -NAs. The structure of PPR with and without a -NA was investigated by calorimetry, X-ray diffraction, and thermomechanical analysis. The results indicated that the relative fraction of crystals (k) in the injection-molded specimens was determined by the type and content of -NA. The relationship between k and the impact toughness was summarized. A critical value for k (0.68) was identified for the brittle-ductile transition of PPR. PPR with -NA having a k greater than 0.68 displayed a higher impact strength than the other mixtures. (c) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42930.
Variation of non‐isothermal crystallization behavior of isotactic polypropylene with varying β‐nucleating agent content
The non-isothermal crystallization behavior, the crystallization kinetics, the crystallization activation energy and the morphology of isotactic polypropylene (iPP) with varying content of β-nucleating agent were investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The DSC results showed that the Avrami equation modified by Jeziorny and a method developed by Mo and co-workers could be successfully used to describe the non-isothermal crystallization process of the nucleated iPPs. The values of n showed that the non-isothermal crystallization of α- and β-nucleated iPPs corresponded to a tridimensional growth with homogeneous and heterogeneous nucleation, respectively. The values of crystallization rate constant showed that the rate of crystallization decreased for iPPs with the addition of β-nucleating agent. The crystallization activation energy increased with a small amount (less than 0.1 wt%) of β-nucleating agent and decreased with higher concentration (more than 0.1 wt%). The changes of crystallization rate, crystallization time and crystallization activation energy of iPPs with varying contents of β-nucleating agent were mainly determined by the ratio of the content of α- and β-phase in iPP (α-PP and β-PP) from the DSC investigation, and the large size and many intercrossing lamellae between boundaries of β-spherulites for iPPs with small amounts of β-nucleating agent and the small size and few intercrossing bands among the boundaries of β-spherulites for iPPs with large amounts of β-nucleating agent from the SEM examination. Copyright © 2010 Society of Chemical Industry