Relaxation behavior of shear-induced crystallization precursors in isotactic polypropylene containing sorbitol-based nucleating agents with different nucleating abilities

Abstract

The nature of shear-induced crystallization precursors, especially their relaxation behaviour, is an important issue in polymer chemical physics. In our work, relaxation behavior of shear-induced crystallization precursors in isotactic polypropylene containing various sorbitol-based nucleating agents (NAs) with different nucleating abilities was investigated by using both rheological and in situ small angle X-ray scattering (SAXS) methods. Rheological crystallization kinetics results showed that the amount of shear-induced precursors, calculated separately from the total nuclei, decayed exponentially with relaxation time in both pure and nucleated iPP. By fitting the decay of shear-induced precursors with relaxation time, the relaxation rate of precursors in nucleated iPP was found to be slower than that in pure iPP. Interestingly, it further decreased with the increase in the nucleating ability of sorbitol-based NAs. Meanwhile, the life-time of precursors was prolonged in nucleated iPP with increasing nucleating ability. Similar results were also testified by in situ SAXS measurements. By investigating the life-times at different temperatures, the activation energy for the relaxation of precursors was calculated and found to increase with stronger nucleating abilities. Our results demonstrated that sorbitol-based NAs could stabilize the iPP precursors and the effect of stabilization enhanced with the increase in nucleating ability. We believe that our work can not only help better reveal the relaxation behavior of shear-induced precursors but also provides a new perspective for understanding the role of NAs in real processing.

Thermal Stability of Shear-Induced Precursors and Their Effect on the Crystalline Structure of β-Nucleated Isotactic Polypropylene

“…For both pure iPP and nucleated iPP, the sheared samples present the fastest crystallization rates, while the quiescent samples present the slowest crystallization rates. The relaxation treatment at 170°C obviously decelerates the crystallization rates of either iPP or β-nucleated iPP compared with samples that have not been relaxed, and longer relaxation time makes the crystallization rates of relaxed samples closer to the quiescent samples, which is in 2 International Journal of Polymer Science agreement with previous studies that also found that relaxation caused attenuation of shear-enhanced crystallization because of the dissolution of shear-induced nucleation precursors [17,18]. The relaxation temperature was set close to the processing temperature in realistic production which was lower than the equilibrium melting point of iPP to mimic thermal treatment in real processing.…”

“…This result is close to the value of the iPP precursor (ca. 200 kJ/mol) proposed by Janeschitz-Kriegl [24], 210 kJ/mol calculated by Cavallo et al [17], and 160 kJ/mol calculated by Fan et al [18] and is much larger than the energy of activation for viscous flow (∼44 kJ/mol for iPP) [25] suggesting that the detachment of polymer chain segments from the oriented nuclei is the limiting step in the process of shear-induced precursor relaxation not diffusion of the detached stem into the melt.…”

“…With respect to the structure of shear-induced precursors, many earlier studies have achieved a general description by means of rheometry and in situ wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), small-angle light scattering (SALS), optical microscopy, and electron microscopy [10]. For the shear-induced precursors with a shish-kebab structure, many previous experiments [16][17][18] demonstrated that it can survive for long under quiescent condition, even when temperature was higher than the melting point of a semicrystalline polymer. Thereby, we consider that figuring out the relationship of thermal stability of shear-induced precursors and its effect on the crystalline structure of β-nucleated iPP may be a primary step to regulate the content of β-crystals.…”

Structure evolution upon heating and cooling and its effects on nucleation performance: A review on aromatic amide β‐nucleating agents for isotactic polypropylene

“…For a representative semi‐crystalline polymer, isotactic polypropylene (iPP), the use of NAs is of special significance because of its relatively slow crystallization rate. By introducing various NAs, not only the crystallization kinetic could be accelerated, which shortens the processing cycle time, but also the physical properties of iPP including stiffness, toughness, and transparency have been remarkably improved. Among diverse NAs for iPP, β‐NA is one of the most extensively studied and widely used NAs .…”

Unique crystallization behavior of isotactic polypropylene in the presence of l‐isoleucine and its inhibition and promotion mechanism of nucleation

“…Generally, nucleation is the slow and limiting step and thus requires a higher degree of supercooling . Of the considerable efforts made to accelerate the nucleation of iPP and promote the crystallization of iPP in previous studies, dispersing nucleating agents (NAs) into the polymer matrix during melt processing is one of the most efficient and precedented . NA reduces the free energy barrier of nucleation by providing a suitable surface for the polymer to adhere to and grow into desired morphologies, such as that of spherulites.…”