Postdoctoral Scholar , California Institute of Technology
Study the Mechanism of Flow-Induced Shish Structure
Polymers are made up of many many molecules all strung together to form really long chains. The polymer material is everywhere in our daily life like plastic wrap/container, tire, automotive parts, health care supplies, textiles, etc.. This material is increasingly replacing traditional materials due to their attractive performance-to-cost ratio, recyclability, ease of processing, and a diversity of material properties. During processing, these polymer melts are subjected to strong flow field, which accelerates the crystallization from days to seconds and influences the morphology dramatically, and the properties of products of these polymers strongly depend on the final morphology.
A remarkable feature that makes shear-induced crystallization ubiquitous: Flow-induced supramolecular structure, which gives “shish-kebab” morphology. It greatly improves the material property. A typical example is the world’s strongest fiber made by Polyethylene such as Spectrum of Honeywell and Dyneema of DSM. A bright white fiber with high resistance to chemicals, water, and ultraviolet light Stronger than steel and 40 percent stronger than aramid fiber Capable of withstanding high-load strain-rate velocities.
The understanding of mechanism could control material properties tailored to the application, However, the mechanism of shish-kebab formation remains unclear and under debate for about 50 years. Previous data allowed us to build a framework to explain the morphology evolution as time and rate varied. Our present goal is to extend our understanding to the molecular level and explain the role of ultra-long polymer chains, which dramatically enhances the shish formation.
The difficulty lies in the literature is it is hard to know what happened during shear. Most of the data were obtained after shearing is stopped; however, at that moment shish structure has already been induced, researchers need to guess what happened during the shear flow. Our instrument (birefringence measurement) gives us a chance to obtain data not only during the shear but also with the time resolution to the millisecond. Such advantage immediately tells us what happened during the shear by comparison the concentration and pressure dependence of the ultra-long chain molecules under shear flow. Together with rheological measurement, we can give understand the role of the role of ultra-long polymer chains in enhancing the shish-formation
Abstract: In this paper, the effect of shear on the flow-induced crystallization (FIC) of several polypropylenes of various macrostructures was studied using rheometry combined with polarized microscopy. Generally, an increase in strain and strain rate or decrease of temperature is found to decrease the thermodynamic barrier for crystal formation and thus enhancing crystallization kinetics at temperatures between the melting and crystallization points. Secondly, popular models based on suspension theory which are used to relate the degree of crystallinity to normalized rheological functions (such as viscosity) are validated experimentally. For this purpose, the space filling of crystals in the polarized micrographs determined from image processing was plotted as a function of normalized viscosity under various shear rates. It is found that the constant(s) of various suspension models should be dependent on the flow parameters in order for the suspension models to describe the effect of shear on FIC, particularly at higher shear rates.
Pub.: 31 May '14, Pinned: 04 Jul '17
Abstract: The quiescent and shear-induced isothermal crystallization behavior of linear and long-chain branched (LCB) polylactides (PLAs) was investigated at a temperature of 130 °C. LCB-PLAs were produced by the reaction with a multifunctional chain extender, Joncryl©. In quiescent crystallization, the presence of the LCB structure accelerated the nucleation process and reduced the induction time, depending on the level of branching. The impact of shear strain and shear rate on crystallization was also examined. The shear-induced crystallization of the linear and LCB-PLAs was affected by both the total shear strain and shear rate. The crystallization kinetics of the LCB-PLAs was more affected by shear than that of the linear PLA. The crystalline morphology of the linear and LCB-PLAs under quiescent and step shear rate conditions was examined using a Linkam optical shearing system. An increase in the spherulite density was observed in the strained melt of both linear (33 %) and LCB-PLAs (15 %), in comparison with those of unstrained counterparts. Optical micrographs confirmed that the crystal nucleation was affected by the shear flow. Long-chain branching significantly promoted the nucleation density (6.7 times), although it diminished the crystal growth rate from 4.4 to 2.0 μm/min.
Pub.: 05 Sep '15, Pinned: 04 Jul '17
Abstract: Effect of weak and strong shear flow on the crystallization of poly(lactic acid), PLA, with different molecular weights, (Mw), was investigated by rheological measurements. To study the effect of the weak flow, PLA samples were heated up to 190 °C to erase thermal history, then rapidly cooled down to 130 °C and finally subjected to a shear flow with shear rates ranging between 0.01 and 0.1 s−1. Effect of shear rate and strain on the shear-induced crystallization of PLA was examined. A remarkable effect of Mw on the shear-induced crystallization of PLA with increasing MW was observed. It was also found that higher levels of strain were required to initiate crystallization with increasing deformation rates. The effect of the strong shear flow on the crystallization kinetics of PLA was also examined. For this, different molecular weight samples were pre-sheared and then crystallized at the same temperature for a specific period of time. Evolution of crystallization was monitored by the variation of storage modulus, G′, versus time, t. Thermal behavior and the microstructure of the sheared samples were studied by Differential Scanning Calorimetry, DSC, and Wide-Angle X-ray Diffraction, WAXD.
Pub.: 06 Feb '17, Pinned: 04 Jul '17
Abstract: It is widely accepted that the role of the high molecular weight (HMW) component is cooperative in shear-induced crystallization, owing to entanglements among long chains. However, this paper demonstrates that the HMW component has a novel effect on structural evolution during the process of multi-melt multi-injection molding (M(3)IM), organized as follows. First, the appropriate HDPE system with an incremental concentration of HMW tails was established. Second, the crystalline morphologies and orientation behaviors of the M(3)IM samples were characterized using a combination of scanning electron microscopy (SEM) and two-dimensional small angle X-ray scattering (2D-SAXS), and these suggested that the amount of shish-kebabs was not monotonically promoted with an increasing content of HMW tails but tended to reduce at a certain value. Third, in order to explain this phenomenon, the special temperature and shear profiles of M(3)IM were depicted subsequently, and finally the mechanism of hierarchical structure formation with the influence of various amounts of HMW tail chains was discussed, based on the classical rheological viewpoint.
Pub.: 29 Jun '17, Pinned: 04 Jul '17
Abstract: Shear and pressure fields unavoidably coexist in practical polymer processing operations, but their combined influence on the crystalline structure of poly(l-lactic acid) (PLLA) has never been studied due to the limit of experiment device. In the current work, we utilized a homemade pressuring and shearing device to study the crystalline morphology and structure of PLLA under the coexistence of shear and pressure. Interestingly, we obtained almost exclusive β-form directly from PLLA melt crystallization at our experimental condition (shear 13.6 s–1, pressure 100 MPa, and crystallization temperature 160 °C). Undoubtedly, abundant β-form is helpful to tackle the major shortcoming of PLLA performance, i.e., poor toughness. This meaningful result is different from the common viewpoints that PLLA β-form can usually be obtained by hot-drawing or solid coextrusion under a high tensile ratio, suggesting that PLLA β-form can be obtained through shear-induced crystallization. In addition, the fraction of β-PLLA strongly depends on supercooling and shear intensity. A higher supercooling (pressure 150 MPa and crystallization temperature 160 °C) could also induce predominant β-form even under a very low shear rate of 1.0 s–1. While, under a lower supercooling (pressure 50 MPa and crystallization temperature 160 °C), we did not observe any trace of β-form. In the heating experiment to investigate crystal form transformation, we also found that partial β-form transformed into α-form through melting–crystallization, and meanwhile some β-form crystals melted directly without transformation. These results could beyond doubt help to comprehend the relationship between crystallization condition and inner crystal structure and thus afford guidance in practical processing to toughen final PLLA products via controlling crystalline structure.
Pub.: 05 May '16, Pinned: 04 Jul '17
Abstract: Understanding the complex crystallization behavior of isotactic polypropylene (iPP) in conditions comparable to those found in polymer processing, where the polymer melt experiences a combination of high shear rates and elevated pressures, is key for modeling and therefore predicting the final structure and properties of iPP products. Coupling a unique experimental setup, capable to apply wall shear rates similar to those experienced during processing and carefully control the pressure before and after flow is imposed, with in situ X-ray scattering and diffraction techniques (SAXS and WAXD) at fast acquisition rates (up to 30 Hz), a well-defined series of short-term flow experiments are carried out using 16 different combinations of wall shear rates (ranging from 110 to 440 s–1) and pressures (100–400 bar). A complete overview on the kinetics of structure development during and after flow is presented. Information about shish formation and growth of α-phase parents lamellae from the shish backbones is extracted from SAXS; the overall apparent crystallinity evolution, amounts of different phases (α, β, and γ), and morphologies developing in the shear layer (parent and daughter lamellae both in α and γ phase) are fully quantified from the analysis of WAXD data. Both flow rate and pressure were found to have a significant influence on the nucleation and the growth process of oriented and isotropic structures. Flow affects shish formation and the growth of α-parents; pressure acts on relaxation times, enhancing the effect of flow, and (mainly) on the growth rate of γ-phase. The remarkably high amount of γ-lamellae found in the oriented layer strongly indicates the nucleation of γ directly from the shish backbone. All the observations were conceptually in agreement with the flow-induced crystallization model framework developed in our group and represent a unique and valuable data set that will be used to further validate and implement our numerical modeling, filling the gap for quantitatively modeling crystallization during complicated processing operations like injection molding.
Pub.: 12 May '17, Pinned: 04 Jul '17
Abstract: Structural and morphological transitions of flow-induced crystallization (FIC) in poly(1-butene) (PB-1) melt have been studied by combining extensional rheology and in situ synchrotron radiation ultrafast wide- and small-angle X-ray scattering (WAXD/SAXS) measurements. Unexpectedly, metastable Form III is crystallized directly from the PB-1 melt by high-speed extension, which has a short lifetime of several tens of milliseconds and manifests the thermodynamic and kinetic competition among Form III, Form II and melt under flow. Relative crystallinity evolution of Form II after extension reveals a crystal melting dominated process within the observation time of 120 s even under high supercooling. This is opposite to the common case of FIC but supports the idea that flow alters the obtained crystal size and its thermodynamic stability. Additionally, a morphological transition from a flow-induced network to shish is observed by SAXS with increasing extension temperature from below to above the melting point of Form II. With above observations, we construct nonequilibrium structural and morphological diagrams of FIC in strain rate-temperature space, which may guide the industrial processing of the PB-1 material.
Pub.: 28 Apr '17, Pinned: 04 Jul '17
Abstract: The oriented crystallization of long chain hyper-branched polypropylene (LCB-PP) induced by shear deformation in the pre-crystallization regime was investigated through simultaneous time-resolved synchrotron wide-angle and small-angle X-ray scattering (WAXS and SAXS) measurements and compared with that of linear PP (L-PP). Highly oriented crystallization was induced in LCB-PP by the application of a step-shear in the pre-crystalline regime; in contrast, the crystalline lamellae of L-PP were slightly oriented. In addition, LCB-PP formed daughter lamellae whose a-axis was oriented along the shear direction. The oriented crystallization of LCB-PP induced by the step-shear was related to its primary structure in the following manner. Since the existence of branched chains increased the relaxation time of the LCB-PP chains, the elongated LCB-PP chains were remained for longer after the step-shear deformation. In addition, the branching points of LCB-PP aided the formation of point-like precursors for crystallization. Consequently, these point-like precursors were arrayed along the shear direction and transformed into thread-like precursors. This caused the rapid formation of highly oriented shish-kebab structures. Furthermore, since short chains that caused the formation of daughter lamellae were covalently tethered to the kebab parts in LCB-PP, the growth of the daughter lamellae were spatially restricted owing to which the a-axis of the daughter lamellae in LCB-PP was oriented along the shear direction.
Pub.: 06 Feb '17, Pinned: 04 Jul '17
Abstract: The sequence and coupling of intra- and interchain orderings in flow-induced crystallization (FIC) of partially cross-linked isotactic polypropylene (iPP) is studied with in situ Fourier transform infrared spectroscopy (FTIR) and synchrotron radiation X-ray scattering techniques, which reveal that multiscale structural intermediates emerge prior to the onset of crystallization. Upon imposing flow, intrachain conformational ordering or coil–helix transition (CHT) occurs first, which is directly correlated with external stress. As helical content is built up at large strain, density fluctuation happens, and sufficient long helices may result in orientation ordering before FIC. The results demonstrate that stress induced intrachain CHT is the essential structural intermediate in FIC, which can be further coupled with interchain orientation and density providing either helical content or length meets the criterions for the phase transitions. We propose that coupling among external stress, intrachain conformational, and interchain orientation and density orderings to be the molecular mechanism for FIC of polymer forming helical structures.
Pub.: 20 Feb '17, Pinned: 04 Jul '17
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.
Pub.: 25 Feb '16, Pinned: 04 Jul '17