Critical Strain for Shish-Kebab Formation

A combined melt-stretching and quenching setup for experimental studies of polymer crystallization under complex flow-temperature environments

A combined melt-stretching and quenching setup is designed and developed to allow experimental investigations of polymer crystallization under the complex flow-temperature environments comparable to those encountered in the actual industrial processing. The melt-stretching proceeds by two drums rotating in the opposite directions with simultaneous recording of a stress-strain curve, where the Hencky strain and strain rate (≤233 s^-1) are adjustable over a large range. After stretching, liquid N₂ is used as a cooling medium to quench the free-standing melt, which is sprayed directly to the deformed melt driven by an electric pump. To ensure a high cooling efficiency, a three-way solenoid valve is employed to execute a sequential control of the liquid N₂ flow direction to reduce the boil-off of liquid N₂ before entering the sample chamber. The melt cooling rate depends on the liquid N₂ flow rate controlled by a flow valve, which is up to 221 °C/s when quenching the isotactic polypropylene (iPP) melt with a thickness of 0.28 mm at 150 °C. Two independent temperature control modules are designed to meet the requirements of different stages of melt-stretching and quenching. To verify the capability of the setup, we have performed the melt-stretching and quenching experiments on iPP samples. The setup is demonstrated to be a valuable new tool to study polymer crystallization under coupled flow-cooling fields.

Investigation of the synergistic effect of melt-extension and nanofiller on the crystal-crystal phase transition from form II to I of isotactic polybutene-1

New questions and conjectures are raised on the crystal-crystal phase transition of isotactic polybutene-1 (iPB-1) containing nanofiller in the flow field. In this work, we investigate the phase transition from flow-induced oriented form II to I in iPB-1 blends with multi-walled carbon nanotubes (MWCNTs) with a homemade two-drum extensional rheometer combined with in situ wide-angle X-ray diffraction (WAXD) measurements. The MWCNTs show a limited promoting effect on the phase transition kinetics under quiescent conditions, while the phase transition kinetic is highly accelerated with the impose of melt-extension. When the loading extension strain is 0.5 or 2.0, the half time of phase transition (t_1/2) is shortened from tens of hours to a few hours, depending on the melt-extension strain and the MWCNTs content in iPB-1. When the extension strain increases to 3.5, t_1/2 decreases to about 30 min, which is independent of the MWCNTs content in all iPB-1 blends except in blends with MWCNTs content of 1%, where the phase transition rate in the middle and late stages is restrained. It's speculated that flow-induced molecular orientation or shish-kebab morphology affects the internal stress or stress transfer. The addition of a nanofiller enlarged the effect of melt-extension through strengthening the localized intensity of flow field. In general, the combination of nanofiller and melt-extension can obviously promote the phase transition kinetics.

Preparation and properties of oriented microcellular Poly(l-lactic acid) foaming material

Poly(l-lactic acid) (PLLA) displays simultaneous repair and regeneration properties. Therefore, it is vital for developing bone repair materials while improving their mechanical strength, and biocompatibility is essential for guaranteeing its application. In this manuscript, using solid hot drawing (SHD) technology to fabricate an oriented shish-kebab like structure, furthermore, the interface-oriented grain boundary controlled the nucleation site and cell morphology during low temperature supercritical carbon dioxide (SC-CO₂) foaming process, resulted in an oriented microcellular structure which was similar to load-bearing bone. The tensile strength, elastic modulus, and elongation at break of the oriented microcellular PLLA were 98.4 MPa, 3.3 GPa, and 16.4%, respectively. Furthermore, the biomimetic structure improved osteoblast cells (MC3T3) attachment, proliferation, and propagation. These findings may pave the way for designing novel biomaterials for bone fixation or tissue engineering devices.

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.

Lamellar crystal-dominated surfaces of polymer films achieved via melt stretching-induced free surface crystallization

Lamellar crystal-dominated (LCD) surfaces hold great superiority and broad prospects in polymer surface engineering. The key to this is avoiding the formation of an amorphous phase in the interlamellar region. Here we give a first report of achieving LCD surfaces of polyethylene films via melt stretching-induced free surface crystallization. We demonstrate that the resultant surface is constructed directly by orientated and edge-on lamellae within a surface depth of tens to hundreds of nanometers, while the normally existing amorphous phase is avoided. The crystallization-driven formation of the LCD surface has been ascribed to the heterogeneous chain dynamics of a melt free surface, that is, high chain mobility, low viscosity and loose chain entanglement, which facilitates the complete chain disentanglement during crystallization. In addition, we confirm that the surface morphology is controllable with respect to lamellar orientation, spacing and depth by changing the melt stretching strain or quenching the deformed melt. Meanwhile, owing to a possible kinetics competition between crystallization and chain disentanglement, the structural spacing of surface lamellae holds a positive correlation with the lamellar depth. Since free surface effects are immanent in polymer materials, the currently proposed melt processing strategy is demonstrated to be transferable to other semicrystalline polymers.

Flow-Induced Crystallization in Polyethylene: Effect of Flow Time on Development of Shish-Kebab

The flow-induced formation and relaxation of the representative oriented shish-kebab structure were studied with synchrotron small-angle X-ray scattering (SAXS) method. The flow duration was varied from 2 to 6 s at an identical strain rate to reveal the effect of flow time on stability and dimension of formed shish. It was found that the short flow time of 2 s was able to generate shish during flow, which, however, relaxed during the isothermal process after cessation of flow. An increase in flow time can improve the shish stability and the long flow time of 6 s can generate the stable shish that nucleate the growth of kebab lamellae. In addition, the quantitative analysis of SAXS results showed that with increasing flow time from 2 to 6 s, the shish length increased from 242 to 574 nm, while the shish diameter remained around 34 nm. This detailed information of the formed shish-kebab structure can be used to shed light on their evolution that occurred during flow from 2 to 6 s, where shish grew at a longitudinal speed of around 80 nm/s, and there was an improvement in the stability and nucleation capability for kebab lamellae.

Polyethylene oxide enhances the ductility and toughness of polylactic acid: the role of mesophase

A lack of understanding of the structure-property relationship of the polylactic acid (PLA)-based polymer composite system makes it a challenge to manufacture products with optimized mechanical performance by precisely regulating the microscopic structure and morphology. Herein, we chose the PLA/polyethylene oxide (PEO) blend as a model to investigate the structural reason for the enhanced ductility and toughness of this kind of material. We have demonstrated that a considerable amount of the PLA mesophases exist in the melt quenched films that display high ductility and toughness, in contrast to the PLA crystals in their counterparts of slowly cooled films that are dominated by brittle fracture. The mesophase formed by melt quenching is attributed to a moderate acceleration of PLA chain mobility due to the plasticizing effect of the flexible PEO. In situ experiments have revealed the further formation of oriented mesophases induced by tensile deformation, which presents a high consistency between the content increase and the tensile stress intensification. We illustrate that the mesophases directly develop into a microfibrillar morphology to transmit the external stress and prevent crack propagation under deformation. This work emphasizes the essential role of the PLA mesophase in acquiring the enhanced ductility and toughness of the PLA/PEO composite films, which may be generalized to other similar PLA-based polymer composite materials.

Poiseuille and extensional flow small-angle scattering for developing structure-rheology relationships in soft matter systems

As the rheoscattering community has grown, so has the breadth of experiments both in terms of materials systems, and also in terms of flow types and analysis approaches. In this review, we seek to highlight important recent developments in rheoscattering that go beyond simple shear measurements. In particular, this review will focus on Poiseuille flow and extensional flow small-angle scattering and results from recent experiments that lead to the development of structure-function relationships in a wide variety of soft materials.

A portable extruder for in situ wide angle x-ray scattering study on multi-dimensional flow field induced crystallization of polymer

We have designed and constructed a portable extruder with a rotatable mandrel, which can be employed to study the multi-dimensional flow field (MDFF) induced crystallization of polymer combined with in situ wide angle x-ray scattering (WAXS). With the piston driving the melt sample to flow along the channel, a direct axial shear field is achieved. At the same time, the central mandrel keeps rotating under a stable speed, providing the sample with an additional circumferential shear field. By presetting different proportions of the two shear fields, namely, axial and circumferential, various flow states of the sample can be obtained, which makes it capable of investigating the effects of MDFF on polymer crystallization. We have performed an in situ WAXS experiment of MDFF induced crystallization of isotactic polypropylene based on the portable extruder at the beam line BL16B in Shanghai Synchrotron Radiation Facility. The rheological and structural information is collected simultaneously, which manifests the viability of the portable extruder on regulating MDFF and can provide guidance for polymer processing.

Rheological Link Between Polymer Melts with a High Molecular Weight Tail and Enhanced Formation of Shish-Kebabs

Presence of an ultra high molecular weight (UHMw) fraction in flowing polymer melts is known to facilitate formation of oriented crystalline structures significantly. The UHMw fraction manifests itself as a minor tail in the molar mass distribution and is hardly detectable in the canonical characterization methods. In this study, alternatively, we demonstrate how the nonlinear extensional rheology reveals to be a very sensitive characterization tool for investigating the effect of the UHMw-tail on the structural ordering mechanism. Samples containing a UHMw-tail relative to samples without, exhibit a clear increase in extensional stress that is directly correlated with the crystalline orientation of the quenched samples. Extensional rheology, particularly, in combination with linear creep measurements, thus, enables the conformational evolution of the UHMw-tail to be studied and linked to the enhanced formation of oriented structures.

Influence of HMW tail chains on the structural evolution of HDPE induced by second melt penetration

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 (M³IM), 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 M³IM 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 M³IM 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.

Self-nanofibrillation strategy to an unusual combination of strength and toughness for poly(lactic acid)

The self-nanofibrillation strategy paves a new way to an unprecedented combination of strength and toughness for pure PLA.

A Phase Field Technique for Modeling and Predicting Flow Induced Crystallization Morphology of Semi-Crystalline Polymers

Flow induced crystallization of semi-crystalline polymers is an important issue in polymer science and engineering because the changes in morphology strongly affect the properties of polymer materials. In this study, a phase field technique considering polymer characteristics was established for modeling and predicting the resulting morphologies. The considered crystallization process can be divided into two stages, which are nucleation upon the flow induced structures and subsequent crystal growth after the cessation of flow. Accordingly, the proposed technique consists of two parts which are a flow induced nucleation model based on the calculated information of molecular orientation and stretch, and a phase field crystal growth model upon the oriented nuclei. Two-dimensional simulations are carried out to predict the crystallization morphology of isotactic polystyrene under an injection molding process. The results of these simulations demonstrate that flow affects crystallization morphology mainly by producing oriented nuclei. Specifically, the typical skin-core structures along the thickness direction can be successfully predicted. More importantly, the results reveal that flow plays a dominant part in generating oriented crystal morphologies compared to other parameters, such as anisotropy strength, crystallization temperature, and physical noise.

Shear-induced enhancements of crystallization kinetics and morphological transformation for long chain branched polylactides with different branching degrees

The effects of long chain branching (LCB) degree on the shear-induced isothermal crystallization kinetics of a series of LCB polylactides (LCB PLAs) have been investigated by using rotational rheometer, polarized optical microscopy (POM) and scanning electron microscopy (SEM). Dynamic viscoelastic properties obtained by small-amplitude oscillatory shear (SAOS) tests indicate that LCB PLAs show more broadened relaxation time spectra with increasing LCB degree. Upon a pre-shear at the shear rate of 1 s⁻¹ LCB PLAs show much faster crystallization kinetics than linear PLA and the crystallization kinetics is enhanced with increasing LCB degree. By modeling the system as a suspension the quantitative evaluation of nucleation density can be derived from rheological experiments. The nucleation density is greatly enhanced with increasing LCB degree and a saturation in shear time is observed. Crystalline morphologies for LCB PLAs observed by POM and SEM demonstrate the enhancement of nucleation density with increasing LCB degree and a transformation from spherulitic to orientated crystalline morphologies. The observation can be ascribed to longer relaxation time of the longest macromolecular chains and broadened, complex relaxation behaviors due to the introduction of LCB into PLA, which is essential in stabilizing the orientated crystal nuclei after pre-shear.