Showing papers by "Liangbin Li published in 2022"

Polymer crystallization under external flow

Abstract: 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.

Strain-Rate-Dependent Phase Transition Mechanism in Polybutene-1 during Uniaxial Stretching: From Quasi-Static to Dynamic Loading Conditions

Abstract: With the assistance of an ultrafast time-resolved synchrotron radiation wide-angle X-ray diffraction technique and a homemade hyphenated high-speed tensile apparatus, structural evolutions in the crystalline domain of polybutene-1 (PB-1) are elucidated even within several milliseconds. The stretching-induced phase transition mechanism of PB-1 consisting of form II crystals is investigated in three strain-rate regions (A, B, and C) spanning six orders of magnitude (from 0.005 to 100 s–1). During the quasi-static loading process in region A (0.005 s–1 < ε̇ < 0.5 s–1), metastable form II crystals progressively transform into the stable form I ones. This classical transition is ascribed to the stress-induced change of the long-range chain position and conformation. Under dynamic loading conditions, in region B (1 s–1 < ε̇ < 10 s–1), besides the form II to I transition, several form II crystals are directly melted with increasing stress. In region C (ε̇ > 50 s–1), not only the form II crystals but also a large amount of the form I crystals are melted when the imposed stress reaches the threshold value. This abnormal stretching-induced phase transition of PB-1 is related to both the high strain rate and accompanied heating effect. When the lattice is subjected to an ultrahigh rate of energy input, the appearance and growth of conformational defects, which are related to the change of short-range contour shape of chains, can be involved. These defects lower the energy barrier of phase transition between the crystalline and amorphous structures significantly. For the crystals containing a large number of defects, they tend to be melted with increasing stress rather than undergoing a phase transition in the crystalline domain as those in the quasi-static loading region.

Advanced Amidoximated Polyethylene Nanofibrous Membranes for Practical Uranium Extraction from Seawater

Abstract: As the key material of the adsorption method used for practical uranium extraction from seawater, the adsorbent should have a high uranium adsorption capacity and long service life. Polyethylene nanofibrous membranes (PENFMs) using a one-step grafting strategy to prepare scalable membrane adsorbents are reported with supreme uranium uptake capacity and long service life. A series of amidoximated polyethylene nanofibrous membrane (called amidoximated membrane or AM) adsorbent materials with different degrees of grafting (DGs) are modified by one-step radiation-induced graft polymerization (RIGP) of acrylonitrile (AN) and acrylic acid (AA) and an amidoximation reaction. The AM adsorbents prepared from the PENFM show outstanding hydrophilicity, good adsorption capacity, and excellent affinity for uranium. The highest uranium uptake capacity of AM adsorbents is 115.32 mg/g in simulated seawater for 24 h and 6.03 mg/g in natural seawater for 10 weeks. The AM adsorbents are reusable for more than 10 cycles of continuous adsorption–desorption–regeneration tests, showing the long service life. In addition, the AM adsorbent adsorption results in natural seawater indicate that only prolonging time to pursue high adsorption capacity is not conducive to the acquisition of benefits. The AM adsorbents have broad potential application in the industrialization of uranium extraction from seawater by choosing the best adsorption efficiency to achieve high adsorption amount.

Entanglement on Nucleation Barrier of Polymer Crystal

Abstract: We propose a theoretical approach to quantitatively account for the role of entanglement in the nucleation of polymer melts, which is the unique feature of polymer differentiated from small molecules. By performing molecular dynamics simulations, we obtain the nucleation barriers of polymer systems with different entanglement densities, which exhibits an opposite trend compared to the prediction of the classic nucleation theory (CNT). To amend the deficiency of the CNT in polymer crystallization, we introduce the entanglement free energy to reflect the role of entanglement in polymer nucleation. Specifically, the polymer nucleation not only involves free energies of monomers inside and on the surface of a nucleus as considered in the CNT, but also affects the entanglement network around the nucleus. Our theoretical approach provides a reasonable interpretation for the unsolved nucleation phenomena of polymers in simulations and experiments.

Zero–Zero Birefringence Cellulose Acetate-Based Optical Films by Benzoylation

Abstract: Zero–zero birefringence film is one of the key components in optical displays, and its design and preparation remain a challenge nowadays. Here, we report the synthesis of benzoylated cellulose acetate (BCA), the preparation of BCA films by solution casting, and the birefringent properties of unstretched and stretched BCA films. It is found that zero–zero birefringence films can be fabricated using both unstretched and stretched BCA films. As the degree of substitution of benzoyl (DSBz) increases, the out-of-plane birefringence (Δnth) gradually decreases and changes from positive to negative. Particularly, for DSBz = 0.511, both the in-plane birefringence (Δnin) and out-of-plane birefringence (Δnth) of stretched BCA films are close to zero. The orientations of the acetyl and benzoyl groups under stretching are measured to interpret the underlying compensation mechanisms. Finally, we show that the wavelength dispersion of BCA films with DSBz = 0.511 varies significantly with the increase in draw ratio, and formulize the normalized birefringence with the relative orientation degree of the benzoyl and acetyl groups. Our results suggest that the birefringence and wavelength dispersion of cellulose acetate-based optical films can be well regulated by a combination of chemical modification and stretching, and have potential significance to the industrial production of zero–zero birefringence films.

Recyclable Nacre‐Like Aramid‐Mica Nanopapers with Enhanced Mechanical and Electrical Insulating Properties

Abstract: The emerging aramid‐mica nanopapers, composed of aramid nanofibers (ANFs) and mica nanosheets (Mica), exhibit superiority in the field of electrical insulation compared with commercial aramid‐mica micropapers. Unfortunately, their mechanical and electrical insulating properties are still less than ideal due to insufficient control of their microstructures. Herein, it is presented that by integrating ANFs and Mica into nacre‐like aramid‐mica nanopapers with improved structural orderliness, densification, and interlayer interaction, simultaneously improved mechanical and electrical insulating properties are achieved. Their maximum tensile strength and breakdown strength reach ≈292 MPa and ≈176 kV mm−1, respectively, which are superior to those of the state‐of‐the‐art ANFs‐based nanopapers. Particularly, the aramid‐mica nanopapers show high resistance to high‐temperature (250–300 °C) and oil‐bath (100 °C) environments commonly involved in practical applications, and can be recycled many times, demonstrating their great potential as high‐performance sustainable electrical insulating papers to be applied in advanced electrical equipment.

Recent progress in flow‐induced polymer crystallization

Abstract: During industrial processing, such as film blowing and injection molding, semicrystalline polymers have to be subjected to external flow. Flow changes the nucleation rate, crystal morphology and polymorphism of polymers. As flow can be tunable, polymeric products with different macroscopic performance and versatile applications can be obtained. Understanding polymer crystallization with the presence of flow, or namely flow-induced crystallization (FIC), is crucial to optimize polymer processing parameters and to understand phase transition under far-from equilibrium conditions, as FIC is essentially a typical nonequilibrium process. Current review aims to summarizes recent achievements of FIC during last five years from three aspects: theory, experiment and simulation. First, we present the FIC model and theory, including the classical conformational entropy reduction model, the newly developed POLYdisperse STRain Accelerated Nucleation Dynamics (PolySTRAND) Model and uFIC Model. Then we discuss the FIC experiments under both well-controlled flow and complex processing conditions. Finally, we show the recent advance of FIC in computer simulation. With the fast development in modeling efficiency, more detailed nucleation and structure transition processes during FIC can be obtained, which promote the study of molecular mechanisms of FIC.