Polymer Features in Crystallization

Self-Nucelation Effect on Crystallization of Double Crystalline Polypropylene-b-Polybutene Block Copolymers

Self-nucleation effect of crystalline polymers originates from the intrinsic in-complete relaxation of crystallization-associated ordered structure and plays a crucial role in accelerating crystallization without adding the extra nucleating agents. In this work, the polypropylene-b-polybutene block copolymers are designed and synthesized with the sequential polymerization method. The in situ wide-angle X-ray diffraction characterization demonstrates that polybutene (PB) and polypropylene (PP) sequences are both crystallizable, which crystallize into the mixture of tetragonal and trigonal phases, and monoclinic phase, respectively. In the meanwhile, The PP sequences in block copolymers exhibit the self-nucleation effect similar to the homopolymer, of which the PP crystallization kinetics is accelerated with decreasing self-nucleation temperatures (Ts). Unexpectedly, this self-nucleation effect of PP blocks on PB crystallization exhibits a rather complex and non-monotonous dependence on Ts. It is interesting to see that as Ts is lowered to Domains II and III to largely accelerate crystallization of PP, PB crystallization is suppressed. However, as the further decreased Ts only anneals PP crystallites, the suppressed PB crystallization could recover. This complex self-nucleation on crystallization is interpreted by the constraint of evolved PP crystallites on PB sequences.

Striving to Disclose the Electrochemical Processes of Organic Batteries

ConspectusOrganic/polymeric materials are promising as electrode materials for batteries because of their advantages of flexibility, high specific capacity due to the possible multielectron transfer, low cost from green natural resources, and weak intermolecular interactions that enable the storage of low-cost large-sized or multivalent metal ions. However, the development of organic electrode materials (OEMs) and organic batteries and the understanding of the electrochemical process face great challenges in the characterization of polymers and the charge storage mechanisms: (1) the charged and/or discharged states of OEMs are often air unstable, which makes the ex situ characterizations susceptible to the interference of air. (2) OEMs, particularly polymeric materials, are designed to be insoluble to deliver high cyclability, which makes it difficult for them to be separated from the electrode. (3) Possible multielectron transfer makes it difficult to determine whether the proposed charge storage mechanism or the experiment results are wrong when the actual capacity mismatches with the theoretical capacity based on the proposed mechanisms. (4) It is difficult to achieve single crystals of polymers, and hence, it seems impossible to know the actual locations of the stored ions in the polymers. (5) The typical methods for characterization of insoluble polymers are only qualitative, and it is challenging to quantify the amount of stored ions. (6) Even for most in situ characterizations, they can only give the tendency of qualitative structural evolution.In this Account, we give an overview of the significance of organic batteries and the challenges related to the characterization and charge storage mechanisms of organic electrode materials. Then, we summarize our efforts in recent years to reveal the charge storage mechanisms and insights into the electrochemical process. Focusing on the complexity of polymer materials, we proposed a strategy to control the reaction kinetics in order to obtain high-quality single crystals or microcrystals of polymers. The chemical structure and reaction mechanism of polymers could be successfully revealed by single crystal structure analysis. To avoid the inconvenient characterizations brought by the insolubility of polymers, soluble monomers or oligomers were studied under the same conditions to simulate and analyze the electrochemical process of polymers. We also proposed the synthesis of isomers for a deep understanding of the structure-property relationships of OEMs. On the other hand, traditional qualitative characterization instruments or techniques were reconsidered to give more information or even quantitative results via insightful analysis of the data or smart design of experiments. In addition, by introducing internal standard substances, it was also possible to realize quantitative characterizations. Strategies to convert the black box of different charged/discharged states into detectable materials or signals were also developed. This Account provides a summary of our recent progress in understanding the electrochemical process of OEMs and prospects of future development of rechargeable organic batteries.

Morphology and Electrical Properties of AC Electric-Field-Assisted Low-Density Polyethylene

The direct current (DC) electrical properties of insulating polymers (e.g., low-density polyethylene, LDPE) are very important for the development of high-voltage direct current (HVDC) cables. The electric field assist can improve the DC electrical properties of the electrical insulating polymers. However, there are rarely reports about the effect of the alternating current (AC) electric field on the electrical properties of insulating polymers. In this article, the effects of the AC-assisted electric field on the DC electrical properties and the morphology of LDPE are studied. LDPE is prepared under the AC-assisted electric field of 0, 0.3, 0.5, 1.0, and 2.0 kV/mm, respectively. The DC conductivity, space charge distribution, breakdown strength, surface potential decay, and morphology of LDPE are characterized. Experimental results show that the AC-assisted electric field not only reduces the DC conductivity and space charge accumulation but also increases the breakdown strength of LDPE. The AC-assisted electric field increases the nucleation rate of LDPE and contributes to forming more spherulites, which introduces more deep traps to improve the DC electrical properties of LDPE. Notably, compared with the untreated LDPE, the LDPE prepared with the AC-assisted electric field of 1.0 kV/mm decreases by 96% in conductivity and reduces by 52.6% in average space charge density, whereas its DC breakdown strength is enhanced by 35%. This work provides a foundation for the improvement of the electrical properties of the insulating polymers.

Interfacial Polarization Strategy to Electroactive Poly(lactic acid) Nanofibers for Humidity-Resistant Respiratory Protection and Machine Learning-Assisted Monitoring

The advancement of intelligent and biodegradable respiratory protection equipment is pivotal in the realm of human health engineering. Despite significant progress, achieving a balance between efficient filtration and intelligent monitoring remains a great challenge, especially under conditions of high relative humidity (RH) and high airflow rate (AR). Herein, we proposed an interfacial stereocomplexation (ISC) strategy to facilitate intensive interfacial polarization for poly(lactic acid) (PLA) nanofibrous membranes, which were customized for machine learning-assisted respiratory diagnosis. Theoretical principles underlying the facilitated formation of the electroactive phase and aligned PLA chains were quantitatively depicted in the ISC-PLA nanofibers, contributing to the increased dielectric constant and surface potential (as high as 2.2 and 5.1 kV, respectively). Benefiting from the respiration-driven triboelectric mechanisms, the ISC-PLA demonstrated a high PM0.3 filtration efficiency of over 99% with an ultralow pressure drop (75 Pa), even in challenging circumstances (95 ± 5% RH, AR of 85 L/min). Furthermore, we implemented the ISC-PLA with multifunction respiratory monitoring (response time of 0.56 s and recovery time of 0.25 s) and wireless transmission technology, yielding a high recognition rate of 83% for personal breath states. This innovation has practical implications for health management and theoretical advancements in respiratory protection equipment.

Ultra-fast supercritically solvothermal polymerization for large single-crystalline covalent organic frameworks

Crystalline polymer materials, e.g., hyper-crosslinked polystyrene, conjugate microporous polymers and covalent organic frameworks, are used as catalyst carriers, organic electronic devices and molecular sieves. Their properties and applications are highly dependent on their crystallinity. An efficient polymerization strategy for the rapid preparation of highly or single-crystalline materials is beneficial not only to structure-property studies but also to practical applications. However, polymerization usually leads to the formation of amorphous or poorly crystalline products with small grain sizes. It has been a challenging task to efficiently and precisely assemble organic molecules into a single crystal through polymerization. To address this issue, we developed a supercritically solvothermal method that uses supercritical carbon dioxide (sc-CO2) as the reaction medium for polymerization. Sc-CO2 accelerates crystal growth due to its high diffusivity and low viscosity compared with traditional organic solvents. Six covalent organic frameworks with different topologies, linkages and crystal structures are synthesized by this method. The as-synthesized products feature polarized photoluminescence and second-harmonic generation, indicating their high-quality single-crystal nature. This method holds advantages such as rapid growth rate, high productivity, easy accessibility, industrial compatibility and environmental friendliness. In this protocol, we provide a step-by-step procedure including preparation of monomer dispersion, polymerization in sc-CO2, purification and characterization of the single crystals. By following this protocol, it takes 1-5 min to grow sub-mm-sized single crystals by polymerization. The procedure takes ~4 h from preparation of monomer dispersion and polymerization in sc-CO2 to purification and drying of the product.

Crystallinity, Rheology, and Mechanical Properties of Low-/High-Molecular-Weight PLA Blended Systems

As semi-crystalline polyester (lactic acid) (PLA) is combined with other reinforcing materials, challenges such as phase separation, environmental pollution, and manufacturing difficulties could hinder the benefits of PLA, including complete biodegradability and strong mechanical properties. In the present investigation, melt blending is utilized to establish a mixture of low- and high-molecular-weight polylactic acids (LPLA and HPLA). The crystallinity, rheology, and mechanical properties of the combination were analyzed using rotational rheometry, differential scanning calorimetry, X-ray diffraction, polarized optical microscopy, scanning electron microscopy, and universal testing equipment. The results demonstrate compatibility between LPLA and HPLA. Moreover, an increase in LPLA concentration leads to a decrease in the crystallization rate, spherulite size, fractional crystallinity, and XRD peak intensity during isothermal crystallization. LPLA acts as a diluent during isothermal crystallization, whereas HPLA functions as a nucleating agent in the non-isothermal crystallization process, promoting the growth of LPLA crystals and leading to co-crystallization. The blended system with a 5% LPLA mass fraction exhibits the highest tensile strength and enhances rheological characteristics. By effectively leveraging the relationship between various molecular weights of PLA's mechanical, rheological, and crystallization behavior, this scrutiny improves the physical and mechanical characteristics of the material, opening up new opportunities.

Colossal electrocaloric effect in an interface-augmented ferroelectric polymer

The electrocaloric effect demands the maximized degree of freedom (DOF) of polar domains and the lowest energy barrier to facilitate the transition of polarization. However, optimization of the DOF and energy barrier-including domain size, crystallinity, multiconformation coexistence, polar correlation, and other factors in bulk ferroelectrics-has reached a limit. We used organic crystal dimethylhexynediol (DMHD) as a three-dimensional sacrificial master to assemble polar conformations at the heterogeneous interface in poly(vinylidene fluoride)-based terpolymer. DMHD was evaporated, and the epitaxy-like process induced an ultrafinely distributed, multiconformation-coexisting polar interface exhibiting a giant conformational entropy. Under a low electric field, the interface-augmented terpolymer had a high entropy change of 100 J/(kg·K). This interface polarization strategy is generally applicable to dielectric capacitors, supercapacitors, and other related applications.

Highly Efficient Blue Organic Light Emitting Diodes Based on Cyclohexane-Fused Quinoxaline Acceptor

Exploring blue organic light emitting diodes (OLED) is an important but challenging issue. Herein, to achieve blue-shifted emission, cyclohexane is fused to quinoxaline to weaken the electron-withdrawing ability and conjugation degree of the acceptor. As a result, blue to cyan fluorescent emitters of Me-DPA-TTPZ, tBu-DPA-TTPZ, and TPA-TTPZ were designed and synthesized with donors of diphenylamine and triphenylamine, which exhibit high photoluminescence quantum yields and good thermal stability. In OLEDs with emitters of TPA-TTPZ, the sensitized and nonsensitized devices demonstrate deep-blue (449 nm) and blue (468 nm) emission with maximum external quantum efficiency and CIE coordinates of 6.1%, (0.15, 0.10) and 5.1%, (0.17, 0.22), respectively, validating their potential as blue emitters in OLEDs.

Personal Perspective on Strain-Induced Polymer Crystallization

Semicrystalline polymer materials are commonly strong yet tough after processed through fiber spinning, film stretching (or blowing), and plastic molding (or foaming), which are fundamentally related with strain-induced crystallization. This paper provides a personal perspective on thermodynamics and kinetics aspects of strain-induced polymer crystallization, mainly based on the author's recent research experience. The thermodynamic studies include homopolymers, random copolymers, solution polymers, and blend polymers. The kinetic studies cover three sequential crystallization stages, i.e., crystal nucleation, crystal growth, and postgrowth. The thermodynamic driving forces join with the kinetic barriers to determine the crystal nucleation mechanisms and the structure evolution at the molecular level, which yield unique polymer crystal morphologies from lamellar crystals to shish-kebab crystals and eventually fibril crystals. The resulting semicrystalline structures were discussed with their implications for the mechanical properties of products. Some future studies were briefly proposed.

Dynamic Monte Carlo Simulations of Strain-Induced Crystallization in Multiblock Copolymers: Effects of Asymmetric Block Rigidity

Thermoplastic elastomers such as polyether-b-polyamides (or -polyesters), polyurethanes (or with -urea) and olefin block copolymers are commonly processed through a stretching process for achieving high elasticity and high toughness in their products, while the size diversity of semicrystalline microdomains of hard blocks appears as the key factor. By means of dynamic Monte Carlo simulations of strain-induced crystallization of locally concentrated and diluted crystallizable blocks alternatingly connected with noncrystallizable blocks in diblock and tetrablock copolymers, we have studied the size diversity of semicrystalline microdomains presumably raised by local concentration fluctuations of crystallizable blocks and found the dilution effects to persist from diblock to tetrablock copolymers. In the present work, we continued to study the effects of asymmetric block rigidity between crystallizable and noncrystallizable blocks on strain-induced crystallization of concentrated and diluted crystallizable blocks in diblock copolymers. The results showed that when crystallizable blocks hold higher thermodynamic rigidity than noncrystallizable blocks, the large semicrystalline domains become larger and the small semicrystalline domains become more, enhancing their size diversity. However, asymmetric kinetic rigidity has little effect. Our observations imply that industrial stretching processing could enhance the toughness of semicrystalline thermoplastic elastomers when their crystallizable blocks hold a higher thermodynamic rigidity relative to noncrystallizable blocks. Our integrated approach paved the way for a better understanding of the structure-property relationship in thermoplastic elastomers.