Effect of chain entanglement on the melt-crystallization behavior of poly(l-lactide) acid

Entanglements of Macromolecules and Their Influence on Rheological and Mechanical Properties of Polymers

Flexible macromolecules easily become entangled with neighboring macromolecules. The resulting network determines many polymer properties, including rheological and mechanical properties. Therefore, a number of experimental and modeling studies were performed to describe the relationship between the degree of entanglement of macromolecules and polymer properties. The introduction presents general information about the entanglements of macromolecule chains, collected on the basis of studies of equilibrium entangled polymers. It is also shown how the density of entanglements can be reduced. The second chapter presents experiments and models leading to the description of the movement of a single macromolecule. The next part of the text discusses how the rheological properties change after partial disentangling of the polymer. The results on the influence of the degree of chain entanglement on mechanical properties are presented.

Self-Healing Binder for High-Voltage Batteries

Lithium-ion batteries are core components of flexible electronic devices. However, deformation types, such as impinging, bending, stretching, folding, and twisting, can cause internal cracks and, eventually, damage these batteries. The cracks separate the active particles from the conductive particles and the binder, as well as the electrode from the collector. Self-healing binders can alleviate this mechanical damage and improve the stress response of active material particles during high rates of charging and discharging of these batteries and the operation at a high voltage, thereby enhancing their cycle performance. In the present study, a thermoplastic intrinsic self-healing polymer (TISP) binder is proposed. The TISP is obtained by polymerization of butanediol (2,3-BDO), propylene glycol (1,3-PDO), succinic acid (SuA), sebacic acid (SeA), and iconic acid (IA). The hydroxyl and ester groups in its structure can form diverse bonds including the hydrogen and ion-dipole with active particles and the current collector, thereby producing elevated adhesion. Its properties, including a low glass transition temperature (-60 °C), amorphous structure, and low cross-link density, improve the mobility of polymer chains at 40 °C, and this facilitates structural recovery and the maintenance of strong adhesions. Owing to its higher occupied molecular orbital (HOMO) level than the electrolyte solvent, the TISP is likely oxidized before the main component of the electrolyte during charging. This decomposition produces a chemical passivation interphase on the cathode which reduces side reactions of LiCoO₂ and the electrolyte under high-voltage conditions. Tests reveal that a LiCoO₂ electrode battery using the TISP as a binder retains 162.4 mAh g^-1 after 349 cycles at 4.5 V, and this represents an 86.5% capacity retention. In addition, heating (40 °C, 1 h) of a scratch-damaged electrode can recover a specific capacity of 156.6 mAh g^-1 after 349 cycles at 4.5 V. Relative to a battery without any mechanical scratch, this capacity recovery represents approximately 96%, and this demonstrates the importance of the TISP to the high-voltage damaged electrode.

Young's modulus of the different crystalline phases of poly (l-lactic acid)

Young's modulus of α'- and α-crystals of poly (l-lactic acid) (PLLA), more precisely, of aggregates of isotropically arranged lamellae, has been estimated based on dynamic-mechanical analysis of sets of isotropic film samples containing largely different though well-defined amounts of crystals. Evaluation of the modulus of elasticity of these film samples yielded the dependence of Young's modulus as a function of the enthalpy-based crystallinity, increasing with the crystal fraction in the assessed range, from zero to about 75% crystallinity. Extrapolation towards 100% crystallinity suggests values of Young's modulus of around 3.7 and 4.6 GPa for isotropic aggregates of α'- and α-crystals, respectively, being only slightly higher than the modulus of the unaged glassy amorphous phase of 3.0 GPa. Noting the inherent anisotropy of the crystal modulus, suggested in the literature, the average modulus determined in this work seems to be controlled by weaker interchain secondary bonding but not the modulus in chain direction. Great effort has been undertaken to minimize errors by keeping the lamellar thickness in samples of different crystallinity constant, and by providing evidence for independence of the moduli on the spherulitic superstructure.

Effect of PMMA Molecular Weight on Its Localization during Crystallization of PVDF in Their Blends

In miscible crystalline/amorphous polymer blends, the exclusion behaviors of the latter with various molecular weights during the crystallization of the former were investigated by the combination of SAXS and DSC by taking a PVDF/PMMA blend as an example. The ratio between internal crystallinity from SAXS and overall crystallinity of the entire blend from DSC was employed to characterize the exclusion of PMMA. Our results indicate that the molecular weight of the amorphous component produces a remarkable influence on the diffusion coefficient (D) and the crystal growth rate (G) of the crystalline component. There are both inter-lamellar and inter-fibrillar structures when PVDF blended with lower-molecular-weight PMMA. With increasing molecular weight of PMMA, the decrease in crystal growth rate (G) dominates the enhanced exclusion behaviors of PMMA, resulting in bigger pores after extraction. Our results are significant not only for the basic understanding of crystallization in polymer blends, but also for the fabrication and structure control of porous structures based on crystallization templates.

Role of Chain Entanglements in the Stereocomplex Crystallization between Poly(lactic acid) Enantiomers

Stereocomplex (SC) crystallization between polymer enantiomers has opened a promising avenue for preparing high-performance materials. However, high-crystallinity SCs are difficult to achieve for high-molecular-weight (HMW) enantiomeric blends of chiral polymers [e.g., poly(lactic acid)]. Despite extensive studies, why HMW enantiomeric blends have difficulty in SC crystallization has not been clarified. Herein, we chose the HMW poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) 1/1 blend as the model system and demonstrated the crucial role of chain entanglement in regulating SC crystallization. PLLA/PDLA blends with various entanglement degrees were prepared by freeze-drying. We observed that disentangling promoted not only the crystallization rate but also the crystallinity of SCs in both the nonisothermal and isothermal processes. The less-entangled samples crystallized exclusively as the high-crystallinity SCs at different temperatures, in contrast to the predominant homocrystallization that occurred in the common entangled samples. This study provides deep insight into the SC crystallization mechanism of polymers and paves the way for future research attempting to prepare SC materials.