Flow-induced crystallization of piezoelectric poly(L-lactide) fibers by a one-step melt-spinning process

The Combined Effects of Optical Purity, Chain Orientation, Crystallinity, and Dynamic Mechanical Activation as Means to Obtain Highly Piezoelectric Polylactide Materials

“…The curve in Figure a (bottom) reveals that the overall chain orientation of the as-drawn PLA film increases with the l -isomer content, indicating an enhanced chain mobility of optically pure PLA chains during the cold drawing process. Less constraining d -segments led to higher efficiency of chain alignment during cold drawing, which is in accordance with the reported decreasing Rouse relaxation time with increasing optical purity in PLA …”

“…The cold crystallization of PLA L99 observed at temperatures just above the T g indicates the presence of highly oriented molecules that can facilitate nucleation, thus promoting cold crystallization immediately above the devitrification process at T g . This T cc shift toward the T g has been reported for cold-drawn PLLA films with high chain orientation . The melt temperature ( T m ) increased with the optical purity, as expected.…”

“…The background signal from air scattering was calibrated according to standard procedures. The azimuthal intensity profiles of the (110/200) diffraction, I ∼ φ (the direction on the detector parallel to the draw direction was defined as φ = 0), was used to calculate the Hermans orientation function for crystal orientation ( f c ), as already presented in a previous work . The f c is 0 for the isotropic structure and 1 for the perfect uniaxial orientation, where the chain axis is parallel to the reference axis.…”

Mechanisms of Flow-Induced Multistep Structure Evolution in Semicrystalline Polymers Revealed by Molecular Simulations

“…In a quiescent state, it is theorized that polymer chains crystallize via intramolecular chain-folding nucleation model, − ,− yielding folded-chain lamellar crystals within systems. − ,− However, when subjected to shear or stretching forces, various other crystalline morphologies emerge, such as oriented crystals, shish-kebab crystals, or extended-chain crystals. − ,,,,,,,,,− ,− ,− The formation of these distinct crystalline structures is likely governed by different nucleation modes. − ,− , For instance, Hu’s group analyzed polymer nucleation modes during stretching at different temperatures using dynamic MC simulations. − ,,,,,, In Figure a, adjacent chain-folding probability in small crystallites under different temperatures remains almost unchanged, but decrease after some critical strains . At low strains, polymer chains crystallize predominantly via the nucleation mode of intramolecular chain-folding but change to the nucleation mode of intermolecular fringed-micelle above critical strains.…”

“…Moreover, it has been found that crystal structures of polymers could be tuned by adjusting the processing conditions of semicrystalline polymers. ,− ,− During the polymer processing, polymer melts experience external flow, which affects the subsequent crystallization process. ,− ,− For example, polymer melts undergo shear forces in extruders or injection molding machines, leading to chain segment orientation and elongation of the entire chains. − ,,,,,− ,,,− These oriented polymer melts exhibit significantly different crystallization behaviors compared to the melts under quiescent state, including faster crystallization rates, higher orientation degree of crystals, and appearance of new crystalline morphologies (e.g., shish-kebabs). − ,,,…”