Chain heterogeneity of poly(lactic acid) and its influence on crystallization kinetics

Temperature rising elution fractionation (TREF) approach was used to separate a biodegradable poly(lactic acid) (PLA) resin into ten fractions and completely establish the relationship between chain microstructure and properties. The main fractions were mainly eluted at 100, 110, 114, and 118 °C, and their mass percentages were 7.98 wt%, 44.83 wt%, 19.64 wt%, and 11.90 wt%, respectively. Through the use of successive self-nucleation/annealing (SSA) thermal fractionation, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and 13C-nuclear magnetic resonance spectroscopy (13C NMR), the intermolecular and intramolecular differences of PLA were further explored. Fractions eluted at 90, 110, 118, and 126 °C were also chosen to research the non-isothermal cold crystallization kinetics, and fractions eluted at 110, 118, and 126 °C were chosen to explore the non-isothermal crystallization kinetics in order to simulate the real process. The findings demonstrated that the Liu-Mo approach were more suited the non-isothermal crystallization and non-isothermal cold crystallization kinetics of PLA. As the elution temperature increased, so did the stereoregularity of the fractions, the crystallization rate, the crystallization capacity, and the lamellar thickness. These will lay a foundation for its basic research and industrial application.

Thermal Fractionation of Polyolefins: Brief History, New Developments and Future Perspective

Abstract

For semi-crystalline polymer materials, the difference in chain structure often leads to different physical properties; therefore, in-depth analysis of the chain structure is of great significance. With the continuous development of advanced instruments, many research means have emerged to characterize the structure of molecular chains. Among them, fractionation techniques provide effectively structural information on inter- and intra-molecular comonomer distribution, branching degree, and sequence length, etc. This work briefly presents the history of developments of various classical fractionation means such as temperature-rising elution fractionation, stepwise crystallization and successive self-nucleation and annealing, while focusing on the present and future of their applications.

Crystalline Characteristics, Mechanical Properties, Thermal Degradation Kinetics and Hydration Behavior of Biodegradable Fibers Melt-Spun from Polyoxymethylene/Poly(l-lactic acid) Blends

A series of polyoxymethylene (POM)/poly(l-lactic acid) (PLLA) blends were prepared by melt extrusion, and their spinnability was confirmed by rheological characterizations, successive self-nucleation, and annealing thermal fractionation analysis. The bicomponent fibers were prepared by means of the melt-spinning and post-drawing technologies using the above-obtained blends, and their morphology, crystalline orientation characteristics, mechanical performance, hydration behavior, and thermal degradation kinetics were studied extensively. The bicomponent fibers exhibited a uniform diameter distribution and compact texture at the ultimate draw ratio. Although the presence of PLLA reduced the crystallinity of the POM domain in the bicomponent fibers, the post-drawing process promoted the crystalline orientation of lamellar folded-chain crystallites due to the stress-induced crystallization effect and enhanced the crystallinity of the POM domain accordingly. As a result, the bicomponent fibers achieved the relatively high tensile strength of 791 MPa. The bicomponent fibers exhibited a partial hydration capability in both acid and alkali media and therefore could meet the requirement for serving as a type of biodegradable fibers. The introduction of PLLA slightly reduced the thermo-oxidative aging property and thermal stability of the bicomponent fibers. Such a combination of two polymers shortened the thermal lifetime of the bicomponent fibers, which could facilitate their natural degradation for ecological and sustainable applications.