Comparative studies of thermal and mechanical properties of macrocyclic versus linear polylactide

Influence of carboxy-terminated hyperbranched polyester and polyethylene glycol on the mechanical and thermal properties of polylactic acid/straw flour composites

Polylactic acid (PLA) wood-plastic composites have a significant advantage over traditional petroleum-based plastics due to their biodegradability. However, PLA has several shortcomings, including high brittleness, low heat resistance, slow crystallization, and poor compatibility with biomass materials, which have limited its potential applications. In this paper, we investigated the effects of carboxy-terminated hyperbranched polyester (CHBP) on the mechanical, crystalline, and thermal properties of PLA/straw flour (SF) blends through extrusion injection molding. Additionally, we added the traditional plasticizer polyethylene glycol (PEG) to synergize with CHBP to enhance the toughness of PLA/SF composites. Our results showed that the appropriate addition of CHBP effectively improved the interfacial bonding between PLA and straw flour. The incorporation of CHBP also improved the tensile strength, bending strength, impact strength, elongation at break, thermal stability, and crystallization rate of the composites. Furthermore, the addition of both CHBP and PEG significantly improved the impact strength of the composites compared to using PEG alone. This method also improved the heat resistance of the material and reduced the migration of plasticizers. Our study demonstrates the feasibility of using hyperbranched polymers and plasticizers to enhance the toughness, thermal stability, and crystalline properties of PLA wood-plastic composites, providing a new approach to improving the properties of these composites.

Accelerating the crystallization kinetics of linear polylactides by adding cyclic poly (L-lactide): Nucleation, plasticization and topological effects

Polylactide is one of the most versatile biopolymers, but its slow crystallization limits its temperature usage range. Hence finding ways to enhance it is crucial to widen its applications. Linear and cyclic poly (L-lactide) (l-PLLA and c-PLLA) of similarly low molecular weights (MW) were synthesized by ring-opening polymerization of L-lactide, and ring-expansion methodology, respectively. Two types of blends were prepared by solution mixing: (a) l-PLLA/c-PLLA, at extreme compositions (rich in linear or in cyclic chains), and (b) blends of each of these low MW materials with a commercial high MW linear PLA. The crystallization of the different blends was evaluated by polarized light optical microscopy and differential scanning calorimetry. It was found, for the first time, that in the l-PLLA rich blends, small amounts of c-PLLA (i.e., 5 and 10 wt%) increase the nucleation density, nucleation rate (1/τ₀), spherulitic growth rate (G), and overall crystallization rate (1/τ_50%), when compared to neat l-PLLA, due to a synergistic effect (i.e., nucleation plus plasticization). In contrast, the opposite effect was found in the c-PLLA rich blends. The addition of small amounts of l-PLLA to a matrix of c-PLLA chains causes a decrease in the nucleation density, 1/τ₀, G, and 1/τ_50% values, due to threading effects between cyclic and linear chains. Small amounts of l-PLLA and c-PLLA enhance the crystallization ability of a commercial high MW linear PLA without affecting its melting temperature. The l-PLLA only acts as a plasticizer for the PLA matrix, whereas c-PLLA has a synergistic effect in accelerating the crystallization of PLA that goes beyond simple plasticization. The addition of small amounts of c-PLLA affects not only PLA crystal growth but also its nucleation due to the unique cyclic chains topology.