Nanocellular Foaming Behaviors of Chain-Extended Poly(lactic acid) Induced by Isothermal Crystallization

Abstract

Recently, the fabrication of semicrystalline polymer foams with a nanocellular structure by supercritical fluids has been becoming a newly developing research hotspot, owing to their peculiar properties and prospective applications. In this work, a facile and effective isothermal crystallization-induced method was proposed to prepare nanocellular semicrystalline poly(lactic acid) (PLA) foams using CO 2 as a physical blowing agent. Styrene–acrylonitrile–glycidyl methacrylate (SAG) as a chain extender (CE) was introduced into PLA through a melt-mixing method to improve the crystallization behavior and melt viscoelasticity of PLA. The chain extension reaction between PLA and SAG occurred successfully as well as the branching and micro cross-linking structures were generated in chain-extended PLA (CPLA) samples, which were confirmed by Fourier transform infrared spectra, gel fraction, and intrinsic viscosity measurements. Owing to the nucleation effect of branching points and the restricted movement of PLA molecular chains by the formation of branching and/or microcross-linking structures, a large number of small spherocrystals were generated in CPLA samples, which was beneficial to produce nanocells. Nanocellular CPLA foams were prepared successfully, when the foaming temperature was 125 °C. As the SAG content increased, the cell size of various PLA foams decreased from 364 ± 198 to 249 ± 100 nm and their volume expansion ratio increased from 1.15 ± 0.05 to 2.22 ± 0.01 times, gradually. When the foaming temperature increased from 125 to 127 °C, an interesting transition from nanocells to microcells could be observed in CPLA foam with the CE content of 2 wt %. Finally, the formation mechanism of nanocells in various PLA foams was proposed and clarified using a schematic diagram.

Simple and feasible strategy to fabricate microcellular poly(butylene succinate) foams by chain extension and isothermal crystallization induction

“…For all the PBS samples, the η* gradually decreased with the increasing ω due to the shear‐thinning behavior, as displayed in Figure (a) . It could be seen clearly in Figure (a) that the η* of various PBS samples increased with the increase of PE‐ g ‐GMA content, resulting from the formation of branching and/or micro crosslinking structures of various PBS samples, which restricted their chain slippage and enhanced their melt viscosity . As aforementioned in Table , with increasing PE‐ g ‐GMA content, the enhancement of gel fraction in various PBS samples should also contribute to enhance the η* of various PBS samples.…”

“…The crosslinking degree of polymers played an important role in their foaming behaviors, which also could provide an indirect proof for the chain extension . Micro crosslinking in PBS was helpful to improve its melt strength and viscoelasticity, but excessive crosslinking structures in PBS would have a negative effect on the cell nucleation and growth in the PBS foaming process and the degradation of PBS . Gel fraction was measured to confirm the crosslinking reaction in various PBS samples during melt mixing.…”

“…This should be attributed to the enhancement number of entanglements between PBS molecular chains by the chain extension. These entanglements could be served as the crosslinking points to increase the melt viscoelasticity of various PBS samples …”

“…On the one hand, the increment in η* of various PBS samples [Figure (a)] would restrict the cell growth and prevent cell coalescence. On the other hand, the formed crystalline region in various PBS samples could not be biaxially stretched during the foaming process, which was adverse to their cell growth, leading to the decrement in their cell size and VER …”

Investigation on compatibility of PLA/PBAT blends modified by epoxy-terminated branched polymers through chemical micro-crosslinking

“…The former is related to the intermolecular friction, and the latter is related to the molecular structure of polymers. Upon addition of ETBP, the formed micro-crosslinking network structure strengthened the macro-molecular chains entanglement and interweaving of PLA and PBAT, leading to the change of molecular structure (from linear structure to micro-crosslinking structure) and the increase of intermolecular friction, thus the elasticity and melt viscosity increased(G' and G'' of the PLA/PBAT/ETBP blends increased) (33).…”