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.