Effect of the crystallization of modified polybutylene terephthalate on its foaming using supercritical CO2: Transition from microcellular to nanocellular foam

A method for regulating crystallization was developed to prepare micro- and nano- cellular foams of polybutylene terephthalate (PBT) using supercritical CO2 (scCO2). This method involves the formation of branching and crosslinking structures in PBT. After modification, the crystallization temperature of PBT increased from 191.7 to 195.8 degrees C, while its enthalpy of crystallization decreased from 46.8 to 41.3 J/g and disordered lamellar crystals were formed, particularly, in the case of PBT with a crosslinking structure. In addition to enhanced viscosity and heterogeneous nucleation owing to lamellar crystals, a transition from microcellular foam to nanocellular foam was achieved after the modification of PBT. Interestingly, the transition was only observed in PBT with a crosslinked structure because of its significantly reduced crystallinity, which prevented the appearance of large unfoamed areas. As a result, microcellular PBT foams transformed to nanocellular PBT foams with a cell size of 0.33 mu m and cell density of 6.37 x 1013 cells/cm3 upon optimizing the polymer chain structure and supercritical CO2 foaming conditions such as the saturation temperature and saturation time.