Cell Structure Variation in Poly(ether-mb-amide) Copolymer Foams Induced by Chemi-Crystallization

Abstract

The objective of this study was to clarify the cell structure variation of the poly(ether-mb-amide) (PEBA) copolymer with the chemi-crystallization induced via thermal treatment at the melt state. Thermal annealing was conducted using a compression molding machine at the melt state for various times (i.e., 3, 30, 60, 120, and 300 min) to promote chemi-crystallization. The chemical structure change was studied with Fourier transform infrared spectra, and the viscosity change was investigated using a rheometer. The crystallization behavior was studied with differential scanning calorimetry and X-ray diffraction. The PEBA samples were foamed using a two-step batch foaming process, and the cell structure was investigated with a scanning electron microscope. It was found that the chemical structure of PEBA was hardly changed during the thermal treatment, while the complex viscosity was decreased. It was speculated that degradation occurred probably via a random chain scission mode. Thus, the chemi-crystallization was promoted for the crystalline segments because of the less suppression from the noncrystalline segments ascribed to the shorter chains. Consequently, three types of cell structures were observed in PEBA: uniform cell structure, bimodal cell structure, and micro/nanosized cells formed within the spherulites. A scheme was presented to interpret the relationship between the cell structure variation and the crystalline states in PEBA samples. The amorphous, partially melted, and unmelted regions were classified during foaming to interpret the formation of the three types of cell structures.