Structural characterization of hierarchical polymer foams by combining X-ray micro-computed tomography and scanning electron microscopy

Complementary structural characterization methods are useful for studying the hierarchical cellular morphology of polymer foams. In this study, we employed scanning electron microscopy (SEM) and X-ray micro-computed tomography (micro-CT) to characterize the hierarchical cellular morphology of poly(methyl methacrylate) (PMMA) and polystyrene (PS) foams. The polymer foams were prepared using pure CO 2 gas and CO 2 –chlorodifluoromethane (HCFC-22) gas mixtures as blowing agents. Depending on the type of polymer and HCFC-22 concentration, hierarchical cellular structures consisting of nanocells, microcells, and macrocells were obtained. The size distribution of the nanocells was determined by high-magnification SEM, while the size, shape, and spatial distribution of the microcells and macrocells in three dimensions were determined by micro-CT. Moreover, a well-designed micro-CT experiment enabled a brightness comparison between the foams and relative local density mapping of the foams based on the brightness. The results clearly showed the formation of a dense skin layer at the air interface of both PMMA and PS foams and dense matrix around the large macrocells in the PMMA foams. Thus, combining SEM and micro-CT provides a deeper understanding of the formation mechanism of the hierarchical cellular structure of polymer foams. • Polymers were foamed using pure CO 2 gas and CO 2 –HCFC-22 gas mixtures. • Polymer foams prepared as model samples had hierarchical structures. • A structural characterization scheme combining SEM and micro-CT images was proposed. • Micro-CT images were used to evaluate the relative densities of polymer foams in three dimensions. • High-density layer in the skin and around the large macrocells were clearly visualized by micro-CT images.