The sample prepared by compression molding polystyrene (PS) beads and pellets was saturated in compressed carbon dioxide (CO₂). After reaching equilibrium, the sample was foamed by temperature soaking method. Intriguing layered foam morphology was obtained. The effect of weak chain entanglements on the bubble nucleation, cell morphology, cell size and distribution were investigated by scanning electron microscopy (SEM) and the solubility measurement of CO₂ in PS. The results indicated that the weak entanglements of chains were introduced into the interfaces between the beads in bead PS during the compression molding process. Due to the different chain entanglement ability in the matrix, the bubble growth and nucleation of CO₂ gas in different regions were not the same. By controlling the parameters of foaming process, unique and novel structures with layered foams were generated.

Key words: carbon dioxide, polystyrene, layered foam morphology