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The objective of the present work is to verify the competitive effect between the heterogeneous and gas-diffusion cell nucleation in foaming of polystyrene/poly­(methyl methacrylate) (PS/PMMA) blends with PS or PMMA as the dispersed phase after structural evolution driven by phase separation. Four PS/PMMA blends were thermally annealed for various times. Scanning electron microscopy (SEM) was employed to study the phase morphology. The domain size and domain density were thus obtained, and the interface area was then calculated. The samples were foamed via a batch foaming process, and SEM was used to study the cell structure. It was found that with thermal annealing, the phase domain size was increased, the domain density was decreased, and the interface area was decreased. For PS20/PMMA80 and PS30/PMMA70, the cell size was increased and cell density was decreased, while for PS80/PMMA20 and PS70/PMMA30, the cell size was decreased and cell density was increased. The expansion ratio was decreased for all of the samples. It was considered that the competition between the heterogeneous and gas-diffusion cell nucleation determines the cell structure variation. For PS20/PMMA80 and PS30/PMMA70, the heterogeneous cell nucleation was the dominant factor, while for PS80/PMMA20 and PS70/PMMA30, the gas-diffusion cell nucleation played a governing role.

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