CO2 foaming of poly(ethylene glycol)/polystyrene blends: Relationship of the blend morphology, CO2 mass transfer, and cellular structure

When polymer blends are foamed by physical foaming agents, such as CO2 or N2, not only the morphology and viscosity of the blend polymers but also the solubility and diffusivity of the physical foaming agents in the polymers determine the cellular structure: closed cell or open cell and monomodal or bimodal. The foam of poly(ethylene glycol) (PEG)/polystyrene (PS) blends shows a unique bimodal (large and small) cellular structure, in which the large-size cells embrace a PEG particle. Depending on the foaming condition, the average size of the large cells ranges from 40 to 500 (mu)m, whereas that of small cells becomes less than 20 (mu)m, which is smaller than that of neat PS foams. The formation mechanism of the cellular structure has been investigated from the viewpoint of the morphology and viscosity of the blend polymer and the mass-transfer rate of the physical foaming agent in each polymer phase. The solubility and diffusivity of CO2, which determine the mass-transfer rate of CO2 from the matrix to the bubbles, were measured by a gravimetric measurement, that is, a magnetic suspension balance. The solubility and diffusivity of CO2 in PS differed from those in PEG: the diffusion coefficient of CO2 in PEG at 110°C was 3.36 × 10-9 m2/s, and that in PS was 2.38 × 10-10 m2/s. Henryʹs constant in PEG was 5600 cm3 (STP)/(kg MPa) at 110°C, and that in PS was 3100 cm3 (STP)/(kg MPa). These differences in the transport properties, morphology of the blend, and CO2-induced viscosity depression are the control factors for creating the unique cellular structure in PEG/PS blends.