Chemistry of Materials

Effect of porogen molecular architecture and loading on structure of porous thin films

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The effects of molecular architecture and loading of a porogen material on the structural characteristics of methylsilsesquioxane (MSQ)-based porous films are examined using X-ray porosimetry (XRP) and small-angle neutron scattering (SANS). Either linear (NG) or star-shaped (TP) polymers are used as the porogen here and blended with the MSQ material at loadings from 20 to 50% by mass, resulting in porosities between 30 and 60% by volume, regardless of porogen architecture. This indicates that the MSQ matrix material contains approximately 10% by volume inherent microporosity. The average pore size increases with increased porogen loadings in a way that depends on the porogen architecture. The NG porogen tends to aggregate, more than tripling the average pore size as the porogen loading increases from 20 to 50% by mass (22 to 76 Å in diameter). By contrast, the TP porogen has less propensity to aggregate, leading to a smaller increase in terms of percent change in the average pore size for the same porogen loading range (76 to 132 A in diameter). Changes in the porogen type or loading do not influence the wall density (1.47 ± 0.01) g/cm 3 of the material between the pores, whereas the coefficient of thermal expansion (CTE) is approximately 1.5 times greater for the NG samples than for the TP samples at all porogen loadings. © 2008 American Chemical Society.