Molecular origin of the foam structure in model linear and comb polystyrenes: II. Volume expansion ratio

Abstract

The effect of the molecular architecture of a series of anionically synthesized linear and comb atactic polystyrenes (PS) on their foam properties including cell size, cell density, and volume expansion ratio (V.E.R.) was investigated. The comb-PS had the same molecular weight of the backbone, Mw,bb ≈ 290 kg/mol, Zbb ≈ 20 entanglements, and branches, Mw,br ≈ 44 kg/mol, Zbr ≈ 3 entanglements, but different numbers of branches, 3 ≤ Nbr ≤ 190. Batch foaming of well-purified linear and comb-PS using CO2 resulted in cell densities about 109 and 4 × 109 cell/cm3, respectively, which shows that LCB has no distinct effect on the cell density, whereas a small amount of residual impurity in linear PS reduced the cell density to ~108 cell/cm3. For a comb-PS series with the same entangled backbone, Mw,bb ≈ 290 kg/mol, Zbb ≈ 20 entanglements, and similar branches, Mw,br ≈ 44 kg/mol, Zbr ≈ 3 entanglements, but different numbers of branches, 3 ≤ Nbr ≤ 190, an increase in the Nbr to 120 with densely grafted comb conformation gradually increased the highest achievable V.E.R. close to a theoretical limit given by the CO2 solubility. The further increase of Nbr to 190 with a bottlebrush conformation reduced the V.E.R. of the foam. From a rheological point of view, this optimum Nbr was related to a comb-PS which showed the maximum strain hardening factor (SHF ≈ 200) in uniaxial extension, and simultaneously the minimum in the zero shear viscosity, η0. The optimum Nbr = 120 for this comb-PS series corresponds to an average spacing distance between two neighbor branch points of about Zs ≈ 0.2 entanglements.