Molecular origin of the foam structure in model linear and comb polystyrenes: I. Cell density

The effect of the molecular architecture of anionically synthesized linear and comb atactic polystyrenes (PS), as well as lab-scale emulsion polymerized PS and commercial PS on the cell density of foam (number of cells per unit volume of neat polymer) was investigated using CO2 as foaming agent in a batch foaming setup. The effects of molecular weight, Mw, dispersity (Ð), low molecular weight components, i.e. oligomers and residual surfactant, and the number of long chain branches per molecule, Nbr, in a series of comb-PS 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, on the cell density were investigated. Specimens for foaming have been prepared with three different methods, which resulted in non-purified, treated, and purified samples. Well-purified samples produced foams with cell density above 109 cell/cm3, while foams out of the non-purified PS had one to three orders of magnitude lower cell density. However, artificial addition of 6 wt% oligomer PS or 3 wt% surfactant to the purified samples did not reduce the cell density significantly. Treated samples prepared by only dissolving the PS in a solvent followed by removing the solvent, produced a foam with slightly lower cell density, ~5 × 108 cell/cm3, than the purified PS. Lower cell density in non-purified PS was supposed to be related to the formation of a continuous ionic thin film layer (e.g. surfactant) rather than only the presence of low molecular weight components as impurity in the polymer. In the absence of such low molecular weight components, other molecular parameters, i.e. Mw, Ð, and Nbr in these series of comb-PS had surprisingly no distinct effect on the cell density.