Molecular weight is defined as the average weight of the molecules that compose a polymer and is connected with the length of the polymeric chains. Several properties like glass transition temperature or some mechanical ones, such as stiffness, are affected by an increment in the molecular weight. In addition, viscosity of a molten polymer increases as molecular weight rises until it reaches a critical value. In industrial processes, the parameter used to correlate viscosity with molecular weight is the melt flow index. Viscosity is also a critical parameter during the foaming behavior of a polymer. However, the effect of molecular weight on the foaming behavior of polystyrene has not been previously studied. This paper is focused on this topic analyzing in detail how the modification of the molecular structure of the polymer strongly affects important properties of the material in foaming process such as solubility and diffusivity and the cellular structure of the foams produced.
In the present work, three types of polystyrenes with different melt flow indexes (that is, different molecular weights) have been used. The molecular weight of the three polymers has been estimated by using the Williams-Landel–Ferry and Arrhenius models in dynamic shear measurements. The glass transition temperature has been also evaluated by using differential scanning calorimetry (DSC). Finally, the relationship between the strain hardening coefficient and the molecular weight was obtained by extensional rheology measurements.
The foaming experiments were conducted using the gas dissolution foaming process with CO2 as blowing agent. The experimental conditions for saturation were 8 MPa and 40ºC. Solubility and diffusivity measurements were also conducted. Foaming tests were performed by heating the materials at 120ºC once they were removed from the pressure vessel. Foams with very low densities (around 30 kg/m3) were produced. Cellular structure parameters (cell size distribution, average cell size, cell density, anisotropy ratio and open cell content) were measured.
Results show that polymers with a higher value of the melt flow index and hence, a low molecular weight present a higher CO2 uptake. On the other hand, materials that present a higher value of the molecular weight showed a higher diffusivity after saturation. Finally, cellular materials produced with polymers with a low value of the molecular weight showed lower densities and lower cell sizes.