Impact of Topological Parameters on Melt Rheological Properties and Foamability of PS POM-POMs

Abstract

Rheological properties highly impact the foaming behavior of polymers, as a high melt strength and low shear viscosity support cell expansion by reducing cell coalescence and rupture. Since rheological properties originate from molecular topology, topology is consequently a governing parameter for foaming. We synthesized well-defined polystyrene (PS) POM-POM-shaped model systems to study the impact of the topology-originated melt rheological parameters on the physical foaming behavior within purely amorphous polymers without any crystallization effects. Therefore, we systematically varied the topological parameters of the POM-POM topology, i.e., the number of arms q, the arm length Mw,a, and the backbone length Mw,b. We investigated the melt rheological properties of the samples concerning their zero-shear viscosity and elongational strain hardening and correlated their resulting foam structure. Within the sample series, η0 can be reduced by up to 104, compared to linear PS with the same total molecular weight, whereas the maximum achieved strain hardening factor is SHF = 141 at T = 160 °C. The foaming experiments revealed that the mean cell size D and the volume expansion VER can be precisely controlled via molecular topology. By controlling the topological parameters, it was possible to increase VER from VER = 2.0 to 15.6, whereas D varied between D = 1.3 and 17.2 μm within 100–150 °C under a pressure of 500 bar. In comparison, the VER of linear PS increased from VER = 2.6 to 5.4, while D increased from D = 2.3 to 4.8 μm. Moreover, the influence of topology is more pronounced at higher temperatures, indicating that the molecular relaxation times and the applied shear rates need to be related. Hence, it is possible to tailor the cellular structure to a specific application by considering and controlling the polymer topology.