Strain Hardening In Elongation Research Articles

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

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.

Modeling of rheological behavior for polymer nanocomposites via Brownian dynamic simulation

Reptation dynamics of the coarse-grained polymer molecular chain is investigated to predict rheological behavior of polymeric nanocomposites by applying Brownian dynamics simulation to the proposed full chain reptation model. Extensibility of polymer chain and constraint release from chain stretch or retraction are of main concern in describing the nanocomposite systems. Rheological results are well predicted by applying the improved simulation algorithm using stepwise Wiener processes. Strong shear thinning and elongational strain hardening are predicted and compared with the experimental results of polyamide 6/organoclay nanocomposites. The full chain reptation model enables us to predict dynamic motion of the polymer chain segments and understand mechanisms for characteristic rheological behaviors.

Viscoelastic stress calculation in multi‐layer coextrusion dies: Die design and extensional viscosity effects on the onset of ‘wave’ interfacial instabilities

AbstractNon‐isothermal calculation of flow history‐dependent viscoelastic stresses in an annular coextrusion die is performed using the 8‐mode modified Leonov constitutive equation and the deformation rate field from finite element simulations. The study shows how the recently proposed total normal stress difference (TNSD) sign criterion, which quantifies the relative stretching of coextruded layers across an interface, can be used to investigate the role of the die design and elongational viscosities of coextruded materials from the point of view of interfacial instability. It is shown that both the die geometry and the elongational strain hardening have a crucial effect on the interfacial wave instability.