Complex polymer topologies in blends: Shear and elongational rheology of linear/pom-pom polystyrene blends

The shear and elongational rheology of linear and pom-pom shaped polystyrene (PS) blends was investigated experimentally and modeled using constitutive models such as the Doi–Edwards and the molecular stress function (MSF) model. The pom-pom molecule is the simplest topology to combine shear thinning with strain hardening in elongational flow. A PS pom-pom with a self-entangled backbone (Mw,bb = 280 kg mol−1) and 22 entangled sidearms (Mw,a = 22 kg mol−1) at each star was blended with two linear PS with weight average molecular weights of Mw = 43 and 90 kg mol−1 and low polydispersities (Ð < 1.05). A semilogarithmic relationship between the weight content of the pom-pom, ϕpom-pom, and the zero-shear viscosity was found. Whereas the pure pom-pom has in uniaxial elongational flow at T = 160 °C strain hardening factors (SHFs) of SHF ≈100, similar values can be found in blends with up to ϕpom-pom = 50 wt. % in linear PS43k and PS90k. By blending only 2 wt. % pom-pom with linear PS43k, SHF = 10 can still be observed. Furthermore, above ϕpom-pom = 5–10 wt. %, the uniaxial extensional behavior can be well-described with the MSF model with a single parameter set for each linear PS matrix. The results show that the relationship between shear and elongational melt behavior, i.e., zero-shear viscosity and SHF, can be uncoupled and customized tuned by blending linear and pom-pom shaped polymers and very straightforwardly predicted theoretically. This underlines also the possible application of well-designed branched polymers as additives in recycling.