Original Article Influence of molecular structure on the foam- ability of polypropylene: Linear and extensional rheological fingerprint Ebrahim Bahreini 1 , Seyed Foad Aghamiri 1 , Manfred Wilhelm 2 and Mahdi Abbasi 1,2 Abstract The foaming structure and rheological properties of four different isotactic homo- polypropylenes with various molecular weights and an isotactic long chain branched polypropylene were investigated to find a suitable rheological fingerprint for PP foams. The molecular weight distribution and thermal properties were measured using GPC-MALLS and differential scanning calorimetry, respectively. Small amplitude oscilla- tory shear data and uniaxial extensional experiments were analyzed using the frame- works of van Gurp-Palmen plot (d vs. jG*j) and the molecular stress function model, respectively. These analyses were used to find a correlation between the molecular structure, rheological properties and foaming structures of linear and long chain branch- ing polypropylenes. Two linear viscoelastic characteristics, jG*j at d ¼ 60  and j*j at o ¼ 5 rad/s were used as criteria for foamability of these polymers, where decreasing of both parameters by increasing the long chain branching content results in smaller cell size and higher cell density. The molecular stress function model was able to quantify the strain hardening properties of long chain branching blends using small amplitude oscillatory shear data and two nonlinear material parameters, 1    2.2 and 1  f 2 max  600, where the minimum and maximum values of these parameters belong to the linear and long chain branched polypropylene, respectively. Increasing the long chain branched polypropylene content of the PP blends increased strain hardening, and therefore improved the foaming characteristics significantly by suppressing the 1 Chemical Engineering Department, Faculty of Engineering, University of Isfahan, Iran 2 Institute of Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Germany Corresponding author: Mahdi Abbasi, Karlsruher Institut fur Technologie, Engesserstr. 18, 76131 Karlsruhe, Karlsruhe, Baden- Wu ¨rttemberg 76131, Germany. Email: mahdi_abbasi2002@yahoo.com Journal of Cellular Plastics 0(0) 1–29 ß The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021955X17700097 journals.sagepub.com/home/cel