Measuring and Simulating Morphology Gradients in Injection-Molded Talc-Reinforced Isotactic Polypropylene Ha ˚ vard Granlund, 1 Erik Andreassen, 2 Eirik Torbjïrn Bakken Skjïnsfjell, 1 Kristin Hïydalsvik, 1 Ana Diaz, 3 Dag Werner Breiby 1 1 Department of Physics, Norwegian University of Science and Technology (NTNU), Hïgskoleringen 5, 7491 Trondheim, Norway 2 Sintef Materials and Chemistry, Pb. 124 Blindern, 0314 Oslo, Norway 3 Paul Scherrer Institut, Villigen PSI CH-5232, Switzerland Correspondence to: D. W. Breiby (E - mail: Dag.Breiby@ntnu.no) Received 9 April 2014; accepted 6 June 2014; published online 30 June 2014 DOI: 10.1002/polb.23538 ABSTRACT: Injection molded polymer parts are known to exhibit structural gradients of crystallinity, crystallite phases and crystallite orientations. The structural variations depend on the geometry, the material properties, and the processing con- ditions, and affect the mechanical properties of the molded part. We explore the use of raster-scanning small- and wide- angle X-ray scattering (SAXS, WAXS) for mapping the micro- structure in dogbone specimens of an isotactic polypropylene (PP) homopolymer and a talc-reinforced isotactic PP com- pound. The specimens were injection molded with different mold temperatures and injection speeds, and the mapping approach revealed systematic structural heterogeneities and asymmetries. Accompanying numerical simulations of the injection molding process yielded predictions of the flow pat- tern, including the shear rate distribution and the resulting ori- entation of the flake-shaped talc particles. We found a clear correspondence between the experimentally observed data and the simulations, in particular regarding the asymmetry of the orientation distributions relative to the center of the dog- bone cross section, caused by asymmetric flow through the entrance of the mold. Furthermore, the shear rate distribution correlated with the occurrence of a- and b-phases. Subtle dif- ferences in the crystallized structures along the long axis of the dogbones suggest an explanation to the observation that the specimens studied always tended to break at the same position in tensile tests. The results clearly demonstrate the potential of mapping experiments which combine lateral resolution on macroscopic length scales with the molecular-scale resolution from scattering. V C 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1157–1167 KEYWORDS: injection molding; isotactic polypropylene; scan- ning SAXS; scanning WAXS; shear; temperature INTRODUCTION Polypropylene (PP) is a commodity polymer with a wide range of applications due to its easily tailored mechanical properties and good overall performance-cost ratio. 1–4 The properties of PP can be modified by, for exam- ple, changing the molecular weight distribution, by copoly- merization, and by inorganic reinforcement. 1,3,5–13 In talc- filled PP-based compounds for injection molding, the added talc particles reduce the thermal expansion, the cycle time, and the shrinkage and warpage, and increases the modulus and strength. 2,6 The flake-shaped talc particles get oriented during the molding process, 14–17 and act as nucleators for the crystallization of PP, thus modifying the orientation dis- tribution of its crystallites. 6,7,18–21 Cross sections of injection-molded PP specimens are known to exhibit several structural layers. 10,18,22,23 Near the mold surface an oriented “skin layer” is found. The core, crystal- lized after the flow has stopped, has an a-phase spherulitic structure. The spherulite size increases toward the specimen center, 24,25 and the unit cell b-axis is preferentially oriented perpendicular to the main flow direction (MD). 6,26 Talc is a nucleating agent for the PP a-phase, resulting in epitaxial growth of PP crystallites on the flake-shaped talc particles. 19 Another nucleation mechanism is the flow-induced crystalli- zation during molding, in which some PP chains get extended parallel to the flow direction, forming ordered bun- dles of c-oriented PP (shishes), serving as nucleation centers for perpendicularly growing PP lamellae (kebabs). 27 As the resulting so-called shish–kebab structure is flow-induced, its presence naturally depends on the part geometry and the molding conditions, 27–29 and it is often observed in regions with comparably high shear rate during molding, in layers Additional Supporting Information may be found in the online version of this article. V C 2014 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52, 1157–1167 1157 JOURNAL OF POLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER