Looking for precursors in flow induced crystallization: A study combining rheology with simultaneous SAXS/WAXD
L. Balzano, Sanjay Rastogi, Gerrit Peters, and A. E. Terry
Eindhoven University of Technology, Eindhoven, The Netherlands

The mechanism leading to nucleation in polymer melts, subjected to flow, is still not completely understood although many studies have been performed in this area. The aim of this work is to investigate the influence of flow and thermal history in promoting the crystallization of polymer melts, by varying systematically the material and the flow parameters, using a homogeneous shear flow. We have employed an innovative (custom-made) rheometer specially developed to combine rheology with X-rays. Step shear experiments are performed to follow the early stages of crystallization. The possibility to follow structural developments (X-rays) simultaneously with the build up of the modulus in the supercooled melt (rheology) allows us to establish the relationship between the melt structure and rheological properties. Preliminary results suggest that rheology is more sensitive than X-rays in detecting early stages of crystallization, even for measurements at a third generation source like the ESRF, Grenoble. Furthermore, there is evidence of SAXS detecting ordered domains before WAXD. On shearing polypropylene samples (MW=365.000, MWD=5.4, Tm=165ºC), for the range of shear rates studied, even below the melting point, it is not possible to recognize any anisotropy in the ordered domains although the build up of the modulus occurred much earlier than in the absence of shear. To induce longer relaxation times higher molecular weight material has been homogeneously mixed with the existing sample. In these mixtures the development of oriented structures is observed. Our observations are that even in the most favorable experimental conditions anisotropic ordered domains are detected only by SAXS, whereas no structural anisotropy is detected by WAXD.