Mapping of Lamellar Ordering in Injection-Molded Polypropylene
H. Granlund1, J.B. Fløystad1, E.T. Bakken1, P.E. Vullum2, E. Andreassen2, D.W. Breiby1
1 Department of Physics, Norwegian University of Science and Technology
2SINTEF Materials and Chemistry
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Detailed mapping of gradients in nanoscale structures in macroscopic, weakly scattering and opticaly opaque samples, such as certain polymers, has in the past been very difficult and essentially only been viable using synchrotron radiation. However, with the advances of new laboratory-based X-ray sources, optics and zero-electronic-noise detectors in the last decade, this has now become feasible even in home laboratories. Here we shall present our work on injection-molded talc-filled isotactic polypropylene (PP) [1], a semi-crystalline polymer with a wide range of applications, which has been done using state-of-the-art equipment in our home laboratory.
Scanning wide-angle X-ray scattering (sWAXS) [2] measurements have been used to study a cross-section of an injection-molded PP sample. Employing automated analysis routines, maps of molecular ordering gradients within the sample were extracted, and, as expected from literature [3], the analysis shows that the PP b-axis (perpendicular to the polymer backbone) aligns with the talc c-axis, and exhibits a preferred orientation within the cross-section (figure 1a). Highly symmetrically, these axes are oriented perpendicular to the mold walls, which we interpret as a preferred directional growth induced by a combination of temperature gradients present during solidification and flow induced orientation of the talc particles, which act as nucleation centres for PP [4]. Looking at the orientational distribution width (figure 1b), we see that the degree of orientation of the crystalline lamellae remains fairly constant over several millimeters from the surface towards the center. The observed separation of the sample volume into differently ordered domains by well-defined regions of confusion, could affect the mechanical properties of the sample. Detailed knowledge of these effects can influence how these materials are processed in the future, ultimately improving their quality, and we thus see a great potential in further developing this characterization technique, also for high-resolution studies at synchrotron sources.
Figure 1: a) Orientation map of the polymer b-axis in a cross-section with its normal
parallel to the flow direction. b) The orientational distribution width (FWHM, in degrees)
of the molecules.
References
[1]: H. Granlund, E.T. Bakken, J.B. Fløystad, M. Esmaeili, P.E. Vullum, E. Andreassen, D.W.
Breiby In preparation
[2]: O. Bunk et al. 2009 New Journal of Physics 11 123016
[3]: P-W. Zhu, G. Edward 2008 Journal of Material Science 43 6459-6467
[4]: M Fujiyama, T. Wakino 1991 Journal of Applied Polymer Science 42 9-20
[5]: Y. Obata et al. 2001 Polymer Engineering and Science 2001 41 408-416