SIMULATIONS OF POLYMER SPHERULITES GROWTH AND SMALL-ANGLE X-RAY-SCATTERING SPECTRA

Mechanical properties of semi-crystalline polymers are closely related to material microstructure. The latter is usually made of spherulites , within which are radially disposed crystalline lamellae, separated b y amorphous regions. A better description of this structure is necessa ry to predict more accuretely mechanical properties. Various physical and chemical characterization techniques can be used, such as scanning electronic microscopy or small-angle X-ray scattering (SAXS). Microsc opy will give a spherulite picture, whereas diffusion technique will b e more specific about the average organization of crystalline lamellae from the interference peak observed on spectrum. In order to relate i nformations given by these two techniques, a modelling approach was ad opted, i.e. writing of a spherulite growth simulation software, the ba se unit being the lamellae, and a SAXS spectra computing program, base d on given structural models. The SAXS computing program has been adap ted from existing IFP (Institut Francais du Petrole) softwares, allowi ng computing of wide-angle X-ray scattering spectra. Several spherulit e growth parameters have been adjusted, so as to obtain a good agreeme nt with scanning electronic micrographs. Also, the simultaneous growth of several spherulitic entities has been simulated, in both instantan eous and homogeneous nucleation cases. The boundaries between spheruli tes at the end of the growth are very similar to those observed in opt ical microscopy. The relative orientations and positions of nuclei are of great importance for impingement, interlocking and organization of spherulites, and therefore for mechanical properties. The diffusion s pectrum of simulated spherulitic structures did not present an interfe rence peak. Some spherulite branches are in fact composed of a set of crystalline lamellae, which explains the presence of the interference peak. Intensity of the latter strongly depends on lamellae stacking di sorder. Several disorder parameters have been investigated : number of lamellae in a stack, inter-lamellar distance variations around an ave rage value, and parallelism of lamellae in a stack. The systematic com puting with all combined levels of these parameters allowed the evalua tion of the effect of each parameter on the interference peak intensit y. Parallelism of lamellae in the stack is by far the most important p arameter. It can be derived that SAXS spectra analysis based on the in terference peak position gives information on the most organized zones in the material solely and therefore does not necessarily reflect the entire polymer structure.