A method to obtain the quantitative orientation of semicrystalline structures in polymers by atomic force microscopy

Abstract

Molecular orientation can determine the final properties in polymer parts during processing: in optoelectronic devices, the emission efficiency is strongly dependent on the orientation of the emitter materials; mechanical performances in polymer parts depend on the orientation and dimension of crystalline structures. A simpler and faster method to obtain the quantitative orientation of crystalline structures, based on atomic force microscopy, is introduced as a powerful alternative to the techniques mentioned above. This method is based on the acquisition of topographical maps along with the sample thickness and applying the directionality analysis to each map to obtain the distribution of orientation on the map. Such a distribution was analyzed following two approaches: the first one is based on Herman’s analysis; it is quite similar to the one adopted for calculating the Herman’s factor from the wide-angle X-ray scattering. The second one is simpler; it is based on the standard deviation of the distribution. Both approaches allowed the determination of an orientation parameter: the orientation parameter was close to 1 in the regions where a high number of oriented fibrils were found, vice versa, the orientation parameter was close to zero where spherulites were found. The orientation parameter was found highly consistent with Herman’s factor for injection molded samples obtained with different mold temperatures, thus with different distributions of orientation and morphology.

Keywords

• material testing
• polypropylene
• orientation
• atomic force microscopy
• directionality