Design of hybrid nanoparticles for electrophoretic ink displays

E-Paper is a reflective bistable display technology, recognized for his paper-like readability and extremely low power consumption. The technology relies on electrophoresis: when an electric field is applied, black and white pigment particles, oppositely charged, move between two planar electrodes, thus generating visual contrast.¹ Although this technology is now welle-established, current e-readers, such as those offered by Kobo©, Kindle© or PocketBook©, still provide only monochromatic screens. As a result, research efforts is now focused on integrating color into electrophoretic display (EPD) devices.^{2, 3}

The objective of this PhD thesis is to develop polychromatic electrophoretic inks incorporating both white and colored pigments. To ensure pigment stability and a fast response under the influence of an electric field, we propose to develop a robust pigment encapsulation method in organic medium. This approach will allow for precise control over the density and surface properties of the particles, as well as optimal pigment dispersion in organic solution, through the use of functional polymers as stabilizers. These polymers will be synthesized via controlled radical polymerization (e.g., RAFT or NMP) and will contain acidic or basic groups, thereby promoting charge formation in apolar media.

Initially, model latex particles, bearing acidic or basic groups on their surface, will be synthesized by polymerization-induced self-assembly (PISA) of block copolymers in an organic medium.⁴ This step will allow for the study of surface charge formation mechanisms in the presence of charge-control agents (CCAs), which facilitate charge dissociation by stabilizing the counterions into reverse micelles.⁵ The same polymers will then be used to encapsulate the pigment particles. This approach will first be applied to titanium dioxide (TiO₂) pigments, before being extended to hybrid pigments composed of TiO₂ nanoparticles and colored organic components (cyan, magenta, or yellow dyes—the subtractive colors of the CMYK model). These hybrid pigments will be obtained through an innovative co-atomization and drying process recently developed by one of the project partner.

The behavior of the inks will be evaluated through electrophoretic measurements, complemented by optical contrast tests conducted on a demonstrator.

The proposed strategy aims to overcome the limitations of current encapsulation methods by providing better control over the shape, colloidal properties, and surface properties of pigment particles. A deep understanding of the mechanism of charge formation in apolar media is crucial for the development of high-performance electrophoretic display devices. The knowledge gained from this research should enable the creation of bicolor (and ultimately multi-color) electrophoretic inks offering high contrast, fast response times, and improved stability in final devices.

References

1.    Ota, I.; Ohnishi, J.; Yoshiyama, M. Electrophoretic image display (EPID) panel. Proceedings of the IEEE 1973, 61, 832.

2.  Heikenfeld, J.; Drzaic, P.; Yeo, J.-S.; Koch, T. Review Paper: A critical review of the present and future prospects for electronic paper. J. Soc. Inf. Display 2011, 19, 129.

3.  Fang, Y.; Wang, J.; Li, L.; Liu, Z.; Jin, P.; Tang, C. Preparation of chromatic composite hollow nanoparticles containing mixed metal oxides for full-color electrophoretic displays. J. Mater. Chem. 2016, 4, 5664.

4.  Gyorgy, C.; Armes, S. P. Recent Advances in Polymerization-Induced Self-Assembly (PISA) Syntheses in Non-Polar Media. Angew. Chem. Int. Ed. 2023, e202308372.

5.  Gacek, M. M.; Berg, J. C. Investigation of surfactant mediated acid-base charging of mineral oxide particles dispersed in apolar systems. Langmuir 2012, 28, 17841.

The thesis, which will cover both fundamental and applied aspects, will be carried out at the CP2M laboratory, with external collaborations with Manuel Gaudon (ICMCB, University of Bordeaux) and Cyril Brochon (LCPO, University of Bordeaux), who will provide their expertise in the synthesis of inorganic colored pigment and electrophoretic ink display systems, respectively. The PhD student will be immersed in a highly multidisciplinary environment encompassing polymer synthesis, pigment encapsulation, analysis of their physicochemical properties and testing on a prototype. Short stays at partner laboratories are also planned, providing the PhD student with the opportunity to develop complementary skills and fully benefit from this collaborative environment.

Supervision. The selected candidate will be co-supervised by Elodie Bourgeat-Lami (Research Director, CNRS) and Fabrice Brunel (Associate Professor, UCBL), both expert in controlled radical polymerization, hybrid materials synthesis, and the physicochemistry of polymers and dispersed systems. . 

The candidate must hold a Master’s degree in Chemistry and/or Materials Science (or equivalent). He/she should possess excellent communication skills, be rigorous and demonstrate high motivation, curiosity and teamwork abilities. A good knowledge of polymer chemistry (and associated techniques), as well as experience in colloidal chemistry and polymerizations in dispersed media, will be considered an advantage.