Two-step reprecipitation method with size and zeta potential controllability for synthesizing semiconducting polymer nanoparticles

A two-step reprecipitation method for synthesizing semiconducting polymer nanoparticles (NPs) has been developed that overcomes a problem with conventional reprecipitation: the difficulty in controlling the size of smaller particles (<50 nm). First, uniform droplets are prepared by mixing polymer solution with de-ionized (DI) water, and then particles are formed by adding poor solvent. The number of polymers confined to a single droplet is regulated, and the NP size is tuned by adjusting the initial polymer concentration. The synthesis yield and size reproducibility are higher than with conventional reprecipitation. Moreover, a large negative zeta potential (−30 mV) can be obtained, which enables the agglomeration to be effectively controlled without using surfactant. The large zeta potential is apparently due to electrostatic charges caused by friction between the droplets and the DI water during the strong mixing. This method was used to synthesize NPs of poly(3-hexylthiophene) (P3HT) with sizes ranging from 10 to 65 nm. Investigation of the effect of the particle size on various physical properties, including film morphology, absorbance, photoluminescence, and crystallinity, showed that these properties were drastically different at around 50 nm and that they were similar below 30 nm. These results demonstrate the importance of precisely controlling NP size.