Silicon-based organic polymers, known as silicones, give way for interesting technologies such as wearable electronics, highly compatible biomedical devices, and stretchable solar cells to become realities. In some architectures of these devices, luminescent materials such as semiconductor nanocrystals are dispersed within the polymers for optical absorption and emission effects – but creating these composites has implications for the mechanical behavior of the nanocrystal/polymer systems. Focusing specifically on polydimethylsiloxane (PDMS), some groups have observed that composites of luminescent silicon nanocrystals (SiNCs) and PDMS exhibit altered elastomeric mechanical properties due to reduced cross-linking sites available during PDMS curing. Here, we present our work on modeling the mechanical properties of the SiNCs/PDMS composites based upon the length of functionalizing ligands attached to the SiNCs, as established via experimental measurements on these composites.
We synthesized hydrogen-terminated SiNCs via an all-gas phase, non-thermal plasma reactor with silane, argon, and hydrogen as reactants. We then surface-functionalized the SiNCs via thermal hydrosilylation within an air-free environment using a Schlenk line. We chose 1-dodecene and 1-octadecene as functionalizing ligands to probe the effects of alkyl chain length on the mechanical properties of the resulting PDMS/nanocrystal composites. These functionalized nanocrystals were suspended and dispersed within the PDMS pre-polymer solution, cured, and cooled. The optical properties of the composites were characterized using photoluminescence spectroscopy and scanning confocal fluorescence spectroscopy combined with histological techniques to understand the distribution of the SiNCs within the PDMS matrix. We then evaluated the mechanical behavior of the composites using uniaxial tensile testing to determine the elastic modulus of the composite systems. Next, we employed a homogenization model to understand how altering the surface of silicon nanocrystals impacts the mechanical properties of the composite. The results of this work have implications for predicting the behavior of polymer/NC systems, pointing towards creating novel stretchable and flexible devices and sensors.