Elastic Behavior of Nanoparticle Chain Aggregates (NCA): Effects of Substrate on NCA Stretching and First Observations by a High-Speed Camera

Chain aggregates of ∼7 nm titania particles exhibit elastic behavior when observed in the transmission electron microscope (TEM); 1,2 under tension produced by an expanding hole in the film, these nanoparticle chain aggregates (NCAs) stretch, and when the tension is relaxed, they contract. The film that coats the TEM grid on which the NCAs are deposited has an amorphous carbon side and a Formvar side. When the NCAs are deposited on the carbon side, the strained NCAs detach easily from the carbon layer and stretching does not occur. On the other hand, NCAs deposited on the Formvar side exhibit stretching and contraction probably because of the adhesion of the Formvar (poly(vinyl formal), mol wt 70 000−150 000) molecules to the ends of the particle chains. A similar phenomenon may explain the reinforcement of rubbery polymers by particulate fillers. Earlier observations of NCA behavior were made by using a conventional video camera with a speed of 30 frames/s (FPS). This was fast enough to follow the stretching but not the contraction process. An attempt was made to follow contraction using a high-speed camera (9000 FPS). The titania NCA stretched by about 10% and then contracted to 78% of the initial length. Inspection of the aggregate shape showed that chain segments folded after contraction. The length of the NCA oscillated in the direction of motion during stretching (∼2 kHz) and contraction (∼1 kHz).