The research on mechanical properties of small size polymer materials is of great importance, due to the rapidly improvement of the fabrication of small devices. However, the study on the material properties of these small size polymer materials is still limited. The material properties of these small size ones could be of great difference compared with the bulk due to the size effect. Without a clear knowledge of the change of material properties in the materials, inaccurate designs of the devices will appear to influence the reliability.

In this work, Parylene-C thin film was chosen as an example of semi-crystalline polymers to study the size dependent properties. The Parylene-C thin film microbridges with different microbridge thicknesses, widths and lengths were fabricated w...[ Read more ]

The research on mechanical properties of small size polymer materials is of great importance, due to the rapidly improvement of the fabrication of small devices. However, the study on the material properties of these small size polymer materials is still limited. The material properties of these small size ones could be of great difference compared with the bulk due to the size effect. Without a clear knowledge of the change of material properties in the materials, inaccurate designs of the devices will appear to influence the reliability.

In this work, Parylene-C thin film was chosen as an example of semi-crystalline polymers to study the size dependent properties. The Parylene-C thin film microbridges with different microbridge thicknesses, widths and lengths were fabricated with MEMS techniques. By implementing the microbridge method, Young’s modulus and residual stress of Parylene-C thin films could be simultaneously determined and the size effect was investigated. Our work experimentally verified the extrinsic sample dimension such as microbridge width and the microbridge thickness significantly contributing to the decrease of the Young's modulus. Also, the glass transition temperatures measured by both microbridge tests and coefficient of thermal expansion tests revealed the dimension dependent behavior. This change in glass transition temperature causes the Young’s modulus-temperature curve shifting to influence the Young’s modulus at room temperature. The degree of crystallinity of the Parylene-C thin films also exhibited a size dependent behavior. The existence of free surfaces with less dense microstructure is the main reason that causes these size dependent behaviors. The surface layers, which have a smaller Young’s modulus, lower glass transition temperature, less crystallite regions, reduce the mechanical properties, the glass transition temperature and the crystallinity of Parylene-C microbridges. These surface layers were verified their existence by testing the distribution of the degree of crystallinity along the thickness direction, modulus mapping of the thin film cross section and HRTEM observation of the thin film cross section. On the other hand, by electrospinning method, the polyethylene terephthalate (PET) nanofibers were fabricated to the microbridges after bonding on a silicon substrate. The results from microbridge tests on PET fibers show that, the Young’s modulus has the size dependent behavior as well as the glass transition temperature in an opposite tendency. The Young’s modulus versus temperature curves explain these two tendency: the reduction of the fiber diameter causes the Young’s modulus versus temperature curve changes its shape. So that the statement “the smaller the stronger” may not be always true especially in high temperature. The increase of the Young’s modulus can be explained by the alignment of the molecule chain inside the fiber.

This study is expected to contribute to the research on the size dependent mechanical and physical properties of semi-crystalline polymers and has both the scientific contribution and practical significance at the same time.