A systematic study of the process-structure relationship for building polymer interphase regions in carbon nanotube composites.

摘要

As the world's demands for new high-performance materials continuously grow, reinforcing polymer materials with carbon nanotubes (CNT) to produce lightweight composites with high properties has been pursued since their discovery. However, to date composites with the anticipated superior properties have not been achieved yet. This is mainly due to the lack of fundamental understanding regarding processing these materials as well as manipulating the nano-scale assemblies, which would dictate the desired macroscopic performance.;This dissertation work focuses on understanding the development of interfacial zones in polymer/CNT composites, which is necessary for producing mechanically outstanding composite materials. Highly crystalline and confined interphase structure plays a significant role for (i) interfacial stress transfer (affecting the overall composite properties), and (ii) structural evolution during heat treatment (enabling low-temperature graphitization of polyacrylonitrile (PAN)). For this reason, control of the structural development in the interphase regions during composite processing is a key to success.;Good nanotube dispersion in terms of both bundle exfoliation and length preservation is a prerequisite for polymer-CNT interfacial interactions. Nano-scale assembly via polymer-CNT epitaxy has enabled extended-chain templating of flexible backbone polymers (i.e., PAN and polyvinyl alcohol (PVA)), which is the most desired conformation needed for high-modulus high-strength fibers. Slight variations in the crystallization parameters result in distinctly different morphologies of the same polymer formed on CNT at the interphase, including folded-chain crystals and amorphous globules. Processing methods for polymer/CNT materials without precise controls will lead to heterogeneity in the interphase morphologies, if any, which can result in low composite properties. Therefore, fundamental understanding of polymer interphase growth is conducted in the present study to resolve these issues. Toward this effort, the quiescent homogeneous crystallization as well as the heterogeneous crystallization in dilute PAN/CNT solutions/dispersions under shear is investigated. The crystallization temperature, degree of undercooling, polymer/CNT concentration, flow field, and solvent removal mechanism are tailored for the optimal interphase growth. Going beyond interfacial crystallization process-structure studies, bulk-free PAN/CNT composites (i.e., consisting only of interphase PAN and CNT) are fabricated by incorporating the crystallization procedures into the processing. These materials are subsequently characterized for the structure and morphology, and assessed for the mechanical and thermal properties to derive the structure-property relationship.;Manipulating nano-structures does not only enhance the mechanical properties of the composites, but also largely affects the structural evolutions during thermal treatments. Low-temperature (up to 1100 °C) graphitization of the interphase PAN has been successfully performed for the bulk-free PAN/CNT composite precursors, which are intended to be used as the next-generation carbon precursor. At a much lower temperature (~1500 °C lower than that for commercial graphitization), graphite was formed predominantly in the structure. This is attributed to the novel-design of the interfacial structure in the precursor. The carbonized structure is analyzed using electron microscopy and spectroscopy. It is found that different types of CNT interact to various extents with the same PAN polymer during heat treatments. Therefore, coupling both the precursor processing and heat treatment is necessary to achieve the optimal carbon structure in the final composites.