Liquid-solid phase separation control of polymer/nano-carbon interaction and blending for composite manufacturing

Abstract

Carbon nanotubes (CNTs) and other nanomaterials in general are promised to furnish polymer-based composites with striking enhancements in multifunctional properties. However, the promised composite performances are far below theoretical predictions. Prior work has shown that the limitations primarily come from the poor dispersion during processing and weak polymer-CNT interaction, which lead to defective structures in the composite. In this dissertation work, the fundamental issue of dispersing CNTs within a polymer matrix is addressed by studying a non-solvent induced liquid-solid phase separation process in polyacrylonitrile (PAN)/CNT composite systems. By employing phase separation, uniform dispersion of CNT in polymer was achieved and interacting PAN-CNT phase was separated from the rest of non-interacting bulk PAN polymer and CNT aggregates in the form of "paste". More specifically, polymer/CNT buckypapers were first formed through filtration. The composite film morphology gradually changed from a CNT-rich to a polymer-rich layer. Examination of the layered structure revealed CNTs with specific bundle size were uniformly dispersed within the polymer-rich layer due to preferred polymer-CNT interaction during phase separation. Experimental, theoretical, and molecular dynamics (MD) studies were performed to show the fundamental mechanism behind layer formation and to understand the specificity of preferential polymer-CNT interactions. A geometric dependence described by a 'cylinder-in-sphere' model was established between the critical CNT bundle size and polymer radius of gyration, which dictates preferential polymer-CNT interactions. This model represents the interactive relationship required to form a blended polymer-CNT phase in the system under the phase separation conditions used. Understanding the use of phase separation as well as this geometrical dependence between filler and polymer is important to resolve CNT dispersion issue. The harvested PAN-CNT blended phase was further incorporated into high-performance composite fibers with customized spinning dope preparation procedure. Detailed analyses regarding the fabricated composite fiber and film microstructures were performed to fundamentally understand the structure-process-property relationship. The contribution of the presented work provides new insights into fabrication of high-performance polymer/CNT composites.