On the formation of potential polymer-nanotube blends by liquid-solid phase separation

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In this work, the fundamental issue of dispersing carbon nanotubes (CNTs) within a polymer matrix is addressed by studying a non-solvent induced liquid-solid phase separation process in polyacrylonitrile/CNT composite systems. To visualize the effect of phase separation, hybrid polymer/CNT buckypapers were formed through filtration. The hybrid film morphology gradually changes from a CNT-rich to a polymer-rich layer. Examination of the layered structure reveals 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 studies were performed to show the fundamental mechanism behind layer formation in the composites and to understand the specificity of preferential polymer-CNT interactions. To this end, a geometric dependence described by a ‘cylinder-in-sphere’ model was established and shows a link 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 pinpoint nano-filler dispersion limits. Identifying these limits is critical toward the processing of superior polymer-based composites which fully utilizes the nano-filler reinforcement.