Abstract
A series of degradable block copolymers, poly(acrylonitrile)-b-poly(ε-caprolactone) (PAN-PCL), have been synthesized by sequential living polymerization in this study. Well-defined, microphase-separated PAN-PCL microdomains were efficiently achieved in bulk by using appropriate solvents for casting. The microphase-separated lamellar samples were then used as templates to produce mesoporous carbon at which large amounts of porous texture in carbonized PAN matrix were formed after the degradation of PCL due to randomly oriented lamellar texture (namely, interconnection of PCL microdomains). Mesoporous carbon materials might be prepared as demonstrated by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and small-angle X-ray scattering. In contrast to the thermal stability of the carbonization of PAN homopolymers, notably, the carbonization procedure can be achieved in PAN-PCL BCPs system regardless of the stretching process (i.e., an essential process to improve the thermal stability of PAN during carbonization). We speculate that this unique feature for the carbonization of PAN-based copolymers might be attributed to the stretching chain conformations of PAN molecules under microphase-separated structures. To further examine the methods for induction of chain stretching under nanoscale dimension, carbonization approach without chemical linkage (namely PAN homopolymers under confinement) were studied. PAN homopolymers were filled into inorganic hundred-nanometer-sized templates, anodized alumina oxides (AAOs), via direct capillary force. With a specific pore-filling process, a solvent-annealing process, graphite-like nanotubes can be fabricated from PAN pore-filling AAO templates. Thus, AAO templates acted as scaffolds supporting the formation of carbon nanotubes through carbonization, and were subsequently etched away, leaving behind the carbon nanotubes. To achieve appropriate molecular packing for carbonization, the anisotropic molecular orientation of PAN chains was induced by solvent annealing due to the capillary-filling process. Consequently, hundred-nanometer-sized carbon nanotubes formed after carbonation, as demonstrated by TEM and FESEM observations. Importantly, the carbon nanotubes with a graphite-like structure and high crystallinity can be fabricated, as demonstrated by selected area electron diffraction results. Notably, these carbon nanotubes cannot be fabricated using the conventional solution-wetting process. The formation of a core-shell cylinder texture was also demonstrated via multiple filling processes. Herein, facile methods for nanoscale PAN carbonization were demonstrated. In a microphase-separed system, the stretching chain conformation for carbonization could be achieved by the immiscibility of each block. For homopolymers, the stretching chain conformation could be formed by capillary force and preserved by controlling the evaporation dynamically. These approaches for nanoscale PAN carbonization can provide a convenient and promising way to fabricate carbon materials with various nanostructures, well-controlled sizes and high crystallinity.