Abstract
In this study, we present a new approach to well-organized nanostructured carbon materials, based on the carbonization of block copolymers containing poly(acrylonitrile) (PAN). The distinguishing feature of the method presented here is that it relies on the use of block copolymers, PAN-PCL, in which the carbonization precursor (PAN) is pre-organized into a well-defined nanostructure through self-assembly, induced by the presence of a immiscible block, poly(ε-caprolactone) (PCL), which could be removed by various ways (i.e. hydrolysis). The copolymer was prepared by Atom Transfer Radical Polymerization (ATRP). The first step was the preparation of ε-caprolactone based macroinitiators by ring-opening polymerization and then ATRP was used for PAN-b-PCL. The nanostructured precursor phase serves then as a template of the target carbon material, which is obtained by pyrolysis accompanied by volatilization of sacrificial phase. The key to success of such a strategy is the survival of the original nanostructure through pyrolysis. This might appear extremely challenging, since a high extent of graphitization typically requires heating the material to temperatures as high as 800oC and above. Herein we demonstrate that this can be accomplished utilizing the process of thermal stabilization used in the manufacturing of carbon fibers. During this step the material is heated to 260 oC in the presence of air, which leads to conversion of PAN into a cross-linked, ladder polymer and to stabilization of the nanostructure. In order to identify the microphase-separated structure of PAN-b-PCL and the reaction mechanism of carbonization procedure, scanning probe microscope (SPM) and Fourier transform infrared (FTIR) are used, respectively. The detail carbonization procedure condition would be identified by Thermal Gravimetric Analysis (TGA) and FTIR. Interestingly, the final remaining weight is relative to the stretching ratio and the degree of orientation of PAN molecules in microphase-separated PAN-PCL from TGA results. Comparison of without pre-stretched PAN homopolymer which is random-coil, the microphase-separated PAN possesses much more orientated molecules and lower degree of chain scission. Therefore, microphase-separated PAN molecules have better stability and more remaining weight of carbon nanofiber during carbonization procedure than PAN homopolymers. Furthermore we also prove that this system has various ways to remove the PCL crystal domain high selectively. Since PCL is a degradable polymer, we could remove PCL via hydrolytic degradation by NaOH/Methanol mixed solution or thermal degradation at high temperature. PCL domain could be hydrolytic degraded by NaOH(0.5M):CH3OH=3:2 (V/V) solutions at above PCL melting point temperature, or thermal degradation with pyrolysis of PAN. For different procedure requirements, there are different ways to be chosen, i.e. when manufacturing carbon nanoarray structure, in order to prevent the microstructure disappearing at higher treatment temperature, hydrolytic degradation of PCL domain is necessary, otherwise when manufacturing a mesoporous carbon template, thermal degradation of PCL domain after stabilization could improve the stability of template.