Cellulosic precursor, molecular structure

Figure 3.22. (a) Molecular structure of a cellulosic-type precursor (Byrne and Marsh, 1995). (b) A possible structure intermediate between cellulose and that of a resultant carbon (Byrne and Marsh, 1995). (c) Possible highly defective carbonaceous structures which, when interconnected, create microporosity (Byrne and Marsh, 1995). (d) Possible highly defective carbonaceous structures which, when interconnected, create microporosity, this model showing locations of adsorbate molecules (Byrne and Marsh, 1995). 

When Herman Mark first evaluated the crystal structure of rubber (with E. A. Hauser) and cellulose (with J. R. Katz) in 1924 and 1925, it was generally accepted that these materials were low molecular weight or monomeric. The unusual properties of these substances, now known to be related to high molecular weight, were then attributed to aggolomeration or "association" of the low molecular weight precursors. A common explanation for the associations were secondary forces such as Johannes Thiele s partial valences. 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

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