Tuesday, April 16, 2013

Solvent Spun Rayon: Prof Turbak's view of the early history (Part 2)

The second extract of Prof. Alban Turbak's chapter on dissolution of cellulose:

Other Cellulose Solvent Systems
In each and every instance of dissolution of cellulose ... thermodynamic factors can be delineated and shown to have a significant and controlling effect on the success of the dissolution process.  In each case the solvent system swells the cellulose and then modifies the molecule either by derivatizing, proton weakening or removal, or by  complexing, to give  a new intermediate specie that can more readily react with the liquid solvent system.

Thus, all cellulose solvent systems can be categorized under 4 mechanisms:
a)    Cellulose as an acid
b)    Cellulose as a base
c)    Cellulose unstable derivative
d)    Cellulose stable derivatives
This chapter will review selected solvent systems involving  areas a, b, and c. Stable derivatives (d) will not be discussed.  All compounds can be considered to be acids or bases relative to some other stronger specie. For example;

       (A) is an Acid      relative to       Base  (B)
       HCl, ­­­H2SO4                              Carboxylic Acids
       Phosphoric                              Acetic, Cellulose
       Phenol                                     NaOH, Sodium Carbonate
       Alcohol                                   NaOH, sodium Metal
       Cellulose                                NMMO, N2N4
       Li(+)/DMAc                              Cellulose
      Cellulose-OH                           Unsolvated  anhydrous Cl(-)
      Hydrocarbons,(isobutane)           NaNH2

It may seem odd to realize that cellulose can act either as an acid or a base, yet this is exactly the case; especially in anhydrous or limited aqueous systems .

Water is an excellent swelling liquid for cellulose and opens millions of internal pores  in the cellulose  microfibrillar structure .  However, H2O is itself both an acid or a base depending on the other molecular specie. Therefore, water normally reacts more rapidly than the cellulose with incoming molecules and this totally disrupts the effectiveness of the incoming potential dissolving moiety.

See Also  "Solvent Spun Rayon" and "Regenerated Cellulose Fibres"

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