Structures of cellulose in solution

Cellulose and their derivatives only rarely form molecularly dispersed solutions. Colloidal aggregates of yet not fully explored supramolecular structure remain preserved even at the highest dilution. The present contribution is concerned with the determination of the number of aggregated chains per colloid particle. In some cases the aggregation number could be directly determined but for the non-ionic, partial substituted cellulose-ethers the degree of polymerization of the non-aggregated cellulose was not sufficiently well known. A detailed analysis of the angular dependencies of scattered light revealed for the large aggregates a star-shaped structure. The smaller ones were better described by a worm-like structure. These observations led to the suggestion of a fringed micelle model where several chains are laterally assembled in a rigid stem leaving at both ends coronas of f/2 dangling chains. Recent SANS experiments support this model. The estimated chain length of a single strand was found with DPw = (4–20)×103 much too high and led to the conclusion that 4–20 chains must be co-linearly (staggered) associated to form one strand. The obtained overall aggregation numbers are close to those found for cellulose in NMMNO and vary from 10 to 800. The similarity of the supramolecular structures in aggregation numbers and dimensions is interpreted as reminiscent of the semi-crystalline structure of the parental native cellulose fibers. Much lower aggregation numbers were found for cellulose xanthogenates (viscose) in NaOH solution and for cellulose 2.5-acetate in acetone than for cellulose in NMMNO.