Association in nonionic cellulose ether solutions due to microcrystallites

Cellulose derivatives have a strong tendency to associate in solution and at interfaces and this underlies many applications. Nonionic cellulose ethers are often of the clouding type, i.e. they phase separate at higher temperatures due to a worsening of the solvency conditions. The association due to the presence of more or less strongly hydrophobic groups and due to solvency conditions in general leads to important rheological effects and gives the basis for uses as associative thickeners. This association is short-lived and reversible and has been extensively investigated. In this report we suggest, based on different recent observations for solutions of ethyl(hydroxyethyl)cellulose (EHEC), that there are also very long-lived associations formed and we suggest that this can be referred to structures similar to those in the crystalline state of cellulose. This association due to 'microcrystallites' is strongly dependent on the distribution of substituents along the polymer chain. It is strongly promoted by 'blockiness', i.e. by an inhomogeneous substitution pattern of hydroxyethyl and ethyl groups along the polymer chains in a way such that unsubstituted parts are separated by more substituted regions. We suggest that the unsubstituted parts of the EHEC chains are likely to be strongly associated for the same reasons as native cellulose, which is insoluble in water, is arranged in crystalline arrays. The presence of crystallinity has been indicated by measuring X-ray diffraction on a sample (very) concentrated in EHEC (20 wt.%). In agreement with the packing distance in native cellulose, a diffuse ring corresponding to 7.0-13.5 Angstrom was observed. (C) 1999 Elsevier Science B.V. All rights reserved.