Association in nonionic cellulose ether solutions due to microcrystallites

Cellulose derivatives have a strong tendency to associate in solution and a t interfaces and this underlies many applications. Nonionic cellulose ether s are often of the clouding type, i.e. they phase separate at higher temper atures due to a worsening of the solvency conditions. The association due t o the presence of more or less strongly hydrophobic groups and due to solve ncy conditions in general leads to important rheological effects and gives the basis for uses as associative thickeners. This association is short-liv ed and reversible and has been extensively investigated. In this report we suggest, based on different recent observations for solutions of ethyl(hydr oxyethyl)cellulose (EHEC), that there are also very long-lived associations formed and we suggest that this can be referred to structures similar to t hose in the crystalline state of cellulose. This association due to 'microc rystallites' is strongly dependent on the distribution of substituents alon g the polymer chain. It is strongly promoted by 'blockiness', i.e. by an in homogeneous substitution pattern of hydroxyethyl and ethyl groups along the polymer chains in a way such that unsubstituted parts are separated by mor e 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 diffrac tion on a sample (very) concentrated in EHEC (20 wt.%). In agreement with t he 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 res erved.