Effects of the thermal history and concentration on the aggregation of Erwinia gum in an aqueous solution

The aggregation of Erwinia (E) gum in a 0.2 M NaCl aqueous solution was investigated by multi-angle laser light scattering and gel permeation chromatography (GPC) combined with light scattering. The GPC chromatograms of five fractions contained two peaks; the fractions had the same elution volume but different peak areas, suggesting that aggregates and single chains coexisted in the solution at 25 degrees C. The apparent weight-average molecular weights (M-w) of the aggregates and single chains for each fraction were all about 2.1 x 10(6) and 7.8 x 10(4), respectively. This indicates that the aggregates were composed of about 27 molecules off gum in the concentration range used (1.0 x 10(-6) to 5.0 x 10(-4) g/mL). The weight fraction of the aggregates (w(ag)) increased with increasing concentration, but the aggregates still existed even in an extremely dilute solution. The fractionation process and polymer concentration hardly affected the apparent aggregation number but significantly changed w(ag). The E-gum M-w decreased sharply with an increase in temperature. When the E-gum solution was kept at 100 degrees C, w(ag) decreased sharply for 20 h and leveled off after 100 h. Once the aggregates were decomposed at a higher temperature, no aggregation was observed in the solution at 25 degrees C, indicating that the aggregation was irreversible. (C) 2000 John Wiley & Sons, Inc.