CONFORMATION AND PHYSICAL GELATION OF TELECHELIC POLYELECTROLYTES IN SOLUTION

摘要

A special kind of associating polyelectrolytes (AP), telechelic polyelectrolytes (TP), composed of a flexible linear polyelectrolyte and short hydrophobic blocks at both ends, are investigated by means of Monte Carlo simulations in aqueous media. These novel materials will ionize into polyions and their counterions when dissolved in water> whose properties strongly depend on pH and salt addition. The PH-tunable property makes its wider applications in drug delivering and targeting.1-3 The interplay between hydrophobic attraction and the long range electrostatic interaction as well as the counterion distribution exerts a major influence on the chain conformations(two basic conformations: loop and non-loop), association behavior (loop association contributes to loop conformation; free, dangling and bridge chains contribute to non-loop conformations) and the forming of clusters. At a concentration close to the overlapping concentration ?, the clusters are found to be further connected by bridge chains under strong hydrophobic interaction, forming a 3D network(See Figure l(a)). Percolation model is used to probe the gelation process. Figure l(b) plots the fraction of corresponding chain association types during this sol-gel transition process. Increasing hydrophobic interaction arouses a maximum of dangling fraction due to the completion of the two transitions, the free into dangling type and dangling into bridge and loop types. In addition, the loop fraction shows no significant change after ε>7 due to the completion of hydrophobic attraction and electrostatic repulsion. The effect of counterion condensation4'5 on chain conformations and the forming of physical gel is also presented. Increasing Coulomb interaction strength is basically in favor of the formation of gel network(See Figure 2(a)). As it is shown in Figure 2(b), with increasing Coulomb interaction strength, firstly chains tend to extend and then collapse into loop conformation due to the screening of the counterion condensation layer. However, bridge chains under strong hydrophobic interactions are more stable than those under small e. In a word, compared with neutral telechelic polymers,6 the gelation of telechelic polyelectrolytes is more complicated.