TRIBLOCK COPOLYMER BASED THERMOREVERSIBLE GELS .2. ANALYSIS OF THE SOL-GEL TRANSITION

Syndiotactic poly(methyl methacrylate) (sPMMA)-polybutadiene (PBD)-sPM MA triblock copolymers of various chemical compositions and different molecular weights have been studied in the presence of o-xylene, which is a selective solvent for the central PBD block. Thermoreversible ge ls can be formed in a suitable curing temperature range (<35 degrees C ) and at a high enough concentration (>1 wt %). The time dependence of the storage (G') and the loss (G '') moduli has been measured in a fr equency range of 0.08-1 Hz. The, static and dynamic properties of the gels have been discussed on the basis of the scaling theory. At the ge l point, where the loss angle (tan delta(c) = G ''/G') is independent of frequency, typical power laws G'(omega) similar to G ''(omega) simi lar to omega(Delta) have been observed. The scaling exponent Delta has been found equal to 0.70 +/- 0.02 independently of the PBD and PMMA m olecular weight, i.e., 36 000 < <(M)over bar (n)> (PBD) < 100 000 and 20 000 < <(M)over bar (n)> , (PMMA) < 51 000. This exponent is also in dependent of the copolymer concentration and temperature in the invest igated range, i.e., 2-7 wt % and 8-24 degrees C, respectively. This va lue of Delta agrees with theoretical predictions as well as with exper imental values reported for some chemical gels; it is however differen t from the experimental values published for most physical gels. A PBD -PMMA diblock copolymer also forms a gel in o-xylene, although at high er concentration and lower temperature compared to the parent triblock copolymer. The scaling exponent Delta is then somewhat smaller, i.e., 0.61.