Triblock Copolymer Based Thermoreversible Gels. 2. Analysis of the Sol−Gel Transition

Syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA 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 gels can be formed in a suitable curing temperature range (<35 °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 frequency 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 gel point, where the loss angle (tan δ c = G‘‘/G‘ ) is independent of frequency, typical power laws G‘ (ω) ∼ G‘‘ (ω) ∼ ω Δ have been observed. The scaling exponent Δ has been found equal to 0.70 ± 0.02 independently of the PBD and PMMA molecular weight, i.e., 36 000 < M̄ n (PBD) < 100 000 and 20 000 < M̄ n (PMMA) < 51 000. This exponent is also independent of the copolymer concentration and temperature in the investigated range, i.e., 2−7 wt % and 8−24 °C, respectively. This value of Δ agrees with theoretical predictions as well as with experimental values reported for some chemical gels; it is however different from the experimental values published for most physical gels. A PBD−PMMA diblock copolymer also forms a gel in o -xylene, although at higher concentration and lower temperature compared to the parent triblock copolymer. The scaling exponent Δ is then somewhat smaller, i.e., 0.61.