Triblock copolymer based thermoreversible gels. 3. Effect of stereocomplexation with iPMMA

Thermoreversible gelation of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA, triblock copolymers in o-xylene has been studied in the presence of isotactic PMMA (iPMMA) in a large range of syndio/iso mixing ratio (s/i). Differential scanning calorimetry (DSC) shows that two types of association contribute to the gel formation, i.e., the self-aggregation of the sPMMA outer blocks and the stereocomplexation of the sPMMA outer blocks with iPMMA. At low iPMMA contents (s/i = 30/1), self-association of the outer blocks accounts for the gelation process, although stereocomplexation occurs. Stereocomplexation becomes the dominant gelation mechanism at a high enough iPMMA content (s/i = 2/1). Thus, gelation is observed at a copolymer concentration as low as 1 wt %. The time dependence of the storage (G‘) and loss (G‘‘) moduli has been measured in the 0.08−1Hz frequency range in order to probe the gelation process. At the gel point, where the loss angle (tan δc = G‘‘/G‘) is independent of the probing frequency, the sample fits in the typical power law G‘(ω) G‘‘(ω) ωΔ. While the gelation time is greatly influenced by the s/i mixing ratio, the scaling exponent Δ is essentially constant in the range of 0.70−0.75. The Δ value agrees with theoretical predictions as well as with experimental values reported for some chemical gels but it is different from the experimental values reported for most physical gels. The Δ value is also independent of the PBD microstructure and it also holds for sPMMA-polystyrene (PS)−PBD−PS−sPMMA (MSBSM) pentablock copolymers. Therefore, the value of Δ appears to be a typical exponent for thermoplastic gels based on triblock copolymers of the ABA type in o-xylene.