Triblock copolymer based thermoreversible gels .4. Effect of the midblock and characterization of the sol-gel transition

Thermoreversible gelation has been studied in o-xylene for poly(methyl methacrylate) containing 80% syndiotactic triads (sPMMA) and block copolymers of the MXM type, where M is sPMMA and X is either polybutadiene (PBD), hydrogenated PBD (FEB), poly(styrene-b-butadiene-b-styrene) (SBS) triblock, or the hydrogenated version of this triblock (SEBS). In o-xylene, which is a selective solvent for the central X block, sPMMA forms thermoreversible gels provided that the molecular weight is high enough. When sPMMA is the outer block of MXM triblock copolymers, the midblock X appears to favor the gelation and it considerably improves the thermal stability of the matured gels. This thermal stability is, however, largely independent of the actual nature of the midblock. The dynamic properties of solutions and gels have been analyzed and discussed on the basis of scaling assumptions. At the gel point, where the loss angle tan delta(c) = G''/G' is independent of the probing frequency, the sample obeys the typical power law G'(omega) similar to G''(omega) similar to omega(Delta). The scaling exponent Delta is found in the 0.65-0.75 range for both sPMMA and MXM block copolymers, independent of the nature of the midblock. Modulus-frequency master curves have been built by using appropriate reaction time dependent renormalization factors for the individual frequency and modulus data. The scaling of these factors with reaction time has allowed us to calculate the static scaling exponents for the increase observed in both modulus and viscosity. The accordingly calculated values agree with the scalar elasticity percolation model.