Characterization of highly branched poly(methyl methacrylate) by solution viscosity and viscoelastic spectroscopy

Highly branched poly(methyl methacrylate) with an estimated degree of branching (DB) over bar = 0.074 (i.e., 3.7 branchpoints per 100 monomer units) was prepared using self-condensing group transfer copolymerization (SCVCP) of methyl methacrylate (MMA) and 2-(2-methyl-1-triethylsiloxy-1-propenyloxy)ethyl methacrylate (MTSHEMA) and fractionated by means of preparative SEC. The fractions were characterized in solution by SEC-viscosity coupling and in the melt by viscoelastic spectroscopy. In THF solution, a Mark-Houwink exponent of alpha = 0.40 was determined for the branched polymer, which is considerably lower than that of linear PMMA (alpha = 0.688). In the region between the relaxation times of the chain, tau (c), and of the segments, tau (s), of the viscoelastic spectrum of the branched polymer, both the storage and the loss moduli (log G' and log G ") are nearly equal, and the complex viscosities show a practically linear dependence on log omega with a slope of -0.54. This is attributed to a broad distribution of relaxation times and the absence of entanglements, similar to near-critical gels. The normalized chain relaxation times scale with the molecular weight with an exponent of alpha = 2.61, which again is considerably lower than the value for the linear case (alpha = 3.39).