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

Highly branched poly(methyl methacrylate) with an estimated degree of branc hing (DB) over bar = 0.074 (i.e., 3.7 branchpoints per 100 monomer units) w as prepared using self-condensing group transfer copolymerization (SCVCP) o f 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 e xponent of alpha = 0.40 was determined for the branched polymer, which is c onsiderably lower than that of linear PMMA (alpha = 0.688). In the region b etween the relaxation times of the chain, tau (c), and of the segments, tau (s), of the viscoelastic spectrum of the branched polymer, both the storag e and the loss moduli (log G' and log G ") are nearly equal, and the comple x viscosities show a practically linear dependence on log omega with a slop e of -0.54. This is attributed to a broad distribution of relaxation times and the absence of entanglements, similar to near-critical gels. The normal ized chain relaxation times scale with the molecular weight with an exponen t of alpha = 2.61, which again is considerably lower than the value for the linear case (alpha = 3.39).