Melt Rheology of Dendritic Hyperbranched Polyacrylates Synthesized by Controlled Radical Polymerization: Evidence of...

Dendritic hyperbranched poly(methyl acrylate)s (HBPMAs) with controlled molecular weight and low dispersity (Đ ≤ 2) were synthesized by radical copolymerization of MA and a branch-inducing monomer, evolmer 2b, in the presence of organotellurium chain transfer agent (CTA) 1, by targeting theoretical generation (G_n) of 3-5 and branch molecular weight (M_BL) of 0.25M_e-2.0M_e, with M_e (= 11,000) being the entanglement molecular weight of PMA. The control of the self-similar branch layer structure in HBPMAs thus synthesized was examined by measuring their linear viscoelastic moduli (G* = G' + iG″). For all G_ns having M_BL = 0.25M_e, the storage modulus G' was lower than the loss modulus G″ at all ω examined. This unentangled feature was observed even when the portion of the chain spanning the two outmost ends of the branches had a molecular weight of 2.0M_e, possibly because of a lack of deep penetration among those G_n chains. In contrast, for M_BL ≥ M_e, G' was higher than G″ in a given range of ω, and this range became wider and the relaxation mode distribution therein became broader on an increase of M_BL, which is characteristic of entangled branch polymers. For the entangled HBPMA chains having the same M_BL but different G_n, hierarchically self-similar relaxation based on the branch retraction mechanism was confirmed for the G' and G″ data: The data for higher G_n agreed with the data for lower G_n after a shift along the ω axis according to this self-similarity. For the unentangled BPMA chains of different G_n, the G' and G″ data obeyed the Rouse-type relationship based on the self-similarity but without the hierarchy. These results support good control of the dendritic branch structures of the HBPMAs synthesized with the current method.