Melt Rheology of Dendritic Hyperbranched Polyacrylates Synthesized by Controlled Radical Polymerization: Evidence of Self-Similar Branch Structure Formation

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 (Gn) of 3-5 and branch molecular weight (MBL) of 0.25Me - 2.0Me, with Me (= 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 Gns having MBL = 0.25 Me, 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 the molecular weight of 2.0Me, possibly because of a lack of deep penetration among those Gn chains. In contrast, for MBL ≥ Me, 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 MBL, which is characteristic of entangled branch polymers. For the entangled HBPMA chains having the same MBL but different Gn, hierarchically self-similar relaxation based on the branch retraction mechanism was confirmed for the G' and G" data: The data for higher Gn agreed with the data for lower Gn shifted according to this self-similarity. For the unentangled BPMA chains of different Gn, 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 and branch structure of the HBPMAs synthesized with the current method.