Rheology of Three-Arm Asymmetric Star Polymer Melts

Abstract

We present experimental and theoretical results for the linear rheology of melts of entangled, three-arm asymmetric polyisoprene stars. Asymmetric three-arm stars, in which two arms have the same length and the third is shorter, cross over from starlike to linear-like stress relaxation as the length of the third arm varies. We combine recent theories of stress relaxation in symmetric stars and in linear melts to predict the dynamic modulus of the asymmetric stars. For stars with short arm molecular weights of a few entanglement lengths, our theory underestimates the effective drag caused by the short arm, even when polydispersity effects are included. This unexplained discrepancy does not appear in a recent comparison of a related theory with measurements on polyisoprene H-polymers.

Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers

“…The ingredients for the three models are the same: (i) the molecular characteristics provided by the synthesis process (molar masses and polydispersities, number of branches), (ii) rheological parameters from the experiments (G e and M e , τ e ), (iii) the scaling exponent for dynamic dilution (M e =M e φ -α or G e =G e φ α+1 with φ being the polymer volume fraction and α=1 or 4/3) [12,70] and the fraction p 2 of tube diameter for hopping of the branch point in polymers with two or more branch points (typical values for p 2 are 1/40 for BoB, 1/12 for HM and 1 for TMA) [9,77]. Here we note that this issue merits further investigation, and recent simulations and analysis [53,[78][79][80] as well as selective experiments probing the branch point region (e.g., with neutron scattering) [81] have been very useful in the direction of assessing the exact role of branch point friction.…”

“…This "barrier" may be related to the often-neglected solvent friction (or equivalently the non-bonded stress [35,46]), which also controls the glass transition. There may be an analogy with the well-known property of small branches to have disproportionate friction-like effects in asymmetric stars [52,77]. As the microscopic origin of this yielding process remains elusive, this idea has not yet been formulated into a theory, and must still be reconciled with the clear and physical stress maximum due to over-orientation predicted by Doi and Edwards, which is "tamed" by CCR.…”

Definitions of entanglement spacing and time constants in the tube model

“…An error of a factor of two was introduced into Eq. 30 by Milner and McLeish (1998), but corrected in Frischknecht, et al (2002; see their footnote 24). However one missprint has been carried persistently from one paper to another, which is in factor of s 2 in denominator of eq.…”

Chain Architecture as an Orthogonal Parameter To Influence Block Copolymer Morphology. Synthesis and Characterization of Hyperbranched Block Copolymers: HyperBlocks

“…Predominantly these studies have been aimed at understanding the impact of chain branching upon the melt rheology of such polymers. Commonly studied branched architectures include the simplest branched structure, star polymers [1][2][3][4][5] , and increasingly complex architectures such as miktoarm stars 6,7 , graft/comb polymers [8][9][10][11][12][13] , H-shaped polymers [14][15][16][17] and dendritically long-chain branched polymers [18][19][20][21][22][23][24][25][26][27][28] . Chain-branching in block copolymers has also been explored with a view to understand the influence of architecture upon phase separation.…”