Viscoelasticity of Monodisperse Comb Polymer Melts

Abstract

The experimentally determined zero-shear viscosity of entangled branched polymers shows dramatic variation due to the topological arrangements of the branches in branched polymer melts. The position of the branch points, the arm length, and number of the arms are essential to defining the rheological behavior. Recent advances in molecular tube models have led to a much greater understanding of the linear rheology of linear, star, H-shaped, pom-pom, and comb polymers. We correct and extend existing molecular theories for the linear viscoelasticity of comb polymer melts, especially in accounting for (1) polydispersity and (2) the path length of backbone extremities. We compare the predictions with linear rheological data of nearly monodisperse polybutadiene combs. We then predict the zero-shear viscosity for monodisperse comb polyethylenes with varying arm lengths, backbone lengths, and number of arms. For a fixed molecular weight, we find that combs with the longest arms but few branch points give the highest predicted zero-shear viscosities and that they obey an exponential dependence on the length of the arms in the same way as star polymers. We find that combs with short arms, under four entanglements, lie below the 3.4 power law obeyed by linear polymers. All other comb topologies are bounded by these extremes.

Linking Models of Polymerization and Dynamics to Predict Branched Polymer Structure and Flow

“…Theory and experiment have since been extended to more complex single-molecule topologies: H-shapes (11), combs (12), and multi-arm polymers (13,14), confirming the predictions of universality over chemistry, and the central role of molecular topology. The hierarchical relaxation of entangled star polymers generalises to more complex architectures: once free ends retract back to the outermost layer of branch-points, these become mobile, activating deeper retractions towards the second layer, and so on (15).…”

Branch-Point Motion in Architecturally Complex Polymers: Estimation of Hopping Parameters from Computer Simulations and Experiments

“…Different versions of hierarchical tube-based models make different choices on these two characteristics of the branchpoint hopping motion. 11,[15][16][17]19,22,30 Secondly, the appropriate value of p 2 is normally determined by inference from rheological data, rather than by measuring the branchpoint motion directly. The data are interpreted by making use of a suitable (hierarchical) rheological model.…”

Anionic Synthesis and Rheological Characterization of Poly(p‐methylstyrene) Model Comb Architectures with a Defined and Very Low Degree of Long Chain Branching

“…The observation of a second minimum in the loss angle (Figure 2b) is a signature for the presence of a second relaxation process. This second relaxation process is attributable to the arm retraction of the entangled side chains, implying a hierarchical relaxation [11,12,15,39,40] of the whole comb molecule as is nowadays understood for the relaxation of branched polymer melts. In the extensional measurements the polymer with linear topology follows the linear viscoelastic behavior at nearly all times, with only a negative deviation ('strain softening') being observed at longer times, especially for the lower strain rates.…”