Synthesis and linear and nonlinear melt rheology of well-defined comb architectures of PS and PpMS with a low and controlled degree of long-chain branching

摘要

Well-defined, monodisperse homopolymer comb architectures with varied number and length of the branches under linear and nonlinear deformation were synthesized and examined to determine the effect of branching on different rheological properties. The correlation of the rheological properties with the comb topology is of special interest for the determination of the degree of branching. Therefore, well-defined polystyrene-based comb polymers with systematically varied number and molecular weight of the branches, narrow polydispersities, and a controlled, but low, number of branches (typically 0.1-1 mol % branches per backbone) were synthesized and compared with data from polystyrene combs of the Roovers series that have a higher number of branches (>1 mol % branches per backbone). To investigate the rheological properties in detail, various linear and nonlinear techniques were applied. Within the linear regime, the reduced van Gurp-Palmen plot (δ vs |G*|/GN _0) was used to identify critical points that illustrated the influence of the branch molecular weight and number of branches on the resulting rheological properties. In the nonlinear regime large amplitude oscillatory shear (LAOS) measurements were performed to obtain the nonlinear parameter Q_0(ω) via a quadratic scaling law from FT-rheology. An intrinsic nonlinear master curve based on the Q_0(ω) parameter reflected the relaxation hierarchy and was shown to be a sensitive method to extract information on the different relaxation time scales. The nonlinear shear measurements were complemented by uniaxial extensional measurements to quantify the strain hardening effect and how the strain hardening was affected by branch relaxation. The results obtained from the uniaxial extensional measurements could be correlated to relaxation times obtained from the intrinsic nonlinear master curve Q_0(ω). Pom-pom constitutive model predictions were performed for the comparison with experimental data for extensional rheology with focus on the strain hardening behavior and for LAOS with focus on the nonlinear parameter Q_0(ω) as a function of increasing number and molecular weight of the branches in the pom-pom molecule. A comparison of the applied rheological methods - (1) small amplitude oscillatory shear (SAOS) in the linear regime, (2) LAOS in combination with FT-rheology, and (3) extensional rheology in the nonlinear regime - illustrated the detection limits as well as the advantages and disadvantages of each technique toward the investigation of rare, but entangled branched comb polymer topologies.