Small and Medium Amplitude Oscillatory Shear Rheology of Model Branched Polystyrene (PS) Melts

Abstract

Linear and nonlinear rheological properties of model comb polystyrenes (PS) with loosely to densely grafted architectures were measured under small and medium amplitude oscillatory shear (SAOS and MAOS) flow. This comb PS set had the same length of backbone and branches but varied in the number of branches from 3 to 120 branches. Linear viscoelastic properties of the comb PS were compared with the hierarchical model predictions. The model underpredicted zero-shear viscosity and backbone plateau modulus of densely branched comb with 60 or 120 branches because the model does not include the effect of side chain crowding. First- and third-harmonic nonlinearities reflected the hierarchy in the relaxation motion of comb structures. Notably, the low-frequency plateau values of first-harmonic MAOS moduli scaled with M w − 2 (total molecular weight), reflecting dynamic tube dilution (DTD) by relaxed branches. Relative intrinsic nonlinearity Q0 exhibited the difference between comb and bottlebrush via no low-frequency Q0 peak of bottlebrush corresponding to backbone relaxation, which is probably related to the stretched backbone conformation in bottlebrush.

Comb and Branch‐on‐Branch Model Polystyrenes with Exceptionally High Strain Hardening Factor SHF > 1000 and Their Impact on Physical Foaming

“…In previous studies of our group, Abbasi and co-workers [18][19][20] used anionic polymerization to synthesize a series of monodisperse atactic LCB comb PS with the same backbone length (M w,bb = 290 kg mol −1 ) and similar, well-entangled branch length (M w,br = 44 kg mol −1 ≈ 3 M e ), but varying number of branches per backbone, N br , from 3 to 190. Those comb PS covered a wide range of conformational regimes from loosely grafted to densely grafted combs and loosely grafted bottlebrush conformations.…”

“…[22,23] Furthermore, the dynamic tube dilution was expected to decrease the effective number of entanglements along the backbone. [18,19] Therefore, even though branching is known to have a positive impact on strain hardening, it was shown that, for the given model system above a certain number of side chains, the elongational performance in uniaxial extension as well as foam expansion is negatively affected. Consequently, excessive side chain crowding will not further increase the SHF.…”

Poly(ethylene glycol) Hydrogel Crosslinking Chemistries Identified via Atmospheric Solids Analysis Probe Mass Spectrometry

“…29,31 Hydrogel formation is typically verified by inverted tube tests or small angle oscillatory shear rheology (SAOS); the latter technique probes the transition of material properties at the gelation point, which change from a primarily viscous to a primarily elastic response. 32 SAOS, tensile testing, and compression tests are also used to determine the mechanical properties, which are often compared to swelling and smallangle neutron scattering data to investigate the gel microstructure. 29,31 None of these techniques, however, provides information about specific chemical crosslinking within the material.…”

Distinguishing between Linear and Star Polystyrenes with Unentangled Arms by Dynamic Oscillatory Shear Tests

“…Next, we investigated the nonlinear behavior of unentangled star PS under MAOS flow. Recently, the MAOS test has been utilized to characterize polymers with different topologies, such as linear, 3-arm star, comb, and bottlebrush structures. Herein, the MAOS test was utilized to distinguish between unentangled star PS and linear PS, because its results reflect the information on relaxation spectra in a complex manner. , Several material functions are available to describe the material response in a MAOS regime.…”