The extrusion flow instabilities of two commercial styrene‐butadiene rubbers are investigated as they vary in isomer content (1,4‐cis, 1,4‐trans, and 1,2 conformation) of the butadiene monomer and the molecular architecture (linear, branched). The investigated samples have similar multimodal molecular weight distribution. Two geometries of extrusion dies, slit and round capillary, are compared in terms of the type and the spatial characteristics of the flow instabilities. The latter are quantified using three methods: a highly pressure sensitive slit die, online and offline optical analysis. The highly pressure‐sensitive slit die has three piezoelectric pressure transducers (Δt ≈ 10−3 s and Δp ≈ 10−5 bar) placed along the die length. The characteristic frequency (fChar.) of the flow instabilities follows a power law behavior as a function of shear rate to a 0.5 power for both materials,  fChar.∝trueγ˙app.0.5. A qualitative model is used to predict the spatial characteristic wavelength (λ) of the flow instabilities from round capillary to slit dies and vice versa. Slip velocities (Vs) are used to quantify the slippage at slit and round capillary dies as well.

Chain extension/branching by reactive processing is a well‐known method to enhance the rheological properties of polymers. In this study, pyromellitic dianhydride, poly(glycolic acid), triglycidyl isocyanurate, and bisphenol A diglycidyl ether were used as chain extender/branching agents to produce branched Polyethylene terephthalate (PETs) with four different molecular structures. According to the linear rheological characterizations, the storage modulus and complex viscosity of modified PET samples enhanced significantly after branching. The shear viscosities of modified PET show a pronounced shear‐thinning behavior and a remarkable increase at low frequencies, which can be an indication of the existence of long‐chain branches (LCBs) in the molecular structure of polymer and broadening the molecular weight distribution. Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) analysis were used to investigate the effect of branching agents on the chemical structure and thermal properties of PET, respectively. DSC results show that higher amounts of LCBs lead to lower melting and crystallization temperatures.

The extrusion flow instabilities of three commercial styrene-butadiene rubbers (SBR) are investigated as a function of molecular weight distribution (MWD); molecular architecture (linear, branched); and temperature. The samples have multimodal MWD, with the main component being SBR and a low amount, less than 10 wt. %, of low-molecular weight hydrocarbons. Deviation from the Cox–Merz rule at high angular frequencies/shear rates becomes intense as the amount of medium-molecular weight component increases. Optical analysis is used to identify and quantify spatial surface distortions, specifically wavelength (λ) and height (h), of the different types of extrusion flow instabilities. Qualitative constitutive models are reviewed and used to fit the experimental data for the spatial characteristics of extrusion flow instability. The fitting parameters as obtained by the models are correlated with molecular properties of the materials. It is found that the characteristic spatial wavelength (λ) increases as the extrusion temperature decreases. Hence, the influence of temperature on the spatial characteristic wavelength is investigated and an Arrhenius behavior is observed.

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The POM−POM architecture is the simplest yet defined branched architecture, showing both strain hardening in elongation and strain softening in shear. The molecular structure consists of q side chains at each end of a backbone segment. To study the rheological and mechanical properties, we synthesized low-disperse POM−POM-shaped polystyrenes (PS) with welldefined molecular properties via anionic polymerization and grafting-onto method. All samples had a backbone with a weightaverage molecular weight of M w,b ≅ 100 kg mol −1 and approximately similar numbers of side chains per star q = 11−14. We varied the side chain length systematically from unentangled up to highly entangled side chains (M w,a = 9−300 kg mol −1 , 0.5−18 entanglements). The POM−POMs having M w,a ≈ 3M e ≈ M c have a maximum decrease in zero-shear viscosity η 0 of over 3 decades compared to linear PS with the same molecular weight, together with the highest strain hardening factor of SHF = 43. Moreover, POM−POMs having M w,a > 5M e displayed enhanced mechanical fatigue resistance beyond those of linear, ultrahigh-molecularweight PS, by up to a factor of 10.