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Shape recovery of a dispersed droplet phase and stress relaxation after application of step shear strains in a polystyrene/polycarbonate blend melt J. Rheol. 43, 951 (1999); 10.1122/1.551035Melt elongation and recovery of polymer blends, morphology, and influence of interfacial tension SynopsisNew data of shear startup on branched styrene-butadiene random (SBR) copolymers are reported, where the novelty consists in repeating the startup run after different rest times at zero stress. Here, the aim is one of exploring the "damage" introduced by the first run, as well as the subsequent recovery, if any, upon waiting increasingly long times. Differently from a linear sample, our branched melts show multiple peaks during the first run, as previously reported by Bacchelli [Kautschuk Gummi Kunststoffe 61, 188-191 (2008)] for similar SBR samples, and, more recently, by Snijkers et al. [ACS Macro Lett. 2, 601-604 (2013)] for a well-characterized comblike polystyrene melt. The repeated runs show an intriguing novel feature with respect to the case of linear polymers, namely, the first peak goes up initially, instead of down. The second peak goes down and seemingly recovers only after an extremely long time, longer than the largest relaxation time practically accessible to linear viscoelasticity, the latter not reaching the terminal behavior. All such features of nonlinear viscoelasticity of highly branched polymers are interpreted by using a simple theory inspired by the well-known pompom model. V C 2014 The Society of Rheology.

Extensional deformations represent an effective stimulus to explore the rich rheological response of branched polymers and elastomers, enabling the design of polymers with specific molecular structure. However, probing the polymer behavior at large deformations is often limited by the experimental devices. We here present an alternative use of the Sentmanat Extensional Rheometer (SER) that allows Hencky strain units much larger than the maximum value achievable, ∼3.6. The proposed procedure consists of an oblique positioning of the sample in the measuring area. If a small inclination of the sample is used, the departure from the ideal uniaxial flow is negligible at Hencky strains <1, and nearly zero for larger values. Experimental results in the linear viscoelastic regime are compared with the double reptation model in order to discern polydispersity and branching effects, whereas the extensional rheology data are contrasted with the molecular stress function theory (MSF), revealing important information about the polymer structure, especially on the long-chain branching (LCB). Finally, the analysis of sample failure upon elongation allowed us to correlate the polymer structure to the rheological behavior during mixing processes.

Fiber spinning experiments are conducted with a capillary rheometer and a Rheotens tester on linear styrene‐isoprene‐styrene copolymer samples by varying extrusion temperature and drawdown velocity in a wide range of values, also covering the occurrence of instability phenomena. Tensile stress is measured during the experiences, and the experimental time series are then analyzed by means of a new methodology. The proposed approach is based on Neural Network modeling of the time series, coupled with Principal Component Analysis postprocessing of the results. The methodology is able to identify and quantify the effects of process condition on the dynamical behavior of the system. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

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