1,721,164 research outputs found
A practical guide to mitigate edge fracture instability in sheared polymer melts
No full textThe measurement of nonlinear shear response of viscoelastic materials is often hindered by edge fracture instabilities. The phenomenon was first addressed theoretically by Tanner and Keentok and ever since has attracted the interest of experimentalists and theorists alike. Despite progress, accounting for or mitigating edge fracture remains a challenge, in particular when dealing with strongly viscoelastic materials such as entangled polymer melts. Here, we present and compare different experimental attempts to delay edge fracture in a cone-and-plate (CP) geometry, including the use of an immiscible fluid bath around the sample (that reduces the stress and interfacial gradients in comparison with the sample/air interface), a cone-partitioned plate (CPP) fixture, and an outer collar attached to the sample's edge (in a CP or CPP fixture). Focusing on the torque signal, we find that the combination of CPP and collar provides the best results. This may indeed help measuring highly elastic materials over an extended range of shear rates and, importantly, contribute to reliably measuring the normal stress coefficients in a cone-partitioned plate tool. It is, therefore, hoped that this simple idea will be further pursued in the direction of improving our current rheometric capabilities
Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers
No full textWe briefly review the recent advances in the rheology of entangled polymers and identify emerging research trends and outstanding challenges, especially with respect to branched polymers. Emphasis is placed on the role of well-characterized model systems, as well as the synergy of synthesis-characterization, rheometry and modeling/simulations. The theoretical framework for understanding the observed linear and nonlinear rheological phenomena is the tube model, which is critically assessed in view of its successes and shortcomings, and alternative approaches are briefly discussed. Finally, intriguing experimental findings and controversial issues that merit consistent explanation, such as shear banding instabilities, multiple stress overshoots in transient simple shear and enhanced steady-state elongational viscosity in polymer solutions, are discussed, and future directions such as branch point dynamics and anisotropic monomeric friction are outlined
Viscoelastic behavior of semicrystalline thermoplastic polymers during the early stages of crystallization
No full textUsing rheological techniques, we investigate the evolution of the microstructure evolution during the early stages of crystallization of poly(1-Butene). In performing the measurements, use is made of an innovative experimental protocol, called “inverse quenching”, which allows stopping the crystallization process and producing a stable biphasic (crystalline/amorphous) system. In this way, very low frequency measurements at fixed degrees of crystallization are made possible. We find that crystallization, evidenced as a Liquid-to-Solid Transition (LST) under isothermal conditions, with characteristics of critical gel behavior, takes place at surprisingly low degrees of crystallinity (below 1.5%). The critical gel properties, which are found to depend on both crystallization temperature and molecular weight, can be reduced to a single master curve when the gel strength is plotted as a function of the relaxation exponent.. More importantly, the LST it is preceded by the development of a long, but finite relaxation process. This latter process, although not fully understood, brings analogies to the slow dynamics observed in hybrid colloid-polymer systems (block copolymer micelles or multiarm star polymers) as well as the recently suggested presence of dormant nuclei. It is clear, however, that the connectivity among crystallites, apparently via the amorphous segments, plays a key role in this new proces
Wall slip in primitive chain network simulations of shear startup of entangled polymers and its effect on the shear stress undershoot
No full textIn some recent experiments on entangled polymers of stress growth in the startup of fast shear flows, an undershoot in the shear stress is observed following the overshoot, i.e., before approaching the steady state. Whereas tumbling of the entangled chain was proposed to be at its origin, here, we investigate another possible cause for the stress undershoot, i.e., slippage at the interface between the polymer and solid wall. To this end, we extend the primitive chain network model to include slip at the interface between entangled polymeric liquids and solid walls with grafted polymers. We determine the slip velocity at the wall, and the shear rate in the bulk, by imposing that the shear stress in the bulk polymers is equal to that resulting from the polymers grafted at the wall. After confirming that the predicted results for the steady state are reasonable, we examine the transient behavior. The simulations confirm that slippage weakens the magnitude of the stress overshoot, as reported earlier. The undershoot is also weakened, or even disappears, because of a reduced coherence in molecular tumbling. Disentanglement of grafted chains from bulk ones, taking place throughout the stress overshoot region, does not contribute to the stress undershoot
Stress growth and relaxation of dendritically branched macromolecules in shear and uniaxial extension
Full text linkWe present unique nonlinear rheological data of well-defined symmetric Cayley-tree poly(methyl methacrylates) in shear and uniaxial extension. Earlier work has shown that their linear viscoelasticity is governed by the hierarchical relaxation of different generations, whereas the segments between branch points are responsible for their substantial strain hardening as compared to linear homopolymers of the same total molar mass at the same value of imposed stretch rate. Here, we extend that work in order to obtain further experimental evidence that will help understanding the molecular origin of the remarkable properties of these highly branched macromolecules. In particular, we address three questions pertinent to the specific molecular structure: (i) is steady state attainable during uniaxial extension? (ii) what is the respective transient response in simple shear? and (iii) how does stress relax upon cessation of extension or shear? To accomplish our goal we utilize state-of-the-art instrumentation, i.e., filament stretching rheometry (FSR) and cone-partitioned plate (CPP) shear rheometry for polymers with 3 and 4 generations, and complement it with state-of-the-art modeling predictions using the Branch-on-Branch (BoB) algorithm. The data indicates that the extensional viscosity reaches a steady state value, whose dependence on extension rate is identical to that of entangled linear and other branched polymer melts. Nonlinear shear is characterized by transient stress overshoots and the validity of the Cox-Merz rule. Remarkably, nonlinear stress relaxation is much broader and slower in extension compared to shear. It is also slower at higher generation, and rate-independent for rates below the Rouse rate of the outer segment. For extension, the relaxation time is longer than that of the linear stress relaxation, suggesting a strong ‘elastic memory’ of the material. These results are 2 described by BoB semi-quantitatively, both in linear and nonlinear shear and extensional regimes. Given the fact that the segments between branch points are less than 3 entanglements long, this is a very promising outcome that gives confidence in using BoB for understanding the key features. Moreover, the response of the segments between generations controls the rheology of the Cayley trees. Their substantial stretching in uniaxial extension appears responsible for strain hardening, whereas coupling of stretches of different parts of the polymer appears to be the origin of the slower subsequent relaxation of extensional stress. Concerning the latter effect, for which predictions are not available, it is hoped that the present experimental findings and proposed framework of analysis will motivate further developments in the direction of molecular constitutive models for branched and hyperbranched polymers
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
