The most astounding of all gelation phenomena, in my opinion, is the universality of the “critical gel” rheology for a multiplicity of complex materials at the gel point. Rheology is generally known to strongly depend on the internal structure of a material. However, independent of their individual structure that may cause gelation, all these materials look the same rheologically at the gel point, at least at small strain and in long-time experiments: the long-time relaxation modulus is a powerlaw G(t)= S t-n. Its format is universal while values for S and n vary from material to material. Really puzzling is the physical dimension of S [Pa sn], which tells us that S should somehow be divided into a modulus G0 and a characteristic time t0, and the relaxation modulus should then be written as G(t)= G0 (t/t0)-n. A criterion needs to be found for defining G0 and t0. Furthermore, S and n depend on each other. Large S was found for small n and vice versa. But why? Further puzzles concern the temperature dependence, the slope of G’(t,w) and G”(t, w) at gel point, the applicability of the time-cure superposition, diverging rheological functions near gel point, physical as compared to chemical gelation, coping with fast gelation, large strain behavior, and the competition with other solidification mechanisms such as crystallization, jamming and the glass transition, to name a few. Data will be shown to define many of the open questions that still need to be addressed before we can claim that gelation rheology may be understood.

Miraculous Gelation - Unresolved Problems and Open

Questions of Gelation Rheology

H. Henning Winter

Department of Chemical Engineering, University of Massachusetts,

Amherst, MA 01003, USA

Simultaneous In-situ Analysis of Instabilities and First Normal StressDifference during Polymer Melt Extrusion Flows

Roland Kádár1,2, Ingo F. C. Naue2 and Manfred Wilhelm2

1 Chalmers University of Technology, 41258 Gothenburg, Sweden2 Karlsruhe Institute of Technology - KIT, 76128 Karlsruhe, Germany

ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 24, 2016

ABSTRACTA high sensitivity system for capillaryrheometry capable of simultaneously de-tecting the onset and propagation of insta-bilities and the first normal stress differ-ence during polymer melt extrusion flowsis here presented. The main goals of thestudy are to analyse the nonlinear dynam-ics of extrusion instabilities and to deter-mine the first normal stress difference inthe presence of an induced streamline cur-vature via the so-called ’hole effect’. Anoverview of the system, general analysisprinciples, preliminary results and overallframework are herein discussed.

INTRODUCTIONCapillary rheometry is the preferredrheological characterisation method forpressure-driven processing applications,e.g. extrusion, injection moulding. Themain reason is that capillary rheometry isthe only method of probing material rheo-logical properties in processing-like condi-tions, i.e. high shear rate, nonlinear vis-coelastic regime, albeit in a controlledenvironment and using a comparativelysmall amount of material.1 Thus, it isof paramount importance to develop newtechniques to enhance capillary rheome-ters for a more comprehensive probing ofmaterial properties. Extrusion alone ac-counts for the processing of approximately35% of the worldwide production of plas-tics, currently 280⇥ 106 tons (Plastics Eu-rope, 2014). This makes it the most im-portant single polymer processing opera-

tion for the industry and can be found ina variety of forms in many manufacturingoperations. Extrusion throughput is lim-ited by the onset of instabilities, i.e. prod-uct defects. Comprehensive reviews on thesubject of polymer melt extrusion insta-bilities can be found elsewhere.4,6 A re-cent method proposed for the detectionand analysis of these instabilities is that ofa high sensitivity in-situ mechanical pres-sure instability detection system for cap-illary rheometry.8,10 The system consistsof high sensitivity piezoelectric transducersplaced along the extrusion slit die. In thisway all instability types detectable, thusopening new means of scientific inquiry. Asa result, new insights into the nonlinear dy-namics of the flow have been provided.9,14

Moreover, the possibility of investigatingthe reconstructed nonlinear dynamics wasconsidered, whereby a reconstructed phasespace is an embedding of the original phasespace.2,14 It was shown that a positive Lya-punov exponent was detected for the pri-mary and secondary instabilities in lin-ear and linear low density polyethylenes,LDPE and LLDPE,.14 Furthermore, it wasdetermined that Lyapunov exponents aresensitive to the changes in flow regime andbehave qualitatively different for the iden-tified transition sequences.14 It was alsoshown that it is possible to transfer thehigh sensitivity instability detection sys-tem to lab-sized extruders for inline ad-vanced processing control and quality con-trol systems.13

A very recent possibility considered

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