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Abstract
Recent research on the stability of entanglements of polymer melts and on the correlation between viscosity improvement during processing and entanglement stability led to the discovery of a new property of the liquid state of polymers which is not explained by the current models in polymer physics: it is called “sustained orientation.” In simple terms, by manipulation of the stability of entanglements, it is possible to create and maintain quasi-stable at high temperature in an amorphous polymeric melt (say 120°C above Tg) a certain state of orientation that was induced by a mechanical deformation. The manipulation of entanglements was done by Rheo-Fluidification. In our experiments, the viscosity of a melt (e.g., PMMA) was measured at the exit of a Rheo-Fluidification treatment where the melt was submitted to a combination of shear thinning and strain softening via the use of cross-lateral shear vibration superposed on pressure flow (originated by an extruder feed). The exiting melt was frozen into pellets and the rheological properties of those pellets were studied. Under certain Rheo-Fluidification processing conditions, the viscosity reduction of the melt induced by the combination of shear thinning and strain softening could be preserved in the pellets granulated at the exit of the disentangling processor. These “treated” pellets displayed sustained orientation, i.e., a lower viscosity when they were reheated in a melt flow indexer, or in a dynamic rheometer after they had been compressed into disks. Yet, the molecular weight, Mw, was hardly changed (∼3%) to justify the viscosity reduction, and it was also observed that the viscosity only gradually (sometimes over periods of hours) returned to the value it should have at the corresponding temperature (the Newtonian value), indicating that the changes were reversible. These results suggest that the classical concept of Me to describe entanglements is too simplistic and its usefulness is probably limited to the linear range of viscoelasticity. We suggest that these type of experimental results, recently presented, have resonance in our fundamental understanding of the interactions between the macromolecules, inviting us to redefine the nature of entanglements, the nature of molecular motions and flow, and reformulate the equations underlying rheology, crystallization, glass formation, and the stability of the interactions at different temperatures and under stress. The whole description of polymer physics appears to be impacted by this new understanding of entanglements, and, to summarize the research objective ahead, we introduce the New School Polymer Physics which emerges from a new understanding of polymer interactions: “the grain-field statistics.”
Acknowledgments
This article is based on a keynote talk presented by the author at the International PPS-29 meeting which was held in Nuremberg, Germany in August 2012. The author would like to thank Prof. P.H. Geil for his useful editing comments.
Funding
The author would like to thank the Fulbright Foundation and the Ikerbasque Foundation for their respective awards. This work was made possible by the attribution of the Marie-Curie award by the European Research Council (FP7).
Notes
1 Interestingly, to the credit of P.G. de Gennes’ scientific integrity, H. Mendil was granted her Ph.D with High Honors, and was subsequently given an Award by the University of Paris for the quality of her research, presumably under the recommendations of de Gennes himself. Unfortunately, this impeccable scientific attitude against censorship of a work exposing the challenging deficiencies of his accepted theory, has not been followed by certain other members of the panel and by other accepters of the reptation interpretation who have been reported to have systematically blocked or delayed the publication of her work in major scientific journals Citation[22].
2 This does not mean that the molecular dynamics interpretation of linear viscoelasticity has been validated (see the next section).
3 In , the onset of shear-thinning can be defined as the intercept of the horizontal “Newtonian line” and of the “power law” line, correctly crossing at λω = 1, or it could be determined at a much lower value, e.g., λω = 0.05.
4 Eq. 7.46 of Ref. 9 provides τp as a function of τR, Rouse's relaxation time: with ξo the monomer friction coefficient, N the number of mers per chain and k the Boltzman's constant.
5 3 families are needed for the Doi-Edwards’ model, corresponding to mode A, mode B and mode C (reptation).Citation[9]
