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Trouble with Polymer Physics: Sustained Orientation : Ground breaking experimental research shakes the present understanding of the liquid state of polymers

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Trouble with Polymer Physics: "Sustained Orientation": Ground breaking experimental research shakes the present understanding of the liquid state of polymers
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   Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)   TROUBLE WITH POLYMER PHYSICS: “SUSTAINED - ORIENTATION” GROUND BREAKING EXPERIMENTAL RESEARCH SHAKES THE CURRENT UNDERSTANDING OF THE LIQUID STATE OF POLYMERS J.P Ibar  POLYMAT INSTITUTE, University of the Basque Country UPV/EHU, Donostia- San Sebastian, Spain  –   Tolosa Avenue 72, Donostia 20017, Spain,  jpibar@alum.mit.edu  Abstract  - Recent research [1-4] 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 o C above T g ) a certain state of orientation that was induced  by a mechanical deformation. The manipulation of entanglements is done by Rheo-Fluidification [4-6]. In our experiments, the viscosity of a melt (e.g. PMMA) is measured at the exit of a Rheo-Fluidification treatment [4] where the melt is submitted to a combination of shear-thinning and strain-softening via the use of cross-lateral shear vibration superposed to pressure flow (srcinated by an extruder feed). Furthermore, 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 display ed 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 was hardly changed (~3%) to justify the viscosity reduction, and it was also observed that the viscosity gradually 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. The whole foundation of polymer physics, based on its understanding of entanglements, appears to be shaken by the type of experimental results presented recently [2-5]. Keywords : Rheo-Fluidification; Entanglement Instability;Sustained-Orientation;New School Polymer Physics. Introduction Advances in rheology for the last 40 years have led to a  better understanding of the influence of the chain configuration on its flow characteristics, such as viscosity [7-11]. The concept of entanglement has been debated all these years and led to many theoretical models as reviewed by Graessley [12]. The concept explains the change of slope (from 1 to 3.4) that characterizes the molecular weight dependence of the  Newtonian viscosity, as well as the width of the rubbery plateau modulus [12] as M increases. More recently, modified versions by M. H. Wagner [11], G. Marrucci et al [10], of the srcinal reptation models by de Gennes [8], Doi & Edwards [9] are admitted to describe fairly well the flow behavior of entangled chains. The reptation model of entanglement is in its full maturity stage. Yet, many questions remain for a full understanding of what an entanglement is, in  particular to explain the challenging results obtained by submitting a melt to conditions of Rheo-Fluidification which combine shear, extensional flow and melt oscillation in order to compensate the effects of shear-thinning and strain softening in ways triggering “su stained- orientation”. In this presentation we show that it is possible to create and maintain quasi-stable at high temperature in an amorphous polymeric melt (say 120 o C above T g ) a certain state of orientation that was induced by a mechanical deformation history. The melt is not branched nor cross-linked when this sustained-orientation occurs, it is simply brought out of equilibrium with respect to the thermodynamic state that represents the most stable configuration of the interactions between the bonds belonging to the macromolecules [2]. The relative stability of the non-equilibrium state is what makes this research very challenging to the admitted concepts of entanglement [12], including the reptation models [7-11]. We found that, under certain conditions, the sustained-orientation could be preserved for hours at high temperature before the entanglement state recovered its equilibrium value. Experimental   The Rheo-Fluidification Processor  . In a Rheo-Fluidification processor a melt extrudes continuously through treatment zones where it is submitted to a combination of shear-thinning and strain-softening via the use of cross-lateral shear vibration superposed to pressure flow (srcinated by an   Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)   extruder feed). Fig. 1 shows 2 treatment stations (11) and (22) for a melt flowing from left to right to exit at the end of the processor, where its viscosity is measured continuously by an in-line rheometer and it is water cooled, granulated into pellets. Fig.1 Schematic of a Rheo-Fluidizer with two stations. Th e “treatment” in stations (1 1) and (22) is sketched in Fig.2: the melt flows through (from left to right) a gap “3” where the  upper gap surface is static and the lower surface is rotated and (optionally) oscillated. Both surfaces contain small ribs, “12”,  detailed in Figs 3a and 3b, which create local vibrational extensional flow  by squeezing and un-squeezing the melt as it is being swept forward helicoidally. Fig.2 Details of the “inside” of one station in Fig. 1  Shear thinning is controlled by the shear rates, which add up vectorially from all types of flow (longitudinal and cross-lateral, vibratory or not), and strain softening is controlled by frequency and the strain amplitude of the cross-lateral shear component. The rotation of the rotor in station 1 and station 2 were in opposite directions, a situation which we casually called “comb to the left- comb to the right”, referring to the sweeping induced by the ribs in relation to the rotation direction. Sustained Orientation. Fig. 4 shows the viscosity of the exiting melt (PMMA)  just after it has been “treated”, i.e. at the end of the second station of the two-station Rheo-Fluidizer shown in Fig. 1. Fig. 3a The surfaces touching the melt have a network of “ribs’ to induce vibration as the melt passes through. Fig. 3b The design of the ribs determines the local vibrational  profile both in shear and in extension (for ribs on divergent conic surfaces). Fig.4 Trace of the viscosity of the melt at the exit die rheometer. Current theories predict no viscosity change (see text). Temperature profiles are different in both stations. Rotation speeds and vibrational frequencies are also different in both treatment stations.The rheometer that measures the exiting melt viscosity is not far from the last treatment zone, but still, it takes the melt about 2 minutes to get there, and, at that temperature, which is 120 o C above the T g  of the polymer, the melt relaxation time is very small, of the order of 0.001 sec for that molecular weight. The x-axis in Fig. 4 is the extrusion time, different parameters being tested until a “successful processing window” is apparently found, seen as the final value of viscosity, 500 Pa-s , which is 1/3 of the extrapolated “un - sheared” in -line viscosity 1,500 Pa-s. In Fig. 4,   Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)   we observe a substantial drop of the melt viscosity at t= 20 min and another one at t=75 min, which is obtained  by just changing the processing parameters of vibration in the treatment zones. Even if it is understood from theory that the melt viscosity could be decreased in the treatment zone under a different set of shear-thinning and strain softening conditions, the same theory  predicts that the thermal-mechanical history should be erased totally in the 2 minutes times it takes to reach the rheometer, 2 minutes being 120,000 times greater than the longest relaxation time. One should not be able to observe any viscosity change at the exit of the Rheo-Fluidizer unless the orientation of the melt induced by shear-thinning and strain softening can be sustained 120 o C above T g , an impossible proposition according to our current understanding of polymer  physics! Results and Discussion Pellets were made of the treated melt by passing it through a strand die, and cooling the strands in water  before pelletization. Two batches of 75 kg each of “treated” pellets were prepared by this procedure and sent for testing. By this comment we mean to say that the process was steady and could produce continuously “disentangled melts” that turned into “disentangled  pellets” (th e way we casually summarized the experiment). The melt flow index (MFI) of these frozen-in treated  pellets is plotted in Fig. 5 versus the in-line Rheometer viscosity value which varies as a function of the chosen  processing parameters. Fig.5 MFI of treated pellets showing sustained-orientation vs in-line viscosity measured at the rheometer at the exit of the treatment. The correspondence is remarkable and means that, indeed, it is possible to retain in the pellets a large  portion of the viscosity reduction observed in the melt due to the Rheo-Fluidification process. The orientation of the melt induced by shear-thinning and strain softening has been preserved and has survived reheating in the MFI barrel still maintaining a 100% lower viscosity than the reference (the melt with no treatment). Furthermore, the rheological properties of those pellets were studied, after the pellets were compressed into new samples as if they were a new  polymer grade. Disks were molded to be studied by dynamic rheometry. Many of those results are given in Ref. 2 for polycarbonate. The viscosity reduction observed at the exit of the Rheo-fluidizer could survive a new heating in a molding press, and, additionally, a study of viscosity vs time in a rheometer at a high temperature. Yet, the molecular weight was hardly changed (~3%) to justify the viscosity reduction and it was also observed that the viscosity gradually returned to the value it should have at the corresponding temperature (the Newtonian value), indicating that the changes were reversible. Actually, it took 24 hours at 235 o C for the viscosity of the treated PMMA to return to its srcinal value!. Again note that the sample in Figs 4 and 5 is a linear PMMA deformed 120 o C above its Tg, and thus, in terms of the conventional understanding of the rheology of polymer melts in the terminal zone, our results imply that the polymer has  been retaining its orientation for a time 86.4 million times longer than its “longest relaxation time” (calculated from the cross-over point in a frequency sweep at the same temperature). This is totally incomprehensible in terms of our present understanding   of “entanglement”, the corner stone of long chain physics.  These experiments could be interpreted by stating that the entanglement became unstable producing an increase of Me, the molecular weight between entanglement (thus the wording which continues to be used: “disentanglement”). The instability of the  entanglement lasted 24 hours! But there is no explanation to why Me can vary independently of the relaxation time, and be increased (or decreased) by relatively low shear forces; there is no explanation in the current theories for an unstable   entanglement network resulting in an unstable liquid state for polymers, and for how it could be correlated to non-linear   viscoelastic effects.  Table 1 provides some MFI improvement results for  polymers that were successfully “disentangled” by the Rheo-Fluidification processor of Fig. 1 with the viscosity reduction benefits preserved in the pellets. In these results the MFI found experimentally is corrected to account for the small molecular weight decrease (2-5%) that comes as a collateral consequence of the treatment that exposes the polymer to a high processing temperature where it can degrade by thermal degradation or oxidation etc. See details in Refs 2, 4. We discussed in another publication [2] the rheological  properties of the polycarbonate and the PMMA pellets from this Table. The pseudo-plasticity of the treated resin can be varied by Rheo-Fluidification as well as the value of the Newtonian viscosity (proportional to the inverse of the MFI). As the melt of the treated resin   Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)   is annealed at high temperature, frequency sweep tests show a gradual return to a normal pseudo-plasticity and Newtonian viscosity. Table 1 Flow Improvement for pellets showing sustained-orientation Discussion Our research has two goals: a theoretical, fundamental understanding of the stability of the network of entanglement and a practical goal: how Rheo-Fluidification proceeds by way of “ disentanglement of the chains ” , and how we can predict the processing  parameters responsible for its triggering (temperature, strain, strain rate in both shear and extension) that will  produce sustained-orientation. The theoretical objective raises fundamental questions regarding our  present understanding of the interactions between the macromolecules which give rise to entanglements, these “physical cross - links”. Our experiments a t least 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. The practical goal of this program will be achieved when we will be able to predict the processing  parameters for a successful Rheo-Fluidification treatment, yielding any chosen value of sustained-orientation, given a polymer melt of known molecular weight characteristics, topology and chemical structure. In simple terms, for a given throughput of melt flow, what should be the Rheo-Fluidification  processing temperatures in the stations, the value of combined strain rate (from pressure and drag flow, oscillatory and rotational), the value of extensional flow and strain rate, the value of pressure in the gap, and what will be the amount of sustained-orientation obtained?. Further, what will be the stability of the new entangled state (how fast will it re-entangle at any given temperature and pressure)? Therefore, the theoretical and fundamental part of this research is to  provide a science base for molding processes under shear-thinning and strain-softening controls , so that the  process result can be achieved rationally based on scientific laws rather than only on experience. All these operating questions can only be answered after entanglement, shear-thinning and strain softening are understood from a molecular base, i.e. after rheological and polymer dynamic studies have provided the correlations between the degree of “disentanglement” (melt degree of out-of-equilibriumness, the time of “ re-entanglement ” (equilibrium recovery), and relaxation times (reptation and tube renewal time  d  and Rouse time  R  ). In attempting to do such a characterization of the entanglement instability from classical visco-elasticity parameters, we came to realize that the linear viscoelasticity theory itself may not be the correct base to extrapolate from to characterize the non-linear  phenomena leading to entanglement instability. The “molecular dynamic” description of viscoel astic data in terms of a family of discrete relaxation times generated  by a terminal time that varies with a local friction coefficient (thus providing the temperature dependence), and with topology (to explain the M or M3.4 dependence) is an elegant and simple mathematical tool to compare the effect of structural and chemical parameters on melt properties, but, as we started to realize [1], perhaps too simplistic to have any value in terms of the physics of deformation of a set of long chains. The Rouse’s or reptation models (de Gennes [7,8], Doi-Edwards [9]), based on such a spectrum of relaxation times, may not correctly describe the basic deformation process giving rise to viscoelastic effects (shear-thinning, normal stresses, extensional flow and the numerous other phenomena observed in non-linear deformation) at very high shear rate, or at high amplitude of strain, causing melt yielding, melt fracture and astonishing memory effects [1,2]. We suggested that the current models’ shortcoming was probably deeply rooted in the misunderstanding of the concept of chain entanglement, and of the entropy of the melt deformation process [3]. A new physics of the interactions between the bonds of the macromolecules had to be found to explain viscoelasticity (linear and non-linear) and the spectroscopic behavior of polymers above and below the glass transition [13].   We propose a different model of understanding of the coupling  between the bonds of polymer macromolecular chains, creating a novel statistics (“Grain -Field- Statistics”) that does not singularize individual chains embedded in a sea of mean field interactions [14]. Instead, we consider the global set of chains of conformers with a re-definition of their conformational statistics to account for the coupling of their inter and intra molecular interactions. This new statistics provides a new understanding of entanglements, describes viscoelasticity and flow properties in a completely srcinal way and suggests that the entanglement network can become unstable [2,3, 14]. Conclusions In this paper we have described processing conditions triggering melts to behave in novel ways contradicting the currently admitted models of polymer rheology. For instance, the viscosity of a polymer melt could be greatly reduced and the reduction of viscosity maintained at high temperatures for times 100,000 to several million times greater than the longest relaxation time. The instability of the network of entanglement could be induced by coupling shear-thinning and strain   Proceedings of the Polymer Processing Society 29th Annual Meeting ~ PPS-29 ~ July 15-19, 2013, Nuremberg (Germany)   softening in what we called a Rheo-Fluidification  processor, which we used to “ disentangle ”  polymers, subsequently “ re-entangle ”  them in a different specific way, creating new exciting materials. We suggested that this research leads to re-visit our understanding of the concept of entanglement in polymer physics in order to clarify “sustained orientation”, a condition which we have empirically observed in melts. In future work, the theoretical simulation of the stability of a network of conformers in collective interactions will be done using the equations of the Grain-Field Statistics to understand “sustained - orientation” , in parallel to a series of experiments conducted to repeat conditions of sustained orientation in plastics, this time with a design dedicated to test the idea that plastics, perhaps recycled  plastics, could be used as batteries to store energy [14]. Acknowledgements 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). References 1. J.P Ibar,  J. of Macrom. Sci., Part B, Phys.  48: 6, 1143-1189, 2009. 2. J.P. Ibar,  J. of Macrom. Sci., Part B, Phys. 49, 1148 -1258 ,2010. 3. J.P. Ibar,  J. of Macrom. Sci., Part B, Phys.  52:222-308, 2013. 4. J.P.Ibar,  J. of Macrom. Sci., Part B, Phys. 52:411-445, 2013. 5. J.P. Ibar,  J. of Macrom. Sci., Part B, Phys. 52:446-465, 2013. 6) J.P. Ibar,  Macromolecular Symposia , Special Issue, 11th International European Symposium on Polymer Blends, 2012. Volume 321-322, Issue 1, p. 30-39 7) P.G. de Gennes,  J. Chem. Phys.55 , 572.   1971, Also  8) P.G. de Gennes, Scaling Concept in Polymer  Physics ; Cornell University Press: Ithaca, NY, 1979. 9) M. Doi, S.F. Edwards, The Theory of Polymer  Dynamics ; Oxford University Press: Oxford, 1986. 10) G Marrucci and G. Iannruberto,  Macromolecules,  36, 3934 ,2004. 11) M.H. Wagner,  J. Rheol  ., 45, 1387 ,2001. 12) W.W. Graessley, Advances in Polymer Science, Vol. 16, “ The Entanglement Concept in Polymer  Rheology ,” Springer, 1974.  13)   J.P. Ibar,  J. Macromol. Sci.-Rev. Macromol. Chem.  Phys. , C37  , 389, 1997. 14)https://sites.google.com/a/eknetcampus.com/mission-and-objectives/
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