Water‐soluble polymers. LXXXII. Shear degradation effects on drag reduction behavior of dilute polymer solutions

Drag reduction measurements were conducted on extensively characterized poly(ethylene oxide) and poly(acrylamide) utilizing a fully automated rotating disk rheometer equipped with an optical tachometer, torque transducer, and software allowing real‐time data acquisition. The instrument sensitivity allowed the study of concentrations as low as 0.1 ppm. In addition, previously immeasurable concentration‐ and time‐dependent shear degradation effects were readily observed. A power law equation was shown to adequately correlate the percentage of drag reduction and the volume fraction for each polymer. Furthermore, an empirical shift factor was utilized to superimpose data from all the systems that were studied. By conducting measurements in the proper concentration and time domains, it was possible to extract the minimal concentration for the maximum drag reduction efficiency in the absence of shear degradation. The resulting values were significantly higher than those previously reported by our laboratories for poly(ethylene oxide) and poly(acrylamide). © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1211–1221, 2001     

Mechanical degradation of semi-dilute polymer solutions in laminar flows

Mechanical degradation of a semi-dilute solution of non-hydrolyzed polyacrylamide was studied under laminar flow conditions through fine capillary systems. Using a multi-pass device and capillary tubes of the same diameter and of various lengths we have shown that mechanical degradation (i) occurs at a critical value of the wall shear rate, chosen as a reference deformation rate, which is slightly higher than that of the appearance of high pressure losses in the entrance region of the capillary tube; (ii) is independent of the capillary tube length; (iii) increases with the number of passes N up to a maximum value for a limiting number of passes N_lim which is a decreasing function of deformation rates but does not depend on capillary length. The amount of degradation is expressed in terms of loss of viscous dissipation in shear and transient elongational flow. This last point is determined by studying the total end pressure loss through the capillary tube as a function of the pass number. The high pressure loss is related to viscous dissipation on macromolecules stretched by rapid converging flow. A comparison between a fresh and a fully degraded solution indicates that the degradation affects shear viscosity much less than viscous dissipation in rapid converging flow which is related to the properties of extended macromolecules. Both experimental results and theoretical interpretation suggest that, in our capillary system, the mechanical degradation occurs in the entrance region of the capillary where macromolecules are stretched and consequently submitted to extensional forces which can overcome the C–C bonds strength.     

High shear rheology and shear degradation of aqueous polymer solutions

We have investigated the possibility of replacing hydraulic oils with aqueous polymer solutions having the same rheological properties. We measured the low and high shear rate viscosities of polymer solutions at several moderate concentrations and compared the results to the predictions of a molecular model. We found that viscosities measured at shear rates near 1 million reciprocal seconds are in good agreement with those calculated using the model of Mochimaru.⁶ Shear degradation studies were also conducted using higher molecular weight versions of some of the polymers. Exposure of these to very high shear rate caused a permanent decrease in viscosity and a corresponding change in the molecular weight and molecular weight distribution. Taken together, these results show that very low molecular weight polymers at moderate concentrations are necessary to formulate an aqueous hydraulic fluid with approximately Newtonian behavior at shear rates near 1 million reciprocal seconds, and without long-term polymer degradation.     

Ultrasonic degradation of polymer solutions. 1. Polystyrene revisited

The results of ultrasonic irradiation of polystyrene solutions in toluene, benzene and carbon tetrachloride are described. The effect of a range of conditions such as ultrasonic intensity, solution concentration and varying solvents were investigated. The non-random nature of the degradation process was clearly demonstrated and the effect of different solvents was interpreted in terms of their thermodynamic properties.     

Drag reduction and mechanical degradation in polymer solutions in flow

A list of conclusions from experimental studies of drag reduction (DR) and mechanucal degradation in flow (MDF) is made. A statistical-mechanical model of chain conformations developed by the author⁹ is used, and its consequences for DR and MDF established. Experimental findings are explained in terms of the model, including those considered to be puzzling and contrary to expectations. A relation between the extent of mechanical degradation and flow time is derived. The equation obtained for relative drag reduction in function of time reproduces perfectly the experimental data for polystyrene+toluene solutions reported by Hunston and Zakin¹². Some predictions from the present model have yet to be tested experimentally.     

Shear degradation of polyisobutene

A polyisobutene of M?_w 1.98 × 10⁶, M?_w/M?_n 1.8, was extruded in an Instron capillary rheometer. Shear degradation occurred at high shear stresses, approaching melt fracture, and was more prominent at lower extrusion temperatures for tests at 60–140°C. The capillary was 2.0 in. long with a length/diameter ratio of 66.7 and a 90° entrance angle. Repetitive extrusions at constant shear rate caused a decrease in a molecular weight and a simultaneous narrowing of the molecular weight distribution. Extrudate expansion was measured after each successive capillary pass for tests at 80°C. Extrudate swelling correlated well with (M?_z+1) M?_z/M?_w, except for the two first passes, where melt fracture was pronounced. The correlation with equilibrium extrudate expansion was almost as good for (M?_z/M?_w)^3.7 (Mill's correlation) and for M?_z+1 alone. The efficiency of bond rupture is low, with the energy required to rupture 1 mole of bonds being about 200,000 kcal at 80°C.     

Shear degradation of poly(vinyl acetate) in toluene solutions by high-speed stirring

A poly(vinyl acetate) (PVAc) of M?_w 750,000 and M?_w/M?_n 5.10 in toluene solution was sheared in a Virtis-60 homogenizer. The polymer concentration was 3.0 to 12.0 g/100ml, and test temperature was 10 ± 0.5°C. The extent of degradation was measured by gel permeation chromatography (GPC). It was concluded that on shearing (i) the molecular weight decreases rapidly at the beginning of shearing and thereafter decreases ever more slowly toward a limiting value, (ii) the molecular weight distribution is narrowed, (iii) no degradation occurs up to 5000 rpm and thereafter increases with stirring speed, (iv) degradation is more at lower concentrations but concentration is not a sensitive variable, and (v) the chain scission occurs randomly. The Mark-Houwing relationship for PVAc in THF at 25°C was derived as [η] = 2.47 × 10^?4 × M?_v^0.644.     

Shear degradation of vinylidene chloride copolymers

Thermally induced dehydrochlorination is a well-established and prominent degradation mode for vinylidene chloride copolymers. During extrusion, other processes may represent significant degradation pathways. Under shear in air, both oxidative chain-scission and cross-linking are prominent processes for both vinylidene chloride/vinyl chloride and vinylidene chloride/methyl acrylate copolymers. Both processes are dependent upon shear rate and temperature. The shear-stress dependency can be modeled by a kinetic expression that incorporates shear stress into the Arrhenius preexponential factor. Vinylidene chloride/methyl acrylate copolymers appear to be somewhat more susceptible to oxidative chain-scission than are comparable vinylidene chloride/vinyl chloride copolymers, presumably because of a rapid oxidative attack at exposed methyl acrylate units. Shear-induced degradation of these materials in air is characterized by early predominant chain-scission with cross-linking assuming a greater importance as a function of time. Degradation under shear in a nonoxidative environment is a much simpler process—oxidative chain-scission is suppressed and cross-linking is very similar to that observed in air. © 1994 John Wiley & Sons, Inc.     

Remote inhibition of polymer degradation

Polymer degradation has been explored on the basis of synergistic infectious and inhibitive interaction between separate materials. A dual stage chemiluminescence detection system with individually controlled hot stages was applied to probe for interaction effects during polymer degradation in an oxidizing environment. Experimental confirmation was obtained that volatile antioxidants can be transferred over a relatively large distance. The thermal degradation of a polypropylene (PP) sample receiving traces of inhibitive antioxidants from a remote source is delayed. Similarly, volatiles from two stabilized elastomers were also capable of retarding a degradation process remotely. This observation demonstrates inhibitive cross-talk as a novel interactive phenomenon between different polymers and is consequential for understanding general polymer interactions, fundamental degradation processes and long-term aging effects of multiple materials in a single environment.     

Thermo-mechanical degradation of polymer blends

Polymer blends can undergo, during processing, degradation phenomena because of the presence of both temperature and mechanical stresses. With respect to pure homogeneous polymers, the degradation of polymer blends shows distinct features because of the possible interaction between the different chemical species. These interaction can give rise to both a faster degradation or to the formation of copolymers which act as stabilizing agents. This latter phenomena is particularly important in the processing of condensation polymers     

Characterization of drag reduction and degradation effects in the turbulent pipe flow of dilute polymer solutions

Turbulent drag reduction data were obtained at Re = 9000 in a 0.62-cm-I.D. pipe for five Polyox compounds covering a wide range of molecular weights. The concentration dependence of drag reduction was shown to obey an improved form of Virk's drag reduction equation, which was previously applied only to flows in capillary tubes. The efficiency of the drag-reducing polymer additives on a unit concentration basis at infinite dilution was determined by using a characteristic parameter, DR_m/[c], for each compound. A linear relationship was found to exist between this parameter and polymer molecular weight. The polymer degradation data were analyzed through use of a variable related to the dissipated energy in the wall region. The polymer molecular weight was found to decrease as a hyperbolic function of the dissipated energy function. By examining the change of molecular weight with respect to this function, a degradation index characteristic of the entire Polyox polymer family was established. This index may be of general application and provide a method by which the shear stability of various species of drag-reducing polymers may be meaningfully compared.     

Ternary hydrogen-bonded polymer solutions

Ternary-phase diagrams have been experimentally determined at 100°C for systems containing a series of poly(n-alkyl methacrylates), poly(ethylene oxide) (PEO), and a solvent [4-ethyl phenol (EPh)]. A totally miscible phase diagram is experimentally determined for the poly(methyl methacrylate)/PEO/EPh system, while a closed-loop diagram is observed for the analogous system containing poly(ethyl methacrylate). The corresponding phase diagrams of analogous mixtures containing poly(n-propyl methacrylate) or poly(n-butyl methacrylate) exhibit large heterogeneous areas. Theoretically predicted phase diagrams calculated using an association model developed in our laboratory are in general accord with these observations for ternary hydrogen-bonded polymer/polymer solutions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1265–1271, 1998     

Wetting by polymer solutions

Very small droplets of polystyrene dissolved in toluene were placed on a glass slide and studied under a goniometer microscope. The drops were allowed to spread and dry and their extent of spreading is plotted as a variation of the concentration of the polymer in the solution. It was observed that drops of polystyrene in a volatile solvent such as toluene, spread to some extent and then gets dried up thus preventing the contact line from moving further. When liquid PDMS in non-volatile solvent, HMDS, and in volatile solvent, toluene, was used, the droplets continued to spread. One conclusion among others, is that even if the droplet dries but leaves behind a mobile contact line then the solution spreads out, on the other hand if the droplet dries and the contact line loses its mobility, the spreading automatically comes to a stop.     

Diffusion in polymer solutions

Experiments using diffusion controlled electrolysis have been carried out in flow and in non-flow situations for the measurement of diffusivities in pure solutions and in solutions with polymeric additives. The non-flow cell was found to be unreliable and time consuming in operation and was not capable of yielding data on the effect of shear rate upon the diffusivity.The flow system gave results for pure solutions which were in agreement with previously published information and no effect of shear rate upon diffusivity was observed. With polymeric additives some solutions showed a significant decrease in diffusivity as the shear rate increased. Although more data are required to quantify this effect the results obtained do show the danger of using the common assumption of constant Dμ//T for such solutions.     

Chemorheological studies on polymer degradation

The degradation of dicumyl peroxide-cured natural rubber was studied by the stress relaxation method. Experiments were carried out in air, in nitrogen, and by irradiation of UV light in the temperature range of 30–200°C. We attempted to divide factors of the degradation into two parts: that caused by heat and that by light, and to study the respective degradation quantitatively. The degradation caused by UV light in nitrogen was almost independent of temperature. However, the degradation caused by UV light in air increased with temperature and the rate of degradation caused by the interaction between light and oxygen was greater at high temperature than at low temperature. It was found that scission along main chains occurs for dicumyl peroxide-cured natural rubber by photo-oxidation as well as thermal oxidation.     

Practical applications of polymer solutions

To chemists and engineers working in industry, the most important applications of polymer solutions are the experiments leading to the measurement of molecular weight, molecular-weight distribution, and other characteristic parameters of random-coil polymers. This article extends earlier reviews by describing such practical aspects of these measurements as preferred techniques;; commercially-available equipment; analysis cost, sample size, and time; limitations of the methods; and pitfalls in the interpretation of the data. For molecular-weight measurement, commonly-used techniques include end-group analysis, cryoscropy, ebulliometry, osmometry, “vapor-pressure osmometry”, light scattering, and ultracentrifugation. Measurement of certain transport properties correlating emprirically with molecular weight, such as the intrinsic viscosity, is useful if the limitations of the techniques are recognized. Currently, the more promising and powerful methods polymer fractopmatopm are cpmsodered to be column elution and gel permeation chromatography.     

Ultrasonic degradation of polysiloxane solutions

Fractions of dimethylsiloxane polymer with viscosity-average molecular weights from 2.4 × 10⁵ to 1.3 × 10⁶ were dissolved in toluene in concentrations from 0.01 to 1.0 g./dl. These solutions were subjected to ultrasonic degradation at 20,000 cps for periods extending to as much as 960 min. in a water-cooled reaction vessel. Gel permeation chromatography shows that samples with different initial distributions approach the same distribution after 120 min. of degradation. Dilution of the polymer from 1.0 to 0.01 g./dl. more than triples the degradation per gram of polymer but requires 10 times as much energy per gram of polymer to degrade from a number-average molecular weight of 10⁶ to 10⁵. Increasing the power input from 36 to 72 w. almost doubles the degradation rate per gram of polymer.     

Mechanochemical degradation of an EPDM polymer

A mill-mastication study of an EPDM polymer (DuPont, Nordel 1070) was conducted in the mill-roll temperature range of 68–480°F. The extent of degradation was determined by dilute-solution viscosity measurements. The role of oxygen in the polymer mastication was followed by infrared spectroscopy. The breakdown of the EPDM polymer on the mill is minimum in the temperature range of 185–315°F. Up to 315°F. the increase in temperature leads to a decreased amount of degradation. During cold mastication mechanical breakdown occurs. The use of a free-radical acceptor shows that this type of breakdown is caused by the mechanical rupture of C? C bonds in the polymer chain. At and above 350°F. thermooxidative degradation becomes dominant, the polymer degrading drastically, and the higher the temperature, the greater the extent of degradation for the same period of mastication. Infrared spectroscopy shows that hot mastication results in decreased double-bond concentration and increased amounts of carbonyl and, possibly, anhydride and lactone groups. Of the carbonyl groups formed 30% are due to the oxidation of double bonds in terpolymer and 70% to the oxidation of the main chain. A mechanism is proposed to account for these observations.