Gel permeation chromatography of high molecular weight cellulose trinitrate

Gel permeation chromatography measurements have been made upon cellulose trinitrate in ethyl acetate using samples which have already been well characterized by light scattering and osmometry. Columns were calibrated using polystyrene standards and it has been shown that the hydrodynamic volume calibration applies to cellulose trinitrate provided that all the data are extrapolated to zero concentration. It has also been shown that the resolution of the columns (as measured by the rate of change with count of logarithm of intrinsic viscosity), and the deviation from the true value of the apparent hydrodynamic volume at any given concentration depends only upon the relative viscosity of the sample. The validity of the hydrodynamic volume calibration does not necessarily mean that both polymers are fractionated solely by an exclusion process. It is possible that both react reversibly with the gel, and that there are compensatory hydrodynamic effects.     

Shear and elongational rheology of polyethylenes with different molecular characteristics. I. Shear rheology

Four metallocene polyethylenes (PE), one conventional low density polyethylene (LDPE), and one conventional linear low density polyethylene (LLDPE) were characterized in terms of their complex viscosity, storage and loss moduli, and phase angle at different temperatures. The effects of molecular weight, breadth of molecular weight distribution, and long‐chain branching (LCB) on the shear rheological properties of PEs are studied. For the sparsely long‐chain branched metallocene PEs, LCB increases the zero‐shear viscosity. The onsets of shear thinning are shifted to lower shear rates. There is also a plateau in the phase angle, δ, for these materials. Master curves for the complex viscosity and dynamic moduli were generated for all PE samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Shear induced vortices in high molecular weight polyethylene oxide melt

Summary The vortices in laminar flow of high molecular weight polyethylene oxide melt under shear were studied using small angle light scattering technique and applying the statistical and model aproaches. It was found that vortices are aggregates of large number of molecules with the core of fluctuating density and molecular orientation of surroundings due to this rotation. Perpendicular dimensions of the more dense core is 4.3 m and for oriented surroundings 7,5 m.Longitudinal dimensions are 8 m and 20–35 (depending on shear rate) for the dense and oriented parts, respectively.     

Shear rheology and melt compounding of compatibilized‐polypropylene nanocomposites: Effect of compatibilizer molecular weight

Direct melt compounding was used to prepare nanocomposites of organophilic montmorillonite (o‐mmt) clay dispersed in maleated polypropylenes (PPgMA) as well as nanocomposites of organoclay and polypropylene (PP) modified with various grades of PPgMA compatibilizers. The thermal effect on the rheology and melt compounding was first investigated with a plasticorder. The shear viscosities and the melt flow indices (MFI) of the PPgMA compatibilizers were sensitive to the blending temperature, which had to be varied with the compatibilizer grade to achieve desirable level of torque for extensive exfoliation of organoclay in the plasticorder. However, for low molecular weight oligomer, the clay dispersion was poor because of low shear viscosity and thermal instability. Next, the PPgMA‐modified PP/organoclay nanocomposites were prepared on a corotating twin‐screw extruder. The nanoscale dimensions of the dispersed clay platelets led to significantly increased linear viscoelastic properties, which were qualitatively correlated with the state of exfoliation in the nanocomposites. The relative viscosity (relative to the silicate‐free matrix) curves revealed a systematic trend with the extent of clay exfoliation. Furthermore, the degree of clay dispersion was found to increase with the loading of compatibilizers; however, high loading of compatibilizer compromised the final moduli of the nanocomposites. POLYM. ENG. SCI. 46:289–302, 2006. © 2006 Society of Plastics Engineers     

Shear and elongational rheology of polyethylenes with different molecular characteristics. II. Elongational rheology

The elongational viscosities of polyethylenes with different molecular characteristics were measured at different Hencky strains and temperatures with a capillary rheometer by the replacement of the capillary cylindrical die with a hyperbolic converging die. The hyperbolic shape of the die established a purely elongational flow field at a constant elongational strain rate throughout the die. The effects of molecular characteristics such as the molecular weight, molecular weight distribution, and long‐chain branching and processing conditions such as the temperature and Hencky strain on the elongational rheology of the polyethylene samples were studied. Good master curves were generated for temperature and Hencky strain shifting and simultaneous shifting with respect to both the temperature and Hencky strain. Both the molecular weight distribution and long‐chain branching seemed to promote strain rate thinning and reduce the elongational viscosity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1184–1194, 2007     

The effect of molecular weight and weight distribution upon polymer melt rheology

A major objective in polymer rheology is to predict a fluid's response to a general deformation from molecular information. A method has been developed which allows one to predict the viscoelastic properties of polymer melts from a limited amount of rheological and molecular data for the polymer. The input parameters are: (a) zero-shear viscosity; (b) molecular weight distribution; (c) temperature and density; and (d) constants relating Graessley's relaxation time to the Rouse relaxation time. The technique then “simulates” a discrete relaxation spectrum using G′ and G″ data from the Rouse theory and finally requires that a continuum model of polymer viscoelasticity be fit to shear viscosity data predicted by Graessley's theory. Examples of the utility of the procedure are given to illustrate the role of molecular weight and weight distribution in determining rheological behavior.     

Polymer rheology: Influence of molecular weight and polydispersity

Literature data on the non-Newtonian flow of bulk polymer and of polymer solutions are correlated on the basis of a four-parameter equation, η = η_∞ + (η₀ ? η_∞)/[1 + (τD)^m], η being the viscosity at shear rate D, and η₀ and η_∞ limiting values at D = 0 and D = ∞, respectively. The parameters η₀, η_∞, and τ all show dependence on molecular weight, and in general there is good correlation between τ and η₀. There is evidence that τ is related to a molecular weight higher than the weight-average. The exponent m shows dependence on molecular weight distribution and approaches an upper limit of unity for a monodisperse linear polymer. For linear unblended polymers it may be expressed empirically by m = (M?_n/M?_w)^1/5.     

Viscosity‐molecular weight relationship for cellulose solutions in either NMMO monohydrate or cuen

The intrinsic viscosities, [η], of nine cellulose samples, with molar masses from 50 × 10³ to 1 390 × 10³ were determined in the solvents NMMO*H₂O (N‐methyl morpholin N‐oxide hydrate) at 80°C and in cuen (copper II‐ethlenediamine) at 25°C. The evaluation of these results with respect to the Kuhn–Mark–Houwink relations shows that the data for NMMO*H₂O fall on the usual straight line in the double logarithmic plots only for M ≤ 158 10³; the corresponding [η]/M relation reads log ([η]/mL g⁻¹) = –1.465 + 0.735 log M. Beyond that molar mass [η] remains almost constant up to M ≈ 10⁶ and increases again thereafter. In contrast to NMMO*H₂O the cellulose solutions in cuen behave normal and the Kuhn–Mark–Houwink relation reads log ([η]/mL g⁻¹) = −1.185 + 0.735 log M. Possible reasons for the dissimilarities of the behavior of cellulose in these two solvents are being discussed. The comparison of three different methods for the determination of [η] from viscosity measurements at different polymer concentrations, c, demonstrates the advantages of plotting the natural logarithm of the relative viscosities as a function of c. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Rheology of 1‐butyl‐3‐methylimidazolium chloride cellulose solutions. I. Shear rheology

In this article, shear rheology of solutions of different concentrations obtained by dissolution of cellulose in the ionic liquid (IL) solvent 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl) was studied by measuring the complex viscosity and dynamic moduli at different temperatures. The obtained viscosity curves were compared with those of lyocell solutions and melt blowing grade polypropylene melts of different melt flow rates (MFR). Master curves were generated for complex viscosity and dynamic moduli by using Carreau and Cross viscosity models to fit experimental data. From the Arrhenius plots of the shift factors with respect to temperature, the activation energies for shear flow were determined. These varied between 18.99 and 24.09 kCal/mol, and were compared with values for lyocell solutions and different polymeric melts, such as polyolefins, polystyrene, and polycarbonate. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Controlled rheology of polypropylene: Modeling of molecular weight distributions

A mathematical model for the controlled degradation of polypropylene is presented in this article. A previous model of this process was extended to predict the whole molecular weight distribution of the modified resin. Probability generating functions were applied to transform the infinite set of mass balance equations of both polymer and radicals. The integration of the transformed set of equations yielded the probability generating function transforms. These transforms were then inverted with two different inversion algorithms, recovering the molecular weight distributions of the polymer. The model predictions were compared with our experimental data and other information taken from the literature. Good agreement was obtained. The approach presented here is also useful for other polymerization and postpolymerization processes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1676–1685, 2003     

The effect of molecular weight blending on PVC melt rheology

The effect of molecular weight blending on melt flow characteristics has been studied with a 50/50 mixture of suspension PVC resins with the respective M _w of 56,300 and 123,000. The dynamic shear measurements were made with the Rheometrics Visco-Elastic Tester at angular frequencies of 0.1 to 40 radians/s. In the temperature range of 160 to 215° C, all samples showed three distinct flow regions marked by three different values of the activation energy. The high molecular weight fraction introduced a relatively strong influence on the melt flow characteristics of the blend due to the effect of its relatively high crystalline content. These samples also failed to show a Newtonian flow behavior at 190°C at an extremely low shear rate corresponding to 10^?4 radians/s., possibly reflecting the effect of the remnant crystallinity of the material.     

Rheological and rheo-optical properties of high molecular weight syndiotactic and atactic polyvinylalcohol solutions

The rheological and rheo-optical properties of solutions of high molecular weight polyvinylalcohol (PVA) with different syndiotactic diad contents in dimethylsulfoxide (DMSO) were investigated in terms of tacticity, molecular weight, and degree of saponification. Tacticity played a significant role in rheological behavior. Over the frequency range of 10^?1 to 10² rad/s PVA with syndiotactic diad content of ～53% (atactic PVA) exhibited almost Newtonian flow behavior whereas PVA with syndiotactic diad content of ～63% (syndiotactic PVA) exhibited Bingham flow behavior. On the plot of storage modulus (G′) against loss modulus (G″) atactic PVA gave slopes of ～2 while syndiotactic PVA gave slopes of ～1. With syndiotactic PVA, an increase of shear rate notably increased flow birefringence (Δn_f) at shear rates higher than 5 sec^?1. On the other hand, only a slight increase in Δn_f was observed in the case of atactic PVA. The effects of molecular weight and degree of saponification were discussed as well.     

The influence of molecular weight on the melt rheology of polypropylene

Melt viscosity and melt elasticity data were obtained over a broad range of temperatures and shear rates on a series of four polypropylenes of different molecular weight but approximately the same molecular weight distribution. The superposition technique was used with both temperature and molecular weight to shift flow curves for all four materials at three temperatures each along the shear rate axis to generate a master flow curve at a given temperature and molecular weight. For polypropylenes of this type, and molecular weight distribution shift, factors which can be used to extend the useful range of experimentally obtained flow data were determined. The dependency of apparent viscosity on weight average molecular weight at shear stresses as high as 10⁶ dynes/cm² is shown. The dependency of melt elasticity on molecular weight and temperature is discussed.     

Analysis of molecular weight distribution of purified cellulose pulp

The molecular weight distribution of purified cellulose pulp is analyzed by using empiric distribution functions. The data are also analyzed by the direct binomial method and the results are compared with those obtained by the method of Beall.     

Melt rheology of segmented polyamides: Effect of block molecular weight

Segmented polyamides, also known as polyether‐ester‐amides, are composed of polyether and polyamide structural units. The rheological behavior of segmented polyamides with respect to the variations in the molecular weight of hard and soft blocks has been studied using a Monsanto Processability Tester. These systems exhibit pseudoplastic flow behavior. The shear viscosity of the segmented polyamides decreases with a decrease in hard block molecular weight up to 1500. However, at low shear rates, the shear viscosity shows marginal change with an increase in soft segment molecular weight. The equilibrium die swell increases with an increase in shear rate, but decreases with increasing temperature. The stress relaxation study of the segmented polyamides reveals that the stress developed during extrusion relaxes exponentially for all the systems. The equilibrium die swell at a fixed temperature and shear rate, the time required to relax a fixed amount of stress and the stress developed after a certain time interval decrease with a decrease in hard block molecular weight up to 1500, but increase with an increase in soft segment molecular weight. The activation energy of the melt flow process increases with the rate of shear in most of the cases. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1739–1747, 1999     

Thermal properties,rheology and sintering of ultra high molecular weight polyethylene and its composites with polyethylene terephthalate

The addition of polyethylene terephthalate (PET) fibers in ultra high molecular weight polyethylene (UHMWPE) may be a promising approach to achieve improved wear properties in artificial joints. Since UHMWPE/PET composites are processed by compression molding, which involves compaction and sintering of polymeric powders, this article investigates their rheology, thermal properties, and sintering behavior to aid in the identification and selection of optimum processing conditions. Isothermal crystallization kinetics studies have revealed that crystallization of UHMWPE proceeds via heterogeneous nucleation and is governed by two‐dimensional growth. The crystallization rates of the composites were lower than those of the neat material, whereas their ultimate crystallinities were higher. The UHMWPE/PET composites had higher viscosity and elasticity than the neat resin. In the presence of PET fibers the onset of sintering took place at higher temperatures but proceeded at substantially higher rates as compared with pure UHMWPE. A marked discrepancy between the Eshelby‐Frenkel model and experimental sintering data suggests that viscous flow is not the prevailing mechanism for coalescence but rather that enhanced surface area, attributed to the highly developed internal morphology of UHMWPE particles, is the controlling factor. POLYM. ENG. SCI., 45:678–686, 2005. © 2005 Society of Plastics Engineers     

Unusual rheology behaviour of ultra high molecular weight polyethylene/kaolin composites prepared via polymerization‐filling

The rheological behaviour of ultra high molecular weight polyethylene (UHMWPE)/Kaolin composites prepared by a polymerization‐filling process was investigated by conducting capillary and dynamic rheology tests. The results showed that the addition of Kaolin could significantly improve the processability of UHMWPE composites. The common viscoelastic flow was discovered for UHMWPE composites in capillary extrusion testing. The further discussion showed that the unique microstructure provided by polymerization‐filling of UHMWPE/Kaolin composites led to this unusual rheology behaviour. Copyright © 2003 Society of Chemical Industry     

Light-scattering studies on solutions of high and low molecular weight polystyrene mixtures used as models of microgel-containing solutions

Diluted solutions of linear polystyrene (PS) in toluene and dioxane were studied by the light-scattering method. The solutes were mixtures of high-M?_w and low M?_w PS. The dissolved PS mixtures were regarded as polymer solutions containing microgels, the high-M?_w PS being looked upon as the microgel counterpart. The calculation method as proposed by Strazielle¹ and Burchard² was used to evaluate the microgel percentage and particle size, whereby the method could be verified against mixtures with well-known weight composition and \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {\left( {r_g ^2 } \right)} ^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $\end{document}. The \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {\left( {r_g ^2 } \right)} ^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $\end{document} values evaluated for the mixtures from the experimental data were compared with those estimated from the molecular weights of the components, their weight concentrations, and their \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {\left( {r_g ^2 } \right)} ^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $\end{document} values. The method^1,2 was found to be useful for evaluating the microgel content in a sample, but not for \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {\left( {r_g ^2 } \right)} ^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $\end{document} values as calculated by Guinier's procedure nor those calculated by Zimm's procedure; the former were low and the latter were even incongruous. A comparative analysis of the theoretical function P^?1(θ)-versus-sin² (θ/2) and experimental (Kc/R(θ))_c=0-versus-sin² (θ/2) curves allowed to discuss the effect of the course of these curves at samll angles from 0° to 30° on M?_w and \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {\left( {r_g ^2 } \right)} ^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $\end{document} as determined for the high and low molecular weight polystyrene mixtures in toluene as solvent.