Non-Newtonian viscosity and non-linear flow birefringence of cellulose tricarbanilate

The shear rate dependence of the intrinsic viscosity and of the intrinsic property, which can be derived from the extinction angle, was investigated for a sample of cellulose tricarbanilate. Its number-average molecular weight was 4.4 × 10⁵; the solvents used were dioxane at 25°C and phenyl benzoate at 75°C, the latter being known to break most intramolecular hydrogen bonds. The onset of non-Newtonian viscosity was found at a reduced shear rate of about 0.1, whereas non-linear behaviour in the extinction angle was noticed already at 0.02. The obtained data were compared with the theory by Noda and Hearst¹⁰, which deals with the influence of chain stiffness on the viscoelastic properties of polymer solutions. The qualitative outcome of this theory agrees fairly well with the experimental results. However, the onset of non-linear behaviour, especially in the extinction angle, occurs at lower values of the reduced shear rate than predicted. The stress—optical law remains surprisingly valid for this stiff polymer throughout almost the whole accessible range of reduced shear rates. However, some experiments with the highly viscous solvent tri-o-cresyl phosphate at 30°C show appreciable deviations at reduced shear rates larger than 1.0.     

Flow birefringence in polymer rheology

Based on a linear stress-optical relation, off-center and centerline velocities along with the orientation angle measurements (via the birefringence method) can be used to test constitutive equations for polymeric melts without the need to measure viscosity hood stresses directly. For polydimethylsiloxane (Dow Corning type 200 grade 300 Pa · s) melt in a 60° wedge flow cell (inward flow) at a pressure drop of 70 psi, the orientation angles computed by the Power-Law model and Goddard-Miller model with two terms in the Taylor Series and a single relaxation time of 0.005 s were compared with the experimentally measured values. It was found that the Goddard-Miller model agreed quite well with the measured data. The stress-optical coefficient (C) was evaluated from the velocity, birefringence, and orientation data. It was calculated to be 1.44 × 10^?10 m²/N for combined shear and extension using the two-term Goddard-Miller Model. For centerline flow (purely extensional), the zero-extension-rate elongational viscosity (η ₀) was calculated to be 930 Pa · s, slightly greater than three times the zero-shear-rate viscosity (η ₀). The cell can be adapted for transient flow experiments.     

Flow birefringence and viscosity of cellulose solutions in semi-dilute regime

An attempt was made to study the flow birefringence and the viscosity of the systems of cellulose in aqueous sodium hydroxide and cadoxen solutions. For this purpose alkali-soluble cellulose samples with crystal form I (simply denoted as cellulose I sample), prepared from conifer wood pulp by the steam-explosion method, and alkali-soluble cellulose samples with crystal form of cellulose II (cellulose II sample), regenerated from cuprammonium cellulose solution under specific conditions, were used. The extinction angle χ of aqueous alkali solutions of the cellulose I sample is significantly less shear rate (γ) dependent as compared with that of the cellulose II sample. In the latter system the χ versus γ relations for a given cellulose sample shifted to the higher γ side with decrease in the average molecular weight. The viscosity of the cellulose II sample in aqueous sodium hydroxide solutions is approximately twice that of the cellulose I sample in the same solvent if compared at the same molecular weight, same concentration, and same temperature. The latter solution showed a non-Newtonian property at relatively smaller γ than the former solution did. Spin-lattice relaxation time T₁ (by ¹³C-NMR) of cellulose in cadoxen solution was smaller in cellulose I, suggesting the existence of intra- and intermolecular hydrogen bondings at the C₆ position of cellulose molecules in cellulose I solution. A dynamic light scattering study on cellulose in cadoxen showed that in a 5 wt % solution of cellulose I cellulose particles are dispersed with time into smaller particles and the larger particles could be excluded by ultracentrifuge and in cellulose II solutions the cellulose particles had almost the same size during storage. The above findings indicate that in 5 wt% cellulose I solutions in aqueous alkali or in cadoxen, cellulose I is not dissolved molecularly, but a supra-molecular structure of the solid is at least partly reserved in the above solutions.     

Gradient dependence of viscosity of cellulose tricarbanilate in tetrahydrofuran

Summary The dependence of viscosity of cellulose tricarbanilate in tetrahydrofuran on the velocity gradient D <2500 s^–1 for molecular weights up to 1.2×10⁶ was determined, and an empirical equation for the determination of the limiting viscosity number for zero velocity gradient was derived. Errors involved in the viscosity determination from a single concentration using an approximate formula were also ascertained.     

Conformational change studies by intrinsic viscosity of cellulose tricarbanilate as a function of temperature

Summary The experimental variation of the intrinsic viscosity [] as a function of temperature for various molecular weights of cellulose tricarbanilate in solution in dioxane displays a S-shaped character with two plateau regions, respectively at low and high temperatures. The use of the Yamakawa model leads to two distinct values for the chain stiffness q, one which may be_o associated to a rather rigid conformation of the macromolecule (q=139 Å), the other which may correspond to a rather flexible conformation (q = 84Å).     

Preparation of cellulose triacetate and cellulose tricarbanilate by nondegradative methods

A method for obtaining triacetate having a D.S. of 3.0 has been obtained by acetylating the cellulose with acetic anhydride–pyridine reagent containing a catalytic amount of acetyl chloride. The process is rapid and nondegradative. Cellulose tricarbanilate can be prepared without recourse to pyridine solutions by using dimethylformamide (DMF) as solvent and triethylenediamine (1,4-diazobicyclo[2.2.2]octane) as catalyst. The DMF solution of the cellulose tricarbanilate can be cast directly into films or the polymer easily recovered owing to the solubility of the reaction solvent and impurities in water.     

Nitrocellulose: birefringence and molecular conformation

Conformational data on nitrocellulose have been used to predict the variation in birefringence with degree of substitution. Of the three possible positions of the 6-nitric ester group, only the gt and tg structure are consistent with the observed birefringence variations.     

Persistence length of cellulose tricarbanilate by small-angle neutron scattering

Small-angle neutron scattering measurements were made on cellulose tricarbanilate in dioxane at different temperatures. The persistence length evaluated from the Kratky plot was found to be 110 Å at room temperature. A decrease in persistence length with increasing temperature was observed.     

Phase studies of a liquid crystalline polymer: hydroxypropyl cellulose

Phase diagrams for the isotropic-anisotropic phase transition in solutions of a thermotropic cellulose derivative, hydroxypropyl cellulose are presented. The effect of the molecular weight is demonstrated. A comparison to the theory of semi-rigid liquid crystalline polymers is given.     

Electric birefringence of cellulose trinitrate in acetone

Electric birefringence studies have been made on a sample of high molecular weight cellulose trinitrate in acetone. The molecule is shown to have only a permanent dipole moment and no significant anisotropic polarisability in this solvent. Both the rotary relaxation times obtained from transient time dependent effects and the amplitudes of the observed birefringence have been analysed in terms of theories for rigid rods, weakly bending rods, worm-like chains and both flexible and stiff random coils. The study indicates that (a) electric birefringence data are sensitive to molecular flexibility and (b) that in this solvent, nitrocellulose of 12% nitrogen content appears to be a stiff, non free draining coil with a dipole moment of the order of a few debye (i.e. ? 10^?29 C m) per monomer unit.     

The effect of precipitation on the molecular weight distribution of cellulose tricarbanilate

The effect of precipitation on the molecular weight distributions of cellulose tricarbanilate (CTC) samples prepared from α-pulp, hydrolyzed α-pulp, and Avicel was determined using size exclusion chromatography (SEC). Precipitation of the CTC samples in methanol resulted in the loss of a low molecular weight fraction accounting for 26% of the weight of the hydrolyzed α-pulp, 20% of the Avicel, and 5% of the α-pulp. Precipitation in a methanol/water mixture resulted in less fractionation; in this case, however, the reaction byproducts were also precipitated. These results indicate that the molecular weight distribution of precipitated CTC may not accurately reflect the molecular weight distribution of the original cellulose. SEC analysis of the nonvolatile products from the carbanilation reaction offers a simple method for determining the complete molecular weight distribution of this cellulose derivative.     

Molecular weight distribution of cellulose as its tricarbanilate by high performance size exclusion chromatography

Cellulose tricarbanilates (CTCs) were prepared from a range of cellulose samples (cotton linters, wood pulps, Avicel, amorphous cellulose, and cellulose II) for molecular weight distribution (MWD) studies by high performance size exclusion chromatography (HPSEC). The HPSEC columns were calibrated using CTC standards with the aid of a microcomputer. CTCs were prepared by reaction of cellulose samples with phenylisocyanate in pyridine at 80°C. For some samples, e.g., cellulose II, activation with liquid ammonia and pyridine was necessary prior to reaction in pyridine. All samples tested were also derivatised in dimethylsulfoxide at 70°C, although for high molecular weight (MW) cellulose samples some MW reduction occurred in this solvent. Conditions were determined for optimum precipitation of CTCs in aqueous methanol without coprecipitation of low MW impurities.     

Electric birefringence of aqueous solutions of methyl cellulose

Electric birefringence studies have been conducted on six samples of methyl cellulose in the molecular weight range 3 × 10⁴ to 1.5 × 10⁵. Using pulsed electric fields, both the amplitudes and rates of the transient birefringence induced in these solutions have been analysed. For molecular weights below 2 × 10³, the molecules are represented by rigid rods. Above this value increasing flexibility is evident. This can conveniently be seen through reduced electrical parameters, defined herein, which are obtained from a combination of the electrical and geometrical properties of the molecules. These parameters are apparently very sensitive to flexibility in the near-rod regime. It demonstrates the utility of electro-optic experiments in which both the electrical and geometrical data are obtained simultaneously. The persistence length concept does not appear to be adequate when rotary relaxation times are interpreted through it using current equations.     

Solubility of sodium cellulose xanthate in aprotic solvents

Conclusions The solubility of sodium cellulose xanthate of various degrees of substitution in dipolar aprotic solvents has been examined.It has been shown that a xanthate with a low degree of substitution is soluble only in dimethyl sulfoxide, while a highly substituted one dissolves also in dimethylformamide, dimethylacetamide, and N-methylpyrrolidone.A water content in the aprotic solvent improves the solubility of the cellulose xanthate, but a sharp decrease in the degree of polymerization and degree of substitution also takes place.Translated from Khimicheskie Volokna, No. 1, p. 27, January–February, 1984.     

New lignocellulose pretreatments using cellulose solvents: a review

Non‐food lignocellulosic biomass is the most abundant renewable bioresource as a collectable, transportable, and storable chemical energy that is far from fully utilized. The goal of biomass pretreatment is to improve the enzymatic digestibility of pretreated lignocellulosic biomass. Many substrate factors, such as substrate accessibility, lignin content, particle size and so on, contribute to its recalcitrance. Cellulose accessibility to hydrolytic enzymes is believed to be the most important substrate characteristic limiting enzymatic hydrolysis. Cellulose solvents effectively break linkages among cellulose, hemicellulose and lignin, and also dissolve highly‐ordered hydrogen bonds in cellulose fibers accompanied with great increases in substrate accessibility. Here the history and recent advances in cellulose solvent‐based biomass pretreatment are reviewed and perspectives provided for addressing remaining challenges. The use of cellulose solvents, new and existing, provides opportunities for emerging biorefineries to produce a few precursors (e.g. monosaccharides, oligosaccharides, and lignin) for the production of low‐value biofuels and value‐added biochemicals. © 2012 Society of Chemical Industry     

Effects of polymer solvents on the performance of cellulose acetate membranes in methanol/methyl tertiary butyl ether separation

The performances of cellulose acetate membranes prepared with casting solutions, with acetone, dimethylformamide (DMF), and N‐methylpyrrolidone (NMP) as solvents, were studied in a series of methanol/methyl tertiary butyl ether separation experiments. The flux and selectivity of the membrane samples were affected by the type of solvent used to prepare the casting solution. The sample with DMF consistently gave the highest selectivity and lowest flux, followed by the samples with NMP and acetone. The differences in the performances were attributed to the effects of the volatility and evaporation rates of the solvents. Scanning electron microscopy and atomic force microscopy techniques were used for comparing the morphologies of the membranes. In addition, we used Raman spectroscopy as a novel technique to study the sorption selectivities of the membrane samples prepared with the three different solvents. In a parallel study, the relation between the polymer concentration in the casting solution and the morphology and performance of the membrane samples was studied. Under similar preparation conditions, the morphology of the membrane changed from being porous to being dense when the membrane was prepared with casting solutions with increasing polymer concentration. Also, the selectivity increased and the permeability decreased with increasing polymer concentration in the casting solution. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2882–2895, 2001