Preparation, characterization, solution properties and rheological behaviour of polyacrylamide

In this report a survey is given on structure and properties of polyacrylamide homopolymers (PAAm) in solution. However the review is restricted to all those papers, where a molecular characterization of the polymers has been achieved as a basis to correlate this fundamental information with applicational properties.

Different polymerization methods are summarised in brief, the preparation and solution structure of long chain branched polyacrylamides as well as chemical modification reactions of linear PAAm are also mentioned. A number of experimental characterization methods (GPC, ultracentrifugation, intrinsic viscosity, and light scattering measurements) are described with special emphasis on the difficulties of the different procedures including some proposals for properly designed experimental techniques. The state of solution is discussed in view of experimental data obtained with different solvents. Moreover viscosity constant ø is calculated for aqueous solution and the unperturbed dimensions are estimated. All available data on cross correlations (e.g. [η] - M, S₀ - M, 2>^1/2 - M) are collected with the intention to give a survey of established relations and, comparing the given relationships, to suggest the reliable ones of them.

The phenomenon of long-term viscosity decrease of aqueous PAAm solutions has been investigated and discussed with regard to its molecular origin.

The viscoelastic properties are discussed in dependence on molecular weight, concentration, solvent quality, and shear rate (10⁶ · s⁻¹). Based on these data a simple equation was developed oped for the η₀-c-M relationship, which can be applied to other polymer systems as well. It is further described that the elastic nature (first normal stress difference) may overwhelm the viscous nature (shear stress) at relatively low shear rates. This high elasticity can cause deviation from laminar flow conditions. Moreover, it can be demonstrated — based on instationary measurements as well as the comparison of steady shear flow with dynamic rheology — that energetic interactions (H-bonds) strongly influence the rheological behaviour.     

Comparison of the ATP version of the EMTP and the NETOMAC programfor simulation of HVDC systems

This paper investigates the capabilities and limitations of the EMTP and the NETOMAC program as applied to HVDC system simulation. The fundamental differences between the two programs and their effect on simulation results are described. Consistency of the results obtained from these programs is examined through simulation of a test HVDC network. As expected, a very high degree of agreement between the two sets of simulation results proved to be achievable, but only when particular care was taken to overcome internal program differences. Finally, the new advanced stability feature of NETOMAC is briefly discussed and then tested against the complex transient models established in the EMTP and in the NETOMAC transients program section     

Viscoelastic properties of linear chain macromolecular gels. Growth of stress at the onset of steady shear flow

The growth of stress (shear stress and normal stress) at the onset of steady shear flow was investigated for linear chain macromolecular fluids to determine nonlinear viscoelasticity. The polyacrylamides (PAAm) were dissolved in H₂O, formamide and ethyleneglycol. The polymers were prepared in this laboratory. They were unbranched and free of initiator. The polystyrene (PS) samples from Pressure Chemical Company were studied in decalin solutions. PAAm shows increasingly typical gel character—in all 3 solvents—with increasing molecular weight and polymer concentration. For homogeneous systems gel fracture and undershoot were observed. Further for the first time overshoot due to entangled molecules and overshoot due to energetically associated chains have been distinguished. This behaviour is a strong contrast to the PS/decalin system and demonstrates the strength of the second valence bondings in the PAAm-systems.     

Influence of the glycosidic linkage on the solution conformation of glucans

In order to gain a better understanding of the influence of β-glycosidic linkages on the solution conformation of glucans, a comparison was made between the carboxymethyl derivatives of cellulose (CMC, 100% 1,4-), barley glucan (CMGG, 30 : 70 1,3 : 1,4-) and curdlan (CMCd, 100% 1,3-glycosidic), each with an average degree of substitution of DS = 1.0–1.5 and molar masses of 144 000–720 000 g mol^–1. For this purpose curdlan and barley glucan were chemically modified, whereas carboxymethylcellulose was available as a commercial sample. Via congo red analysis it was proved for the microbial β-1,3-glucans curdlan (unsubstituted) and scleroglucan (one β-1,6-linked glucose side group on every third monomer unit) that substituents in glucan derivatives weaken an existing helical structure but that reduction of the molar mass with ultrasound has no influence on the helices. The molecular parameters M_w, M_n, R_G, L_P and [η] were determined by means of SEC/MALLS measurements and viscometry and the chain mobility was quantified using the ¹³C NMR relaxation times, T₁, of the different polymers. From this it was possible to derive [η]-M_w relationships. For all samples, the exponent ν of the R_G-M_W relationship lay in the range 0.54–0.59, from which it is possible to deduce a coiled solution structure. However, β-1,3-glucans occupy 25% less volume than β-1,4-glucans and exhibit less chain mobility. Hence β-1,4-glucans should be considerably more viscous than β-1,3-glucans, as was indeed demonstrated by rheological flow curves of 6% solutions of the carboxymethylglucan derivatives. The zero-shear viscosity, η₀, decreases by 3 decades in the order of CMC > CMGG > CMCd.     

Rheologische Untersuchungen zum Einfluß von Hydrokolloiden auf die Gefrier-Tau-Stabilität von Stärkekleistern

Rheological Examination of the influence of Hydrocolloids on the Freeze-thaw-stability of Starch Gels. On thawing, frozen food containing starch often exhibits synaeresis, which is clearly related to structural changes. In order to determine the extent of synaeresis generated by freeze-thaw-cycles (FTC), rheological measurements were conducted with potato starch, waxy rice starch and 4 different maize starches. The relative growth of the storage modulus due to an FTC was defined as a measure of the freeze-thaw-stability (FTS). A value of less than 1 indicates an adequate FTS. Comparison of different maize and waxy maize starches revealed no correlation between FTS and amylose content. Potato starch, which exhibits very poor FTS, was chosen as a medium to examine the influence of hydrocolloid addition on the freeze-thaw-properties. First no or only little improvement of FTS was detected when examining tara gum, l-carrageenan and locust bean gum. Maximum FTS was achieved with mixtures of 5%-gels containing 10% of hydrocolloids as either l-carrageenan or locust bean gum. On the basis of the viscoelastic data, the mechanism by which hydrocolloids improve the FTS was deduced as a combination of network coupling and exclusion effect.     

Gas-liquid mass transfer in the bubble column with viscoelastic liquid

The effects of liquid-phase viscosity, pseudoplasticity, and viscoelasticity on gas-liquid mass transfer in a bubble column have been investigated using aqueous solutions of sucrose, xanthan, polyacrylamide, and mixtures of xanthan and polyacrylamide. The elastic properties, considered in the form of the Weissenberg number, decreased the volumetric mass transfer coefficient by a factor of up to 5. The effect was strong even at low Weissenberg numbers encountered in many liquids, the elasticity of which is usually neglected.     

[η]-M-relationship for polyacrylamide in aqueous 0.1 M Na2SO4 solution

Summary The intrinsic viscosity [] and the molecular weight M_W of 8 pure, unbranched polyacrylamide (PAAm) samples — prepared in our laboratory — were determined and lead to the following formula [] = 0,0194 · M_W ^0,70(cm³/g) Solvent: aqeous 0.1 M Na₂SO₄ solution; T=298K; M_W/M_n-2,5 . In addition a comparison to the []-M-relationships of PAAm in other solvents is given.