Maria Bercea

In situ gelation of aqueous solutions of entangled poly(vinyl alcohol)

Rheological behaviour of poly(vinyl alcohol) (PVA) aqueous systems was systematically investigated at 30 °C for different concentrations, molecular weights and hydrolysis degrees. The viscosity of very dilute polymer solutions was studied by means of viscometry, while the concentrated solutions were analyzed by steady shear flow measurements. The limit between the entangled and non-entangled states of PVA aqueous solutions was determined as the crossover of two scaling laws describing the dependence of specific viscosity as a function of coil overlap parameter, c[η]. Then, physical gels with good elastic properties were prepared in situ by freezing/thawing/ageing (200 min per cycle) of entangled PVA solutions. The influence of each stage of the applied cryogenic treatment on the gel formation and elastic properties of the final network was followed by means of dynamic rheological measurements at low strain, in the linear domain of viscoelasticity. It was shown that the gel properties largely depend on the initial state of PVA solutions, as described by the c[η] value, and on the degree of hydrolysis, as well as on the thermal history, i.e., the number of cryogenic cycles, thawing rate, and ageing step. For a given coil overlap parameter, the elastic modulus of cryogels tends to a limiting value, which can be reached faster by adding an ageing sequence to the classical freezing/thawing cryogenic cycles. This maximum value of the elastic modulus increases with increasing the coil overlap value of the initial solution.

Intrinsic viscosity and conformational parameters of xanthan in aqueous solutions: Salt addition effect

The intrinsic viscosity and conformational parameters of xanthan in aqueous solutions were investigated at 25°C as a function of salt nature (NaCl and KCl) and concentration (up to 3×10(-1)mol/L). The viscometric parameters were evaluated by applying semi-empirical equations proposed by Rao and Wolf. The results show that the new model proposed by Wolf provides accurate intrinsic viscosity values comparable with those obtained by using traditional methods. The experimental data were modeled with Boltzmann sigmoidal equation. The stiffness parameter, hydrodynamic volume and viscometric expansion factor were determined and discussed. With increasing salt concentration, the hydrodynamic volume and the viscometric expansion factor decrease and the critical overlap concentration increases, reaching limiting values above a given salt concentration. The high Huggins constant values suggest the existence of aggregates for salt concentrations above 5×10(-2) and 3×10(-3)mol/L for NaCl and KCl, respectively. Stiffness parameter was determined by Smidsrød and Haug method as being 5.45×10(-3), indicating a rigid conformation for xanthan macromolecules in solution.

Viscometric study of extremely dilute polyacrylonitrile solutions

Abstract The critical overlapping concentration c′ which separates the dilute and extremely dilute regimes for polyacrylonitrile in dimethylformamide was studied by viscometry. A scaling law between the reduced viscosity ηsp/c and the concentration for the extremely dilute regime was found and the deviation from the Huggins dependence was discussed.

Poly(vinylpyrrolidone) – A Versatile Polymer for Biomedical and Beyond Medical Applications

The high versatility of poly(vinylpyrrolidone) (PVP) can be explained by its diverse properties including its solubility in water and in a broad range of liquid media, high chemical and thermal resistance, and unique wetting, binding, and film-forming properties. Thanks to biocompatibility, absence of toxicity and high capacity to form interpolymer complexes, PVP is widely used for designing materials for different applications, such as biomaterials for medical and nonmedical uses. This review summarizes a vast diversity of applicative examples showing the tremendous opportunities for future research and developments of PVP based biomaterials. GRAPHICAL ABSTRACT

Effect of Temperature and Aging Time on the Rheological Behavior of Aqueous Poly(ethylene glycol)/Laponite RD Dispersions

The viscoelastic properties of 2% poly(ethylene glycol) aqueous solutions containing Laponite RD from 1% to 4% were investigated by oscillatory and flow measurements in the temperature range of 15-40 °C. The enhancement of the clay content from mixture causes the increase of the viscoelastic moduli and the change of the flow from liquid-like behavior (Maxwellian fluid) to a solid-like one at a set temperature. The longest relaxation times (τ(1)) of the mixtures with low clay concentrations (1% and 2%) are not affected by changes in temperature unlike the samples having high content of clay at which τ(1) increases above 30 °C and below 17.5 °C. The characteristic behavior of the mixtures with the high clay concentration could be explained by considering the effect of Brownian motion on the network structure formed in these dispersions as well as by the poor solubility of poly(ethylene glycol) in water at high temperatures. The flow activation energy was determined and discussed. An abrupt increase of the flow activation energy was evidenced between 2% and 3% Laponite RD. The rheological measurements carried out at different rest times showed a decrease of the gelation time from 1 week to 2 h when the clay concentration increases from 2% to 4%. The aging kinetics of poly(ethylene glycol)/Laponite RD/water mixtures, investigated at 25 °C, revealed the increase of the viscosity-rate kinetic constant by increasing the clay concentration.

Ultrahigh molecular weight polymers in dilute solutions

Abstract The article reviews the thermodynamic aspects of ultrahigh molecular weight polymers in dilute solutions. The investigation of these compounds is much more complicated as compared to the study of usual length polymer–solvent systems. The restrictions applying to the establishment of concentration regimes, short range interactions (theta condition, unperturbed dimensions, chain flexibility, the transition from theta to collapsed regime), and long range interactions (excluded volume effect by intermolecular and intramolecular interactions, conformational characteristics and conformational transitions phenomena) are discussed. Systematic studies, based on the coupling of different experimental techniques, are described for the elucidation of diverse theoretical and experimental contributions.

Synthesis, characterization and solution behaviour of oxidized pullulan

Various amounts of carboxyl groups were introduced at C-6 of the non-ionic, water soluble polysaccharide, i.e. pullulan, by applying the well-established TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), sodium hypochlorite/sodium bromide oxidation protocol, varying the reaction time. The oxidized products, more water-soluble than pullulan, were further characterized by FT-IR, (1)H NMR and (13)C NMR techniques, in order to assess the degree of oxidation. The absolute molecular weight measurements performed using a multiangle laser light scattering molecular weight analyzer, reveals a sharp drop of the molecular weight of the samples oxidized for longer reaction times. The second virial coefficients (A2), increased from unoxidized pullulan to the oxidized samples. Dynamic light scattering (DLS) measurements provide zeta potentials and hydrodynamic radius for all studied samples. The viscosity of the initial and oxidized pullulan dilute aqueous solutions was studied in detail. All oxidized samples except the highest oxidized pullulan sample (OxPu8) showed strong polyelectrolyte behaviour, whereas this effect is less pronounced for OxPu8 due to the high degradation of the chains. The intrinsic viscosity and the interaction parameter were determined at 25 °C as a function of solvent ionic strength according to Wolf approach. The dependence of these parameters on the salt concentration follows Boltzmann sigmoid model.

Synergistic behavior of poly(aspartic acid) and Pluronic F127 in aqueous solution as studied by viscometry and dynamic light scattering.

Pluronic F127/poly(aspartic acid) mixtures were investigated in dilute solutions by viscometry and dynamic light scattering. The two polymers were chosen due to well known applications in biomedical field, taking into account the final purpose (the use of the complex structure as drug delivery systems). The central item was to identify the possibility of complexation between the poly(carboxylic acid) and a non-ionic polymer and to investigate the conditions of the interpolymer complex formation. The ability of Pluronic F127 to form micelle is well known. Poly(aspartic acid), as a polycarboxylic acid with resemblance with polyacrylic acid, can act as dispersant, antiscalant, superabsorber, being also able to form micelles. Due to its functional groups, -COOH and -NH(2), poly(aspartic acid) can make physical and/or chemical bonds with other macromolecular compounds. The intrinsic viscosity and the dynamic light scattering data obtained for PLU/PAS mixtures at 25 °C have shown that interpolymer complex formation occurs around 1/1 wt. ratio between the two polymers.

High molecular weight polystyrene in solvent mixtures. Preferential and total adsorption

Abstract The behaviour of high molecular weight polystyrene in three binary solvent systems, i.e. cyclohexane/benzene (theta solvent/solvent), cyclohexane/acetone (theta solvent/nonsolvent) and benzene/methanol (solvent/nonsolvent) is studied by viscometry and refractometry. The experimental results on the preferential and total adsorption coefficients are discussed and compared to those predicted by different theoretical treatments.

Conformational characteristics of oligo- and polyacrylonitrile in dilute solution

Abstract The paper is concerned with the conformational characteristics of oligo- and polyacrylonitrile in dilute solution as evidenced by gel permeation chromatography (GPC) and viscometric data. The effects of chain ends and chain stiffness on the Mark–Houwink dependencies are discussed for a large domain of molecular weights.