Simona Morariu

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.

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.

Salicyl-imine-chitosan hydrogels: Supramolecular architecturing as a crosslinking method toward multifunctional hydrogels

Hydrogels based on chitosan and salicyladehyde were obtained by dynamic covalent chemistry. The unusual chitosan gelling in the presence of the monoaldehyde has been deciphered following and correlating data of NMR, FTIR, single crystal and wide angle XRD, POM and optical measurements. Of significant importance in understanding the crosslinking features was the synthesis of a model compound and the successfully growth as single crystal allowing the study of its supramolecular peculiarities. The hydrogels exhibited in SEM a porous or fibrous morphology, in good correlation with the crosslinking degree. They swelled very fast, similar to the superporous hydrogels of third generation and exhibited self-healing properties. Rheological investigation demonstrated good mechanical properties, thermosensitivity and thixotropy. The paper revealed a hydrogel with suitable properties for use in bio-medical applications, and moreover, revealed a new concept of obtaining chitosan hydrogels using monoaldehydes - which are widespread in nature, cheap and beneficial to the human body.

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.

Chitosan/Poly(Vinyl Alcohol)/LDH Biocomposites With pH-Sensitive Properties

The viscometric parameters of chitosan/poly(vinyl alcohol) mixtures determined at 37°C are sensitive to polymer composition and pH changes, giving information about interpolymer complexes formation. Hybrid composites based on interpolymeric chitosan/poly(vinyl alcohol) complexes and the inorganic matrix of layered double hydroxides were synthesized in the presence of glutaraldehyde. The viscoelastic parameters showed a stronger network formation at pH = 5.5 which is not broken down by the mechanical shear forces. The swelling kinetics of hybrid hydrogels is pH sensitive and the type of diffusion of small molecules into the network was established. GRAPHICAL ABSTRACT

Viscoelastic and structural properties of poly(vinyl alcohol)/poly(vinylpyrrolidone) hydrogels

Physical hydrogels were obtained by freezing/thawing of aqueous solutions of 15% poly(vinyl alcohol) (PVA)/poly(vinylpyrrolidone) (PVP) mixtures with different ratios between the polymers. The rheological behavior was investigated after different numbers of freezing/thawing cycles at 37 °C. An aging effect, which depends on PVP content into the polymer mixture, was evidenced as an increase of the viscoelastic moduli in time. Swelling measurements demonstrated that the hydrogels containing up to 10% PVP show a higher stability in water. Scanning electron microscopy images of the hydrogels reveal a three-dimensional structure with the pores size depending on the ratio between the two polymers. X-ray powder diffraction evidenced that the addition of PVP determines a decrease of the crystallinity of PVA gels obtained by repeated freezing/thawing cycles. Small-angle X-ray scattering measurements indicated that the maximum diameter of the scattering objects reaches a minimum value for 10% PVP in the polymers mixture.

Dual crosslinked iminoboronate-chitosan hydrogels with strong antifungal activity against Candida planktonic yeasts and biofilms.

Chitosan based hydrogels are a class of cross-linked materials intensely studied for their biomedical, industrial and environmental application, but their biomedical use is limited because of the toxicity of different organic crosslinkers. To overcome this disadvantage, a new strategy to produce supramolecular chitosan hydrogels using low molecular weight compounds able to form covalent linkages and H-bonds to give a dual crosslinking is proposed. For this purpose we used 2-formylphenylboronic acid, which brings the advantage of imine stabilization via iminoboronate formation and potential antifungal activity due to the presence of boric acid residue. FTIR and NMR spectroscopy indicated that the gelling process took place by chemo-physical crosslinking forming a dual iminoboronate-chitosan network. Further, X-ray diffraction demonstrated a three-dimensional nanostructuring of the iminoboronate network with consequences on the micrometer-scale morphology and on the improvement of mechanical properties, as demonstrated by SEM and rheological investigation. The hydrogels proved strong antifungal activity against Candida planktonic yeasts and biofilms, promising to be a friendly treatment of the recurrent vulvovaginitis infections.

Free-radical ring-opening polymerization of 2-(o-chlorophenyl)-4-methylene-1,3-dioxolane

Abstract2-(o-chlorophenyl)-4-methylene-1,3-dioxolane was synthesized and polymerized in presence of AIBN or by use of benzoin methyl ether as photosensitizer. Based on the analysis of the resulting polymers, a polymerization mechanism was proposed.