Rheological investigation on the associative properties of poly(vinyl alcohol) solutions (original) (raw)
Related papers
Macromolecules, 2014
Bulk and solution studies revealed a strongly pronounced effect of chain structure on the rheological and relaxation behavior of well-characterized vinyl acetate–vinyl alcohol copolymers of similar composition and polymerization degree. The frequency–temperature superposition principle is fully applicable to the random copolymers, which demonstrate all expected relaxation states, whereas a divergence of the reduced dynamic moduli–frequency dependences is observed for the multiblock copolymers. In the latter case, the terminal zone is sensitive to the self-assembling of vinyl alcohol blocks into (depending on the copolymer composition) crystalline or amorphous microstructures. The monomer unit distribution particularly affects properties of the copolymer solutions in N,N-dimethylformamide (DMF). 5% solutions behave as simple viscoelastic liquids at 20 °C and show viscoplastic behavior at −20 °C, where more blocky chains are characterized by up to 4 orders of magnitude higher yield stress values. The multiblock copolymer solutions demonstrate a pronounced viscosity hysteresis in the heating–cooling cycle, being absent in the random copolymers. 10% solutions of multiblock copolymers are practically gelatinous even at room temperature. The observed effects are explained by examining the peculiarities of hydrogen bonding in vinyl acetate–vinyl alcohol copolymers using FTIR spectroscopy. The multiblock copolymers are characterized by stronger hydroxyl–hydroxyl H-bonds and greater fraction of interchain hydroxyl–acetyloxy H-bonds providing aggregation of chains and high viscosity of the corresponding samples, whereas the random copolymers more strongly interact with the residual solvent. Dynamic light scattering studies prove that the relaxation of concentration fluctuations is completely diffusive, being bimodal in the random copolymers and trimodal in the multiblock ones. The fast mode in the latter case demonstrates anomalous concentration behavior. In the dilute regime, up to very low concentrations, multiblock copolymer chains form stable aggregates, and this fact correlates with an unusual growth of the reduced viscosity in the corresponding rheological experiments.
Rheological properties of binary associating polymers
Rheologica Acta, 2006
Dynamics of associating polymer solutions above the reversible gelation point are studied. Each macromolecule consists of a soluble backbone (B) and a small fraction of specific strongly interacting groups (A or C stickers) attached to B. A mixture of B-A and B-C associating polymers with 1:1 stoichiometric ratio is considered. As a result of AC association, the polymers reversibly gelate above the overlap concentration. It is shown that (1) the network strands are linear complexes (double chains) of B-A and B-C; (2) "diffusion" of the network junction points is characterized by an apparent activation energy, which can be significantly higher than the energy of one AC bond; (3) most importantly, the randomness of sticker distribution along the chain can significantly slow down the network relaxation leading to a markedly non-Maxwellian viscoelastic behavior. The theory elucidates the most essential features of rheological behavior of polysaccharide associating systems (with A = adamantyl moiety, C = β-cyclodextrin, B = either chitosan or hyaluronan) including similar behavior of G and G in a wide frequency range, strong temperature depen-This paper was presented at
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1996
The physical properties of hydrophobically modified polyurethane thickeners have been studied in solution. Such polymers form transient networks via association of their hydrophobic end groups. The rheological response of such systems is characterized by a Maxwellian viscoelastic response. It is shown that this is consistent with a picture in which stress relaxation occurs via disengagement of a chain end from an association followed by rapid Rouse relaxation of the stressed chain. This mechanism in principle allows the preparation of fluids with designed relaxation spectra. In this way it is possible to control the viscosity/shear-rate profile of a system thickened with such macromolecules. The concentration dependence of the shear modulus of such solutions can be described by a simple model in which there is an interplay between chains taking up loop and link configurations. A theory developed on such principles shows excellent agreement with experiment. This model is also shown to shed light on the interaction of associating polymers with surfactants. The phase behaviour of associating polymer mixed with unmodified homopolymer of the same monomer composition is also discussed. The observed phase diagram is well-described by a modified form of Flor~Huggins theory to account for the associations between the hydrophobic end groups of the associating polymer (AT) chains. It is argued that phase separation is driven by entropic rather than enthalpic considerations. This paper highlights the fact that many of the physical features of AT can be accounted for by a simple picture of the network topology combined with an understanding of the mechanism of stress relaxation. It is shown that the capability of association between chains has considerable implications regarding the physical response of polymer solutions.
Industrial & Engineering Chemistry Research, 2020
Molecular dynamics simulations are used to study structure and dynamics of poly(vinyl alcohol) and water in aqueous solution as a function ofconcentration at different temperatures in the range278-338 K.Simulations were performed using multiple oligomeric chains for facilitating inter-chain interactions as well as a direct comparison with experimental data. PVA chains fold and bundle up to form an aggregate in solution. The intermolecular spatial distributions show the structure of aggregate to be ordered. PVA chains show high tendency to form intra-chain hydrogen bonds between adjacent repeating units, instead of interchain Hbonds, indicating hydrophobic effect as the major driving force for aggregate formation. At all temperatures the conformations of a single PVA chain by itself in solution is unstable, going back and forth between extended and folded states. However, interchain interactions among PVA chains in the aggregate stabilize the folded conformation. Increase in temperature gives faster motions and increase in concentration results in slower dynamics.At higher concentration chains adopt a single folded state independent of temperature so that there is insignificant effect on R g .The competition between formation of various hydrogen bonds such as intra-chain, interchain, and PVA-water is the key to understand the solvation behavior of PVA.The activation energy for conformational transition between trans-gauche states of backbone dihedralsobtained from the simulations is 15.73 kJ/mol which is close to the value 13.4 kJ/mol obtained from experiments for 15 wt % PVA solution.Thehydrophobic effect rather than inter-chain PVA hydrogen bonding is the major driving force for aggregation of PVA in water.
In situ gelation of aqueous solutions of entangled poly(vinyl alcohol)
Soft Matter, 2013
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[h]. 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[h] 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.
The role of chain structure in the rheological behavior of vinyl acetate-vinyl alcohol copolymers
Polymer Science Series A, 2014
A comparative study of the viscoelastic properties of melts of vinyl acetate-vinyl alcohol copolymers with equimolar compositions characterized by different statistical distributions of chain units has been performed. It has been shown that the principle of temperature-frequency superposition is obeyed by copolymers close to a random copolymer, but is violated by copolymers with the block distribution of units. Unlike amorphous random copolymers, a multiblock copolymer is characterized by weak crystallinity, the absence of the relaxation flow state, and a more pronounced tendency to form interchain hydrogen bonds both between two hydroxyl groups and between hydroxyl and ester groups.
Journal of …, 2004
The rheological behavior of a model hydrophobically modified alkali soluble emulsion ͑HASE͒ polymer, comprised of a random copolymer backbone of methacrylic acid and ethylacrylate and pendant hydrophobic macromonomers, is examined in cosolvents of water and propylene glycol ͑PG͒ of different proportions. We find the solvent solubility parameter to have a direct impact on both the steady and dynamic behavior of the polymer solutions. In particular, scaling of the relative viscosity (rel) and the elastic modulus (G Ј)at a fixed frequency with the solvent solubility parameter (␦ s) reveals the presence of two distinct regimes with different dependences on ␦ s. In ''water-rich'' solvents, both rel and G Ј show a strong dependence on ␦ s in contrast to ''PG-rich'' solvents, in which there is slight or no dependence on ␦ s. The concentration dependences of both rel and G Ј are also found to be different in water-rich solvents from that in PG-rich solvents. In water-rich solvents, rel and G Ј reveal power-law dependences with exponents of 2.5 and 3.2, respectively, compared to exponents of 1.4 and 2.3 in PG-rich solvents. The different behavior in PG-rich solvents is ascribed to the presence of minimal hydrophobic associations, with the polymer behavior analogous to that of unmodified polymers without hydrophobes. This hypothesis is supported by the similarity in rel scaling with concentration observed for both the HASE polymer in PG-rich solvents and a similar polymer without the hydrophobes in both solvents. The lack of hydrophobic interactions in the PG-rich solvents may be attributed to the observed decrease in polymer coil dimension together with a lower tendency of the hydrophobes to form micelles in less polar media.
The Huggins viscosity coefficient of aqueous solution of poly(vinyl alcohol)
European Polymer Journal, 2001
Although water at room temperature is not a poor solvent for poly(vinyl alcohol) (PVA), the values of the Huggins viscosity coecient k H are typically higher than 1a2. This is attributed to association. Based on the model put forward by Peterlin, the k H values reported in the literature for aqueous solutions of PVA were examined and the equilibrium constant K Ã was estimated. It turned out that it increases with increasing molecular weight and depends on the content of residual acetate units. The eect of temperature seems to be weak.