Polymer Volume Fraction Research Articles

Evolution process of polymethacrylate hydrogels investigated by rheological and dynamic light scattering techniques

Evolving gel formation processes of colloidal hydrogels derived from high solid-content polymethacrylate microemulsions have been studied using rheological and dynamic light scattering (DLS) techniques. The critical gel points, evaluated through oscillatory shear measurement and characterized by a single power-law exponent of G′ and G″ on the frequency (the power-law exponent, n = 0.36–0.65) were influenced more by the volume fraction of polymers than by the gelling temperature. Further topological fractal analysis and rheological studies suggested that reaction-limited cluster aggregation (RLCA) may dominate the aggregation behavior of polymer nanoparticles with the fractal dimension ( d f) = 1.8–2.2. The DLS results showed a fast relaxation mode with a single exponential decay followed by a slow one with a stretched exponential decay, which indicated that the gradual incorporation of polymer nanoparticles or clusters into the gel network leads to the maturity of the homogenous hydrogel network. Comparative studies on a series of polymethacrylate hydrogel systems with different hydrophilicities demonstrated that a slight hydrophilicity favors gel formation. The gelation mechanism was discussed based on the in-situ aggregation of nanoparticles as well as in terms of the colloidal instability of high solid-content nanolatex.

Evaluation of the polymer–solvent interaction parameter χ for the system cured polybutadiene rubber and toluene

One of the most popular cured rubbers used in industrial applications is polybutadiene rubber (BR) and frequently its network structure, in the cured state, is studied by means of swelling in a solvent. Normally, toluene is used as solvent in this test. In order to estimate the crosslink density from the equilibrium volume fraction of rubber in the swollen state, the correct evaluation of the Flory–Huggins interaction parameter χ is necessary. This work covers the swelling behaviour of two types of cured BR in toluene. The rubbers used have sulphur and accelerator in their composition and were vulcanized at 433 K at different times in order to obtain several network structures and crosslink densities. The crosslink densities were evaluated from uniaxial stress-strain data measured at room temperature and analysed in the frame of the conformational tube model. Using this information and the swelling measurements in the frame of the Flory–Rehner relationship, a relationship between χ and v r, the polymer volume fraction at equilibrium (maximum) degree of swelling was established for the cured BR/toluene system. It was found that χ is not a constant but depends on the crosslink density of the sample.

SANS and SLS Studies on Tetra-Arm PEG Gels in As-Prepared and Swollen States

A series of model networks consisting of polyethylene glycol (PEG), tetra-PEG gels, have been prepared and their structure and dynamics have been investigated by small-angle neutron scattering (SANS) and static light scattering (SLS). The Tetra-PEG gels were prepared by cross-end coupling of two types of tetra-arm PEG macromers with molecular weights, Mw, of (5 to 40) × 103 g/mol. In the SANS regime, the structure factors of both as-prepared and swollen gels can be represented by Ornstein−Zernike-type scattering functions and superimposed to single master curves with the reduced variables, ξq and I(q)/ϕ0ξ2, irrespective of the molecular weight of tetra-PEG, where q, ξ, I(q), and ϕ0 are the magnitude of the scattering vector, the correlation length, the scattering intensity, and the polymer volume fraction at preparation, respectively. In the SLS regime, however, a power-law-type upturn was observed, indicating the presence of PEG chain clusters. Interestingly, these inhomogeneities disappear by swelling. ...

Unexpected Scaling Behavior of the Order−Disorder Transition of Nearly Symmetric Polystyrene-block-polyisoprene Diblock Copolymers Dilated with a Nominally Neutral Solvent at Very Low Dilution

The scaling behavior of the order−disorder transition (ODT) temperature for several nearly symmetric P(S-b-I) copolymers has been studied over a polymer volume fraction range of 0.9 ≤ ϕdiblock ≤ 1....

Hindered diffusion of oligosaccharides in high strength poly(ethylene glycol)/poly(acrylic acid) interpenetrating network hydrogels: Hydrodynamic vs. obstruction models

Diffusion coefficients of small oligosaccharides within high strength poly(ethylene glycol)/poly(acrylic acid) interpenetrating network (PEG/PAA IPN) hydrogels were measured by diffusion through hydrogel slabs. The ability of hindered diffusion models previously presented in the literature to fit the experimental data is examined. A model based solely on effects due to hydrodynamics is compared to a model based solely on solute obstruction. To examine the effect of polymer volume fraction on the observed diffusion coefficients, the equilibrium volume fraction of polymer in PEG/PAA IPNs was systematically varied by changing the initial PEG polymer concentration in hydrogel precursor solutions from 20 to 50 wt./wt.%. To examine the effect of solute radius on the observed diffusion coefficients, solute radii were varied from 3.3 to 5.1 Å by measuring diffusion coefficients of glucose, a monosaccharide; maltose, a disaccharide; and maltotriose, a trisaccharide. Both the hydrodynamic and obstruction models rely on scaling relationships to predict diffusion coefficients. The proper scaling relationship for each of the hindered diffusion models is evaluated based on fits to experimental data. The scaling relationship employed is found to have a greater significance for the hydrodynamic model than the obstruction model. Regardless of the scaling relationship employed, the obstruction model provides a better fit to our experimental data than the hydrodynamic model.

Static Structures and Dynamics of Hemoglobin Vesicle (HbV) Developed as a Transfusion Alternative

Hemoglobin vesicle (HbV) is an artificial oxygen carrier that encapsulates solution of purified and highly concentrated (ca. 38 g dL(-1)) human hemoglobin. Its exceptionally high concentration as a liposomal product (ca. 40% volume fraction) achieves an oxygen-carrying capacity comparable to that of blood. We use small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) to investigate the hierarchical structures and dynamics of HbVs in concentrated suspensions. SAXS data revealed unilamellar shell structure and internal density profile of the artificial cell membrane for Hb encapsulation. The SAXS intensity of HbV at scattering vector q > 0.5 nm(-1) manifests dissolution states of the encapsulated Hbs in the inner aqueous phase of the vesicle having ca. 240 nm diameter. The peak position as well as the height and width of static structure factor of Hb before and after encapsulation are almost identical, demonstrating the preserved protein-protein interactions in the confined space. To overcome multiple scattering from turbid samples, we employed thin layer-cell DLS combined with the so-called bruteforce and echo techniques, which allows us to observe collective diffusion dynamics of HbVs without dilution. A pronounced slowdown of the HbV diffusion and eventual emergence of dynamically arrested state in the presence of high-concentration plasma substitutes (water-soluble polymers), such as dextran, modified fluid gelatin, and hydroxylethyl starch, can be explained by depletion interaction. A significantly weaker effect of recombinant human serum albumin on HbV flocculation and viscosity enhancement than those induced by other polymers is clearly attributed to the specificity as a protein; its compact structure efficiently reduces the reservoir polymer volume fraction that determines the depth of the attractive potential between HbVs. These phenomena are technically essential for controlling the suspension rheology, which is advantageous for versatile clinical applications.

Compression elastic modulus of neutral, ionic, and amphoteric hydrogels based on N‐vinylimidazole

AbstractSeveral hydrogels of N‐vinylimidazole and sodium styrenesulfonate have been prepared by radical cross‐linking copolymerization in aqueous solution, using N,N′‐methylene‐bisacrylamide as crosslinker. Depending on composition, these hydrogels were neutral, amphoteric, cationic or anionic. Compression‐strain measurements were performed on samples as‐synthesized and swollen in deionized water or in acid aqueous solutions, with and without salt. It was thus found that the cross‐linking densities determined by compression measurements on as‐synthesized samples are in good accordance with those calculated by means of the model of polymer networks with pendant vinyl groups. A non‐Gaussian parameter (β) was introduced to explain that the elastic moduli (G) of samples swollen at equilibrium are larger than predicted by the Gaussian model. The β values of the neutral or ionized systems increase with swelling and fall into a single curve, which denotes a common behavior. Swelling has two opposite effects on G; on the one hand G decreases because the polymer volume fraction diminish and the system shifts from the affine limit to the phantom one; on the other, β increases and contributes to increasing G. The balance of those two opposite effects determines the variation of G with swelling. The possible contribution of ionic crosslinks to νe for the polyampholyte and for the polycation wearing divalent counteranions was discussed. A peculiar system is poly(sodium styrenesulfonate), whose cross‐linking density is much lower than expected. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1078–1087, 2009

Structure of an Adsorbed Polyelectrolyte Monolayer on Oppositely Charged Colloidal Particles

An adsorbed layer of a cationic polyelectrolyte, poly(diallyldimethyl-ammonium) chloride (PDADMAC) on negatively charged colloidal latex particles was investigated by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). SANS gives a layer thickness of 8 +/- 1 A and a polymer volume fraction of 0.31 +/- 0.05 within the film. DLS gives a somewhat larger thickness of 18 +/- 2 A, and the discrepancy is likely due to the inhomogeneous nature of the layer and the existence of polymer tails or loops protruding into solution. These results show that a highly charged polyelectrolyte adsorbs on an oppositely charged colloidal particle in a flat configuration due to the attractive forces acting between the polyelectrolyte and the substrate.

Effect of synthesis composition on the swelling pressure of polymeric hydrogels

This paper focuses on the determination of the swelling pressure of polymeric hydrogels by variation of the composition of the monomer mixture used for synthesis via free radical polymerization. The method used is based on the principle of determining the swelling pressure at a given final volume of swelling. Data for different types of low swollen hydrogels based on weakly crosslinked poly(acrylic acid)/poly(sodium acrylate) copolymers are presented for swelling in deionised water. For polymer volume fractions ϕP between 0.03 and 0.30, swelling pressures p in the range of 0.20–4.23MPa were obtained. No effect on the swelling pressure could be found by variation of monomer concentration and crosslinker content. In contrast, the content of sodium acrylate had an effect on the swelling pressure. The results are discussed based on the effect of counterion condensation.

Discrimination of the roles of crosslinking density and morphology in the swelling behavior of crosslinked polymers: Poly(N‐vinylimidazole) hydrogels

AbstractThe degree of swelling (S) of crosslinked polymers depends on several variables whose individual contributions are difficult to be distinguished. The aim of this work is to determine the contributions of the morphology to S, discriminating them from the effect of the crosslinking density. Under usual conditions of synthesis, these two characteristics vary simultaneously but here, several samples of chemically crosslinked poly(N‐vinylimidazole) having the same permanent crosslinking density (determined by DSC) and different porous morphologies (determined by SEM) were prepared. It was thus found that the variation of S with porosity, keeping constant the rest of variables, is strongly solvent dependent. Swelling in methanol is almost constant whereas for ethanol, deionized water, and aqueous HCl solutions, S depends on morphological features in a different way for each medium. It was concluded that S increases with inherent porosity and solvent effects arise from the counterbalance of such increment with the dependence of the polymer‐solvent interaction parameter on the polymer volume fraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Characterization of PMMA Composite Dispersed Aluminum Powder Prepared by Solution-Casting to Utilize for Automobile Body Applications

In order to utilize for automobile body applications, poly-methyl methacrylic acid (acrylic resin, PMMA) composites dispersed by Al-powders was prepared by solution-casting, which could easily control the dispersion ratio. Although the Al addition gradually decreased the impact value, it radically enhanced the electrical conductivity at the critical volume fraction in polymer. The jump of electrical conductivity was observed at the critical volume fraction of the Al addition. Based on the percolation theory, the result was explained.

Cavitation and fracture behavior of polyacrylamide hydrogels

The mechanical properties of gels present qualitatively contradictory behavior; they are commonly soft but also notoriously brittle. We investigate the elasticity and fracture behavior of swollen polymer networks using a simple experimental method to induce cavitation within a gel and adapt scaling theories to capture the observed transition from reversible to irreversible deformations as a function of polymer volume fraction. It is shown quantitatively that the transition from reversible cavitation to irreversible fracture depends on the polymer volume fraction and an initial defect length scale. The use of cavitation experiments permits characterization of network properties across length scales ranging from µm to mm. We anticipate that these results may significantly enhance the understanding of mechanical properties of soft materials, both synthetic and biological.

Mechanically Reinforcing Polyacrylate/Polyacrylamide Hydrogels through the Addition of Colloidal Particles

AbstractPolyacrylamide is a popular material for many bio-related applications ranging from electrophoretic separation to cellular supports. A limitation of polyacrylamide-based hydrogels, however, is their mechanical compliance. The current study examines the effect of colloidal particles as a reinforcing filler phase to enhance the mechanical stiffness of polyacrylamide-polyacrylate hydrogels. Measurements with oscillatory rheology show that for a fixed polymer volume fraction, the presence of colloidal particles with various surface modifications generally results in an increase of the shear storage modulus of the hydrogel-particle composite. Interestingly, this study indicated that no discernable trends can be linked between the values of the shear storage modulus and the particle surface characteristics.

Structure Analyses of Swollen Rubber-Filler Systems by Using Contrast Variation SANS

The polymer layers adsorbed on silica particles in rubber−silica systems have been investigated with the contrast variation small-angle neutron scattering (SANS) method. The scattering intensities of specimens swollen by solvents with various scattering length densities were measured. The contrast variation SANS for the specimens yielded partial scattering functions: the scattering function for polymer−polymer correlation SPP(q), the scattering function for silica−silica correlation SSS(q), and the scattering function for polymer−silica correlation SPS(q). The analyses of SSS(q) explored the hierarchical structures formed by silica particles. The analyses of SPS(q) and SSS(q) clarified the existence of dense polymer layers around silica aggregates. Several characteristic parameters are estimated from the analyses, such as the size of aggregates, the thickness of layers, the volume fractions of polymer of layers and matrix, and the correlation length of the matrix network. The contrast variation SANS is found to be a powerful tool for the analyses of the structures of the rubber-filler systems.

Anomalous melting behavior of cyclohexane and cyclooctane in poly(dimethyl siloxane) precursors and model networks

AbstractBuilding on previous observations of anomalous melting behavior of solvents in polyisoprene, we have expanded our insight into the melting behavior of organic solvents in polymers and polymer networks through a calorimetric investigation of cylcohexane and cyclooctane in poly(dimethyl siloxane) (PDMS) precursors and model networks. The results are contrary to general expectations. Besides deviations between the predictions of the Flory‐Huggins model and observed melting point depression of the small molecule organics, it is found that the melting point depression of cyclohexane in model networks is lower than that in the uncrosslinked precursors and unaffected by the molecular weight between crosslinks, which is not consistent with the general observation that higher crosslinking density leads to greater melting point depression. We interpret the observed phenomenon in terms of phase separation. In the case of cyclooctane, it exhibits a double melting peak in the model networks with high molecular weight between crosslinks. Furthermore, the heats of fusion of both cyclohexane and cyclooctane decrease with increasing polymer volume fraction which violates the underlying assumption that the heat of fusion of solvent in the polymer is the same as that in the bulk for both the Flory‐Huggins model and the Gibbs‐Thomson equation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2779–2791, 2008

Universal Scaling of Linear and Nonlinear Rheological Properties of Semidilute and Concentrated Polymer Solutions

Using oscillatory and steady-shear rheometry on polystyrene (PS) solutions in tricresyl phosphate (TCP) with three nearly monodisperse molecular weights at six values of the reduced concentration c/ce, where ce is the entanglement concentration, we show that for each value of c/ce below 2.0 rheological functions are successfully superimposed using de Gennes “blob” scaling laws for the concentration-dependent plateau modulus GN0, and the equilibration time τe,scaling using the literature value of the solvent-quality exponent ν = 0.53 (Solomon, M. J.; Muller, S. J. J. Polym. Sci., Polym. Phys. 1996, 334, 181−192). However, once the polymer volume fraction exceeds the “swelling volume fraction” ϕs, above which the polymer takes a random walk configuration on all length scales even in a good solvent, this universal scaling breaks down and the polymer conformation appears to be governed by Colby−Rubinstein’s scaling laws for Θ solutions. We estimate that all polybutadiene solutions in phenyl octane (a good sol...

Polymer flexibility and turbulent drag reduction

Polymer-induced drag reduction is the phenomenon by which the friction factor of a turbulent flow is reduced by the addition of small amounts of high-molecular-weight linear polymers, which conformation in solution at rest can vary between randomly coiled and rodlike. It is well known that drag reduction is positively correlated to viscous stresses, which are generated by extended polymers. Rodlike polymers always assume this favorable conformation, while randomly coiling chains need to be unraveled by fluid strain rate in order to become effective. The coiling and stretching of flexible polymers in turbulent flow produce an additional elastic component in the polymer stress. The effect of the elastic stresses on drag reduction is unclear. To study this issue, we compare direct numerical simulations of turbulent drag reduction in channel flow using constitutive equations describing solutions of rigid and flexible polymers. When compared at constant phi r2, both simulations predict the same amount of drag reduction. Here phi is the polymer volume fraction and r is the polymer aspect ratio, which for flexible polymers is based on average polymer extension at the channel wall. This demonstrates that polymer elasticity plays a marginal role in the mechanism for drag reduction.

Light Scattering and Phase Separation Studies on Cyclohexane Solutions of Six-Arm Star Polystyrene†

Light scattering and phase separation experiments were performed for four six-arm star polystyrene (6SPS) samples with weight-average molecular weights Mw of 9.62×104 to 1.16×106 in cyclohexane below the theta temperature (34.5 °C). From the former experiment, the apparent second virial coefficient J was obtained as functions of the polymer volume fraction φ and temperature T, along with the spinodals. In the latter experiment, the concentrations of coexisting two phases were determined as functions of T. The critical point Tc determined from the coexisting curve was lower than that for four-arm star polystyrene (4SPS) when compared at the same Mw. As was the case for 4SPS and linear polystyrene in cyclohexane, J for 6SPS at each T in a region of large φ was represented by a universal function of φ/P0.1 regardless of P (the volume of the polymer chain relative to that of the solvent molecule), although it differed from the common function previously found for the other two types of polystyrene. It was concluded that the solubility of 6SPS higher than that of 4SPS in cyclohexane as indicated by the lower critical point is attributable to the chain-end effect.

Nanoparticle Stability in Semidilute and Concentrated Polymer Solutions

The wetting of PDMS-grafted silica spheres (PDMS- g-silica) is connected to their depletion restabilization in semidilute and concentrated PDMS/cyohexane polymer solutions. Specifically, we found that a wetting diagram of chemically identical graft and free homopolymers predicts stability of hard, semisoft, and soft spheres as a function of the bulk free polymer volume fraction, graft density, and the graft and free polymer chain lengths. The transition between stable and aggregated regions is determined optically and with dynamic light scattering. The point of demarcation between the regions occurs when the graft and free polymer chains are equal in length. When graft chains are longer than free chains, the particles are stable; in contrast, the particles are unstable when the opposite is true. The regions of particle stability and instability are corroborated with theoretical self-consistent mean-field calculations, which not only show that the grafted brush is responsible for particle dispersion in the complete wetting region but also aggregation in the incomplete wetting region. Ultimately, our results indicate that depletion restabilization depends on the interfacial properties of the nanoparticles in semidilute and concentrated polymer solutions.

Transport and structural analysis of molecular imprinted hydrogels for controlled drug delivery

Molecular imprinting provides a rational design strategy for the development of controlled release drug delivery systems. We demonstrate that imprinting a hydrogel network results in macromolecular memory for the template molecule, indicated by the two or more times greater partitioning into these networks as compared to non-imprinted networks. Partitioning of drug into networks synthesized from multiple functional monomers was 8 times greater than networks synthesized from single monomers. One-dimensional permeation studies showed that the gel with maximum incorporated chemical functionality had the lowest diffusion coefficient, which was one to two orders of magnitude lower than all other gels studied. All imprinted networks had significantly lower diffusion coefficients than non-imprinted networks, in spite of comparable mesh sizes and equilibrium polymer volume fractions in the swollen state, which to our knowledge, is the first time that such a study has been conducted in the literature. We propose the “tumbling hypothesis”, wherein a molecule tumbling through an imprinted network with multiple, organized functionalities and an appropriate mesh size, experiences heightened interactions with memory sites and shows delayed transport kinetics. Thus, the structural plasticity of polymer chains, i.e. the organization of functional groups into memory sites, may be responsible for enhanced loading and extended release.