Characterization of inhomogeneous polyacrylamide hydrogels

The physical and structural properties of acrylamide gels have been characterized by osmotic deswelling, mechanical compression, and x-ray scattering. These properties vary considerably with the concentration of the crosslinking agent bisacrylamide, at fixed total monomers concentration (10% wt/wt water). In particular, changes in the properties appear more prominent at a crosslinking level of about 5-6% (wt bisacrylamide/wt monomers). The compression modulus of as-prepared and swollen gels passes through a maximum at this level of crosslinking. The swelling pressure curves can be separated into osmotic and elastic contributions of the gel network. The elastic part exhibits similar behavior to the compression modulus. The scaling of the osmotic part with the gel concentration varies with the degree of crosslinking, changing from 2.33 to 3.09. This indicates that the solvent power of water decreases with increasing crosslinking level, towards Φ conditions. The scattering patterns from the gels have been analyzed as arising from additive contributions from a homogeneous gel matrix, and embedded heterogeneities having a higher crosslinking density. These heterogeneities become much more prominent at the same level of crosslinking about 5-6%. Hysteresis observed in the sorption/desorption behavior of polyacrylamide gel suggests that further irreversible structural changes may occur at water activities lower than probed by osmotic deswelling. © 1992 John Wiley & Sons, Inc.     

Osmotic deswelling of weakly charged poly(acrylic acid) solutions and gels

The osmotic pressure of weakly charged aqueous poly(acrylic acid) (PAA) solutions and the swelling pressure PAA gels were studied by osmotic deswelling at different degrees of ionization (α). In solution, the osmotic pressure was found to scale linearly with concentration, whereas the scaling power of the swelling pressure of gels was higher (1.66). The effect of the ionization degree on the osmotic coefficient in PAA solutions was in agreement with the theory of Borue and Erukhimovich [Macromolecules, 21 , 3240 (1988)]. Ionization increases the swelling capacity of the PAA gels until a plateau is reached at about 35% neutralization. The concentration at equilibrium swelling scales as C_e ～ α^?0.6. The contribution of the network to the gel swelling pressure is evaluated by subtracting the osmotic pressure of the polymer solution at the same concentration and degree of ionization. In swollen gels the extended network opposes swelling. As the gel is osmotically deswelled, a state of zero network pressure exists at a certain concentration, below which the network elasticity favors swelling. The crossover concentration shifts to lower values as the degrees of ionization increases. © 1995 John Wiley & Sons, Inc.     

Microstructure and rheology of stimuli-responsive microgel systems--effect of cross-linked density

The effect of cross-linked density on the rheological behavior of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with di-allyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counter-ions trapped in the polymeric network against the ions in bulk solution, which is responsible for the swelling and increase in viscosity. The viscosity exhibits a maximum at approximately 1 wt.% DAP and it decreases to a steady value at 4 wt.% DAP, which is independent of pH and particle concentrations. Static light scattering results confirmed this optimum density as the critical point where sufficient cross-link points are present to produce permanent junctions that permit optimal swelling of the microgel particles. In addition, the variation of relative swelling with cross-linked densities of our model microgel systems agrees with the theoretical scaling law, Q alpha (yalphaN(x))(3/2) for cross-linked densities beyond this optimum point (Q is the swelling ratio, y is the acidic MAA content, N(x) is the average number of monomer units between two cross-linked points, and alpha is the degree of neutralization). By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.     

Swelling,elasticity and structure of hydrogels prepared via bis-macromonomers of poly(ethylene oxide)

The effect of swelling on the shear modulus was studied for hydrogels prepared by radical polymerization of methacrylate-terminated poly(ethylene oxide) (PEO) bis-macromonomers of different molecular weight. Gels made of long chains (M = 12000 or 6000) display classical softening upon swelling, whereas gels made of shorter chains (M = 4000 or 2000) remain rigid or even stiffen. The abnormal behaviour is explained by a specific character of network junctions presented by polymethacrylate chains in which each unit is linked with a PEO network chain. It is assumed that the interactions among densely grafted PEO chains result in their stretching on polymerization and non-affine deformation on swelling, which stiffen the gel. This is verified by the data on copolymer (macromonomers - 2-hydroxyethyl methacrylate) gels that have lesser densities of PEO chains attached to the junctions and show weaker stiffening on swelling. The osmotic pressure of gels was estimated from the swelling pressure and shear modulus. Similar to the mixing pressure of equivalent PEO solutions, it varies as the 9/4 power of polymer concentration. At the same time, it is lower than the mixing pressure. This indicates that the junctions make only quantitative changes in the osmotic properties of macromonomer chains.     

The interaction parameter of crosslinked networks and star polymers

Recent results on blends containing star polymers have revived the interest on the interaction parameters of structures that contain junctions between chains, a matter which can be connected with the earlier studies on the influence of crosslinks on the interaction parameters of polymer networks and gels. Here, we review results on crosslinked networks and star polymer solutions together with the more recent work on star polymer blends. The review covers swelling and elastic deformation of gels, differential vapour sorption between crosslinked and uncrosslinked polymers, osmotic equilibrium of gels and of star polymer solutions, and neutron scattering of polymer blends containing star polymers. In the systems reviewed, the interaction parameters of stars and networks differ from those of linear chains, and the difference is attributed mainly to entropic effects.     

Unique size-change behavior of photo-crosslinked cinnamic acid derivative nanoparticles during hydrolytic degradation

A unique size change of photo-crosslinkable poly[(3,4-dihydroxycinnamic acid)-co-(4-hydroxycinnamic acid)] nanoparticles was observed during hydrolytic degradation depending on the crosslinking degree. The diameter of uncrosslinked nanoparticles decreased from 850 to 300 nm during hydrolysis, whereas that of 75% crosslinked nanoparticles increased from 700 to 950 nm. The diameter changes of crosslinked nanoparticles during hydrolysis might be induced by swelling of the crosslinked networks depending on the crosslinking degree. Moreover, the diameter of the uncrosslinked nanoparticle recovered by additional UV irradiation during hydrolysis. These results suggested that the diameter of the nanoparticles could be controlled even during hydrolysis by UV irradiation.     

Osmotic compression of mixtures of polymers and particles

Aqueous dispersions of nanometric ceria particles have been concentrated through osmotic stress. Mixed dispersions of ceria with small adsorbing macromolecules of poly (vinylpyrrolidone) have been prepared by the same method. The osmotic pressure of pure ceria dispersions results from electrostatic repulsions between particles. The osmotic pressure of dispersions containing a non-saturating amount of PVP is the same as that of pure dispersions, and the colloidal stability is depressed with respect to the pure dispersions. The osmotic pressure of dispersions containing an excess of PVP is increased by the free macromolecules, and the colloidal stability is enhanced. The organization of particles in these dispersions has been examined by small-angle x-ray scattering and cryotransmission electron microscopy. In pure ceria dispersions and in saturated dispersions, a liquid-like short-range order was found; when the concentration is increased, this short-range order follows a three-dimensional swelling law. In dispersions containing a non-saturating amount of PVP, the structure shows an alternance of clusters and voids, and the separations of clusters follow an unusual one-dimensional swelling law.     

Swelling of dextran gel and osmotic pressure of soluble dextran in the presence of salts

The swelling of dextran gels (Sephadex) in salt solutions with a water activity of 0.937, compared with the swelling in pure water, exhibited anion specificity as evidenced by an increased swelling ratio in the following order: Na₂SO₄ < H₂O < NaCl < NaSCN. The swelling ratio showed a good linear correlation with the osmotic pressure of dextran (500 kD) in these solutions. The salt‐concentration difference (imbalance) between the polymer‐solution side of the membrane and the polymer‐free permeate side during the osmotic‐pressure measurements positively correlated with the effect of the salt on the polymer osmotic pressure. These phenomena conform to Hofmeister‐type (or lyotropic) behavior. The diminishing augmentation of dextran osmotic pressure and the change in the salt‐concentration imbalance with rising NaSCN concentration imply a positive preferential interaction and adsorption of the salt onto the polymer. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2740–2750, 2001     

Effect of added free polymer on the swelling of neutral microgel particles: a thermodynamic approach

Microgel particles based on poly (N-isopropylacrylamide) have been shown to display an initial swelling behavior, followed by a collapse, with increasing concentration of added poly(ethylene oxide), PEO, chains. This paper considers the thermodynamic reasons for the observed expansion and subsequent shrinkage of the particles. At low concentrations of PEO, the free chains permeate into the microgel particles and cause an increase in osmotic pressure, expanding the particles. At higher concentrations of PEO, the particles are saturated and an increase in osmotic pressure in the external phase causes the particles to collapse again. The calculated magnitude of swelling and the effect of PEO molecular weight are, at least qualitatively, in agreement with the experimental observations reported elsewhere.     

Sol/gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 1. Features of the i/n transition

The phase behavior of a natural nontronite clay was studied for size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, and rheological experiments. In parallel, the positional and orientational correlations of the particles were analyzed by small-angle X-ray scattering. Aqueous suspensions of nontronite exhibit a true isotropic/nematic (I/N) transition that occurs before the sol/gel transition, for ionic strengths below 10(-3) M/L. In this region of the phase diagrams, the system appears to be purely repulsive. The I/N transition shifts toward lower volume fractions for increasing particle anisotropy, and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic, and gel phases. The swelling law (separation distance vs swelling law) exhibits two regimes. For high volume fractions, the swelling law is one-dimensional as in layered systems and reveals the presence of isolated platelets. At lower volume fraction, distances scale as phi(-1/3), indicating isotropic volumic swelling. Finally, the experimental osmotic pressure curves can be satisfactorily reproduced by considering the interparticle distances between two charged planes whose effective charge is around 10% of the structural charge.     

Microgels - colloidal models for the cross-linked state

This article describes the synthesis of various types of microgels with different cross-linking topologies. The swelling data of the standard systems can be fitted with non-affine theories of rubber elasticity. Light scattering shows the existence of long-range swelling heterogeneities. The dynamic mechanical behaviour shows some peculiarities, e.g. small spherical microgels show viscous flow. Besides statistically cross-linked systems, critically cross-linked as well as macroporous systems can be made. The critical systems exhibit a fractal structure scaling which depends on synthetic conditions. This dependence agrees well with the different dynamic percolation theories. On the other hand, no clear relation between structure scaling and the observed dynamic behaviour can be found. A qualitative scheme which explains various effects observed during cross-linking of microgels is presented.     

Osmotic compression and expansion of highly ordered clay dispersions

Aqueous dispersions of nanometric clay platelets (Laponite) have been dewatered through different techniques: centrifugation, mechanical compression, and osmotic stress (dialysis against a polymer solution). The positional and orientational correlations of the particles have been determined through small-angle neutron scattering. Uniaxial compression experiments produce concentrated dispersions (volume fraction > 0.03) in which the platelets have strong orientational and positional correlations. The orientational correlations cause the platelets to align with their normal along a common axis, which is the axis of compression. The positional correlations cause the platelets to be regularly spaced along this direction, with a spacing that matches the average volume per particle in the dispersion. The swelling law (volume fraction versus separation distance) is one-dimensional, as in a layered system. Changes in the applied osmotic pressure cause the water content of the dispersion to either rise or decrease, with time scales that are controlled by interparticle friction forces and by hydrodynamic drag. At long times, the dispersions approach osmotic equilibrium, which can be defined as the common limit of swelling and deswelling processes. The variation of the equilibrium water content with the applied osmotic pressure has been determined over 1 decade in volume fractions (0.03 < phi < 0.3) and 3 decades in pressures. This equation of state matches the predictions made from the knowledge of the forces and thermal agitation for all components in the dispersion (particles, ions, and water).     

Theoretical analysis of sorption of a binary solvent in a polymer phase. II. Occurrence of maxima and minima in the curve of total sorption

It has been shown that sorption equilibrium in the system polymer–mixed solvent and the total sorption as one of its manifestations can be derived from osmotic equilibrium. The equilibrium relation for the dependence of the osmotic pressure II (as the potential for total sorption) on the composition of the ternary polymer-containing phase also includes the preferential sorption ε. This complication leads to difficulties in the analysis of the dependence of II on composition: for this reason, analysis has been limited to the case of a very dilute polymer phase. If preferential sorption is neglected, the occurrence of an extremum on the curve representing the dependence of the total sorption on the composition of the mixed solvent is affected by the Flory-Huggins parameters as in the occurrence of an inversion in preferential sorption, with the exception that in the former case the difference in the molar volumes of the solvent components contributes a term smaller by a factor of two. In the general case, however, the contribution of preferential sorption to the osmotic pressure (which has always a negative sign) also plays its role. Consequently, while total sorption, at a low preferential sorption and at a large positive value of the solvent–solvent interaction parameter χ₁₂, can exhibit a maximum, a larger preferential sorption can overcompensate the effect of the parameter χ₁₂, so that a minimum appears. The coexistence of a minimum with a maximum on the same curve is possible in some cases with positive χ₁₂. The latter theoretical predictions have not yet been confirmed experimentally. At a negative χ₁₂ the extremum is always a minimum.     

Swelling determination of W/O/W emulsion liquid membranes

Based on the definition of swelling in emulsion liquid membrane (ELM) process, the concepts of apparent swelling and actual swelling are proposed to illustrate the relationship between the emulsion swelling (the osmotic swelling and entrainment swelling) and the membrane breakage, focussing on the effect of the volume change caused by emulsion swelling and membrane breakage on the experimental results. Theoretical analyses indicate that “zero” or “negative” swelling may occur under certain experimental conditions. A bi-tracer method is further proposed and then used to measure the osmotic, entrainment swelling and the membrane breakage simultaneously, only requiring some initial operation conditions and measurements of concentrations of both tracers in the external and internal phases. It has been experimentally proved that this new method is highly applicable in ELM process and provides a useful tool to specify the effects of membrane breakage, osmotic swelling and entrainment swelling in the same experiment. “Negative” swelling may occur under certain operating conditions, particularly when the electrolyte concentration in the external phase is higher than that in the internal phase. The experimental results also indicate that the effect of membrane breakage on the measurement of emulsion swelling should not be neglected to avoid measurement error. Polymeric surfactant LMA is superior to other commercial surfactants as it imparts high membrane stability and small emulsion swelling.     

W／O／W乳液的渗透溶胀与夹带溶胀

研究了W/O/W乳液的溶胀,实验结果表明,渗透溶胀随内外相溶液间的渗透压差、表面活性剂及载体浓度的增大而增加,但随膜粘度的增加而降低,渗透压差较高时,水渗透的影响大于夹带的影响;膜相中含氧化合物对溶胀的影响大于含氮化合物,采用Span 80作乳化剂时,比采用E 644渗透溶胀约高6倍,夹带溶胀也较高;重复聚结再分散使夹带溶胀急剧增加,因而多级混合澄清槽对液膜操作似不适用。     

Swelling pressure and elastic behavior of polymacromonomer networks with different functionalities of junctions

Networks with different junction functionalities as obtained by polymerization of a macromonomer (composed of 20 units) at identical concentrations were studied by computer simulation. The functionality determined by the length of chains produced from the end units of the macromonomer was varied over a wide range by varying the kinetic parameters of polymerization. From the number of collisions of units with the lattice walls at different swelling stages, the network swelling pressure and the osmotic pressure of solution of its fragments obtained by cutting in half interjunction chains were determined. From these data, the osmotic and elastic components of swelling pressure were found, the former was defined as the pressure of solution and the latter was defined as the difference of the network and solution pressures. The osmotic component is a power function of the polymer concentration with the power index increasing from 2.7 to 3.9 with an increase in functionality from 4.8 to 55 in accordance with a change in network topology. The elastic pressure depends on the swelling ratio Q in different manners at a low and a high functionality of junctions. Its absolute value decreases with a growth in Q in the former case, in agreement with the theory of elasticity of phantom networks (～Q ^?1/3), but increases in the latter case. This behavior is consistent with the effect of functionality on the elastic behavior of real polymacromonomer networks and confirms that differences in the character of change in the modulus of such networks during swelling are due to a difference in the functionality of their junctions. Possible mechanisms of the influence of multifunctional junctions on the elasticity of polymer networks are discussed.     

Temperature and crosslinking effects on the swelling of poly(isoprene) in isopiestic conditions

Isopiestic measurements of solvent uptake have been made in the synthetic isoprene/benzene system for both crosslinked and uncrosslinked polymers in order to revisit the question of the swelling activity parameter S. Both the nonzero value of S at zero swelling and the appearance of a peak in S vs. degree of swelling have been observed in some solvent/rubber pairs and we here investigate both the crosslink and temperature dependencies of these phenomena. The data analysis is an extension of prior work from this laboratory using continuum thermodynamics concepts and avoiding molecular models in an attempt to establish the fundamental phenomenology of the process and the validity or lack of validity of the hypothesis that the mixing and elastic contributions to the free energy of networks are separable. We present results from measurements in benzene vapors at temperatures between 10 and 55°C for an uncrosslinked rubber and rubbers crosslinked with 1, 5, 10, and 15 parts per hundred dicumyl peroxide. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 817–826, 1997     

On the Modeling of Polyelectrolyte Gels

Summary: Ionic polymer gels are very attractive actuation materials with a great similarity to biological contractile tissues. They consist of a polymer network with bound charged groups and a liquid phase with mobile ions. Absorption and delivery of solvent lead to a considerably large change of volume. This swelling mechanism results from the equilibrium of different forces such as osmotic pressure forces, electrostatic forces and viscoelastic restoring forces and can be triggered by chemical (change of salt concentration or pH in the solution), thermal or electrical stimulation. In the present work, chemically and electrically stimulated electrolyte polymer gels in a solution bath are investigated. To describe the different phenomena occurring in these gels adequately, the modeling can be conducted on different scales. If only the global macroscopic behavior is of interest, the statistical theory which is capable to describe the global swelling ratio, is sufficient. By refining the scale, the mesoscopic coupled multi-field theory can be applied. Here, the chemical field is described by a convection-diffusion equation for the different mobile species. The electric field is directly obtained by solving the Poisson equation in the gel and solution domain. The mechanical field is formulated by the momentum equation. By further refining the scale, the whole structure can be investigated on the microscale by the discrete element (DE) method. In this model, the material is represented by distributed particles comprising a certain amount of mass; the particles interact with each other mechanically by a truss or beam network of massless elements. The mechanical behavior, i.e. the dynamics of the system, is followed by solving the Newton's equations of motion while the chemical field, i.e. the ion movement inside the gel and from the gel to the solution, is described by diffusion equations for the different mobile particles. All three formulations can give chemical, (electrical) and mechanical unknowns and all rely on the assumption that the concentration differences between the different regions of the gel and between gel and solution form the osmotic pressure difference, which is a main cause for the mechanical deformation of the polyelectrolyte gel film.     

A New Gibbs Energy Model for Obtaining Thermophysical Properties of Aqueous Electrolyte Solutions

In this paper, a new Gibbs energy model is proposed to study the thermophysical properties of aqueous electrolyte solutions at various temperatures. The proposed model assumes that the electrolytes completely dissociate in solution. The model also has two temperature-independent adjustable parameters that were regressed using experimental values of the mean ionic activity coefficients (MIAC) for 87 electrolyte solutions at 298.15 K. Results from the proposed model for the MIAC were compared with those obtained from the E-Wilson, E-NRTL, Pitzer and the E-UNIQUAC models, and the adjustable model parameters were used directly to predict the osmotic coefficients at this temperature. The results showed that the proposed model can accurately correlate the MIAC and predict the osmotic coefficients of the aqueous electrolyte solutions better on the average than the other models studied in this work at 298.15 K. Also, the proposed model was examined to study the osmotic coefficient and vapor pressure for a number of aqueous electrolyte solutions at high temperatures. It should be stated that in order to calculate the osmotic coefficients for the electrolyte solutions, the regressed values of parameters obtained for the vapor pressure at high temperatures were used directly. The results obtained for the osmotic coefficients and vapor pressures of electrolyte solutions indicate that good agreement is attained between the experimental data and the results of the proposed model. In order to unequivocally compare the results, the same experimental data and same minimization procedure were used for all of the studied models.     

同轴静电纺丝法制备具有核-壳纤维结构的多功能敷料

利用同轴静电纺丝制备了具有核壳结构纳米纤维的未交联敷料,其中纤维内核为载有抗菌药物莫匹罗星的聚己内酯(PCL),外壳则由载有麻醉剂利多卡因的胶原构成;通过京尼平将胶原外壳交联后得到交联敷料.用扫描电子显微镜(SEM)和透射电子显微镜(TEM)观察了未交联敷料的表面形貌和纤维的核壳结构.体外药物释放实验结果表明,在2种敷料中,2种药物在1 h内均出现了突释现象,而在随后的60 h中,2种药物均能从敷料中缓慢释放出来,说明2种敷料均具有较好的持续止痛与抗菌性能.二辛可宁酸(Bicinchonininc acid,BCA)蛋白测试结果表明,未交联敷料外壳上的胶原蛋白能够持续地释放出来.体外细胞培养结果表明,与交联敷料相比,未交联敷料能够更好地促进成纤维细胞L929的黏附和生长,具有更好的促进伤口愈合作用.体外抗菌实验结果显示,负载了莫匹罗星的2种敷料的抗菌性能均明显高于对照组,具有良好的抗菌性能.